Kinetics and Benefits of an Unique Assisted Free Bodyweight Squatting System: The DB Method^®TM

Gerard C. Gorniak

Anatomy & Biomechanics, The University of St Augustine for Health Sciences, St. Augustine, Florida, USA.

DOI: 10.4236/ojtr.2022.104016                

Abstract

Squatting movements are used frequently in the activities of daily living and squatting exercises are used to strengthen abdominal core and lower limb muscles. However, many individuals cannot perform a traditional squat. An alternative is to hire a physical trainer or coach for supervision which can be prohibitively expensive. The DB Method^® machine is unique and affordable. A product satisfaction descriptive survey to owners of The DB Method machine describes increased gluteal, core, pelvic floor and lower body strength, an improvement in gluteal region shape, and an increase in activity and energy level. A surprising result of the survey was that this DB Method also seemed to strengthen the pelvic floor muscles and, in some cases, to lessen urinary incontinence. In this article, the results of this survey are described as the kinetics of this machine relative to the lower limbs and the pelvic floor muscles. A comparison between traditional squats and The DB Method and the benefits of using The DB Method are discussed.

Keywords

Squats, Kinetics, Gluteus Maximus, Pelvic Floor, Posterior Pelvic Tilt, Functional Applications

1. Introduction

Squatting movements and the associated muscles actions are used in everyday activities, such as sitting and standing, crouching to pick up items off the floor or under a cabinet, going to the bathroom, stooping to hug a child, and during many recreational activities. As we age, the ability to squat is more difficult because of gluteal and quadriceps muscle weakness, limited joint movement, knee, hip and back pain and decreased balance [1] [2] [3] [4].

While there are many types of squats and squatting techniques that target the muscles of the lower limb, back, abdominal wall and pelvic floor muscles, choosing a type of squat can be confusing and even dangerous [5] [6] [7] [8] [9].

Squats are commonly categorized into: 1) traditional; 2) barbell loaded squats; 3) free standing bodyweight squats, 4) wide and narrow stance; and 5) unilateral ( [8] [10] - [20]; Table 1). The freestanding bodyweight includes assisted free body weight squats that use a machine such as The DB Method. The type of squat is chosen on the basis of training/rehabilitation goals, with each type of squat having different biomechanics that require varying degrees of mobility, strength, balance and coordination. While each squat will generally challenge muscles of the lower body, core and trunk, each variation will have certain emphases ( [8] [9] [11] [12] [14] [15] [18] [19] [21] - [31] Table 1).

The kinematics of proper squatting are complex and vary with the type of squat. If not properly performed squatting “can lead to a wide range of maladies, especially in combination with the use of heavy weights” [5] [8] [9] [16] [20]. These complexities can be especially challenging to home users who do not have the benefit of coaching or personal supervision to ensure correct form and safety, and to those with gluteal muscle and quadriceps weakness, lower limb joint and back pain, and balance problems. While many kinetic analyses have been done for traditional, barbell loaded and unilateral squats, the variations in body and weight positions, the depth and velocity of the squat, and the patterns of squatting movements, these analyses can be confusing and comparisons among these complicated [5] [9] [10] [12] [14] [15] [16] [19] - [38].

A high percentage of females suffer from disorders of the pelvis that have a major effect on a person’s quality of life [10] [39] - [50]. The literature reports that as many as 40% - 50% of the female population had episodes of urinary incontinence but less than half seek treatment. About 16% of males have pelvic dysfunction. There are many exercises to strengthen the pelvic floor muscles to treat urinary incontinence, including Kegel exercises, bridges, bird dog movement, marches, and diaphragmatic breathing [39] [51] - [71]. While squatting is associated mainly with muscle strengthening in athletes, weightlifters, recreational activities and in physical rehabilitation, studies show that modified squats and wall squats squatting can also be beneficial in those with pelvic floor dysfunction, especially urinary incontinence [51] [52] [56] [57] [66] [67] [72] [73] [74] [75] [76]. Studies have also described co-activation of the pelvic floor muscles and abdominal muscles as both are affected by internal abdominal pressure [44] [77] [78] [79] [80]. The kinetics of how squatting strengthens on the pelvic floor are unclear [41] [62] [74] [81].

In this article, the results of a multiple-choice descriptive owners’ product satisfaction survey taken by 191 users of The DB Method are reported. The kinetics of The DB Method assisted free bodyweight machine are described during the descent (eccentric phase) and ascent (concentric phase) of the squat. Based on the kinetics, the actions of the hip, knee and leg muscles are described, and

Table 1. Common types of squats and purpose.

The DB Method is compared to traditional squatting. Finally, a kinetic assessment of how The DB Method may strengthen the pelvic floor is presented and discussed.

2. Methods

2.1. Surveys

A descriptive product satisfaction survey of 18 multiple choice questions and 1 open question were emailed to owners of The DB method (inclusion criteria) using Monkey Survey. There were no exclusion criteria. Two questions asked age and gender, 2 questions asked on the use of The DB method, 2 questions about not using traditional squats, 10 questions on what they experienced using The DB method, 1 recommendation question and 1 question open to general comments. There were 191 respondents (a 2% return) and for each choice in a question the range, mean percentages and standard deviations calculated. For 1 question, the answers were ranked. The open-end questions were grouped and summarized.

2.2. Kinetic Analysis

A kinetic analysis of this assisted free weight machine, the DB Method (Figure 1), during the descent or eccentric phase and ascent or concentric phase determined from sagittal and frontal photographs during the start, mid squat, and deep (end) squat. This analysis included the Center of Mass (CM), Line of Gravity (LOG), Ground Reaction Forces (GRF), Center of Pressure (COP), Weight (resistance) Arm for the hip, knee and ankle, External Moments for the hip, knee and ankle, and the Lines of Actions of 17 hip, knee, and ankles muscles. The muscle lines of Action were used to assess the Internal Moments at each joint.

Because users described strengthening of the pelvic floor and some even the decrease and stoppage of urinary incontinence (Table 2), a kinetic analysis was performed on the effects of The DB Method on the Pelvic Floor Muscles and fascia.

Table 2. Survey results from the DB Method users.

Figure 1. Is a photograph of the DB Method^®TM device showing components and adjustments on which the kinetic analysis was based.

3. Results

Survey results are shown in Table 2. In summary, most users were female between 30 - 59 years of age and use The DB Method 2 - 4 times a week for 4 months to 1 year. 55% could do traditional squats but 44% could not because lower limb joint pain and muscle weakness; balance problems and fear of injury, and confusion about how to properly perform the squat. Between 70% and 85% of the users described increases in gluteal strength and shape, core strength, overall lower limb strength, pelvic floor muscle strength, and improvement in overall activity levels and energy. 99% recommended The DB Method.

There were 98 open question responses that were divided by content into the following 5 categories: 1) 67% were very positive (3 responses described its use by the family and 23% “loved” the method); 2) 8 % were negative (2% about the hardware, 4% said it did not help; 2% complained about the cost apps); 3) 10% were new and inconsistent users with nothing positive or negative to say; 4) 4% asked questions; 5) 11% had no added comments.

3.1. Description of the Muscles Involved in Squatting

During the descent phase of the squat, the gluteus maximus and the quadriceps femoris contract eccentrically to control range of motion while these same muscles contract concentrically during the ascent phase (Figure 2, Figure 3). The hamstrings may show eccentric action during the descent to control the position of the tibia (Figure 4). Hip rotators, adductors and abductors act mainly concentrically to stabilize the hip in the frontal and transverse planes (Figure 5). The muscles of the leg also act concentrically as stabilizers of the ankle (Figure 6).

3.2. Kinetics of Squatting with the DB Method^®TM

Descriptions and kinetics of a squat using The DB Method during the descent from the initial start position, at mid squat and at deep squat and at the ascent

Figure 2. Gluteal muscles actions during the squat.

Figure 3. Sartorius and quadriceps muscle actions during the squat.

Figure 4. Hamstring muscle actions during the squat.

end of the squat are shown (Figures 7-12). Individual are instructed look forward, pull your shoulder back, lightly hold the handrails; place your heels in the center of the foot ramp, keep your knees slightly bent but straight forward; tighten your abdominal muscles; rotate the top of your pelvis back so your tailbone is tucked under you; and then tighten your gluteus maximus.

3.2.1. Description of Kinetics in Figure 7

1) HIP MUSCLES AND MOVEMENTS: Posterior pelvic tilt and the posterior position of the hip joint to the line of gravity and the ground reaction force produce an external flexion moment. As the body moves down, the amount of hip flexion needs to be controlled to avoid falling. This control is produced mainly by eccentric (or lengthening) contractions of the gluteus maximus and posterior part of the gluteus medius muscles. While the hamstrings are also extensors of the hip, the posterior tilt of the pelvis slackens the hamstrings, producing only a small hip flexion controlling force compared to the gluteal muscles.

Figure 5. Hips rotators, abductors, and adductors muscles actions during the squat.

Figure 6. Leg muscle actions during the squat.

Figure 7. Lateral view at the start of the squat descent; COP = Center of Pressure, GRF = Ground Reaction force, LOG = Line of Gravity.

Figure 8. Lateral view at Mid Position during the squat descent.

The hip in this squat position is slightly abducted because of action of the gluteus medius and minimus, piriformis, and tensor fascia latae but the degree of hip abduction is stabilized by the hip adductors.

Figure 9. Lateral Mid Position during the squat descent.

Figure 10. Lateral view at the End Descent and the start of ascent.

Figure 11. Lateral view at the End Ascent.

Figure 12. Frontal view at mid position (ER = External rotation; IR = Internal Rotation): The hip is stabilized by the contraction of the ER and IR muscles of the hip. The knee is stabilized by the ER and IR of the tibial and the patellofemoral joint vastus lateralis and vastus medialis of the quadriceps.

Rotation of the hip is near neutral and stable at this squat starting position if the feet are properly aligned in a straightforward position with the knee and hip. Internal rotation is produced by the gluteus medias and minimus, the piriformis and tensor fascia latae and hip adductors and this internal rotation moment is counterbalanced by an external rotation moment produced by the gluteus maximus, quadratus femoris, obturator internus and internus, and sartorius. If the feet are pointed outwardly, then the hip external rotators are acting with more force than the hip internal rotators. If the feet are pointed inwardly, then the hip internal rotators are more active than the hip external rotators.

2) KNEE MUSCLES AND MOVEMENTS: Hip flexion slackens the rectus femoris muscle so little to no action occurs at the hip and only slight eccentric control of knee flexion at early in the squat. The knee is held in extension because the GRF lies anterior to the knee joint. No quadricep activity is needed to maintain knee extension, but contraction from the vastus lateralis and medialis may stabilize knee rotation and the knee flexes. Contraction of the hamstrings are initially assisting the gluteus maximus in controlling hip flexion and are counterbalancing the extension moment by the GRF at the knee and the vastus lateralis and medialis to stabilize tibial rotation.

3) ANKLE MUSCLES AND MOVEMENTS: With the Center of Pressure (COP) at the heel, the GRF runs posterior to the ankle and places a mechanical plantar flexion moment on the ankle which presses the foot against the angled foot plate. The mechanical ankle position does not need concentric contraction of the gastrocnemius and soleus muscles to produce and hold this position. Slight contraction of the gastrocnemius would help control the extended position of the knee and help stabilize rotation of the femur on the tibia to counteract tibial rotation by the hamstrings and quads.

3.2.2. Description of Kinetics in Figure 8, Figure 9 and Figure 12

1) HIP MUSCLES and MOVEMENTS: At the 90˚ squat, there is still a posterior pelvic tilt which flattens the lumbar spine, opening the intervertebral foramen and decreasing the compression force on the intervertebral or facet joints. The erector spinae muscles are active eccentrically at this time to stabilize the spine. The hip remains in an external flexion moment and the gluteus maximus and the posterior gluteus medius muscles are still eccentrically active, controlling the degree of hip flexion. The weight and force moment arms of the gluteus maximus at a 90˚ squat is about the same as those at the start of the squat. As the gluteus maximus is near its optimal fiber length at 90˚ of flexion and in an area on the length-tension curve that shows the greatest number of cross bridges and highest tension, the force of the gluteus maximus remains high and relatively the same from the start of the squat to a 90˚ squatting position.

The hip is stabilized in abduction by action of the hip abductors (gluteus medius and minimus, piriformis, tensor fascia latae) and concurrent action of the hip adductors (gracilis, adductor longus and brevis and the large adductor magnus). Hip rotation is stabilized by the actions of the internal rotators and the hip external rotators. The degree of hip abduction and rotation varies depending on foot placement and the type of squat.

2) KNEE MUSCLES AND MOVEMENTS: Because the knee is flexed, and the pelvis is in a posterior tilt, the hamstrings are slack. This position results in weak hamstring activity that is primarily associated with stabilizing tibial torsion and in controlling anterior motion of the tibia as the femoral condyles rotate anteriorly (Posterior roll of the femur) and glide anteriorly from 25 - 145 degrees of closed chain knee flexion. If anterior translation of the tibia is not controlled, then the anterior tibial movement would increase tension on the ACL during closed chain knee flexion and may damage the ACL.

The quads are eccentrically contracting to control knee flexion occurring because the GRF lies posterior to the knee producing a mechanical knee flexion moment. The vastus lateralis and medialis stabilize femoral rotation as the hamstrings stabilize tibial rotation. As the knee flexes, compression at the patellofemoral joint increases but so does the areas of force distribution also increase which lessens the unit compression force as knee flexion reaches and passes 90˚ of flexion. (The highest joint reaction forces for non-weight bearing knee flexion are reported from 30 to 70 degrees of flexion, peaking at 45 - 60 degrees and at 90 degrees for weight bearing knee.)

3) ANKLE MUSCLES AND MOVEMENTS: With the COP maintained posterior to the ankle, The GRF continues to apply a mechanical plantarflexion moment on the ankle but with the knee flexed, no activity would be expected from the gastrocnemius and only slight activity form the solus to adjust the position of the heel.

3.2.3. Description of Kinetics in Figure 10 and Figure 11

1) HIP MUSCLES AND MOVEMENTS: At full squat, the hip reached about 125˚ of flexion, and the fiber length has increased resulting in fewer cross bridges and less force production. Further, the weight moment arm in shorter than at 90˚ which also produces a reduction in the magnitude of gluteus maximus force needed to provide an equilibrium state. This reduction in gluteus maximus eccentric force may be a protective mechanism to minimum damage to the muscle or tendon. The hip remains abducted but stable because of the interactions of the hip abductors and adductors. Hip rotation is also stabilized because of the opposing actions of the hip internal and external rotators. But the degree of hip abduction and rotation varies again depending on foot placement and the type of squat.

At the end of full squat, the stretched elastic components of the gluteus maximus and posterior gluteus medius act as a stretched spring and recoil extending the hip, concurrent with concentric (shortening) contraction of the gluteal muscle. This produces an external extension moment at the hip and the ascent stage of the squat begins. The lumbar spine remains flattened but the pelvic begins an anterior tilt. The anterior tilt stretches the hamstrings and concentric activity of the hamstrings produced hip extension, assisting the gluteus maximus.

2) KNEE MUSCLES AND MOVEMENTS: With the knee flexed beyond 90˚, eccentric contraction of the quads increases but patellofemoral unit compression force decreases. At the end of full squat, the stretched elastic components of the quadriceps act as a stretch spring and recoil extending the knee, concurrent with concentric contraction of the quadriceps muscle. This produces an external extension moment at the knee and the ascent stage of the squat begins.

The vastus lateralis and medialis concentrically contract to extend the knee and stabilize femoral rotation and movement of the patella at the patellofemoral joint in the frontal plane. The rectus femoris and vastus intermedius main contribute to knee extension and upward vertical movement of the patella and the patellofemoral joint.

The hamstrings are slack in this position but may provide some weak concentric action to stabilize tibial rotation at the full squat. As the knee extends during the ascent, the hamstrings are stretched, and this passive elastic component will begin move the tibia posteriorly which initially help extend the knee. As the knee further extends and the elastic component of the hamstrings becomes tight, concentric action of the hamstrings will move the tibia future posteriorly helping to extend the knee as well helping to extend the hip which is extending at the same time. The hamstrings will also stabilize rotation at the knee by controlling tibial torsion and counterbalancing the femoral rotation produced by the vastus lateralis and medialis.

3) ANKLE MUSCLES AND MOVEMENTS: With the COP maintained posterior to the ankle, the GRF continues to apply a mechanical plantarflexion moment on the ankle but with the knee flexed, no activity would be expected from the gastrocnemius and only slight activity from the soleus to adjust the position of the heel.

During the ascent, the COP should remain posterior to the ankle and maintain the plantarflexion moment to the end of the ascent. As the trunk rises and the knee and hip extend, the GRF will move closer to the knee which will decrease the mechanical knee flexion moment and assist knee extension until the end of ascent when the GRF now lies anterior to the knee applying mechanical knee extension moment and decrease the need for quadricep muscle concentric contraction. At the hip, the GRF will move more anterior to the hip increasing the mechanical hip flexion moment. This increased moment would place increased resistance to hip extension causing the extensors to produce more force to actively extend the hip. The gluteus maximus and hamstrings would have to work harder to extend the hip, producing a resistive concentric exercise.

3.3. Description of the Pelvic Floor Muscles and Ligaments

Descriptions of the anatomy and functions of the pelvic floor muscles (PFM) are abundant and show complex interconnections among viscera, muscle and supporting fascial elements [24] [31] [46] [49] [54] [82] - [99]. This anatomical complexity and anatomical variations explain some of the difficulties associated with the interpretations, causation, understanding and treatment of pelvic pain and dysfunction [2] [11] [17] [46] [67] [70] [73] [79] [80] [82] [84] [89] [99] - [104].

When bladder, pelvic and abdominal forces are high, urinary continence is maintained by the ligaments of the pelvic floor and viscera, and the muscles of the pelvic diaphragm acting as a unit to hold the bladder, urethra and vagina in an upward and forward position [31] [46] [52] [54] [67] [71] [72] [73] [74] [81] [82] [85] [86] [88] [89] [91] [92] [96] [99] [100] [102] [103] [105]. The levator ani, coccygeus, piriformis and obturator internus muscles form the pelvic floor muscles (Figure 13, Table 3). The levator ani consists of the pubococcygeus, iliococcygeus, and puborectalis muscles. The pubococcygeus is divided into the levator prostatae in males, and pubovaginalis in females. The levator ani and coccygeus form the pelvic diaphragm. The pelvic diaphragm is a thin, flat, membrane-like muscle [4] [11] [17] [29] [69] [84] [88] [90] [93] [94] [97] [98] [100] that is split in the middle by the urethra, vagina, and rectum and attaches laterally to the mobile thin fibrous tendinous arch of the levator ani and medially to the viscera by another tendinous arch of the pelvic fascia. Its only stable bony attachment is to the pubic bone by the pubococcygeus muscle and a fibrous attachment to the mobile coccyx. In union with the muscles of the pelvic floor, the actions of the surrounding and interconnecting connective tissue elements are needed to support the pelvic viscera and maintain continence.

Table 3. Attachments and actions of the pelvic floor muscles.

Figure 13. (Left) Drawing of the pelvic floor muscles showing their main action (arrows) during a squat. (Right Top) Dissection of the pelvic floor. (Right Bottom) Diagram showing the internal left side of the pelvic floor and the main actions (arrows) during a squat.

Contractions of the pubococcygeus and iliococcygeus lift the pelvic floor and tenses the connective tissue network, resulting in stiffening of the pelvic floor and elevation and forward displacement of the pelvic organs [11] [29] [73] [75] [81] [82] [85] [86] [88] [94] [97] [104]. The iliococcygeus may also be important in controlling side to side movement of the pelvic organs that occur during physical activities. Bilateral contraction of the coccygeus closes off the posterior part of the inferior pelvic opening and applies tension to the coccyx to stabilize it. The pubococcygeus and iliococcygeus attach to the coccyx (Table 3). Coccygeal stability allows the levator ani muscles to produce a tight and stiff muscular sheet to support the viscera and resist depression of the pelvic floor with increased abdominal pressure.

The pelvic floor muscles control and delay emptying of the bladder and rectum until it is convenient. Contraction of the pelvic floor muscles lifts the pelvic organs and tightens the openings of the vagina, anus and urethra. Relaxation of the levator ani results in downward movement of the pelvic floor associated with urination and defecation. The levator ani muscle and the pelvic fascia interact to maintain continence and pelvic organ support [11] [17] [31] [46] [51] [52] [69] [74] [81] [82] [86] [89] [96] [99] [102] [104] [106] [107]. The upward displacement of the urogenital organs by the pelvic diaphragm is relatively small. Because maintaining an upward and forward position of these organs for urinary continence is important and because upward displacement of these organs by the pelvic diaphragm is small, the resting position of the urogenital organs is near the elevated holding level needed for urinary continence. This resting position is maintained by several pelvic ligaments (Figure 14, Table 4).

Table 4. Fascial and ligaments support structures.

Figure 14. Diagram of a sagittal view of the female pelvis with ligaments (by GG from J Women’s Physical Therapy 36: 2015).

The tendinous arch of the levator ani and the tendinous arch of the pelvic fascia form a thin, dense connective band on each side of the urogenital hiatus. Tension on this fascia and pelvic ligaments make up a support system that elevates the pelvic organs while slack in this system results in depression of these organs. In females, these ligaments are the urachus, pubovesical, broad ligament of the uterus and the transverse cardinal ligament of the cervix and in males these are the pubovesical, puboprostate and urachus [11] [89] [91] [99] [106]. Stretching of these ligaments during pregnancy, expansion of the abdominal due to weight gains, weakness of the abdominal muscles and surgery or trauma involving the anterior abdominal wall may result in the downward and posterior movement of the bladder. If these ligaments are stretched, increased abdominal pressure may move the bladder into a downward position favorable for urination. The coccyx, anococcygeal raphe (ligament) and sacrococcygeal joint are needed for proper action of the puborectalis at the anorectal junction to control anal continence.

3.4. Kinetics of the Pelvic Floor during Squatting

The kinetic actions of the pelvic floor and the interconnecting connective tissue structures are described and shown in Figures 15-18.

4. Discussion

The survey used in this article was a simple descriptive survey intended to give indications of who, why, and for what reasons The DB Method is being used. The developer of The DB Method could not do traditional squats because of injury and so designed this apparatus that would enable individuals to access the benefits of a squatting, especially those who have difficulty performing traditional squats. This survey was to provide a verification that met the designer’s goals. The results of the survey (Table 2) show that of those using The DB Method, 56% reported being able to perform tradition squats, but 44% could not do so. Thus, the goal to enable those who could not perform and benefit from squatting exercises seems appropriate. The goal of increasing gluteal (85%) and core muscle strength (75%) also seems to have been met. In addition, over 70% described an increase in lower limb strength (84%); pelvic floor strength (70%); an increase in activity level (82%) and an increase in energy (71%). Overall, it was not surprising that 99% of the respondents recommended The DB Method.

Figure 15. Diagram showing the kinetics at the start of the squat descent on the pelvic floor.

Figure 16. Diagram showing the kinetics at the mid position of the squat descent on the pelvic floor.

Figure 17. Diagram showing the kinetics at the end position (deep squat) of the squat descent on the pelvic floor.

Figure 18. Diagram showing the kinetics at the end position at the end of the squat ascent on the pelvic floor.

Table 5 shows a comparison of traditional squats and the DB Method.

There are several structural elements in the design of the DB Method that are important. The handrails and the seat provide the user stability which can overcome balance problems and fear of falling from the apparatus while in use especially by the elderly and those with lower limb weakness because of neurological problems and disuse. The handrails also extend the arms forward moving the scapulae forward and outward which increases the size of the thoracic cavity. This increase in the thoracic cavity decreases the thoracic pressure which improves inhalation that occurs throughout the descent while also reducing intra-abdominal pressure (IAP). This decreases the forces on the pelvic floor [107] [108], making movement and exercising of the pelvic muscles easier and reducing the energy requirements for muscle contraction and motion. This decrease may also prevent damage to the pelvic floor muscles (PFM) and pelvic fascia and the pelvic nerves and blood vessels due to excessive or repetitive increases in IAP.

Table 5. Comparison between traditional squat and the db method.

Another important element of The DB Method is the bilateral foot ramp which positions the feet in front of the knee, places the ankle in a plantarflexion moment, and the COP at the heel of the foot. This position of the foot and the use of the seat and handrail change the kinetics for the squat by shifting the line of gravity away from the ground reaction forces. The design of The DB Method facilitates the GRF to mechanical position the hip, knee and ankle, rather than muscle action, which would decrease muscle energy requirements during the squat.

Posterior pelvic tilt and the posterior position of the hip joint relative to the line of gravity and the GRF produce a mechanical flexion external moment at the hip. As the body moves down, the amount of hip flexion needs to be controlled to avoid falling. This control is produced mainly by eccentric (or lengthening) contraction of the gluteus maximus and tension on the muscle’s passive elastic components. While the hamstrings are also extensors of the hip, the posterior tilt of the pelvis and knee flexion slackens the hamstrings, producing only a small hip flexion controlling force compared to the gluteal muscles.

The posterior pelvic tilt and flattening of the lumbar spine is not a passive event but requires muscle action. The rectus abdominis, external and internal obliques, transverse abdominis, gluteus maximus and hamstrings all have to fire to establish this position. The contractions of the abdominal muscles facilitate core activation at the start of the squat, containing IAP. The flattening of the spine as a result of the posterior pelvic tilt during the squat 1) increases the size of the intervertebral foramen which decreases compression forces on the spinal nerve, 2) separates the facet joint decreasing compression on the joint, and 3) tightens the posterior longitudinal ligament increasing the vertebral canal and providing a barrier for posterior disc motion.

During the squat descent, the GRF lies anterior to the knee joint and provides an extension external moment at the knee. Because the weight of the body is applying a flexion moment at the knee, quadriceps is active eccentrically to control knee flexion. Contraction from the vastus lateralis and medialis stabilizes sagittal plane knee rotation and the position of the patella as the knee flexes. Contraction of the hamstrings is stabilizing anterior translation and medial and lateral transverse rotation of the tibial.

With the Center of Pressure (COP) at the heel, the GRF runs posterior to the ankle and places a mechanical plantar flexion moment on the ankle which presses the foot against the angled foot plate. The mechanical ankle position does not need concentric contraction of the gastrocnemius and soleus muscles to produce and hold this position. With the COP maintained posterior to the ankle, the GRF applies a mechanical plantarflexion moment on the ankle. With the knee flexed, no activity would be expected from the gastrocnemius and only slight activity from the soleus to stabilize the tibia.

During the ascent, the COP should remain posterior to the ankle and maintain the plantarflexion moment to the end of squat ascent. As the trunk rises and the knee and hip extend, the GRF will move closer to the knee which will decrease the mechanical knee flexion moment and assist knee extension until the end of ascent when the GRF now lies anterior to the knee where the GRF applies a mechanical knee extension moment which decreases the need for quadricep muscle concentric contraction. At the hip, the GRF will move more anterior to the hip increasing the mechanical hip flexion moment. This increase moment would place increased resistance to hip extension causing the extensors to produce more force to actively extend the hip. The gluteus maximus and hamstrings would have to apply increased muscle tension work to extend the hip, producing a resistive concentric exercise.

Strength of the gluteus maximus and quadriceps is necessary to perform many activities of daily living safely. An anterior pelvic tilt that occurs during the traditional squat makes contraction of the gluteus maximus muscles less efficient and less able to increase muscle size and strengthen. Starting in a posterior pelvic tilt, eccentric contraction of the gluteus maximus during the descent followed by concentric during the ascent of the squat acts to strengthen the gluteus maximus throughout the squat. Eccentric exercises are more effective at increasing muscle mass than concentric exercises [13] [61] [109] [110] [111] and higher forces are developed during eccentric contraction than concentric contraction. Total force production is greatest for eccentric contraction than for isometric contraction and both types are greater than concentric contraction [109]. The lengthening of muscle fibers during eccentric contraction produces more force because both the active and passive elastic elements of the muscle are involved. In concentric contractions, only the active element is involved. A single muscle can produce about 50% more force during lengthening than during shortening. It is reported that we are 25% - 30% stronger when lowering a weight than when lifting it. Eccentric contraction is more energy efficient than concentric contraction. Eccentric contraction of the gluteus maximus, quadriceps, and pelvic diaphragm produce tension and stretch the elastic components of the muscle during the squat. Eccentric muscle tension stretches the elastic components during eccentric contraction to store potential energy, which is used when the elastic elements recoil, like a spring, when eccentric contraction is followed by a concentric contraction. This example occurs when eccentric contraction of the gluteus maximus transitions to concentric contraction at the end of the descent phase of a squat to the ascending phase. This sequence optimizes the effects of the passive elastic components of eccentric contraction and directly prepares the muscle to contract concentrically at the bottom of the squat to ascend the body. It also optimizes the gluteus maximus contraction during both the eccentric descent and concentric ascent and strengthens the gluteus maximus from the start to the end of the squat. The functional significance of the muscular elastic components is often overlooked in kinetic analyses.

At the end of squat descent, the gluteus maximus continues to lengthen which increases sarcomere length but decreases muscle contraction force. However, muscle tension is maintained by the stretched passive elastic component producing a spring-like tension and high potential energy. At the start of the ascent, the weaker concentric contraction of the gluteus maximus and the quadriceps is aided by the recoil of the muscle elastic component.

During a squat, abdominal pressure changes which moves the pelvic diaphragm upward anteriorly and downward posteriorly. The tension of the pelvic fascia would stimulate fibroblast activity to increase collagen fiber production and with time assist in strengthening the fascia when IAP is high as during coughing, Valsalva maneuvers, jumping, lifting and carrying 30 lbs (13.6 kg) [80] [108]. This response to tension at the fascial attachment sites of the gluteus maximus and quadriceps muscles and would increase the strength of the series and parallel elastic components of the muscle. As muscle tension is a factor of active and passive tensions, an increase in passive tension by the elastic component would increase total muscle tension [13] [109].

The obturator internus muscle stabilizes hip rotation during a squat and its fascia attaches to the Tendinous Arch of Levator Ani (TALA). This Arch is a primary attachment site for the iliococcygeus and pubococcygeus muscles of the pelvic diaphragm. Tension of the obturator internus fascia to the Tendinous Arch of Levator Ani strengthens and secures the attachment site of the levator ani, allowing the pelvic diaphragm function eccentrically and concentrically to move the pelvic floor. This tendinous and connective tissue complex forms the passive elastic forces of the PF which stretches during eccentric contraction of the LA and recoils during concentric contraction of the LA.

The abdominal pressure acts transversely across the urethra, altering the stresses in the wall of the urethra so that its anterior wall is deformed toward its posterior wall, thereby helping to close the urethral lumen and prevent leakage caused by the increase in intravesical pressure.

Prolonged and repetitive elevated abdominal and pelvic forces, associated with pregnancy, abdominal weight gains, coughing, sneezing, heavy laughing, strenuous lifting and other activities, can stretch the ligaments supporting the pelvic organs and the pelvic diaphragm. This stretching can move the resting position of the urinary bladder caudally towards a position of urination. The repetition requires greater displacement and force production by the pelvic diaphragm to reach the continence position. To counter this stretching action, the pelvic diaphragm needs to be strengthened to produce increased force to lift the pelvic viscera and withstand increases in abdominal and pelvic captivity forces.

5. Practical Applications

The requirements of balance, spatial awareness, proprioception and coordination are major barriers to exercise for large segments of the population. A lack of physical support and guidance coupled with a lack of personal supervision (i.e., training or coaching) makes squatting all but impossible to perform for many people. Simply introducing a place to hold on for balance and a seat support during the squat is a major improvement in accessibility of the squat as an exercise.

The DB Method for squatting described here seems to strengthen the gluteus maximus, quadriceps, abdominal core and the pelvic diaphragm. As exercise is an important part of maintaining good cardiopulmonary, musculoskeletal and psychological health, an apparatus that allows more people to exercise is beneficial to many but especially to the elderly and home bound. Individuals with unilateral low limb weakness as from a CVA or peripheral nerve damage may be able to perform squats to increase strength, lower limb coordination, and generally improve their health and way of life.

Pelvic floor exercise by squatting can be beneficial and should be available to any individual that experiences a disruption of continuity of the pelvic fascia, or when the pelvic diaphragm is damaged, as during childbirth, hysterectomies, prostatectomies, prolapses, and trauma [11] [42] [43] [45] [48] [50] [52] [54] [62] [92] [112]. Both fascial and muscle damages produce less stiffness of the supportive layer under the urethra and provides less resistance to deformation during increases in abdominal pressure. Thus, closure of the urethral lumen is not ensured, raising the possibility of stress incontinence. Pelvic diaphragm damage may also increase the connective tissue composition of the muscle and reduce its contractibility so that it is less effective in lifting and holding the pelvic organs to maintain urinary and anal continence [11] [73] [74] [79] [100] [104] [106] [107].

Because the pelvic viscera move with changes in posture and with activities [72] [81] [82] [86] [106], a strong contractile mechanism that produces bilateral contractile forces and controls excessive lateral movements of the pelvic organs is needed. Guided and balanced sagittal plane squatting exercises that can uniformly and bilaterally activate the pelvic diaphragm will provide strong and highly repetitive resistance to abdominal forces that move the pelvic organs laterally. Unilateral movement of the pelvic organs and unilateral tension of the pelvic fascia and diaphragm may compress the neurovascular bundles lying in the fascia, interfere with blood flow and produce pain and possible organ dysfunctions [11] [81] [82] [83] [90]. Because the iliococcygeus is composed mainly of type I muscle fibers, this muscle can act bilaterally for a long period of time as a postural muscle to control the central position of the pelvic organs. Using a squatting method to control and guide repetitive contraction of the pelvic diaphragm would minimize unilateral stretch of the pelvic fascia and compression of the neurovascular structures.

Decreased forces production by the pelvic diaphragm may be of particular concern with a nonactive aged person and a person on prolonged bed rest [1] [2] [3] [4] [46] [88] [98] [100] [104] [109] [110]. Because slow twitch fibers atrophy with disuse [3] [4] [90] [94] [100] [109], the predominance of slow twitch fibers in the pelvic diaphragm would produce a decrease in contractile strength, suggesting that with aging and reduced activity the ability of the pelvic diaphragm to resist increases in abdominal pressure may result in urinary stress incontinence. A repetitive routine of squat exercise would benefit the maintenance pelvic floor muscle strength.

Acknowledgments

I am very grateful to Adam Swartz who in a tutoring session introduced me to The DB Methods^®TM because he wanted to understand the kinetics involved in the positive comments, he was receiving from those using The DB Method. Our many discussions and his initiation and follow through on the survey were of great value to me. My thanks to Josh Rosenzweig and Adam Swartz for their work on the survey and to Adam Swartz and Erika Rayman for their useful comments on the manuscript.

Statement

I am an independent analyst. I am not an employee of The DB Method^®TM, nor do I or many members of my family have a financial investment or shares in the company that manufactures, sells or owns The DB Method^®TM.

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	Booth, F.W., Weeden, S.H. and Tseng, B.S. (1994) Effect of Aging on Human Skeletal Muscle and Motor Function. Medicine & Science in Sports & Exercise, 26, 556-560. https://doi.org/10.1249/00005768-199405000-00006
[2]	Campbell, M.J., McComas, A.J. and Petito, F. (1973) Physiologic Changes in Aging Muscle. Journal of Neurology, Neurosurgery and Psychiatry, 36, 174-182. https://doi.org/10.1136/jnnp.36.2.174
[3]	Thompson, L.V. (2002) Skeletal Muscle Adaptations with Age, Inactivity, and Therapeutic Exercise. JOSPT, 32, 44-57. https://doi.org/10.2519/jospt.2002.32.2.44
[4]	Tomonaga, M. (1997) Histochemical and Ultrastructural Changes in Senile Human Skeletal Muscle. Journal of the American Geriatrics Society, 25, 125-131. https://doi.org/10.1111/j.1532-5415.1977.tb00274.x
[5]	Burgess-Limerick, R. (2003) Squat, Stoop, or Something in Between? International Journal of Industrial Ergonomics, 31, 143-148. https://doi.org/10.1016/S0169-8141(02)00190-7
[6]	Lubans, D.R., Morgan, P.J., Cliff, D.P., Barnett, L.M. and Okely, A.D. (2010) Fundamental Movement Skills in Children and Adolescents. Sports Medicine, 40, 1019-1035. https://doi.org/10.2165/11536850-000000000-00000
[7]	Macrum, E., Bell, D.R., Boling, M., Lewek, M. and Padua, D. (2012) Effect of Limiting Ankle-Dorsiflexion Range of Motion on Lower Extremity Kinematics and Muscle-Activation Patterns during a Squat. Journal of Sport Rehabilitation, 21, 144-150. https://doi.org/10.1123/jsr.21.2.144
[8]	Myer, G.D., Kushner, A.M., Brent, J.L., Schoenfeld, B.J., Hugentobler, J., Lloyd, R.S., et al. (2014) The Back Squat: A Proposed Assessment of Functional Deficits and Technical Factors That Limit Performance. Strength and Conditioning Journal, 36, 4-27. https://doi.org/10.1519/SSC.0000000000000103
[9]	Silvio, L., Ostermann, M., Zeidler, F., Zimmer, P., Jentsch, L., List, R., Taylor, W. and Schellenberg, F. (2018) How to Squat? Effects of Various Stance Widths, Foot Placement Angles and Level of Experience on Knee, Hip and Trunk Motion and Loading. BMC Sports Science, Medicine and Rehabilitation, 10, Article No. 14. https://doi.org/10.1186/s13102-018-0103-7
[10]	Glassbrook, D.J., Helms, E.R., Brown, S.R. and Storey, A.G. (2017) A Review of the Biomechanical Differences between the High-Bar and Low-Bar Back-Squat. The Journal of Strength & Conditioning Research, 31, 2618-2634. https://doi.org/10.1519/JSC.0000000000002007
[11]	Gorniak, G.C. and Conrad, W. (2015) An Anatomical and Functional Perspective of the Pelvic Floor and Urogenital Organ Support System. Journal of Women’s Health Physical Therapy, 39, 65-82. https://doi.org/10.1097/JWH.0000000000000033
[12]	Gullett, J.C., Tillman, M.D., Gutierrez, G.M. and Chow, J.W. (2009) A Biomechanical Comparison of Back and Front Squats in Healthy Trained Individuals. Journal of Strength and Conditioning Research, 23, 284-292. https://doi.org/10.1519/JSC.0b013e31818546bb
[13]	Hou, M. (2018) Force-Length Relation of Skeletal Muscles: From Sarcomeres to Myofibril. Biomechanics and Modeling in Mechanobiology, 17, 1797-1810. https://doi.org/10.1007/s10237-018-1057-0
[14]	Khuu, A., Foch, E. and Lewis, C.L. (2016) Not All Single Leg Squats Are Equal: A Biomechanical Comparison of Three Variations. International Journal of Sports Physical Therapy, 11, 201-221.
[15]	Saeterbakken, A.H. andersen, V. and Van den Tillaar, R. (2016) Comparison of Kinematics and Muscle Activation in Free-Weight Back Squat with and without Elastic Bands. The Journal of Strength & Conditioning Research, 30, 945-952. https://doi.org/10.1519/JSC.0000000000001178
[16]	Schoenfeld, B.J. (2010) Squatting Kinematics and Kinetics and Their Application to Exercise Performance. The Journal of Strength & Conditioning Research, 24, 3497-3506. https://doi.org/10.1519/JSC.0b013e3181bac2d7
[17]	Singh, K., Jakab, M., Reid, W.M., Berger, L.A. and Hoyte, L. (2003) Three-Dimensional Magnetic Resonance Imaging Assessment of Levator Ani Morphologic Features in Different Grades of Prolapse. American Journal of Obstetrics and Gynecology, 188, 910-915. https://doi.org/10.1067/mob.[phone]
[18]	Swinton, P.A., Lloyd, R., Keogh, J.W., Agouris, I. and Stewart, A.D. (2012) A Biomechanical Comparison of the Traditional Squat, Powerlifting Squat, and Box Squat. The Journal of Strength & Conditioning Research, 26, 1805-1816. https://doi.org/10.1519/JSC.0b013e3182577067
[19]	Thompson, S.W., Lake, J., Rogerson, D., Ruddock, A. and Barnes, A. (2002) Kinetics and Kinematics of the Free-Weight Back Squat and Loaded Jump Squat. Journal of Strength and Conditioning Research, 5, 1-31.
[20]	Vakros, J.P., Nitz, A.J., Threlkeld, A.J., Shapiro, R. and Horn, T. (1994) Electromyographic Activity of Selected Trunk and Hip Muscles during A Squat Lift. Effect of Varying the Lumbar Posture. Spine, 19, 687-695. https://doi.org/10.1097/00007632-199403001-00008
[21]	Bryanton, M.A., Kennedy, M.D., Carey, J.P. and Chiu, L.Z. (2012) Effect of Squat Depth and Barbell Load on Relative Muscular Effort in Squatting. Journal of Strength and Conditioning Research, 26, 2820-2828. https://doi.org/10.1519/JSC.0b013e31826791a7
[22]	Gutierrez, A. and Bahamonde, R.E. (2009) Kinematic Analysis of the Traditional Back Squat and Smith Machine Squat Exercises. ISBS-Conference Proceedings, Limerick, 4 p.
[23]	Hartmann, H., Wirth, K. and Klusemann, M. (2013) Analysis of the Load on the Knee Joint and Vertebral Column with Changes in Squatting Depth and Weight Load. Sports Medicine, 43, 993-1008. https://doi.org/10.1007/s40279-013-0073-6
[24]	Israetel, M.A., McBride, J.M., Nuzzo, J.L., Skinner, J.W. and Dayne, A.M. (2020) Kinetic and Kinematic Differences between Squats Performed with and without Elastic Bands. The Journal of Strength & Conditioning Research, 24, 190-194. https://doi.org/10.1519/JSC.0b013e31819b7995
[25]	Kim, S.H., Kwon, O.Y., Park, K.N., Jeon, I.C. and Weon, J.H. (2015) Lower Extremity Strength and the Range of Motion in Relation to Squat Depth. Journal of Human Kinetics, 45, 59-69. https://doi.org/10.1515/hukin-2015-0007
[26]	Larsen, S., Kristiansen, E., Falch, H.N., Haugen, M.E., Fimland, M.S. and van den Tillaar, R. (2022) Effects of Barbell Load on Kinematics, Kinetics, and Myoelectric Activity in Back Squats. Sports Biomechanics, 1-15. https://doi.org/10.1080/14763141.2022.2085164
[27]	Maddox, E.M. and Bennett, H.J. (2021) Effects of External Load on Sagittal and Frontal Plane Lower Extremity Biomechanics during Back Squats. Journal of Biomechanical Engineering, 143, Article ID: 051006. https://doi.org/10.1115/1.4049747
[28]	Schwarz, N.A., Harper, S.P., Waldhelm, A., McKinley-Barnard, S.K., Holden, S.L. and Kovaleski, J.E. (2019) A Comparison of Machine versus Free-Weight Squats for the Enhancement of Lower-Body Power, Speed, and Change-of-Direction Ability during an Initial Training Phase of Recreationally-Active Women. Sports, 7, Article No. 215. https://doi.org/10.3390/sports7100215
[29]	van den Tillaar, R. andersen, V. and Saeterbakken, A.H. (2019) Comparison of Muscle Activation and Kinematics during Free-Weight Back Squats with Different Loads. PLOS ONE, 14, e0217044. https://doi.org/10.12697/akut.2019.25.03
[30]	Yavuz, H.U. and Erdag, D. (2017) Kinematic and Electromyographic Activity Changes during Back Squat with Submaximal and Maximal Loading. Applied Bionics and Biomechanics, 2017, Article ID: 9084725. https://doi.org/10.1155/2017/9084725
[31]	Yiou, R., Costa, P., Haab, F. and Delmas, V. (2009) Functional Anatomy of the Pelvic Floor. Progres en Urologie: Journal de L’association Francaise D’urologie et de la Societe Francaise D’urologie, 19, 916-925. https://doi.org/10.1016/j.purol.2009.09.002
[32]	van den Tillaar, R. (2019) Effect of Descent Velocity upon Muscle Activation and Performance in Two-Legged Free Weight Back Squats. Sports, 7, Article No. 15. https://doi.org/10.3390/sports7010015
[33]	Zawadka, M., Smolka, J., Skublewska-Paszkowska, M., Lukasik, E. and Gawda, P. (2020) How Are Squat Timing and Kinematics in the Sagittal Plane Related to Squat Depth? Journal of Sports Science & Medicine, 19, 500-507.
[34]	Amritashish, B. (2015) An Electromyographical Analysis of Barbell and Smith Machine Squats among Weight Lifters. International Journal of Sports Sciences & Fitness, 5, 293-303.
[35]	Biscarini, A., Benvenuti, P., Botti, F., Mastrandrea, F. and Zanuso, S. (2011) Modelling the Joint Torques and Loadings during Squatting at the Smith Machine. Journal of Sports Sciences, 29, 457-469. https://doi.org/10.1080/02640414.2010.534859
[36]	Cotterman, M.L., Darby, L.A. and Skelly, W.A. (2005) Comparison of Muscle Force Production Using the Smith Machine and Free Weights for Bench Press and Squat Exercises. Journal of Strength and Conditioning Research, 19, 169-176. https://doi.org/10.1519/00124278-200502000-00029
[37]	Sheppard, J.M., Doyle, T.L.A. and Taylor, K. (2008) A Methodological and Performance Comparison of Free Weight and Smith-Machine Jump Squats. The Journal of Australian Strength and Conditioning, 16, 5-9.
[38]	van den Tillaar, R., Kristiansen, E.L. and Larsen, S. (2021) Is the Occurrence of the Sticking Region in Maximum Smith Machine Squats the Result of Diminishing Potentiation and Co-Contraction of the Prime Movers among Recreationally Resistance Trained Males? International Journal of Environmental Research and Public Health, 18, Article No. 1366. https://doi.org/10.3390/ijerph18031366
[39]	Bump, R.C. and Norton, P.A. (1998) Epidemiology and Natural History of Pelvic Floor Dysfunction. Obstetrics and Gynecology Clinics of North America, 25, 723-746. https://doi.org/10.1016/S0889-8545(05)70039-5
[40]	Ciangola, C.I., Capuano, I., Perrone, F. and Franceschilli, L. (2014) Epidemiology and Prevalence of Pelvic Floor Disorders. In: Gaspari, A.L. and Sileri, P., Eds., Pelvic Floor Disorders: Surgical Approach, Springer, Milan, 9-15. https://doi.org/10.1007/978-88-470-5441-7_2
[41]	DeLancey, J.O. (2005) The Hidden Epidemic of Pelvic Floor Dysfunction: Achievable Goals for Improved Prevention and Treatment. American Journal of Obstetrics and Gynecology, 192, 1488-1495. https://doi.org/10.1016/j.ajog.2005.02.028
[42]	Irwin, D., Kopp, Z., Agatep, B., Milsom, I. and Abrams, P. (2011) Worldwide Prevalence Estimates of Lower Urinary Tract Symptoms, Overactive Bladder, Urinary Incontinence and Bladder Outlet Obstruction. BJU International, 108, 1132-1138. https://doi.org/10.1111/j.1464-410X.2010.09993.x
[43]	Kepenekci, I., Keskinkilic, B., Akinsu, F., Cakir, P., Elhan, A.H., Erkek, A.B. and Kuzu, M.A. (2011) Prevalence of Pelvic Floor Disorders in the Female Population and the Impact of Age, Mode of Delivery, and Parity. Diseases of the Colon & Rectum, 54, 85-94. https://doi.org/10.1007/DCR.0b013e3181fd2356
[44]	Melville, J.L., Katon, W., Delaney, K. and Newton, K. (2005) Urinary Incontinence in US Women: A Population-Based Study. Archives of Internal Medicine, 165, 537-542. https://doi.org/10.1001/archinte.165.5.537
[45]	Nygaard, I., Barber, M.D., Burgio, K.L., Kenton, K., Meikle, S., Schaffer, J., Spino, C., Whitehead, W.E., Wu, J., Debra, J. and Brody, D.J. (2008) Prevalence of Symptomatic Pelvic Floor Disorders in US Women. JAMA, 300, 1311-1316. https://doi.org/10.1001/jama.300.11.1311
[46]	Ph Slieker-ten Hove, M., Pool-Goudzwaard, A., Eijkemans, M., Steeger-Theunissen, R., Burger, C. and Vierhout, M. (2009) Pelvic Floor Muscle Function in a General Female Population in Relation with Age and Parity and the Relation between Voluntary and Involuntary Contraction of the Pelvic Floor Musculature. International Urogynecology Journal, 20, 1497-1504. https://doi.org/10.1007/s00192-009-0978-7
[47]	Sibert, L., Rigaud, J., Delavierre, D. and Labat, J.J. (2012) Chronic Pelvic Pain: Epidemiology and Economic Impact. Progrès en Urologie, 20, 872-885. https://doi.org/10.1016/j.purol.2010.08.004
[48]	Sung, V.W. and Hampton, B.S. (2009) Epidemiology of Pelvic Floor Dysfunction. Obstetrics and Gynecology Clinics, 36, 421-443. https://doi.org/10.1016/j.ogc.2009.08.002
[49]	Wang, J., Varma, M.G., Creasman, J.M., Subak, L.L., Brown, J.S., Thom, D.H. and Van Den Eeden, S.K. (2010) Pelvic Floor Disorders and Quality of Life in Women with Self-Reported Irritable Bowel Syndrome. Alimentary Pharmacology & Therapeutics, 31, 424-431. https://doi.org/10.1111/j.1365-2036.2009.04180.x
[50]	Wu, J.M., Vaughan, C.P., Goode, P.S., Redden, D.T., Burgio, K.L., Richter, H.E. and Markland, A.D. (2014) Prevalence and Trends of Symptomatic Pelvic Floor Disorders in US Women. Obstetrics and Gynecology, 123, 141-148. https://doi.org/10.1097/AOG.0000000000000057
[51]	Berg-Poppe, P., Christensen, M., Koskovich, N. and Stephenson, C. (2022) Pelvic Floor Muscle Resting Tone in Children with Dysfunctional Voiding Symptomology Following Simple Gross Motor Exercises. Pediatric Physical Therapy, 34, 28-35. https://doi.org/10.1097/PEP.0000000000000842
[52]	Bø, K. (2004) Urinary Incontinence, Pelvic Floor Dysfunction, Exercise and Sport. Sports Medicine, 34, 451-464. https://doi.org/10.2165/00007256-200434070-00004
[53]	Borello-France, D.F., Downey, P.A., Zyczynski, H.M. and Rause, C.R. (2008) Continence and Quality-of-Life Outcomes 6 Months Following an Intensive Pelvic-Floor Muscle Exercise Program for Female Stress Urinary Incontinence: A Randomized Trial Comparing Low- and High-Frequency Maintenance Exercise. Physical Therapy, 88, 1545-1553. https://doi.org/10.2522/ptj.20070257
[54]	Caetano, A.S., Suzuki, F.S. and Lopes, M.H. (2019) Urinary Incontinence and Exercise: Kinesiological Description of an Intervention Proposal. Revista Brasileira de Medicina do Esporte, 25, 409-412. https://doi.org/10.1590/1517-869220192505213379
[55]	Dorey, G., Speakman, M., Feneley, R., Swinkels, A. and Dunn, C. (2004) Randomized Control Trial of Pelvic Floor Muscle Exercises and Manometric Biofeedback for Erectile Dysfunction. British Journal of General Practice, 54, 819-825.
[56]	Fernandez, A.G. and Petros, P.E. (2019) Nocturnal Enuresis in Children-Literature Review and Anatomical Rationale for a Squatting-Based Pelvic Floor Regime. Pelviperineology, 38, 126-129.
[57]	Garcia-Fernandez, A. and Petros, P.E. (2020) A Four Month Squatting-Based Pelvic Exercise Regime Cures Day/Night Enuresis and Bowel Dysfunction in Children Aged 7-11 Years. Central European Journal of Urology, 73, 307-314. https://doi.org/10.5173/ceju.2020.0044
[58]	George, S.E., Clinton, S.C. and Borello-France, D.F. (2013) Physical Therapy Management of Female Chronic Pelvic Pain: Anatomic Considerations. Clinical Anatomy, 26, 77-88. https://doi.org/10.1002/ca.22187
[59]	Haslam, J. (2002) Pelvic Floor Muscle Exercise in the Treatment of Urinary Incontinence. In: Laycock, J. and Haslam, J., Eds., Therapeutic Management of Incontinence and Pelvic Pain, Springer, London, 63-71. https://doi.org/10.1007/978-1-4471-3715-3_8
[60]	Haslam, J. and Laycock, J. (2007) Therapeutic Management of Incontinence and Pelvic Pain: Pelvic Organ Disorders. Springer Science & Business Media, London. https://doi.org/10.1007/978-1-84628-756-5
[61]	Hody, S., Croisier, J.L., Bury, T., Rogister, B. and Leprince, P. (2019) Eccentric Muscle Contractions: Risks and Benefits. Frontiers in Physiology, 10, Article No. 536. https://doi.org/10.3389/fphys.2019.00536
[62]	Jundt, K., Peschers, U. and Kentenich, H. (2015) The Investigation and Treatment of Female Pelvic Floor Dysfunction. Deutsches Ärzteblatt International, 112, 564-574. https://doi.org/10.3238/arztebl.2015.0564
[63]	Kegel, A.H. (1948) Progressive Resistance Exercise in the Functional Restoration of the Perineal Muscles. American Journal of Obstetrics & Gynecology, 56, 238-249. https://doi.org/10.1016/0002-9378(48)90266-X
[64]	Laycock, J. and Vodusek, D. (2002) Electrical Stimulation. In: Laycock, J. and Haslam, J., Eds., Therapeutic Management of Incontinence and Pelvic Pain, Springer, New York, 83-88. https://doi.org/10.1007/978-1-4471-3715-3_12
[65]	Neumann, P. and Gill, V. (2002) Pelvic Floor and Abdominal Muscle Interaction: EMG Activity and Intra-Abdominal Pressure. International Urogynecology Journal and Pelvic Floor Dysfunction, 13, 125-132. https://doi.org/10.1007/s001920200027
[66]	Petros, P.E. and Skilling, P.M. (2001) Pelvic Floor Rehabilitation According to the Integral Theory of Female Urinary Incontinence—First Report. European Journal of Obstetrics & Gynecology and Reproductive Biology, 94, 264-269. https://doi.org/10.1016/S0301-2115(00)00313-4
[67]	Luginbuehl, H., Baeyens, J.P., Taeymans, J., Maeder, I.M., Kuhn, A. and Radlinger, L. (2015) Pelvic Floor Muscle Activation and Strength Components Influencing Female Urinary Continence and Stress Incontinence: A Systematic Review. Neurourology and Urodynamics, 34, 498-506. https://doi.org/10.1002/nau.22612
[68]	Siegel, A. (2014) Pelvic Floor Muscle Training in Males: Practical Applications. Urology, 84, 1-7. https://doi.org/10.1016/j.urology.2014.03.016
[69]	Siff, L.N., Hill, A.J., Walters, S.J., Walters, G. and Walters, M. (2020) The Effect of Commonly Performed Exercises on the Levator Hiatus Area and the Length and Strength of Pelvic Floor Muscles in Postpartum Women. Female Pelvic Medicine & Reconstructive Surgery, 26, 61-66. https://doi.org/10.1097/SPV.0000000000000590
[70]	Thompson, J.A., O’Sullivan, P.B., Briffa, N.K. and Neumann, P. (2006) Assessment of Voluntary Pelvic Floor Muscle Contraction in Continent and Incontinent Women Using Transperineal Ultrasound, Manual Muscle Testing, and Vaginal Squeeze Pressure Measurement. International Urogynecology Journal and Pelvic Floor Dysfunction, 17, 624-630. https://doi.org/10.1007/s00192-006-0081-2
[71]	Wallace, S.L., Miller, L.D. and Mishra, K. (2019) Pelvic Floor Physical Therapy in the Treatment of Pelvic Floor Dysfunction in Women. Current Opinion in Obstetrics and Gynecology, 31, 485-493. https://doi.org/10.1097/GCO.0000000000000584
[72]	Devreese, A., Staes, F., Janssens, L., Penninckx, F., Vereecken, R. and DeWeerdt, W. (2007) Incontinent Women Have Altered Pelvic Floor Muscle Contraction Patterns. The Journal of Urology, 178, 558-562. https://doi.org/10.1016/j.juro.2007.03.097
[73]	Bush, M.B., Liedl, B., Wagenlehner, F., Yassouridis, A. and Petros, P.E. (2005) Effects of Posture and Squatting on the Dynamics of Micturition. International Urogynecology Journal, 26, 779-780. https://doi.org/10.1007/s00192-015-2643-7
[74]	Law, Y.M. and Fielding, J.R. (2008) MRI of Pelvic Floor Dysfunction: Review. American Journal of Roentgenology, 191, S45-S53. https://doi.org/10.2214/AJR.07.7096
[75]	Lo, S.K., Naidu, J. and Cao, Y. (2003) Additive Effect of Interferential Therapy over Pelvic Floor Exercise Alone in the Treatment of Female Urinary Stress and Urge Incontinence: A Randomized Controlled Trial. Hong Kong Physiotherapy Journal, 21, 37-42. https://doi.org/10.1016/S1013-7025(09)70038-7
[76]	Skilling, P.M. and Petros, P.E. (2004) Synergistic Non-Surgical Management of Pelvic Floor Dysfunction: Second Report. International Urogynecology Journal, 15, 106-110. https://doi.org/10.1007/s00192-004-1122-3
[77]	Sapsford, R.R. and Hodges, P.W. (2000) Voluntary Abdominal Exercise and Pelvic Floor Muscle Activity. Neurourology and Urodynamics, 19, 510-511.
[78]	Sapsford, R.R., Hodges, P.W., Richardson, C.A., Cooper, D.H., Markwell, S.J. and Jull, G.A. (2001) Co-Activation of the Abdominal and Pelvic Floor Muscle during Voluntary Exercises. Neurourology and Urodynamics, 20, 31-42. https://doi.org/10.1002/1520-6777(2001)20:1<31::AID-NAU5>3.0.CO;2-P
[79]	Sapsford, R.R. and Hodges, P.W. (2001) Contraction of the Pelvic Floor Muscles during Abdominal Maneuvers. Archives of Physical Medicine and Rehabilitation, 82, 1081-1088. https://doi.org/10.1053/apmr.2001.24297
[80]	Sapsford, R. (2004) Rehabilitation of Pelvic Floor Muscles Utilizing Trunk Stabilization. Manual Therapy, 9, 3-12. https://doi.org/10.1016/S1356-689X(03)00131-0
[81]	Dietz, H.P. (2010) Pelvic Floor Ultrasound: A Review. American Journal of Obstetrics and Gynecology, 202, 321-334. https://doi.org/10.1016/j.ajog.2009.08.018
[82]	Ashton-Miller, J. and Delancey, J.O.L. (2007) Functional Anatomy of the Female Pelvic Floor. Annals of the New York Academy of Sciences, 1101, 266-296. https://doi.org/10.1196/annals.1389.034
[83]	Ashton-Miller, J.A., Howard, D. and Delancey, J.O.L. (2001) The Functional Anatomy of the Female Pelvic Floor and Stress Continence Control System. Scandinavian Journal of Urology and Nephrology: Supplement, 207, 1-12. https://doi.org/10.1080/003655901750174773
[84]	Bernstein, I.T. (1997) The Pelvic Floor Muscles: Muscle Thickness in Healthy and Urinary-Incontinent Women Measured by Perineal Ultrasonography with Reference to the Effect of Pelvic Training. Estrogen Receptor Studies. Neurourology and Urodynamics, 16, 237-275. https://doi.org/10.1002/(SICI)1520-6777(1997)16:4<237::AID-NAU2>3.0.CO;2-F
[85]	Corona-Quintanilla, D.L., Zempoalteca, R., Arteaga, L., Castelán, F. and Martínez-Gómez, M. (2014) The Role of Pelvic and Perineal Striated Muscles in Urethral Function during Micturition in Female Rabbits. Neurourology and Urodynamics, 33, 455-460. https://doi.org/10.1002/nau.22416
[86]	Constantinou, C.E., Hvistendahl, G., Ryhammer, A., Nagel, L.L. and Djurhuus, J.C. (2002) Determining the Displacement of the Pelvic Floor and Pelvic Organs during Voluntary Contractions Using Magnetic Resonance Imaging in Younger and Older Women. BJU International, 90, 408-414. https://doi.org/10.1046/j.1464-410X.2002.02907.x
[87]	Cundiff, G.W. (2008) Anatomy of the Pelvic Viscera. In: Ostergard, D.R., Bent, A.E., Cundiff, G.W. and Swift, S.E., Eds., Ostergard’s Urogynecology and Pelvic Floor Dysfunction, 6th Edition, Lippincott, Williams & Wilkins, Philadelphia, 3-26.
[88]	Deindl, F.M., Vodusek, D.B., Hesse, U. and Schussler, B. (1993) Activity Patterns of Pubococcygeal Muscles in Nulliparous Continent Women. British Journal of Urology, 72, 46-51. https://doi.org/10.1111/j.1464-410X.1993.tb06455.x
[89]	Delancey, J. (1994) Functional Anatomy of the Pelvic Floor and Urinary Continence Mechanism. In: Schussler, B., Laycock, L., Norton, P. and Stanton, S., Eds., Pelvic Floor Re-Education: Principles and Practice, Springer-Verlag, London, 9-21.
[90]	Dixon, J. and Gosling, J. (1994) Histomorphology of the Pelvic Floor. In: Schussler, B., Laycock, J., Norton, P. and Stanton, S., Eds., Pelvic Floor Re-Education: Principle and Practice, Springer-Verlag, London, 28-33.
[91]	El Sayed, R.F., Morsy, M.M., El Mashed, S.M. and Abdel-Axim, M.S. (2007) Anatomy of the Urethral Supporting Ligamants Defines by Dissection, Histology and MRI of Female Cadavers and MRI of Healthy Nulliparous Women. American Journal of Roentgenology, 189, 1145-1157. https://doi.org/10.2214/AJR.07.2215
[92]	Gonzalez, D.C., Khorsandi, S., Mathew, M., Enemchukwu, E. and Syan, R. (2022) A Systematic Review of Racial/Ethnic Disparities in Female Pelvic Floor Disorders. Urology, 163, 8-15. https://doi.org/10.1016/j.urology.2021.09.018
[93]	Gowda, S.N. and Bordoni, B. (2021) Anatomy, Abdomen and Pelvis, Levator Ani Muscle. In: StatPearls [Internet], StatPearls Publishing, Treasure Island, 1-4. https://www.ncbi.nlm.nih.gov/
[94]	Macchi, V., Porzionato, A., Vigato, E., Stecco, C., Paoli, A., Parenti, A., Dodi, G. and De Caro, R. (2008) Histotopographic Study of the Pubovaginalis Muscle. Pelviperineology, 27, 7-9.
[95]	Maubon, A., Aubard, Y., Berkane, V., Camezind-Vidal, M.A., Mares, P. and Rouanet, J.P. (2003) Magnetic Resonance Imaging of the Pelvic Floor. Abdominal Imaging, 28, 217-225. https://doi.org/10.1007/s00261-001-0189-2
[96]	Morin, M., Bourbonnais, D., Gravel, D., Dumoulin, C. and Lemieux, M.C. (2004) Pelvic Floor Muscle Function in Continent and Stress Urinary Incontinent Women Using Dynamometric Measurements. Neurourology and Urodynamics, 23, 668-674. https://doi.org/10.1002/nau.20069
[97]	Pan, D., et al. (2018) MRI Evaluation on Morphology and Function of Iliococcygeal Muscles in Fertile and Nulliparous Women. Chinese Journal of Medical Imaging Technology, 12, 581-585.
[98]	Sharfik, A., Asaad, S. and Doss, S. (2002) The Histomorphologic Structure of the Levator Ani Muscle and Its Functional Significance. International Urogynecology Journal, 13, 116-124. https://doi.org/10.1007/s001920200026
[99]	Tunn, R., DeLancey, J.O., Howard, D., Ashton-Miller, J.A. and Quint, L.E. (2003) Anatomic Variations in the Levator Ani Muscle, Endopelvic Fascia, and Urethra in Nulliparas Evaluated by Magnetic Resonance Imaging. American Journal of Obstetrics & Gynecology, 118, 116-121. https://doi.org/10.1067/mob.2003.58
[100]	Morkved, S., Salvesen, K.A., Bo, K. and Eik-Nes, S. (2004) Pelvic Floor Muscle Strength and Thickness in Continent and Incontinent Nulliparous Pregnant Women. International Urogynecology Journal and Pelvic Floor Dysfunction, 15, 384-390. https://doi.org/10.1007/s00192-004-1194-0
[101]	Petros, P. and Abendstein, B. (2019) The Mechanics of Urethral Closure, Incontinence and Midurethral Sling Repair. Part 2 Further Experimental Validation. Neurourology and Urodynamics, 38, 814-817. https://doi.org/10.1002/nau.23841
[102]	Piloni, V., Bassotti, G., Fioravanti, P., Amadio, L. and Montesi, A. (1997) Dynamic Imaging of the Normal Pelvic Floor. International Journal of Colorectal Disease, 12, 246-253. https://doi.org/10.1007/s003840050099
[103]	Price, N., Dawood, R. and Jackson, S.R. (2010) Pelvic Floor Exercise for Urinary Incontinence: A Systematic Literature Review. Maturitas, 67, 309-315. https://doi.org/10.1016/j.maturitas.2010.08.004
[104]	Vereist, M. and Leivseth, G. (2007) Force and Stiffness of the Pelvic Floor as a Function of Muscle Length: A Comparison of Women with and without Stress Urinary Incontinence. Neurourology and Urodynamics, 26, 852-857. https://doi.org/10.1002/nau.20415
[105]	Quaghebeur, J., Petros, P., Wyndaele, J.J. and De Wachter, S. (2021) Pelvic-Floor Function, Dysfunction, and Treatment. European Journal of Obstetrics & Gynecology and Reproductive Biology, 265, 143-149. https://doi.org/10.1016/j.ejogrb.2021.08.026
[106]	Herschorn, S. (2004) Female Pelvic Floor Anatomy: The Pelvic Floor, Supporting Structures, and Pelvic Organs. Reviews in Urology, 6, S2-S10.
[107]	Wallner, C., Dabhoiwala, N.F., DeRuiter, M.C. and Lamers, W.H. (2009) The Anatomical Components of Urinary Continence. European Urology, 55, 932-944. https://doi.org/10.1016/j.eururo.2008.08.032
[108]	Gerten, K.A., Richter, H.E., Wheeler II, T.L., Pair, L.S., Burgio, K.L., Redden, D.T., Varner, R.E. and Hibner, M. (2008) Intraabdominal Pressure Changes Associated with Lifting: Implications for Postoperative Activity Restrictions. American Journal of Obstetrics and Gynecology, 198, 306e1-306e5. https://doi.org/10.1016/j.ajog.2007.09.004
[109]	McComas, A. (1996) Skeletal Muscle: Form and Function. Human Kinetics, Champaign.
[110]	Reeves, N.D., Maganaris, C.N., Longo, S. and Narici, M.V. (2009) Differential Adaptations to Eccentric versus Conventional Resistance Training in Older Humans. Experimental Physiology, 94, 825-833. https://doi.org/10.1113/expphysiol.2009.046599
[111]	Roig, M., O’Brien, K., Kirk, G., Murray, R., McKinnon, P., Shadgan, B. and Reid, W.D. (2009) The Effects of Eccentric versus Concentric Resistance Training on Muscle Strength and Mass in Healthy Adults: A Systematic Review with Meta-Analysis. British Journal of Sports Medicine, 43, 556-568. https://doi.org/10.1136/bjsm.2008.051417
[112]	MacLennan, A., Taylor, A., Wilson, D. and Wilson, D. (2000) The Prevalence of Pelvic Floor Disorders and Their Relationship TP Gender, Age, Parity and Mode of Delivery. BJOG: An International Journal of Obstetrics & Gynaecology, 107, 1460-1470. https://doi.org/10.1111/j.1471-0528.2000.tb11669.x