The Effects of Using a Specially Designed Stirrup on Kinetic Energy Absorption by the Knee Joint of 12 Show Jumping/Eventing Riders ()
1. Introduction
Equestrian sport is one of a kind, where rider’s legs play a key role in communication with the horse, as well as ensuring the rider maintains body balance whilst in the saddle. Rider’s legs work hard to keep the rider in an optimal saddle position, and they need to withstand a lot of force—coming mainly from the dynamic interactions with the ground and the horse while riding. These forces are rather exceptional for equestrian sport, and are otherwise rarely experienced in “normal”, daily activities—the closest one comes to such bent knee physical exertion are during squats. It is especially the knee joints of riders that experience considerable and repeated loading, being responsible for a good deal of shock absorption. A number of professional riders, who ride 6 - 8 horses on a daily basis, often experience knee pain [1] . This discomfort is felt by both experienced and less experienced riders, who have yet to attain a well-trained balance whilst in the saddle, as well as taller riders, who might experience greater forces around the knee joint. Additionally, acceleration of the riders’ legs during jumping decreases with increasing experience level of both horse and rider [2] . There is a significant difference in mean stirrups forces between rising and sitting trot (related to the body weight of the rider): 1.2 N/kg and 0.45 N/kg, respectively. Furthermore, such peaks occur with a biphasic nature having a smaller additional peak: 0.33 and 0.38 N/kg in rising and sitting trot, respectively [3] . Martin and colleagues confirmed this biphasic nature during rising trot, although they found higher mean values: 2.9 N/kg in the sitting phase and 7.5 N/kg in the standing phase [4] .
Use of comfortable, innovative solutions is not only of benefit for riders, it also benefits their horses. Correct placement of the leg in the stirrups and the mechanical properties of shock absorption by the stirrups are believed to ensure knee protection and provide a stable seat. It seems that the riders’ limb muscles act as stabilizers and contribute to coordinate his or her movements during a jump since they show a highly constant pattern of activation [5] . As novice riders often struggle to balance their body with the horses movements [6] [7] , overuse due to higher peak forces can be expected. However, by improving a riders balance and response to the horses (back) movement(s), thus attenuating the shock transfer from the horses to the riders back, one experiences an increase in forces at the stirrups [8] . This is further supported by earlier findings from the same group revealing higher level of activity of the M. quadriceps femoris vastus lateralis in advanced riders compared to novice riders [7] . However, one could argue that this effect may be the result of different leg positions of the riders, since van Beek and colleagues [3] did not find a significant difference in stirrup forces between riders of different experience levels. Additionally, improved shock absorption can also be expected to reduce both overuse injuries and associated pain, ultimately protecting the horse from a rider’s asymmetrical “guarding” style of riding, which they may have adopted in order to try and avoid injury.
The aim of this trial was to test the efficiency of stirrups designed for their shock absorbing properties as measured using acoustic myography (AMG) in healthy equestrian athletes while riding in symmetrical and asymmetrical gaits. The hypotheses tested were, 1) Winderen Knee Protection Solution stirrups reduce the amplitude of the AMG signal around the knee joint of riders at low speeds and low levels of riding activity compared with standard iron stirrups, and 2) At high levels of riding activity the Winderen Knee Protection Solution stirrups reduce the time duration of loading forces around the knee joint of riders when compared with standard iron stirrups.
2. Materials and Methods
2.1. Ethical Approval
The method applied was non-invasive, and the trial project followed the guidelines set by the Helsinki Declaration 2013 (https://www.wma.net/wp-content/uploads/2016/11/DoH-Oct2013-JAMA.pdf). This study was likewise approved by the local ethics committee at IVH, Copenhagen University, Denmark.
2.2. Subjects
Twelve healthy and experienced riders (11 female; 1 male), who were in regular training (minimum of 5 times a week for at least 1 hour) were recruited at random for this trial project from riding schools in an around the Copenhagen area. The average weight of the riders was 59.9 ± 7.7 kg and they were aged 31.2 ± 13.0 years (range 13 - 55). The riders were asked to state any knee related injuries or pain, and to identify which leg was affected if so. If the riders reported an injury or pain, then that leg was selected for measurement.
2.3. Measurements
Measurements from two key anatomic structures, responsible for knee joint amortization in a riding position, namely the patellar ligament (also referred to as the patellar tendon) and the vastus lateralis muscle (lateral head of quadriceps femoris muscle) were selected for this trial project.
2.4. Methodology
Acoustic Myography (AMG), which has proven to be non-invasive, facilitating a real-time monitoring of muscle and ligament activity, as well as enabling the assessment of physical performance during exercise (e.g. cycling, jogging, horse riding) was used. Acoustic myography (AMG) is a biomechanical method measuring generated pressure waves from a contracting muscle or connective tissue [9] [10] .
This trial measured each of the riders during periods of walk, rising trot, canter as well as whilst performing a jump. Left- and right-handed circles and riding in a straight line. Initially, the riders own standard iron stirrups were used for a baseline measurement of the three gaits, and thereafter the standard stirrups were exchanged for the Winderen Knee Protection Solution stirrups. Winderen Knee Protection Solution stirrups have been designed with a double shock absorption system (https://www.winderen.com/en/news/92/technology-that-supports-muscles-and-joints.html). In one instance, where a rider was already using the Winderen Knee Protection Solution stirrups, these were exchanged for another riders standard iron stirrups, and the riders own Winderen Knee Protection Solution stirrups, replaced with a set of identical stirrups supplied by the manufacturer for the purposes of this trial. This was done to avoid any difference that might occur between a new and used set of Winderen Knee Protection Solution stirrups.
AMG recordings were carried out with a CURO unit and CURO sensors (CURO-Diagnostics ApS, Denmark; formerly MyoDynamik ApS) and followed in real time on an iPhone (Apple Inc, Cupertino, CA, USA) via the App “CURO Equine” and a specialized data recording system. This allowed us to see the actual wave recordings and the ESTi score while recording. We used one 50 mm and one 20 mm sensor with a frequency recording range of 0.5 - 20 ± 0.5 kHz, for the muscle and ligament, respectively. Sensors were held in place on the skin above the tissue of interest using a hydrogel patch (M863X-T; R & D Medical Products Inc, USA). The sampling rate was 4 kHz. Recorded data were stored to the CURO unit and after completion of measurements transferred to the CURO software for analysis (https://app.myodynamik.com).
The ESTi-score with its three components: 1) efficiency (E-score) 2) spatial fiber recruitment (S-score), and 3) temporal fiber recruitment (T-score) was calculated using the company software [9] [10] . A high value close to 10 represents a good signal, and conversely one that is close to zero, a poor signal. Thus, an S-score of 8 represents a signal with a very small amplitude (Approx. 0.3 V) and indicates a very efficient use of the muscle. The E- and T-scores were calculated in a similar way, also using a score scale from 0 - 10, where for the E-score a value of 0 represents constant activity and 10 no activity, and for the T-score a high value indicates an efficient use of the fibers, and a low value muscle fibers that are working hard.
2.5. Statistical Analysis
Data were analyzed for statistical significance using standard tests (ANOVA; t-tests), and a level of significance of 5% (P < 0.05) was adopted (GraphPad Prism, Version 9.4). Values are the mean ± SD of 12 subjects.
3. Results
The riders had a mean riding experience rating of 3.3 ± 0.9, where 1 represents a low level of experience and 4 is a very experienced rider (range 1 to 4).
For placement of the sensors on the legs of the subjects, see Figure 1.
The AMG data for the vastus lateralis muscle at the walk, trot and canter can be seen in Figures 2(a)-(c), respectively.
It should be noted that in general these activities result in relatively low E-scores compared with the S- and T-scores. Moreover, the use of Winderen Knee Protection Solution stirrups resulted in higher scores for all four parameters (see figures where closed symbols are Winderen Knee Protection Solution stirrups and open symbols are standard iron stirrups). For Figure 2(a) – vastus lateralis at the walk, there was a statistically significant difference for the E-score
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Figure 1. Placement of the 50 mm and 20 mm AMG sensors on the muscle and ligament of the rider’s leg.
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Figure 2. (a)-(c) Muscle vastus lateralis during periods of walk, trot and canter.
(P = 0.03), T-score (P = 0.05) and ST-scores (P = 0.03), with Winderen Knee Protection Solution stirrups giving better scores. This study found a 289% improvement in the E-score, a 33% improvement in the T-score and a 22% improvement in the ST-score when using a set of Winderen Knee Protection Solution stirrups compared with standard iron stirrups.
For Figure 2(b) and Figure 2(c) – vastus lateralis at the trot and canter, no statistically significant differences were noted for any of the parameters whilst trotting, but for the canter both the S-score and the ST-score were significantly improved with the Winderen Knee Protection Solution stirrups (P = 0.008 and P = 0.01, respectively) compared with standard iron stirrups. It is worth noting that similar improvements to those found during walking were noted: trot: 137% improvement in the E-score, a 15% improvement in the T-score and a 17% improvement in the ST-score; canter: 54% improvement in the E-score, a 22% improvement in the T-score and a 14% improvement in the ST-score.
The AMG data for the patellar ligament at the walk, trot and canter can be seen in Figures 3(a)-(c), respectively.
As with the muscle, the patellar ligament measurements show relatively low E-scores compared with the S- and T-scores. Likewise, the use of Winderen Knee Protection Solution stirrups results in higher scores for all four parameters. For Figure 3(a) – patellar ligament at the walk, there was a statistically significant difference for the E-score (P = 0.007) and the T-score (P = 0.05), with Winderen Knee Protection Solution stirrups giving better scores. This study found a 220% improvement in the E-score, a 35% improvement in the T-score
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Figure 3. (a)-(c) The patellar ligament during periods of walk, trot and canter.
and a 20% improvement in the ST-score when using a set of Winderen Knee Protection Solution stirrups compared with standard iron stirrups.
For Figure 3(b) and Figure 3(c) – patellar ligament at the trot and canter, no statistically significant differences were noted for any of the parameters. However, it is worth noting that similar improvements to those found during walking were noted: trot: 54% improvement in the E-score, a 22% improvement in the T-score and a 14% improvement in the ST-score; canter: 55% improvement in the E-score, a 22% improvement in the T-score and a 14% improvement in the ST-score.
Finally, the riders rated the change from their standard stirrups to the Winderen Knee Protection Solution stirrups as having a mean value of 3.6 ± 1.2 where 1 represents no noticeable difference and 5 represents the most considerable and positive difference they could imagine.
4. Discussion
This trial set out to test the hypotheses that; 1) Winderen Knee Protection Solution stirrups reduce the amplitude of the AMG signal around the knee joint of riders at low speeds and low levels of riding activity compared with standard iron stirrups, and 2) At high levels of riding activity the Winderen Knee Protection Solution stirrups reduce the time duration of loading forces around the knee joint of riders when compared with standard iron stirrups.
The data show a statistically significant difference between AMG parameters (E-, T- and ST-scores) at low speeds (walk), which is in agreement with the first and second hypotheses. However, due to the small sample size and lack of data power with this trial, the changes found at higher speeds, whilst supportive of the first and second hypotheses, were not found to be statistically different. Power analysis shows with the current means and variance that a sample size of 34+ would be needed to achieve statistical significance for AMG parameters at the trot and canter.
This trial has found that the E-score improves when a Winderen Knee Protection Solution stirrup is used compared with a standard iron stirrup. Such an improvement signifies that for any given period of activity, less time is spent under stress or strain, both for the ligament as well as the muscle. In terms of the muscle, a Winderen Knee Protection Solution stirrup resulted in a reduction of stress or strain time in the order of 14 seconds per minute of activity whilst walking and some 5 - 7 seconds less whilst trotting or cantering. For the ligament, a Winderen Knee Protection Solution stirrup resulted in a reduction of stress or strain time in the order of 25 seconds per minute of activity whilst walking and some 9 - 10 seconds less whilst trotting or cantering.
It was likewise clear from the data that the force level in the muscle and ligament was greatly reduced when riding with a Winderen Knee Protection Solution stirrup compared to a standard iron stirrup. This was assessed using the mean of the AMG S-spatial summation and T-temporal summation parameters, which have been found to be very closely correlated with force [10] . A higher ST-score is indicative of fewer active motor units (muscle fibers) and a lower firing rate, which simply means the muscle needs fewer fibers to perform its activity, and in all cases an improvement in the ST-score with the Winderen Knee Protection Solution stirrups was noted; 22% at the walk, 17% at the trot and 26% at the canter. For the patellar ligament, similar improvements in the ST-score were noted; 20% at the walk, 14% at the trot and 14% at the canter. In summary, when using the Winderen Knee Protection Solution stirrups, the muscle and ligament experience lower forces exerted on them as a result of riding at different gaits.
The improvements in the ST-score and consequently the reduction in forces around the knee, are mostly seen through an improvement in the T-score, the firing frequency. In muscle this can be explained as a reduction in the need to tense up fibers and thereby stabilize a joint in response to a shock wave or ground reaction force traveling up through the limb from the stirrups. Thus, since Winderen Knee Protection Solution stirrups have been designed to reduce such shock wave or ground reaction forces whilst riding, this improvement in the T-score is not surprising. In the ligament we likewise measure the frequency of shock waves or ground reaction forces as they travel up through the leg. The improvement seen in the ST-score is therefore also to be anticipated, confirming that Winderen Knee Protection Solution stirrups do in fact reduce such shock waves or ground reaction forces around the knee whilst riding.
Our measurements did not reveal statistically significant differences in the AMG signal during trotting and cantering, however, the riders stated that they felt the greatest benefits of the Winderen Knee Protection Solution stirrups at these higher speeds (likewise those few who jumped), lending weight to our observations. A larger sample size would most likely show statistical significance for these higher speeds. However, all things considered it seems advisable to use Winderen Knee Protection Solution stirrups when riding in order to prevent equestrian foot injuries [11] .
5. Conclusion
It would appear from this trial that Winderen Knee Protection Solution stirrups offer a number of advantages over standard iron stirrups. We have noted a reduction of stress or strain time in the muscle and ligaments around the knee during walking, trotting and cantering. These findings are supported by significant improvements in the AMG E-score and ST-score. We have also observed a considerable improvement in rider comfort and feeling of leg stability, compared with the owners current stirrups, whilst riding with Winderen Knee Protection Solution stirrups. Combined, these findings support the manufacturers’ claims that these stirrups do indeed offer a means of protecting riders’ knees from unnecessary forces and strains whilst riding.
Acknowledgements
The authors are indebted to the riders for giving freely of their time, as well as the riding school owners for allowing us access to the riding facilities used in this trial.