Regulation of step frequency in endurance transtibial amputee athletes, using a running-specific prosthesis.

Laura Oudenhoven, Judith Boes
Faculteit der Bewegingswetenschappen, VU Amsterdam
Han Houdijk, Laura Hak

Since the invention of the running specific prosthesis (RSP), running has become very popular among lower limb amputees [1]. Typically, running gait is characterized by spring-like behaviour of the legs during the ground contact phase, where the legs compress in the first half of stance and rebound during push off. In accordance with this spring-like behaviour, leg stiffness is assumed to be an important factor in the regulation of step frequency while running. A higher leg stiffness corresponds with a shorter contact time and therefore results in a higher step frequency. Leg stiffness can be seen as a summation of the stiffness of underlying joints, which on its turn is influenced by the muscles surrounding the joints and can be modulated to accommodate different step frequencies. However, in amputees leg stiffness does not only depend on modifiable joint stiffness, but is also influenced by the RSP stiffness. RSP stiffness is assumed to be fixed, since the prosthesis is a real physical spring. It is currently unknown how the fixed RSP stiffness influences step frequency and step frequency modulation in amputees. Theoretically, running with a step frequency close to the natural frequency of the RSP allows for the largest amount of energy return from the RSP and least amount of work done by the muscles [2]. Therefore, it might be optimal for the residual joints to conform to the natural frequency of the RSP and hence act as stiff elements of the leg. The aim of the present study was to investigate how step frequency is regulated when running on an RSP. The first question was whether the preferred step frequency of an amputee athlete is close to the natural frequency of the RSP. The second question was whether amputees can deviate step frequency from their preferred step frequency (PSF) and how this is regulated in terms of kinetics, kinematics and muscle activity.

Seven unilateral transtibial amputee long distance runners participated in this study. Subjects ran on a treadmill with embedded forceplate on five different step frequencies. Step frequencies were imposed by a metronome, varying around their preferred step frequency, as measured during the last minute of a warming-up trial. To determine the natural frequency of the RSP, subjects hopped on the forceplate on their prosthetic leg, keeping the residual joints extended and as stiff as possible. The difference between the natural frequency of the RSP and the preferred step frequency during running was determined, as well as the effects of imposed step frequency on leg stiffness (Kleg), knee joint stiffness (Kknee), muscle activity and oxygen consumption (V ̇O2). Besides, it was investigated whether these outcome measures differed between legs (prosthetic /intact).
For all subjects, the natural frequency of the RSP was higher than the preferred step frequency during running. When step frequencies were imposed, kleg increased with increasing step frequencies in both legs, but Kleg of the intact leg increased more with high step frequencies than Kleg of the prosthetic leg (p<.05). Although there was considerable variation between subjects, Kknee also increased with higher step frequency, especially at the highest frequency (PSF +15%) in the prosthetic leg. Muscle activity in the upper leg did not show a clear relationship with step frequency. V ̇O2 was lowest around PSF, but V ̇O2 at PSF did only differ significantly from the trial at the lowest step frequency (PSF -15%) (p<0.05).
The subjects did not synchronize their preferred step frequency to the natural frequency of the RSP. Therefore, we cannot assume that the prosthesis dictates the behaviour of the legs and hence dominates step frequency. This was further supported by the fact that we found a large variation in gait kinematics and running style between subjects, although all of them used the same type of RSP. Subjects were able to vary step frequency. A higher step frequency was accompanied by shorter contact times and higher leg stiffness. The effect of step frequency on leg stiffness was larger for the intact leg than for the prosthetic leg, which suggests that step frequency was mainly regulated by adapting the intact leg. For knee kinematics, we found a lot of variation between subjects and in some subjects the knee of the prosthetic leg did not show spring-like behaviour at high step frequencies as it did not flex during stance phase but remained extended. This supports the notion that that the capacity to regulate leg stiffness is limited for the prosthetic leg. Considering energy expenditure, subjects showed comparable effects of step frequency as able-bodied: V ̇O2 was lowest around PSF. This suggests that preferred step frequency is adopted to minimize metabolic cost, but since this deviates from the natural frequency of the RSP the exact influence of the RSP on preferred frequency remains unclear. In conclusion, the preferred step frequency of amputee runners deviates from the natural frequency of the RSP and athletes are able to modify step frequency of both legs, although this capacity is reduced in the prosthetic leg. These findings will be helpful in the future to understand and predict the optimal stiffness of an RSP for individual athletes. References [1] Nolan, L. Foot and ankle surgery (2008). [2] Noroozi et al., Journal of sports engineering and technology (2012).