Analysis of legs stiffnesses in unilateral amputee elite athletes during treadmill running and track sprinting

Leg Stiffness (Kleg) is a common parameter used to characterize leg function during bouncing gaits, like running and sprinting. In people with a lower limb amputation Running-Specific Prostheses (RSP) are designed to replicate the spring-like behaviour of the biological leg. The aim of this study is to analyse the leg stiffness of the affected limb (AL) and the unaffected limb (UL) in unilateral amputee elite athletes, during treadmill running and track sprinting. For this reason, a spring-mass model has been defined and implemented, using a custom MATLAB code, to describe and examine the running action during the stance phase. Moreover, not only the ‘Leg Stiffness’ but also the ‘Foot Stiffness’ of the prosthetic foot and the contribution of the residual limb have been investigated. The data used for the analysis had been acquired from treadmill and track in-vivo experimental sessions, in which the participant, the ‘Gold medallist’ sprinter Ambra Sabatini, performed constant speed running. Each test session required specific protocols and specific instrumentations, including Motion Capture cameras, force platforms and wearable force sensors. The results of the analysis were compared to highlight differences on the Kleg between the affected and unaffected limb, running on different types of surfaces (treadmill and track) and using different prothesis configuration. It has been confirmed that unilateral amputee wearing RSP present bilateral asymmetry in Kleg while running: in fact the Kleg was smaller in the affected limb than the unaffected limb. In addition, Leg Stiffness on track was greater than that on treadmill. Although several studies suggested that prosthetic alignment could influence running performance through the regulation of stiffness in lower-extremity amputees, from the data obtained in this work, not significantly changes has been observed considering the different socket alignments. Since prosthetic leg stiffness is considerably lower than stiffness of the RSP, compliance of the residual leg should not be ignored; in fact, a substantial contribution of the residual leg to total leg stiffness was observed. Due to the numerous assumptions and simplifications of the model, caution must be used in the interpretation of these results: further in-vivo experimental sessions will be taken place to overcome a good part of the limitations of this study and improve the quality of the analysis.