People with transtibial amputation (TTA) using passive-elastic prostheses have greater leg muscle activity and metabolic cost during level-ground and sloped walking than non-amputees. Use of a stance-phase powered (BiOM) versus passive-elastic prosthesis reduces metabolic cost for people with TTA during level-ground, +3° and +6° walking. Metabolic cost is associated with muscle activity, which may provide insight into differences between prostheses. We measured affected leg (AL) and unaffected leg (UL) muscle activity from ten people with TTA (6 males, 4 females) walking at 1.25 m s −1 on a dual-belt force-measuring treadmill at 0°, ±3°, ±6° and ±9° using their own passive-elastic and the BiOM prosthesis. We compared stride average integrated EMG (iEMG), peak EMG and muscle activity burst duration. Use of the BiOM increased UL lateral gastrocnemius iEMG on downhill slopes and AL biceps femoris on +6° and +9° slopes, and decreased UL rectus femoris on uphill slopes, UL vastus lateralis on +6° and +9°, and soleus and tibialis anterior on a +9° slope compared to a passive-elastic prosthesis. Differences in leg muscle activity for people with TTA using a passive-elastic versus stance-phase powered prosthesis do not clearly explain differences in metabolic cost during walking on level ground and slopes.
Passive-elastic prosthetic feet are manufactured with different numerical stiffness categories that are prescribed based on the body mass and activity level of the user, but the mechanical properties, such as the stiffness values and hysteresis are not typically reported by the manufacturer. Since the mechanical properties of passive-elastic prosthetic feet can affect the walking biomechanics of people with transtibial or transfemoral amputation, characterizing these properties would provide objective values for comparison and aid the prescription of prosthetic feet. Therefore, we characterized the axial stiffness values, torsional stiffness values, and hysteresis of 33 different categories and sizes of a commercially available passive-elastic prosthetic foot model, the Össur low-profile (LP) Vari-flex with and without a shoe. We measured axial stiffness from compression and torsional stiffness from dorsiflexing and plantarflexing the prostheses. In general, a greater numerical prosthetic foot stiffness category resulted in increased heel, midfoot, and forefoot axial stiffness values, increased plantarflexion and dorsiflexion torsional stiffness values, and decreased heel, midfoot, and forefoot hysteresis. Moreover, a greater prosthetic foot size resulted in decreased heel, midfoot, and forefoot axial stiffness values, increased plantarflexion and dorsiflexion torsional stiffness values, and decreased heel and midfoot hysteresis. Finally, adding a shoe to the LP Vari-flex prosthetic foot resulted in decreased heel and midfoot axial stiffness values, decreased plantarflexion torsional stiffness values, and increased heel, midfoot, and forefoot hysteresis. In addition, we found that the force-displacement and torque-angle relationships were better described by curvilinear than linear equations. Ultimately, our results can be used to objectively compare LP Vari-flex prosthetic feet to other prosthetic feet in order to inform their prescription and design and use by people with a transtibial or transfemoral amputation.
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