Individuals with transtibial amputations face unique biomechanical constraints when running and sprinting using running-specific prostheses. Additionally, because the use of running-specific leg prostheses is becoming more common, measures of mediolateral (ML) foot placement and its variability may be useful for those seeking simple metrics to assess potential balance problems in runners and sprinters. To this extent, a team of researchers examined the effects of speed and prosthetic legs on ML foot placement and its variability in sprinters with and without transtibial amputations. The results were published January 15 in the online journal PLOS ONE.
The affected leg when fitted with a running prosthesis generates lower ground forces and exhibits less stiffness compared to the biological leg, the distal end of the residual limb lacks the neural specialization that the plantar surface of the foot provides, and running-specific prostheses cannot provide direct sensory information about ground contact. Additionally, using these passive-elastic running prostheses likely challenges a person’s ability to maintain balance while running and sprinting because the design primarily facilitates forward sagittal plane motion. In addition, running-specific prostheses have a fixed stiffness, whereas the human leg can be neurally modulated, allowing able-bodied people to adapt to changes in surface stiffness and speed. As a final point, running-specific prostheses do not provide any proprioceptive feedback about “ankle” joint or foot position. In light of these observations, the researchers explored how well sprinters can modulate foot placement from step-to-step when using running-specific prostheses.
The researchers measured the midline of the body and center of pressure in the ML direction while 12 able-bodied sprinters and seven Paralympic sprinters with transtibial amputations (six unilateral, one bilateral) ran across a range of speeds up to maximum speed on a high-speed force measuring treadmill. They quantified ML foot placement relative to the body’s midline and its variability.
Overall, the study highlights several findings about sprinters with and without transtibial amputations, the study authors wrote. “We infer from our results that 1) when compared to slow speeds, maintaining lateral balance is more challenging at faster running speeds up to maximum sprint speed and 2) sprinters with a unilateral transtibial amputation found maintaining lateral balance just as challenging as non-amputee sprinters. Furthermore, the apparent asymmetries in ML foot placement and its variability suggest that the use of running-specific prostheses results in a compensatory foot placement strategy for maintaining lateral balance in sprinters with a unilateral transtibial amputation. Finally, when compared to all other sprinters in our subject pool, the sprinter with bilateral transtibial amputations exhibited the greatest increases in ML foot placement variability across normalized speed, indicating that maintaining lateral balance was the most challenging for this Paralympic sprinter.”