Effect of prosthetic mass on swing phase work during above-knee amputee ambulation

Recent advances in prosthetic technology have resulted in prosthetic limbs that weigh substantially less than those previously used by amputees. Although one of the clinical expectations associated with lighter limbs was that they would reduce the abnormally high metabolic cost of amputee ambulation, this has not be shown in oxygen consumption studies. This expectation was based on previous studies of normal walking, which showed that the greatest changes in mechanical work occurs in the leg during swing phase acceleration and deceleration. To better understand the relationship between limb mass and mechanical work, this study assessed the effect of varying prosthetic limb masses on the sources and magnitude of the mechanical work required for limb movement during swing. Eight above-knee amputees were studied during over ground walking at their self-selected speeds while wearing identical prosthetic limbs under three weight conditions: unweighted; 0.68 kg of added mass; 1.34 kg of added mass. Using inverse dynamics, the mechanical work from muscle sources and joint transfer sources that was used to accelerate the limb forward during late stance and early swing was determined and compared with the recovery of energy from the limb by hip joint transfer to the trunk during terminal swing deceleration. With the addition of 1.34 kg of mass, there was a combined increase in hip flexor muscle concentric work and mechanical energy transfer across the hip joint of 5.4 J, which was needed to accelerate the heavier prosthetic limb into the swing phase. The increase in acceleration work was balanced by a comparable increase (5.6 J) in the recovery of leg energy during terminal swing deceleration. By effectively conserving the additional mechanical work needed to propel a heavier limb, amputees appear to minimize any adverse effect of prosthetic mass on the mechanical work of walking. This may explain the absence of differences in metabolic cost between limbs of different masses.