EFFECT OF PROSTHETIC MASS ON SWING PHASE WORK DURING ABOVE-KNEE AMPUTEE AMBULATION

Recent advances in prosthetic technology have resulted in prosthetic l imbs that weigh substantially less than those previously used by amput ees. Although one of the clinical expectations associated with lighter limbs was that they would reduce the abnormally high metabolic cost o f amputee ambulation, this has not be shown in oxygen consumption stud ies. This expectation was based on previous studies of normal walking, which showed that the greatest changes in mechanical work occurs in t he leg during swing phase acceleration and deceleration. To better und erstand the relationship between limb mass and mechanical work, this s tudy assessed the effect of varying prosthetic limb masses on the sour ces and magnitude of the mechanical work required for limb movement du ring swing. Eight above-knee amputees were studied during over ground walking at their self-selected speeds while wearing identical prosthet ic limbs under three weight conditions: unweighted; 0.68 kg of added m ass; 1.34 kg of added mass. Using inverse dynamics, the mechanical wor k from muscle sources and joint transfer sources that was used to acce lerate the limb forward during late stance and early swing was determi ned and compared with the recovery of energy from the limb by hip join t transfer to the trunk during terminal swing deceleration. With the a ddition of 1.34 kg of mass, there was a combined increase in hip flexo r muscle concentric work and mechanical energy transfer across the hip joint of 5.4J, which was needed to accelerate the heavier prosthetic limb into the swing phase. The increase in acceleration work was balan ced by a comparable increase (5.6J) in the recovery of leg energy duri ng terminal swing deceleration. By effectively conserving the addition al 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.