Kegel (1978) determined that from the amputees perspective their greatest functional limitations were in running, walking long distances or on rough terrain, and difficulties with being able to return to pre-amputation employment levels. In a subsequent study (Kegel 1980) that focussed more on recreational activities in amputees, the most common factors that limited their activity were perspiration, pain, residual limb sores and fatigue. The level of functional achievement of congenital and younger traumatic amputees may be significantly compromised by the characteristics of the prosthetic limb. The functional limitations can be broadly broken down into the following areas; discomfort and mechanical skin injury, an inability to walk distances and run, and difficulties with fatigue and reduced endurance. Since these studies on the functional limitations of amputees have been published there have been many biomechanical analyses of amputee ambulation performed. The question to be answered is, ''To what extent have these biomechanical studies contributed to the functional enhancement of amputee patients''?Discomfort and Mechanical Skin InjuryBiomechanical research has begun to explore the loading conditions of the intact and residual limbs of the amputee patient. In spite of the loss of the normal neuromuscular control mechanisms of the intact foot and ankle, amputee runners tend to have lower than normal loading of the prosthetic limb compared to the intact limb (Engsberg 1993, Miller 1987, Czemiecki, 1990). In addition, when compared to non-amputees running at the same velocity, amputees exhibit a reduced loading rate and vertical impact peak on their prosthetic limbs but an increased loading rate and impact peak on their intact limbs (Miller 1987, Czemiecki, 1990). The choice of prosthetic component does not seem to affect the loading characteristics of the prosthetic limb (Czerniecki, 1990), however, the abnormal loading of the intact limb can be reduced by the choice of prosthetic component (Czerniecki, 1990). The data in walking also supports these general concepts of lower than normal loading of the prosthetic limb and greater than normal loading of the intact limb (Barth 1992, Engsberg 1991, Powers 1994). From this data it is difficult to determine whether the goals of prosthetic development should be to strive towards a normalization of the impact loading characteristics, which might suggest a more ''normal'' use of the limb, or rather to reduce the impact loads with the goal of reducing tissue injury and pain.Inability to Walk Longer Distances and to RunHistorically biomechanical investigations were limited to descriptive analyses of the kinematics of amputee ambulation (Culham 1984). More recently biomechanical techniques have been advanced that allow us to gain an understanding of the biomechanical mechanisms that underly amputee ambulation (Winter 1983a, 1983b, Robertson 1980). BK amputee runners compensate for the absent or markedly reduced mechanical energy generation in the prosthetic limb by using their hip extensor musculature as an energy absorber and generator during stance phase (Czerniecki 1991). There are few mechanical work adaptations in the intact stance phase and prosthetic swing phase limbs. There is an increase in the mechanical work done on the intact swing phase limb (Czerniecki and Gitter 1992) however, which seems to result in an increased energy transfer to the trunk during terminal swing phase (Czernieck and Gitter 1994). The energy transfer to the trunk may partially compensate for the absence of push off power on the prosthetic foot. The Flex energy storing prosthetic foot appears to result in a normalization of the power output characteristics of the hip and knee during stance phase on the prosthetic limb.Mechanical power output, and mechanical work analyses have similarly significantly increased our understanding of the adaptations BK amputees use in walking (Winter and Sienko 1988, Gitter 1991, Colborne 1992). As with running, the most important compensatory adaptation for the reduced power generation at the prosthetic foot is an increase in mechanical work by the hip extensors on the prosthetic side. The increased work done by the hip extensors including the hamstrings in early and mid stance phase on the prosthetic limb necessitates an increase in co-contraction by the knee extensors. The co-contraction at the knee results in an overall reduction in the calculated net knee moment. Energy storing prosthetic components result in significantly greater energy absorption and energy generation than conventional components but in walking they do not significantly effect the mechanical work characteristics at the hip and knee on the prosthetic limb.Reduced Endurance and FatigueIt is well established that amputees have an increased metabolic cost of ambulation (ml O2/kg/m). There is little known about the biomechanical correlates of the increased energy expenditure of ambulation in amputees. One of the important conceptual frameworks that has been used to explain the increased metabolic energy expenditure has been an excessive excursion of the center of mass of the body (Inman 1981). The increased excursion of the center of mass results from an increased muscle work in the extremities. Other conceptual frameworks that have been used include modeling the swing phase limb as a compound pendulum and determining the effect of modifications in mass and mass distribution of the limb (Holt 1990), and quantification of the amounts of energy transfer, and mechanical work performed (Martin 1992).SummaryBiomechanical studies have significantly increased our understanding of impact loading of the extremities as well as the influence of prosthetic foot design on the impact loading characteristics. Similarly recent developments in biomechanical techniques have assisted us in understanding the underlying mechanisms that allow amputees to walk and run. Further research is required to allow us to determine the underlying mechanisms which result in an increased metabolic cost of ambulation.