Abstract
Flexion/extension moment arms were obtained for the major muscles crossing the hip, knee and ankle joints in the orang-utan, gibbon, gorilla (Eastern and Western lowland) and bonobo. Moment arms varied with joint motion and were generally longer in proximal limb muscles than distal limb muscles. The shape of the moment arm curves (i.e. the plots of moment arm against joint angle) differed in different hindlimb muscles and in the same muscle in different subjects (both in the same and in different ape species). Most moment arms increased with increasing joint flexion, a finding which may be understood in the context of the employment of flexed postures by most non-human apes (except orang-utans) during both terrestrial and arboreal locomotion. When compared with humans, non-human great apes tended to have muscles better designed for moving the joints through large ranges. This was particularly true of the pedal digital flexors in orang-utans. In gibbons, the only lesser ape studied here, many of the moment arms measured were relatively short compared with those of great apes. This study was performed on a small sample of apes and thus differences noted here warrant further investigation in larger populations.
Highlights
As apes are the closest living relatives of humans, their locomotion is of considerable interest to scientists investigating the evolution of human bipedal gait
Variation in muscle–tendon unit (MTU) moment arms may be expected to have a significant effect on the capacity of each MTU to (1) generate and transfer force and (2) to apply that force at speed, over a wide range of joint positions
Moment arms were scaled by segment length so that they could be compared between subjects of varying size
Summary
As apes are the closest living relatives of humans, their locomotion is of considerable interest to scientists investigating the evolution of human bipedal gait. In the first part of this study (Payne et al 2006) we compared the volume and architecture of hindlimb muscles in apes and humans and related the consequences of these features for functional capacity to aspects of locomotor behaviour. The functional capacity of a muscle–tendon unit (MTU) is governed by the volume and arrangement of the constituent muscle fibres and by the leverage of that MTU about a joint at different joint angles. Human bipedal walking utilizes only a small proportion of the range of mobility of most major lower-limb joints. Variation in MTU moment arms may be expected to have a significant effect on the capacity of each MTU to (1) generate and transfer force and (2) to apply that force at speed, over a wide range of joint positions
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