Abstract
A horse's legs are compressed during the stance phase, storing and then returning elastic strain energy in spring-like muscle-tendon units. The arrangement of the muscle-tendon units around the lever-like joints means that as the leg shortens the muscle-tendon units are stretched. The forelimb anatomy means that the leg can be conceptually divided into two springs: the proximal spring, from the scapula to the elbow, and the distal spring, from the elbow to the foot. In this paper we report the results of a series of experiments testing the hypothesis that there is minimal scope for muscle contraction in either spring to adjust limb compliance. Firstly, we demonstrate that the distal, passive leg spring changes length by 127 mm (range 106-128 mm) at gallop and the proximal spring by 12 mm (9-15 mm). Secondly, we demonstrate that there is a linear relationship between limb force and metacarpo-phalangeal (MCP) joint angle that is minimally influenced by digital flexor muscle activation in vitro or as a function of gait in vivo. Finally, we determined the relationship between MCP joint angle and vertical ground-reaction force at trot and then predicted the forelimb peak vertical ground-reaction force during a 12 m s(-1) gallop on a treadmill. These were 12.79 N kg(-1) body mass (BM) (range 12.07-13.73 N kg(-1) BM) for the lead forelimb and 15.23 N kg(-1) BM (13.51-17.10 N kg(-1) BM) for the non-lead forelimb.
Highlights
Large cursorial mammals have lengthened distal limb bones and tendons
Tendinous tissue is elastic and returns about 93% of the energy stored in it (Ker, 1981), the long tendons and muscle aponeurosis are used to store and return elastic energy during the stance phase of locomotion (Alexander, 1988; Alexander and Bennet-Clarke, 1977)
The short fibres reduce the energetic cost of force generation. These adaptations result in a substantial reduction in the energetic cost of locomotion (Minetti et al, 1999; Roberts et al, 1997; Biewener et al, 1998), with the limb acting as a pogo-stick-like, tuned-spring system (Blickhan, 1989; Cavagna et al, 1977; McMahon, 1985; McMahon and Cheng, 1990)
Summary
Large cursorial mammals have lengthened distal limb bones and tendons. The muscles of the distal limb associated with weight bearing have short muscle fibres, a pennate structure and significant passive elastic properties (Alexander et al, 1979, 1982; Biewener, 1998a; Dimery et al, 1986). The limbs of smaller animals (including humans) appear mechanically as compression springs (Farley et al, 1991, 1993; McMahon and Greene, 1979), but much of the length change occurs in muscle. This is energetically expensive due to the greater cost of force generation in long fibres and the requirement for muscle work (force × length change)
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