HORSE ANATOMY AND PHYSIOLOGY

The Mechanics Of The Hind Limb In Equine Locomotion

The Mechanics Of The Hind Limb In Equine Locomotion

Abstract

The forelimb digital flexors of the horse display remarkable diversity in muscle architecture despite each muscle-tendon unit having a similar mechanical advantage across the fetlock joint. We focus on two distinct muscles of the digital flexor system: short compartment deep digital flexor (DDFsc) and the superficial digital flexor (SDF). The objectives were to investigate force-length behavior and work performance of these two muscles in vivo during locomotion, and to determine how muscle architecture contributes to in vivo function in this system. We directly recorded muscle force (via tendon strain gauges) and muscle fascicle length (via sonomicrometry crystals) as horses walked (1.7 m s- 1), trotted (4.1 m s- 1) and cantered (7.0 m s- 1) on a motorized treadmill. Over the range of gaits and speeds, DDFsc fascicles shortened while producing relatively low force, generating modest positive net work. In contrast, SDF fascicles initially shortened, then lengthened while producing high force, resulting in substantial negative net work.

Introduction

The size and speed of large running mammals require substantial energetic demands ([Alexander, 1984] and [Schmidt-Nielsen, 1984]). Among large mammals, horses consume less metabolic energy than would be expected based on these demands, even when moving at relatively fast speeds ([Taylor et al., 1970] and [Taylor et al., 1982]). Much of this extra locomotor capacity may result from effective utilization of elastic strain energy. Substantial elastic energy recovery from cyclic loading and unloading of long tendons has been demonstrated in horses ([Dimery and Alexander, 1985], [Dimery et al., 1986] and [Biewener, 1998]) and other mammals such as camels (Alexander et al., 1982) and macropods ([Alexander and Vernon, 1975] and [Biewener and Baudinette, 1995]). Strain energy recovery is most effective when animals employ bouncing gaits at faster running speeds (Cavagna et al., 1977). Long, gracile tendons also favor elastic strain energy storage and recovery (Biewener and Roberts, 2000).

Elastic energy recovery alone, however, cannot completely account for the apparent reduction of locomotor costs in horses (Heglund et al., 1982b). Overall locomotor economy depends on both tendon and muscle properties (Biewener and Roberts, 2000). Energetically favorable use of strain energy requires that tendons allow appreciable strain energy storage under load, and that the muscles of these tendons produce adequate resisting force in a metabolically economical manner. Thus, the integrated function of muscle-tendon units is critical to locomotor economy and limb-spring capability ([Biewener and Roberts, 2000] and [Roberts, 2002]).

Contrary to the traditional interpretation of limb muscle as a power generating actuator, muscles in the distal limb of a number of avian and mammalian species are specialized to perform little to no net mechanical work during level, steady-speed locomotion ([Roberts et al., 1997], [Biewener et al., 1998] and [Daley and Biewener, 2003]). What is the function of muscles that perform little or no work in locomotion? Certainly there is potential for non-work muscles to serve as joint stabilizers or in positioning coordination. However, the function of such muscles, if their architectural features were appropriate, could also ...