Muscle-tendon stresses and elastic energy storage during locomotion in the horse

The stresses acting in muscle-tendon units and ligaments of the forelimb and hindlimb of horses were determined over a range of speed and gait based on recordings of ground reaction forces and limb kinematics. Maximum stresses of 40-50 MPa were calculated to act in several of the principal forelimb (superficial digital flexor (SDF), deep digital flexor (DDF), ulnaris lateralis (UL) and flexor carpi ulnaris/radialis (FCU/R)) and hindlimb tendons (plantaris, DDF) at the fastest galloping speeds recorded (up to 7.4 m s-1). Smaller stresses were found for the gastrocnemius (GAST) tendon (30 MPa) and suspensory ligaments (S-Ligs) (18-25 MPa). Average peak muscle stresses reached 200-240 kPa during galloping. Tendon and muscle stresses increased more steeply with changes of gait and during galloping, than during trotting. Calculations of elastic strain energy storage based on tendon stress showed similar patterns of increase with change of speed and gait, with the greatest contribution to elastic savings by the DDF tendons of the forelimb and hindlimb. In general, the hindlimb contributed two-thirds and the forelimb one-third to overall energy storage. Comparison of tendon elastic energy savings with mechanical work showed a maximum 40% recovery of mechanical work by elastic savings when the horses changed gait from a walk to a slow trot. Percentage of recovery then decreased with increased trotting speed, but increased again with a change of gait to a gallop, reaching 36% recovery at the fastest measured galloping speed (7.4 m s-1). The long length of horse tendons in relation to extremely short pennate muscle fibers suggests a highly specialized design for economical muscle force generation and enhanced elastic energy savings. However, elastic energy savings in terms of percentage of recovery of mechanical work and metabolic energy is less than that observed in wallabies and kangaroos during hopping, but similar to that in humans during running, and greater than that for dogs during trotting and galloping.