The relationship between mechanical work and energy expenditure of locomotion in horses

Three-dimensional motion capture and metabolic assessment were performed on four standardbred horses while walking, trotting and galloping on a motorized treadmill at different speeds, The mechanical work was partitioned into the internal work (W-INT), due to the speed changes of body segments with respect to the body centre of mass, and the external work (W-EXT), due to the position and speed changes of the body centre of mass with respect to the environment. The estimated total mechanical work (W-TOT=W-INT+W-EXT) increased with speed, while metabolic work (C) remained rather constant. As a consequence, the 'apparent efficiency' (eff(APP)=W-TOT/C) increased from 10% (walking) to over 100% (galloping), setting the highest value to date for terrestrial locomotion. The contribution of elastic structures in the horse's limbs was evaluated by calculating the elastic energy stored and released during a single bounce (W-EL,(BOUNCE)), which was approximately 1.23 J kg(-1) for trotting and up to 6 J kg(-1) for galloping. When taking into account the elastic energy stored by the spine bending and released as W-INT, as suggested in the literature for galloping, W-EL,(BOUNCE) was reduced by 0.88 J kg(-1). Indirect evidence indicates that force, in addition to mechanical work, is also a determinant of the metabolic energy expenditure in horse locomotion.