GRIEGO MARSH, AM; WICKLER, SJ; HOYT, DF; DUTTO, DJ; COGGER, EA; California State Polytechnic University, Pomona: Hind limb muscle function during decline locomotion in the horse

The cost of transport (ml O2/m) decreases substantially on a 10% decline. We hypothesized that this decreased cost was due to increased eccentric activity and, as a result of the force-length relationship, decreased volume of active muscle. Muscle activity (EMG) and strain (sonomicrometry) were measured during stance phase in a knee extensor (vastus), hip extensor (gluteus), and biarticular hip extensor/knee flexor (semitendinosus, ST) in a horse trotting on the level and decline between 3 & 4 m/s. Integrated EMGs (IEMG) were calculated from rectified signals. In estimating the timing of muscle activity we assumed 45 ms electromechanical delay and 45 ms relaxation period . During decline trotting, IEMG activity increased 35% in the vastus, but decreased approximately 70% in the gluteus and stopped completely in the ST. In the vastus, strains during the first 50% of stance were similar on the level and decline (16% early shortening followed by 6% lengthening). However, in the remaining period of muscle activity, the vastus shortened 11% during level locomotion and lengthened 6% on the decline. In the gluteus, initial shortening strains (first 10% of stance) were greater on the level (2.8 vs 1.4%) and during the remaining period of muscle activity the gluteus shortened during level trotting (1%) but lengthened on the decline (2%). The ST actively shortened during level locomotion (net strain = 19%), but was inactive during decline. Increased IEMG in the vastus and decreased IEMG in gluteus & ST indicates increased importance of vastus function on the decline. Overall, increased eccentric activity in two muscles and decreased IEMG in two muscles may explain the reduced metabolic costs of trotting on a decline. Supported by NIH # S06 GM53933 to DFH & SJW and an ARI grant to SJW.