ELLERBY, D.J.*; MARSH, R.L.; HENRY, H.T.; CARR, J.A.; Northeastern University, Boston; Northeastern University, Boston; Northeastern University, Boston; Northeastern University, Boston: How leg muscles respond to back and limb loading in running guinea fowl: clues about mechanical function from blood flow measurements

Surprisingly little direct information is available on the mechanical function of the diverse array of muscles active during running, and on the division of energy use among the variety of mechanical tasks performed. Muscle blood flow is locally regulated and flow is distributed proportional to metabolic rate. Thus blood flow can be used as a measure of energy use. By adding weights to manipulate body weight and limb mass in running guinea fowl (Numida meleagris), we aimed to reveal the mechanical functions of different leg muscles through their metabolic response to loading. We used the microsphere technique to measure the blood flow to 25 leg muscles, under unloaded, back-loaded, and limb-loaded conditions. Limb loading increases the work required to swing the limbs and correspondingly most swing phase muscles showed a significant increase in blood flow during distal limb loading relative to unloaded running. Previous work has hypothesized that supporting body weight is the energetically most expensive function performed during stance. This hypothesis would predict that back-loading should influence the energy expenditure of many muscles active during stance. However, our results indicate that in guinea fowl most of the increased energy expenditure required by back-loading is restricted to a small subset of muscles that could potentially support the increased weight. The femero-tibialis, iliotibialis lateralis pars postacetabularis, fibularis longus, and gastrocnemius medialis account for 90% of the increase in blood flow during back loading. Others, such as the lateral gastrocnemius and digital flexors, are unaffected by back-loading. Supported by NIH AR47337 to RLM.