The energetic costs of trunk and distal-limb loading during walking and running in guinea fowl Numida meleagris II.: Muscle energy use as indicated by blood flow

We examined the changes in muscle energy use in guinea fowl running at 1.5 ms(-1) either unloaded, or carrying trunk loads equal to 23% of body mass, or loads on their distal legs equal to a total of 5% of body mass. We estimated muscle energy use by measuring blood flow to all of the leg muscles using injected microspheres. Total blood flow to the leg muscles increased by approximately 15% under both loading conditions, which matched the percentage increase in net organismal metabolic rate. Significant increases in energy use ( inferred from blood flow) above that found in unloaded birds were found in 12 muscles in trunk-loaded birds, with most of the increases restricted to stance-phase muscles, as predicted. Just three of these muscles, the femerotibialis, the iliotibialis lateralis pars postacetabularis and the fibularis longus accounted for 70% of the increased energy use. Noticeably absent from the group of muscles that increased energy use during trunk loading were several large biarticular muscles that have extensor actions at the hip or ankle, but flexor actions at the knee. We concluded that the low energetic cost of carrying trunk loads in guinea fowl may rely on the activation of a group of muscles that together provide support and propulsion across all the major joints, without producing opposing moments at other joints that could potentially waste energy. The specific leg muscles responsible for the increase in metabolism during trunk loading also suggest that the energy cost of producing mechanical work may be an important determinant of the cost of carrying extra mass on the trunk. During distal-limb loading, eleven leg muscles had significant increases in energy use, but unlike during trunk loading, both stance- and swing-phase muscles had large increases in energy use. This distribution of energy use between stance and swing agrees with the prediction that increased mechanical work determines the cost of limb loading, because a substantial fraction of the increased segmental work during distal-limb loading in guinea fowl has been found to occur during stance.