MARSH, R.L.*; RUBENSON, J.; HENRY, H.T.; DIMOULAS, P.M.; Northeastern University, Boston, MA: Muscle energetics measured by blood flow in guinea fowl during uphill running

Running uphill requires more energy than running at the same speed on the level. We used measurements of muscle blood flow with microspheres to partition the energy cost of uphill running among the individual hindlimb muscles. We measured whole animal energetics and blood flow under 4 conditions: 1) rest; 2) level running at 1.5 m/s; 3) uphill running at 1.5 m/s on a 15 %slope; and 4) running at 2.34 m/s. Running uphill at 1.5 m/s and running at 2.34 m/s both resulted in an ~30% increase in metabolic rate and total leg blood flow over the values for level running at 1.5 m/s. Based on blood-flow, ~85% of the increase in leg-muscle energy use between level and uphill running occurred in stance phase muscles. This increase in stance cost was not distributed uniformly across the stance phase muscles. At least 60% of the increase in stance costs was accounted for by increases in blood flow to the hip extensors, compared to approximately 27% in ankle extensors, and only 13% in knee extensors. Among the hip extensors just 3 muscles accounted for almost the entire increase within this group. The increase in energy expenditure to swing phase muscles in uphill running (~15% of the total increase) may seem surprising given that these muscles cannot contribute to lifting the body. However, uphill running requires larger angular excursions of the joints during swing phase, and may also require increased swing-phase work against gravity to lift the limb. The distribution of energy use among the muscles is different when comparing uphill running to running at 2.34 m/s, which requires the same total metabolic rate from the hindlimb muscles. This difference suggests a mechanism to explain differences in maximal metabolic rate found previously during level and uphill running. Supported by NIH AR47337.