MECHANICAL DETERMINANTS OF THE MINIMUM ENERGY-COST OF GRADIENT RUNNING IN HUMANS

The metabolic cost and the mechanical work of running at different spe eds and gradients were measured on five human subjects. 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 t he external work (W-ext) due to the position and speed changes of the body centre of mass in the environment. W-ext was further divided into a positive part (W-ext(+)) and a negative part (W-ext(-)), associated with the energy increases and decreases, respectively, over the strid e period. For all constant speeds, the most economical gradient was -1 0.6 +/- 0.5% (S.D., N=5) with a metabolic cost of 146.8 +/- 3.8 ml O-2 kg(-1) km(-1). At each gradient, there was a unique W-ext(+)/W-ext(-) ratio (which was 1 in level running), irrespective of speed, with a t endency for W-ext and W-ext to disappear above a gradient of +30 % and below a gradient of -30 %, respectively. W-int was constant within ea ch speed from a gradient of -15 % to level running. This was the resul t of a nearly constant stride frequency at all negative gradients. The constancy of W-int within this gradient range implies that W-int has no role in determining the optimum gradient. The metabolic cost C was predicted from the mechanical experimental data according to the follo wing equation: C = W-ext(-)-el(-) / eff(-) + W-ext(+)-el(+) / eff(+) W-int / eff(i), where eff(-) (0.80), eff(+) (0.18) and eff(i) (0.30) are the efficiencies of W-ext(-), W-ext(+) and W-int, respectively, an d el(-) and el(+) represent the amounts of stored and released elastic energy, which are assumed to be 55 J step(-1). The predicted C versus gradient curve coincides with the curve obtained from metabolic measu rements. We conclude that w(ext)(+)/w(ext)(-) partitioning and the eff (+)/eff(-) ratio, i.e. the different efficiency of the muscles during acceleration and braking, explain the metabolic optimum gradient for r unning of about -10%.