MECHANICAL DETERMINANTS OF GRADIENT WALKING ENERGETICS IN MAN

1. The metabolic cost and the mechanical work at different speeds duri ng uphill, level and downhill walking have been measured in four subje cts. 2. The mechanical work has been partitioned into the internal wor k (W(int)), due to the speed changes of body segment with respect to t he body centre of mass (BCM), and the external work (W(ext)), related to the position and speed changes of the BCM in the environment. 3. W( ext) has been further divided into a positive part (W(ext)+) and a neg ative one (W(ext)), associated with the energy increases and decreases , respectively, over the stride period. 4. For all constant speeds the most economical gradient has been found to be -10.2% (+/- 0.8 S.D.). 5. At each gradient there is a unique W(ext)+/W(ext)- ratio (= 1 in le vel walking), regardless of speed, with a tendency for W(ext)- and W(e xt)+ to vanish above +15% and below -15% gradient, respectively. 6. W( int) is constant at each speed regardless of gradient. This is partly explained by an only slight decrease in stride frequency at increasing gradient. W(int) constancy implies that it has no role in determining the optimum gradient. 7. A linear multiple regression relating W(ext) + and W(ext)-to the metabolic cost at different gradients showed that negative (eff-) and positive (eff+) efficiencies decrease with increas ing speed (from 0.912 to 0.726, and from 0.182 to 0.146, respectively) . The eff-/eff+ ratio, however, remains rather constant (4.995 +/-0.12 5 S.D.). 8. We conclude that the measured W(ext), the W(ext)+/W(ext)-p artitioning and eff-/eff+ ratio, i.e. the different efficiency of the muscles used as force and brake generators, can explain the metabolic optimum gradient at about -10%.