Ae. Minetti et al., MECHANICAL DETERMINANTS OF THE MINIMUM ENERGY-COST OF GRADIENT RUNNING IN HUMANS, Journal of Experimental Biology, 195, 1994, pp. 211-225
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%.