Adiabatic transformability hypothesis of human locomotion.

It is hypothesized that metabolic and mechanical changes in human locomotion associated with changes in speed v are constrained by two attractive strategies: Qmetab = 1 and delta Qmetab/delta v = a positive definite constant. Qmetab = delta Eks-1/ml O2s-1 where delta Eks-1 is the summed increments and decrements per unit time in the translational and rotational kinetic energies of the body's segments and ml O2s-1 is the rate at which chemical energy is dissipated. The expected constancy of delta Qmetab/delta v was derived from an extension of Ehrenfest's adiabatic hypothesis by which transformations (increases, decreases) in locomotion v can be considered as adiabatic, even though the biological conditions are nonconservative and non-rate-limited. The expected significance of Qmetab = 1 was derived from stability considerations of the symmetry per stride of stored and dissipated energy. An experimental evaluation was provided by collecting metabolic and mechanical measures on walking (10 subjects) and running (9 subjects) at progressively greater treadmill speeds but within the aerobic limit. Results revealed that walking was restricted to Qmetab < or = 1, with a nonlinear trajectory in v x Qmetab coordinates shaped by Qmetab = 1 (primarily) and the constancy of delta Qmetab/delta v. Running satisfied Qmetab > 1, with a linear trajectory in v x Qmetab coordinates conforming to delta Qmetab/delta v = a constant, with the constant predicted from invariants in the mechanical space v x delta Eks-1. Results also suggested that the metabolic costs of running might be predictable from measures made in the v x delta Eks-1 space.