The spring-mass model and the energy cost of treadmill running.

During running, the behaviour of the support leg was studied by modelling the runner using an oscillating system composed of a spring (the leg) and of a mass (the body mass). This model was applied to eight middle-distance runners running on a level treadmill at a velocity corresponding to 90% of their maximal aerobic velocity [mean 5.10 (SD 0.33) m x s(-1)]. Their energy cost of running (Cr). was determined from the measurement of O2 consumption. The work, the stiffness and the resonant frequency of both legs were computed from measurements performed with a kinematic arm. The Cr was significantly related to the stiffness (P < 0.05, r=-0.80) and the absolute difference between the resonant frequency and the step frequency (P < 0.05, r=0.79) computed for the leg producing the highest positive work. Neither of these significant relationships were obtained when analysing data from the other leg probably because of the work asymmetry observed between legs. It was concluded that the spring-mass model is a good approach further to understand mechanisms underlying the interindividual differences in Cr.