. Energetics and mechanics of human running on surfaces of different stiffnesses. J Appl Physiol 92: 469-478, 2002; 10.1152/ japplphysiol.01164.2000.-Mammals use the elastic components in their legs (principally tendons, ligaments, and muscles) to run economically, while maintaining consistent support mechanics across various surfaces. To examine how leg stiffness and metabolic cost are affected by changes in substrate stiffness, we built experimental platforms with adjustable stiffness to fit on a force-plate-fitted treadmill. Eight male subjects [mean body mass: 74.4 Ϯ 7.1 (SD) kg; leg length: 0.96 Ϯ 0.05 m] ran at 3.7 m/s over five different surface stiffnesses (75.4, 97.5, 216.8, 454.2, and 945.7 kN/m). Metabolic, ground-reaction force, and kinematic data were collected. The 12.5-fold decrease in surface stiffness resulted in a 12% decrease in the runner's metabolic rate and a 29% increase in their leg stiffness. The runner's support mechanics remained essentially unchanged. These results indicate that surface stiffness affects running economy without affecting running support mechanics. We postulate that an increased energy rebound from the compliant surfaces studied contributes to the enhanced running economy. biomechanics; locomotion; leg stiffness; metabolic rate IN THEIR GROUNDBREAKING work, McMahon and Greene (29) investigated the effects of surface stiffness (k surf ) on running mechanics. Their study sought to determine whether it was possible to build a track surface that would enhance performance and decrease injury. Their work showed that a range of k surf values existed over which a runner's performance was enhanced by decreasing foot-ground contact time (t c ), decreasing the initial spike in peak vertical ground reaction force (f peak ), and increasing stride length. Tracks built within this enhanced performance range at Harvard University, Yale University, and Madison Square Garden have been shown to increase running speeds by 2-3% and to decrease running injuries by 50% (29). Despite the success of these "tuned tracks," the mechanisms underlying the performance enhancement are not clearly understood.A major assumption of McMahon and Greene's (29) was that the running leg and surface could be represented as a simple spring and mass (Fig.

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