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MARTIN, PHILIP E. Mechanical and physiological responses to lower extremity loading during running. Med. Sci. Sports Exerc., Vol. 17, No. 4, pp. 427-433, 1985. Fifteen highly trained men performed treadmill running at 12 km · h-1 to determine the effect of lower extremity loading on measures for seven temporal and kinematic descriptors of the running cycle, the mechanical work done on the lower extremity, oxygen consumption (O2), and heart rate. Five load conditions (no added load and loads of 0.50 kg and 1.00 kg added to either the thighs or feet) were examined. The results demonstrated that O2 and heart rate increased as load was increased on both the thighs and feet. All changes were statistically significant except for the heart rate changes due to thigh loading. The increases in O2 due to foot loading—approximately 7.2% per kg of load— were nearly twice as great as those due to thigh loading. The results also demonstrated that 1.00 kg added to the feet produced small but significant increases in stride length (1.4 cm), swing time (9 ms), and flight time (6 ms) and a decrease in peak ankle velocity (0.23 m · s-1). No other load condition resulted in significant changes in any of the temporal and kinematic variables. The results for mechanical work demonstrated that significant increases in the work done on the leg were produced by the loading but that these increases were limited to the loaded segments. Consistent with the data for oxygen consumption and heart rate, mechanical work was increased to a greater extent by foot loading than by thigh loading. It was concluded from these results that the increased physiological demand was directly related to the mechanical work increases, which in turn were attributed to the increased inertia of the loaded segments rather than modifications in the kinematics of the lower extremity movements.