This study examined the effect of adding load to either the feet or thighs on biomechanical and physiological measures of running performance. Submaximal oxygen consumption and heart rate, selected temporal and kinematic descriptors of the running cycle, and measures of mechanical work were quantified for five load conditions to determine the effect of both load magnitude and load position on running performance.

Fifteen subjects completed an eight-minute run on a treadmill at 12.0 km/hr for each of the following load conditions:​ 1) no added load, 2) 0.25 kg added to each thigh, 3) 0.25 kg added to each foot, 4) 0.50 kg added to each thigh, and 5) 0.50 kg added to each foot. Oxygen consumption and heart rate data were collected and high speed films were taken at 100 fps for each load condition. The films were analyzed to yield values for a series of temporal and kinematic descriptors of the running patterns of the subjects including stride length, single leg support time, swing time, and flight time. Also measured from film were the mechanical work done on the foot, shank, thigh, and total leg, and the contributions of the joint reactions forces and joint moments of the lower extremity to these work measures. The work analysis was limited not only to the lower extremity, but also to the swing phase of the running cycle.

The results demonstrated that oxygen consumption increased nearly linearly as load was increased on both the thighs and feet. Foot loading, however, resulted in oxygen consumption increases that were approximately twice as great as those due to thigh loading. The increase in oxygen consumption was about 0.7% for each 100 g of load applied to the feet. The results for heart rate were similar to those for oxygen consumption. Heart rate, however, was a less sensitive measure of the increased demand due to the addition of load rather than oxygen consumption.

The results for the temporal and kinematic descriptors indicated that there was little change in these variables produced by adding load to the feet or thighs. Based on these results, it is believed that the basic movement pattern for running is resistant to change when small amounts of load are applied to the lower extremities. This has interesting implications not only when considered from mechanical and physiological perspectives, but also from a motor control perspective.

When the effect of adding load to the feet or thighs on the mechanical work done on the segments of the leg was evaluated, the results demonstrated that the work values increased as load was increased. These changes, however, were limited to the segment which was loaded. While the increases in work were proportional to the magnitude of the load, they were considerably greater for the feet than for the thighs. With respect to the relative contributions of the joint reaction forces and joint moments to the work done on the lower extremity segments, the results indicated that the joint reaction forces played a greater role in doing work during the swing phase than the joint moments. Nevertheless, both the hip and knee moments made important contributions to the work done on the thigh and shank. The function of the hip moment was primarily one of energy generation to the thigh early in the swing;​ whereas, the knee moment acted primarily to dissipate energy from the thigh and shank.

When considered in combination, the results of this study indicated that a direct relationship exists between the changes in oxygen consumption and changes in the work done on the lower extremity during the swing phase as load was increased. Because there was little change in the basic pattern of movement, the increases in work were due almost totally to the increases in the inertia of the loaded segment. The increases in oxygen consumption were then attributed to greater involvement of the musculature. This increased musculature involvement manifested itself in the form of increased joint forces and moments which were needed to overcome the increased inertia due to segment loading.