The objective of this study was to quantify individual muscle function differences between level walking and backpack load carriage at the same speed by using a muscle-actuated forward dynamics simulation. As experimental investigations have revealed that backpack loads of up to 64 per cent of an individual's body mass have little effect on the sagittal plane gait kinema-tics, further biomechanical analyses are necessary to investigate the contributions of individual muscle coordination strategies to achieve a given motor task by mechanical power generation, absorption, and transference to each body segment. A biomechanical framework consisting of a musculoskeletal model actuated by 18 Hill-type musculotendon actuators per leg and a non-linear suspension model of a backpack equipped with shoulder straps and waist belt was utilized to perform the simulation study. An optimization framework based on minimizing the muscle energy consumption was employed to investigate the muscle load sharing mechanism during simulation of the movements under investigation. Estimated muscle activations were in good agreement with the salient features of the corresponding electromyographic recordings of the major lower extremity muscles. Furthermore, simulated joint kinematics closely tracked experimental quantities with root-mean-squared errors less than one degree. Segmental power analysis for individual muscles was performed to elucidate the muscle's contribution to body support and forward progression in load carriage. Comparing muscle functions during the activities under investigation illustrated the different functional performance of the lower extremity muscles and the capability of the joints and segments to reduce the transmission of force during load carriage.
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