Contributions of individual muscles to forward motion at different walking speeds

Understanding how muscles produce forward motion during human walking is a difficult task, due in large part to muscle redundancy and the fact that muscles can accelerate joints they do not span. Several investigators have combined optimization techniques with forward dynamic models to identify how individual muscles contribute to support and forward progression during walking. However, each of these studies simulated a single walking speed, and it is likely that the muscle contributions change with walking speed.

The mechanisms by which humans modulate walking speed are poorly understood. Knowing how muscles are used at different walking speeds is important because individuals with gait disorders usually walk slowly. However, treatment decisions are often based on comparisons of a patient's slower gait to the faster gait of unimpaired individuals. An improved understanding of how humans generate slower walking speeds may be helpful in selecting appropriate treatments. Therefore, the purpose of this study is to determine how individual muscles contribute to support and forward progression at different walking speeds.

Three-dimensional joint kinematics, kinetics, and EMG data will be collected for each of ten healthy young subjects during walking. Subjects will walk at slow, free, and fast speeds. A generic three-dimensional dynamic model that characterizes the bone geometry, joint kinematics, muscle properties of the lower extremity will be used to analyze the movement kinetics. The model includes the major muscles crossing the hip, knee, and ankle.

A tracking algorithm will be used to estimate a set of muscle activations that drive a forward dynamic model to match each subject's walking kinematics and ground reaction forces. Static optimization will be used to resolve muscle redundancy at each time step within the simulation. An induced acceleration analysis of the simulation will be used to compute the muscle-induced ground reaction forces, and horizontal and vertical accelerations of the center-of-mass.

Individual muscle contributions to support and forward progression during different walking speeds will be identified for each subject. A muscle contributes to support if the force it produces adds to the vertical ground reaction force and the vertical acceleration of the body center of mass. Similarly, a muscle contributes to forward progression if the force it produces adds to the anterior ground reaction force and the forward acceleration of the body center of mass. Muscle contributions to net joint torques will also be identified. These analyses will be performed for the subject data recorded at different speeds to determine how muscle contributions are modulated with changing velocity.

This research is significant because it will clarify the relationship between walking speed and muscle function. An improved understanding of how unimpaired individuals modulate speed will enable better evaluations of abnormal gait patterns that are often associated with diminished speed. The results of this study may have important implications in treatment planning for individuals with gait disorders.

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