Neuromusculoskeletal model that walks and runs across a speed range with a few motor control parameter changes based on the muscle synergy hypothesis

Abstract

Humans walk and run, as well as change their gait speed, through the control of their complicated and redundant musculoskeletal system. These gaits exhibit different locomotor behaviors, such as a double-stance phase in walking and flight phase in running. The complex and redundant nature of the musculoskeletal system and the wide variation in locomotion characteristics lead us to imagine that the motor control strategies for these gaits, which remain unclear, are extremely complex and differ from one another. It has been previously proposed that muscle activations may be generated by linearly combining a small set of basic pulses produced by central pattern generators (muscle synergy hypothesis). This control scheme is simple and thought to be shared between walking and running at different speeds. Demonstrating that this control scheme can generate walking and running and change the speed is critical, as bipedal locomotion is dynamically challenging. Here, we provide such a demonstration by using a motor control model with 69 parameters developed based on the muscle synergy hypothesis. Specifically, we show that it produces both walking and running of a human musculoskeletal model by changing only seven key motor control parameters. Furthermore, we show that the model can walk and run at different speeds by changing only the same seven parameters based on the desired speed. These findings will improve our understanding of human motor control in locomotion and provide guiding principles for the control design of wearable exoskeletons and prostheses.

Figure 1

Schematic model of muscle synergy: (A) Muscle activities are explained by the linear combination of a small number of basic waveforms. In most cases, human walking and running can be explained by five waveforms, but only three waveforms are shown here to simplify the illustration. (B) Hypothetical CPG motor control model producing five activation pulses for motor commands. Depending on the gait, the model shifts the activation timing of the second pulse.

Figure 2

Neuromusculoskeletal model for human walking and running: (A) musculoskeletal model, (B) motor command in the movement generator composed of the linear combination of five rectangular pulses based on hypothetical motor program, (C) muscles activated by each of the five rectangular pulses, and (D) seven motor control parameters to produce walking and running through the musculoskeletal model.

Figure 3

Simulated locomotor behavior for 1.6 m/s of desired speed for walking and running: (A) simulation stick diagrams (also see Movies S1 and S2) and comparison of simulated and measured data for (B) joint movements, (C) ground reaction forces, and (D) muscle activities. Vertical dotted lines indicate the liftoff timing in the simulated locomotion. Increasing joint angle corresponds to joint flexion. Measured data were obtained at a belt speed of 1.85 m/s. R is correlation coefficient and S is cosine similarity.

Figure 4

Simulated COM movement and horizontal speed for 1.6 m/s of desired speed for (A) walking and (B) running. Estimated horizontal COM speeds from the measured data at a belt speed of 1.85 m/s are also shown in the right panels. Gray regions indicate simulated double-stance phase for walking and flight phase for running. R is correlation coefficient and S is cosine similarity.

Figure 5

Simulated joint torques by muscles for 1.6 m/s of desired speed and estimated joint torques from the measured data at a belt speed of 1.85 m/s for (A) walking and (B) running. Vertical dotted lines indicate the liftoff timing in the simulated locomotion. Increasing joint angle corresponds to joint flexion. R is correlation coefficient.

Figure 6

Simulated change of gait speed: (A) gait cycle duration, (B) onset phase of second activation pulse for desired speed, (C) amplitudes of five activation pulses relative to those for a desired speed of 1.6 m/s, and (D) generated speed.