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Human Motor Control

If fundamental principles of legged dynamics and control exist, they should apply to human locomotion as well and, in effect, should have a strong impact on human motor control. Little attention has however been paid to this connection between principles and motor control. I am interested in understanding it.

Figure 1

A muscle reflex that generates and stabilizes compliant leg behavior

The natural candidates for a connection between principles of legged dynamic systems and human motor control are muscle reflexes. Muscle reflexes can map sensory information that represents legged mechanics directly onto the activation of leg muscles bypassing central motor input.

We showed in a simulation study that the compliant leg behavior fundamental to the dynamics of walking and running can be generated and stabilized with a simple reflex: positive force feedback of the leg extensor muscles. This muscle-reflex system behaves like a mechanical spring in steady-state, but automatically thrusts or brakes in response to ground disturbances. Key to this self-adaptive control is the interplay between the positive feedback and the nonlinear muscle dynamics.

Figure 2

More details about this research can be found in

H Geyer, A Seyfarth, R Blickhan. Positive force feedback in bouncing gaits? Proc R Soc B 270: 2173–2183, 2003. [PDF]

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Systematic expansion into neuromuscular human model that encodes principles of legged mechanics

The previous example suggests that principles of legged dynamic systems can effectively be encoded in human motor control by muscle reflexes. To clarify whether this principled approach to human motor control can be generalized, we developed a more detailed neuromuscular human model.

Figure 3 illustrates the evolution of this human model. Starting from the reliance on compliant leg behavior as a key principle of legged locomotion, the model evolves into a seven segment system that is actuated by fourteen muscles. Throughout this evolution, the control of these muscles is mainly based on autonomous reflex arcs which encode principles of legged dynamics and control.

Figure 3

In consequence, this human model neither has central rhythm generators nor does it follow target joint trajectories. It is blind and entirely depends on the interplay of its muscle reflexes with the environment. Yet this model stabilizes into a walking gait, tolerates ground disturbances, and adapts to slopes without parameter interventions (hover mouse pointer over figure 4).

Figure 4

Moreover, the muscle activities predicted by the model closely match observed activities for some muscles (shown for ankle muscles in figure 5). These results suggest not only that the interplay between mechanics and motor control is essential to human locomotion, but also that human motor output could, for some muscles, be dominated by neural circuits that encode principles of legged dynamics and control.

Model-Predicted vs Observed Motor Output at Ankle

Figure 5

The neuromuscular model reveals the power of the principle-based approach to understanding human motor control. I am interested in intensifying this approach in two core areas: swing leg behavior and trunk balance. For both, the fundamental principles of dynamics and control are less well understood than for the stance leg behavior. Nor does a clear understanding exist of how these principles shape human motor control of the swing-leg and trunk motions.