E4 Neuromuscular Control and Coordination

Topic

Motor control coordinates the temporal and spatial activation of muscles to produce the desired movement pattern with the lowest possible metabolic cost and the greatest possible stability.

In mammals, this motor control of locomotion relies on central pattern generators located in the spinal cord, which produce locomotor rhythms autonomously, without requiring step-by-step direction from the brain. These central pattern generators are, in turn, modulated from above by signals descending from the cerebellum and motor cortex, and from below by sensory feedback from proprioception and the vestibular system, ensuring the autonomous rhythm constantly adjusts to actual movement conditions.

This sensory feedback that adjusts the locomotor rhythm is neither received nor processed instantaneously; there is a neuromuscular reaction time between the moment a disturbance is detected and the moment the muscle responds with the corresponding corrective force, and this reaction time limits the system's speed in correcting an actual disturbance. An animal's balance response to a disturbance is described as a correction proportional to the magnitude of the deviation and the rate at which that deviation is changing, applied after that same time delay.

The force with which the muscle responds—whether correcting a disturbance or producing normal movement—entails a metabolic cost that follows a simple relationship: the energy expenditure of an isometric contraction is directly proportional to the muscle's activation level multiplied by the maximum force that muscle is capable of generating. The higher the required activation level, the greater the metabolic cost, even if the muscle does not shorten or produce any actual movement. By simultaneously activating two antagonist muscles around the same joint, the motor control system can adjust the limb's effective stiffness without altering its position, as the forces exerted by the two muscles oppose and cancel each other out regarding their effect on position. This co-activation increases joint stiffness—helping to stabilize the limb against unexpected perturbations—but also raises the metabolic cost of activating both muscles, without that additional expenditure translating into any net mechanical work at the joint.

In quadrupedal mammals, this same motor control mechanism also coordinates the activation of all four limbs; this inter-limb coordination gives rise to distinct gait patterns—such as walking, trotting, and galloping—which are distinguished by the temporal symmetry of each limb's contact with the ground relative to the others. Ultimately, each of these gait patterns represents a different solution to the problem introduced at the beginning of this description: coordinating muscle activation in time and space to achieve the desired movement with the lowest possible metabolic cost and the greatest possible stability.

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