E5 Neuromuscular Control

Topic

The central and peripheral nervous system controls the final activation of the muscle—the very muscle modeled elsewhere in this framework—by modulating the firing frequency of active motor units and recruiting additional motor units within that same muscle.

Each motor unit consists of an alpha motoroneuron and the muscle fibers it innervates; Henneman's principle dictates that the nervous system recruits these units in ascending order of size—starting with small, slow units and progressing to large, fast units only when the muscle needs to generate maximal force. If the muscle requires even greater force, the nervous system recruits an even larger and faster motor unit within that muscle.

The muscle's electrical signal—the same signal depicted in electromyographic models—reflects the spatial and temporal summation of action potentials from all active motor units within the muscle. Consequently, the muscle's final force can be modeled as the product of its normalized neural activation and its maximal force—the latter being defined by the Hill curve model.

The stretch reflex—a monosynaptic reflex arc involving the nervous system and the organism's lower limb—provides proprioceptive feedback that stabilizes joints during ground contact; this mechanism is also linked to the reflex inhibition associated with neuromuscular dysfunction in injured muscles. Consequently, this model—characterized by the specific pattern of muscle activation controlled by the nervous system—dictates that the organism's total reaction time (the sum of neural and electromechanical components) limits the final speed of muscle force generation during a sudden impact. This co-activation between agonist and antagonist muscles—a phenomenon also addressed in a separate model regarding energy dissipation within a kinetic chain—increases joint stiffness; however, this comes at the cost of reducing both the muscle's net force and its metabolic efficiency (the latter being the subject of yet another model in this series). Greater co-activation between the two muscles results in even higher joint stiffness, albeit at the expense of the net force and metabolic efficiency that conclude this model.

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