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Muscles may contain several hundreds of thousands of individual muscle fibers. These fibers are grouped into functional units (10 – 1000 fibers) activated by a single motor neuron. A single motor neuron and all the muscle fibers that it activates is a called a motor unit (MU). Activation of one MU will cause its muscle fibers contract and relax in in unison (or, “twitch”) producing one-unit of force.

Force can be increased by recruiting additional MUs (recruitment), or by increasing the number of times per second that each MU is activated (rate-coding). Activating MUs more frequently allows twitch-forces to overlap in time to a greater degree and sum up more efficiently. When the muscle starts contracting, sometimes there is a brief period of unusually short inter-pulse intervals between successive MU firings (doublets). The summation of twitch-forces can also be affect by peripheral conditions in the muscle. A moderate to intense contraction can activate biochemical processes within the muscle proteins that augment MU twitch-forces, for the subsequent contraction, termed post-activation potentiation (PAP). The nervous system decreases MU firing rates in the presence of PAP, to accommodate the larger twitch-forces.

The interaction between PAP and MU firing may be mediated by muscle length. Joint angle can increase or decrease compliance within the muscle-tendon unit (MTU) by changing its length, accordingly. Temporal summation of twitch-forces is less efficient at shorter muscle lengths when the compliance is greater. Potentiation is, by coincidence, greatest under these conditions. The situation is complicated at longer muscle lengths. Compliance decreases within the MTU at longer muscle lengths, which is optimal for temporal summation. However, electrical stimulation of the muscle has shown that PAP can still be present at longer muscle lengths and interfere with temporal summation. Thus, there is the possibility that PAP can affect MU activity patterns during voluntary control at any muscle length. Our attempts to understand the neural control of muscle during fatiguing contractions is also affected by the relationship between PAP and MU activity. The decrease in MU firing rates observed in the presence of PAP also resembles MU synchronization, where MUs are recruited simultaneously to superimpose their twitch forces. The result resembles a single potentiated MU twitch. When muscle electrical activity is measured from the skin surface, both PAP and MU synchronization can produce changes in the muscle electrical signals that resemble another potential mechanism: the progressive recruitment of more MUs, then as they fatigue, they simply stop firing. This proposal outlines a layered approach with each successive study to disentangle central and peripheral factors that regulate force in non-fatiguing and fatiguing contractions, to understand motor commands to the muscle.