The basic unit of contraction of a skeletal muscle is the contraction of the group of muscle fibers making up a single motor unit, which consists ofa single motor neuron and all the muscle fibers receiving synaptic connections from that neuron. Whenever the motor neuron fires an action potential, all the muscle fibers in that motor unit twitch together. The magnitude of the contraction generated by activation of a motor unit depends on the number of muscle fibers in that motor unit. The number of fibers in a unit, and hence the magnitude of the tension produced by activating the unit, varies considerably among the set of motor neurons innervating a particular muscle.

The type of contraction produced by activation of a whole muscle depends on the load against which the muscle is contracting. If the load is too great for the muscle to move, the length of the muscle does not change during the contraction, which is then called an isometric contraction. If the tension is sufficient to overcome the weight of the load, the contraction will be accompanied by a shortening of the muscle. During the shortening, the tension in the muscle remains constant and equal to the weight of the load. Such a contraction is called isotonic. The overall tension developed by a muscle depends on how many motor units are activated and on the frequency of action potentials within a motor unit. Increasing muscle tension by increasing the number of active motor neurons is called motor neuron recruitment. When the frequency of action potentials within a motor unit is increased, the resulting muscle tension increases until a steady plateau state, called tetanus, is reached. Normally, during a maintained contraction all the motor neurons of a muscle are not active simultaneously; rather, the activity of individual motor neurons is restricted to periodic bursts that occur asynchronously among the pool of motor neurons controlling a muscle. This helps reduce fatigue in the muscle by allowing individual motor units to rest periodically during a maintained contraction.

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