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Muscle Twitch and Tetanus

The molecular events during the process of muscle contraction just described can now be considered in more detail (i.e., at the level of the entire muscle). In response to a single stimulus of adequate strength, a momentary rise in tension known as a twitch is produced. A muscle twitch has three phases.

The latent period is defined as the brief delay, lasting approximately 15 milliseconds, during which the muscle maintains a constant length without force production. This is the period of muscle depolarization, Ca2+ release from the SR, and cross-bridge formation before sarcomere shortening occurs.

The contraction period is that period during which sarcomere shortening occurs because of rapid succession of myosin–actin cross-bridge cycling. The relaxation phase is defined by restoration of resting myoplasmic Ca2+ concentrations, release of cross-bridges, and relaxation of muscle to its original length.

If a second stimulus reaches the muscle fiber after the relaxation phase of a twitch, no increase in muscle tension occurs, and another twitch of identical tension takes place. If, however, a second stimulus of adequate intensity reaches the fiber before completion of the relaxation phase, an increase in tension above that of the first twitch is possible.

This summation is known as an unfused tetanus. The temporal summation of twitch tension increases proportionately with the stimulation frequency. As the stimulation frequency increases, the amount of tension produced is increased proportionally until reaching a maximum level known as a fused tetanus.

The rate of muscle shortening depends on both the muscle fiber type and the weight of the load being lifted. Fibers can be Type I (red, or slow twitch) or Type II (white, or fast twitch). Type I fibers have a relatively long time period before they reach peak tension, whereas Type II fibers have a short time to peak tension. If the force that is generated by the contraction is greater than the resisting load, the muscle shortens.

Another term for a shortening contraction is a concentric contraction. If the resisting load is greater than the amount of tension being produced, there will be a lengthening contraction, which is commonly referred to as eccentric contraction.

The force of the muscle contraction depends not only on the frequency of motor unit stimulation but also on the absolute number of motor units that are stimulated for contraction. The quantity of motor units being stimulated with each contraction is adjustable through a method known as  recruitment.

The two entities, stimulation frequency and recruitment, are believed to act in concert with each other to effect an optimal combination for force production and limb movement.

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