Types of Muscle Contractions

Muscle Twitch The response of a muscle to a single stimulus is called a twitch. The period between the time of the electricla stimulus and the beginning of the contractionis called the latent period. This is the time for the calcium to be released from the sarcoplasmic reticulum. The upward deflection of the tracing is called the contraction phase. It usually lasts 10-80msec. The downward deftlectionis called the relaxation phase. The length of time of the relaxation phase is also around 10-80msecs.

In the same manner as for nerve tissue, after depolarisation, a muscle fibre cannot be depolarises again for about 5 msec. This is called the refractory period and is divided into an absolute and relative refractory periods. It should be noted that the refractory period in a skeletal muscle lasts for a fraction of the time of muscle twitch (average = 5%). A single muscle fibre also obeys the all or none law - a minimal threshold stimulus would produce a maximal response i.e. the muscle fibre would contract fully or not at all. On the other hand in a whole muscle, graded contractions are possible e.g. one needs a stronger contraction to carry a 50Kg weight than to carry a 5 gm weight. These gradations depend on the frequency of stimulation and on the number of motor units stimulated.