Muscular contraction.

 When a muscle is stimulated a short latent period follows, during which it is taking up the stimulus. It then contracts when it becomes short and thick and finally it relaxes and elongates.

  In the case of striped (voluntary) muscle fiber the contraction lasts only a fraction of a second and each contraction occurs in response to a single nerve impulse. Each single contraction is of the same force. The force with which a whole muscle contracts is adjusted by varying the number of the fibers contracting and the frequency with which each fiber contracts. When contracting vigorously the individual fibers may contract more than 50 times each second.

 Certain factors influence force with which a muscles fiber contracts. It contracts more forcibly when it is stretched and when it is warm. Fatigue and cold weaken the power to contract.

Unstriped muscles fibers contact much more slowly and are not dependent on nervous impulses, although these alter the force of contraction.