Graduation of contraction

Graduation of contraction refers to the strength of contraction exerted by the muscle. It depends on:

  1. The number of motor units stimulated or recruited. If only a few of the motor units within the muscle are recruited, the strength of contraction will be weak; whereas for maximal contraction to occur, all motor units must be stimulated
  1. The frequency of the stimuli (wave summation). For a motor unit to maintain a contraction it must receive a continuous string of impulses. Usually a frequency of 80-100 stimuli per second is required. Slow twitch fibres have a lower threshold for activation than fast twitch fibres and, therefore, tend to be recruited first
  1. Timing of the stimuli to various motor units (synchronisation or spatial summation). If all the motor units are stimulated at exactly the same time, then maximum force can be applied. If, however, a muscle needs to work over a long period, fatigue can be delayed by rotating the number of motor units being stimulated at any one time