The greater the strength required, the greater the number of motor units (and therefore the number of muscle fibres) that contract. For example, more motor units will be recruited in the biceps brachii when performing the bicep curl with a heavy dumbbell, than when performing the same exercise with an egg.
This process can sometimes deceive us. For instance, when lifting a box that appears to be light, not enough motor units are recruited and the box cannot be lifted. When trying a second time, the box is easily lifted because this time enough motor units have been recruited. Conversely, when attempting to lift a box that appears to be heavy (but in fact is not), an explosive movement often occurs as too many motor units have been recruited for the task.
The force output of a muscle can vary over a wide range, a graduation that is essential for smooth, coordinated patterns of movement. The strength of contraction exerted by the muscle depends on:
- 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.
- The frequency of the stimuli (wave summation). For a motor unit to maintain a contraction it must receive a continuous string of impulses. Slow twitch fibres have a lower threshold for activation than fast twitch fibres and, therefore, tend to be recruited first. If a motor unit is activated once, the twitch that arises does not produce a great deal of force. Increasing the frequency of activation so that the twitch forces are allowed to summate (add on to each other) results in greater force developed by the motor unit. This method of varying force output is generally used in small muscles, such as those of the hand. Even at low forces, most of the motor units are activated, though at a lower frequency. Force output of the whole muscle is intensified by increasing the frequency of firing of the individual motor units.
- 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.