Motor Unit Summation

How then does the muscle know how to exert the correct amount of force to lift a light or heavy object? A muscle is made up of many bundles of muscle fibers. Each of these bundles of fibers is called a motor unit. Each unit has a motor nerve which branches out at its tip. Each muscle fiber, therefore, has its own nerve ending to stimulate it. An electrochemical impulse is transmitted by chemicals from the nerve ending to the fiber, causing the fiber in that motor unit to contract. Depending on the type of load to be lifted, a differing number of motor units will be recruited into the contraction process. So, for the feather lift, only a few motor units would be required, whereas for the cement many units would be needed. So, the central nervous system is constantly making decisions as to how many motor units are required to lift a certain item. The more fibers that contract the bigger and harder the muscle will be. For instance lifting your bicep to scratch you nose doesn't require the recruitment of many motor units. Consequently your bicep will remain soft. But, if you curl a weight up to your shoulder, your bicep will contract forcefully and become hard and will bulge.

The finesse with which we are able to control our muscular strength is dependant on the number of fibers within each motor unit. The fingers have relatively few fibers within their motor units - sometimes less than 10 - and this allows fine fingertip control. The large thigh muscles of the upper leg, however, are not capable of such delicate control. Typically each motor unit will contain in excess of 100 fibers.