The muscle spindle fiber (MSF) is like a trigger wire. When excited by a stimulus, it can force the muscle into rapid contraction. For example, when an object is placed in someone’s hand and the object is much heavier than expected the biceps muscle contracts strongly and immediately to bear the load and prevent tearing of the muscle or tendon. The MSF is actually a bundle of several fibers that are both muscle and nerve in origin. Because of its unique physiology, the MSF can measure the rate and degree of a muscle contraction. Within the MSF are both sensory and motor capabilities. Nowhere else in the body is the connection between the stimulus and the reaction so closely tied. The MSF has the power to both feel and act at the level of the spinal cord rather than the brain creating a shortened feedback loop. Thus, it can impose an excitatory action upon a muscle very quickly.

The Golgi Tendon Organ (GTO) is found in the musculotendinous junction where the two tissues interface. It specifically measures the tension or amount of load on a muscle so that it can trigger the muscle to shut-down in the face of overload. For example, this is often seen in marathon running when an exhausted runner, maybe only steps away from the finish, collapses as the muscles of the legs shut down to prevent fatal overheating. The action of the GTO on the muscle tissue is therefore inhibitory.

The MFS and GTO, two specialized fibers, support one another although they may create opposite actions within the muscle. Together they determine the mass of an object by measuring that object’s resistance to movement.