How does a muscle "know" how much force is required?
Muscles have sensation, as do tendons and joint capsules. Each contains sensory receptors whose activation informs the motor system about the rate of change of muscle length, tension levels in tendons, and stresses in ligaments. This information tells the motor system whether its components are moving appropriately.
The most important sensory receptors in muscles are muscle spindles (Smith, Weiss, & Lehmkuhl, 1996, pp.96-103) and Golgi tendon organs (pp. 94-96).
Students can learn more about muscle spindles and Golgi tendon organs at useful web sites maintained by Northeastern University's PT department, and by the King's College (London, UK) Division of Physiology.
Memory and experience also teach the motor system to anticipate the amount of force it must produce to perform certain activities. For instance, we adjust our posture before we move, not just during movements.
Our experience can work against us, as in cases when we:
- pick up an empty container when we think it is full
- fail to anticipate one last stair in a dark stairway
RECIPROCAL INHIBITION (Smith, Weiss, & Lehmkuhl, 1996, Fig. 3-15, p.102)
involves reflexes, mediated by chains of neurons in the spinal cord, that link agonists with their antagonists. Activity in an agonist produces a volley of neuronal activity that inhibits its antagonist. Reflex inhibition is not absolute, but is one of many neuronal factors that influence muscle activity. Nevertheless, the effect of reciprocal inhibition is that agonists and antagonists are generally not active at the same time.
Reference:
Smith, L.K., Weiss, E.L. & Lehmkuhl, L.D. (1996). Brunnstrom's clinical kinesiology (5th ed.). Philadelphia: F.A. Davis.