Exercises Based on Reciprocal Innervation
Related to the balancing of opposing muscle groups is a method for getting muscles to relax based on the concept of reciprocal innervation. Reciprocal innervation refers to the phenomenon, discovered by neurophysiologists in the nineteenth century, that the contraction of one set of muscles will inhibit the contraction of the opposing group -- something we can see when we flex the arm at the elbow. Flexion at the elbow is produced by the biceps which, by shortening or contracting, moves the arm. Since this movement would be stopped if the extensors of the arm contracted at the same time, the action of the opposing muscles is inhibited at the same time as the flexor muscles are excited.
Based on this concept, many exercise and somatic awareness methods suggest that if the back muscles are overworking, then contracting the opposing abdominal muscles will inhibit the activity in the back muscles, causing them to relax. Referring to the extensors and flexors of the neck, for instance, Moshe Feldenkrais, founder of the Feldenkrais method, asserts that "the two sets of muscles are antagonistic, and the contraction of the front ones would reflectively reduce the contraction in the back muscles," and recommends using this mechanism as a technique for releasing muscles: "...prolonged contraction of the flexor muscles of the abdomen increases the tonus of the extensors of the back."
The problem with this use of this concept is that reciprocal innervation applies to normal function and cannot be utilized correctively. If it could, all you would have to do to release shortened muscles would be to contract the opposing muscles. In fact, it doesn't work that way because we're always tightening muscles, and this has no effect whatsoever on opposing muscles that are chronically contracted. Even if you could somehow balance opposing groups of muscles in this manner, the effect would be to further imbalance the larger system, since the body always works as a whole and cannot be put in order by relaxing specific muscle groups.
The fact is, reciprocal innervation is a normative function; it simply does not work in a corrective context. In spite of this mistake, however, this concept has been around for decades, and many doctors and physical therapists still advocate exercises largely based on reciprocal innervation without realizing that these exercises simply don't work and without questioning their efficacy.