Antagonists versus Synergists

Antagonistic muscles are those whose actions are in mechanical opposition. If one muscle flexes the elbow, for example, its antagonist extends the elbow. Synergistic muscles are those acting at the same time in the production of movement. The term is sometimes restricted to the muscles aiding the principal muscle, but, since this distinction cannot always readily be made, I prefer to extend the definition to all muscles helping to produce the movement. Even so the classification is not always fixed. A muscle acting synergistically with regard to another muscle may act antagonistically to the same muscle in another movement . This probably never happens in motion in a single plane, that of a hinge-joint, but it frequently takes place at the rotary joints which results from the fact that the change in the direction of the skeletal parts results in a change in the direction of the pull, since the origin of the muscle is fixed.

The reality of muscular antagonism can be experimentally shown in the case, for example, of a spinal animal (also known as a vertebrate), that is one from which the brain has been carefully removed. Here any movement is necessarily a spinal reflex. Under such conditions the stimulation of one set of muscles will be accompanied by relaxation in the antagonists. This relationship forms the basis of coordinated movement, and at the same time points out the reflex character of coordination, a fact that makes it unwise to attempt the teaching of a movement by calling attention to the actual muscles used. We do not feel or will the particular contraction. If the proper resistance is present the proper contraction, in normal cases, will follow as a mechanical necessity. The fact that many untalented pupils do not adopt the proper muscular adjustment is an indication of mental deficiency in this respect. There are individual variations in the sensitivity and adaptation of the kinesthetic sense just as real as individual differences in vision or the ability to hear. Of this the various ataxias^[1] give unmistakable evidence.

[1]ataxia: Ataxia describes a lack of muscle control or coordination of voluntary movements, such as walking or picking up objects. A sign of an underlying condition, ataxia can affect various movements and create difficulties with speech, eye movement and swallowing.