Spatial Relationships

The problem is somewhat complicated by the fact that the spatial relationships in both types of movement remain the same. The points of muscular origin and insertion, therefore, are alike, and yet the condition of the intervening muscle differs. Much confusion on this question has arisen through the popular comparison of a muscle to an elastic band. The analogy holds only to a limited extent. If the property of elasticity, by virtue of which a body returns to its original shape when the force responsible for its preceding alteration ceases to act, were inherent in the muscle, any stretching of a muscle would be accompanied by a constantly increasing resistance, equal exactly to the force responsible for the stretching.
This is the principle upon which all

1. spring balances, (see Figure 1)
2. sling-shots, and
3. similar devices

are based. It is entirely opposed to the principle of relaxation which is discussed in the module on Properties of Muscles, and also to that of coordination, discussed in module 9.

The same opposition is found when the distance between the points of origin and insertion is decreased. If elasticity were an inherent muscular property, the muscle would contract directly with diminution of this distance, whether the movement be made actively or passively. But muscular contraction involves work, and work is always the result of an expenditure of energy. Hence we should have the muscle, in passive movement, doing the same amount of work as in active movement. It is a condition flatly opposed to the law of least effort .