Striated muscle is easily stretched, and, if the extension be not carried too far, recovers very completely its original length. That is to say, the elasticity of muscle is small or weak, but very perfect. When a muscle is stretched to a given extent by a weight - say of 1 gramme - if another gramme be then added, it will not stretch the muscle so much as the first did; and so on if repeated gramme weights be added one after the other, each succeeding gramme will cause less extension of the muscle than the previous one; so that the more a muscle is stretched the more force is required to stretch it to the given extent, or, in other words, the elastic force of muscle increases with its extension.

If a tracing be drawn, showing the extending effect of a series of equal weights attached to a fresh muscle, it will be found that a great difference exists between it and a similar record drawn by inorganic bodies or an elastic band of rubber.

When a weight is applied to a muscle, it does not immediately stretch to the full extent the weight is capable of effecting, but a certain time, which varies with circumstances, is required for its complete extension. The rate of extension is at first rapid, then slower, until it ceases. As a muscle loses its powers of contraction from fatigue, it becomes more easily extended. Dead muscle has a greater but less perfect elasticity than living, i. e., it requires greater force to stretch it, but does not return so perfectly to its former shape. The importance of the elastic property of muscle in the movements of the body is noteworthy. The muscles are always in some degree on the stretch (as can be seen in a fractured patella, the fragments of which remain far apart and cause the surgeon much anxiety), and brace the bones together like a series of springs, the various skeletal muscles being so arranged as to stretch others by their contraction. When one muscle - for example, the biceps - contracts, it finds an elastic antagonist already tense, and has to shorten this antagonist as it contracts itself. The triceps in this case acts as a weak spring, opposing the biceps, and it gently returns to its natural length when the contraction of the biceps ceases. The muscles are kept tense and ready for action by their mere elasticity, and have to act against a gentle spring-like resistance, so that the motions occur evenly, and there is no jarring or jerking, as might take place if the attachments of the inactive muscles were allowed to become slack.