In Figure 31, this difference is photographically shown. One picture illustrates the third finger being lifted by an outside agency, and the absence of any ridge across the back of the hand, proves the absence of the muscular contraction.
Whereas in figure 31 b) a voluntary lift of the same finger is shown, and the well-defined ridge across the back of the hand can be seen, representing the tension of the tendon leading to the muscle.

The rubber-band simile is thus seen to be inadequate in the study of muscular reaction. The muscle does not contract when the distance between the skeletal and muscular attachments merely decreases, but only if the decrease demands overcoming external resistance. Slack is taken up only when opposed by muscular activity in the antagonists. Then the give and take is very nicely adjusted, leading to very fine degrees of coordinated movement . When the movement is caused by external force alone, the purpose of any "balanced" reaction obviously vanishes, since the organism no longer has anything to do with the aim of the movement.