Hmmm... I took it differently:Here protract is being used as the converse of contract. Collins's definition is poor and confusing. It implies that muscles can extend themselves. Unfortunately, muscles cannot extend themselves and thus cannot be said to "protract" themselves (although they do contract themselves.)-> muscles can contract or relax: muscle A can be said to be protracted (stretched) by the action of an opposing muscle, B.
Movement in your body (other than by gravity or exterior force) is caused by muscles contracting -> obviously, some muscles must also relax to allow this. All contraction of muscles will cause movement in the attached body part.