What is the direction in which muscles can act?

Muscles primarily function by contracting and relaxing, a fundamental principle of muscle physiology. When a muscle contracts, it generates force and shortens, which can result in movement of the skeletal system. In this sense, muscles can only actively shorten and create tension; they cannot extend or lengthen by themselves. Instead, the relaxation phase allows the muscle to return to its original length passive by the force exerted by antagonistic muscles or gravity.

The nature of muscle action is such that they can only exert force through contraction, focusing on the pulling action rather than pushing. For example, when you perform a bicep curl, the bicep muscle pulls the forearm upwards; it does not push the arm downwards, illustrating the pull mechanism effectively. Hence, understanding that muscles act through contraction and relaxation is key to grasping how movement is achieved in the body.