Muscles act in opposite directions in order to cause a bone to move. The muscle fibers in opposing muscle groups are arranged in a way that allows them to work together to produce movement. For example, when the biceps muscle in the upper arm contracts, it pulls the forearm towards the upper arm. However, the triceps muscle, which is the opposing muscle group, must relax in order for the biceps to contract and move the arm. Similarly, when the triceps muscle contracts, the biceps must relax to allow movement. This coordinated action of opposing muscle groups enables us to perform a wide range of movements, such as bending and straightening our arms or lifting and lowering our legs.