Why must skeletal muscles work in pairs to move a bone?

Skeletal muscles typically work in pairs to move a bone due to the arrangement of their fibers and the way they are connected to the bones they control. Here are the reasons why:

1. 拮抗作用 (antagonistic action): Skeletal muscles are often arranged in pairs or groups with opposing functions. These muscles work as synergists and antagonists to control the movement of a joint. For example, in the case of the elbow joint, the biceps brachii muscle flexes the elbow, while the triceps brachii muscle extends it. When the biceps contracts, the triceps relaxes, and vice versa. This antagonistic action provides balanced and controlled movement.

2. Smooth and precise movements: Working in pairs allows for smoother and more precise movements. When one muscle contracts, its antagonist relaxes, creating a smooth transition in the movement. This coordination ensures that movements are not jerky or abrupt.

3. Stability and control: Working in pairs provides stability and control to the joints. When a muscle contracts, it stabilizes the joint by pulling on the bone in a specific direction. The opposing muscle then counteracts this pull, preventing excessive movement and providing a balanced force. This coordination is especially crucial for activities requiring precision and stability, such as lifting objects or maintaining a steady posture.

4. Strength and power: Working in pairs allows for greater strength and power generation. When two muscles work together, they can generate more force than if they worked independently. This combined effort is beneficial for tasks that require significant force, such as lifting heavy objects or performing strenuous exercises.

It is important to note that not all skeletal muscles work in antagonistic pairs. Some muscles may have the same function and work synergistically, while others may have more complex arrangements that involve multiple muscles working together to produce a specific movement.