Muscle Action around a Joint

Muscles function by contracting and relaxing, allowing for bodily movements and maintaining posture.
The contraction or shortening of muscle is known as concentric action, while the lengthening under tension is called eccentric action.
Another muscle action is the isometric action, where the muscle length stays the same but tension is increased. This is common in resistance exercises.

Every movement around a joint involves two groups of muscles called the agonist (or prime mover) and the antagonist.
The agonist is the muscle that contracts to create the desired movement.
The antagonist is the muscle that relaxes to allow the movement to occur. However, it can provide resistance to control the speed and range of the agonist’s movement.
For example, in arm bending (flexion), the biceps are the agonist muscles and the triceps are the antagonist muscles. During arm straightening (extension), the roles are reversed.

Agonist and antagonist muscles often operate in pairings. When one muscle contracts (agonist), the other muscle in the pair relaxes (antagonist) to facilitate smooth and controlled movement.
This concept is known as reciprocal inhibition, which prevents both muscles from contracting at the same time, which would impede movement.
Other examples of muscle pairings include the quadriceps and hamstrings in the legs, and the abdominal and lower back muscles.

Muscles also play a crucial role in maintaining joint stability. By contracting, they keep bones and joints in alignment and prevent dislocations and other injuries.
For example, the rotator cuff muscles in the shoulder help to stabilise the shoulder joint.

In summary, muscle actions around a joint are complex, involving contraction and relaxation of agonist and antagonist muscles, respectively.
These actions allow for controlled, smooth movements, maintain body posture, and provide joint stability. Understanding them is key to understanding human physiology and movement.