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Muscular System

There are more than 600 muscles in the body, which together account for about 35 to 40% of a person’s weight.
Together with bones and joints, skeletal muscles form the musculoskeletal system, which holds our body together and controls its movements.

What is Muscle Contraction?

Muscle contraction is the generation of tension within a muscle fiber. Although the term ‘contraction’ implies shortening, in physiology, muscle contraction does not necessarily mean shortening of the muscle.
Under tension, the muscle may shorten [*concentric contraction], lengthen [*eccentric contraction] or remain the same, as when holding a position [*isometric contraction]. Opposite of *muscle relaxation, which is a return of the muscle fibers to their low tension-generating state.


Concentric contraction occurs when the muscle shortens under tension, meaning that the force generated by the muscle exceeds the load acting against it. For example, the upward phase of a biceps flexion is a concentric contraction. Opposite of *eccentric.


In eccentric contraction, the tension generated is insufficient to overcome the external load, and the muscle fibers lengthen while remaining under tension. The muscle acts to decelerate the movement or control the repositioning of the load. Opposite of *concentric.



In isotonic contraction, the tension in the muscle remains constant, whilst the muscle’s length changes. There are two types of isotonic contractions: concentric and eccentric. Opposite of *isometric.


Agonist muscle, also known as “primary movers”, is a muscle or group of muscles that takes the lead role in a movement, ensuring strength and control of the action. Opposite of *antagonist muscle.


Antagonist muscle is a muscle or group of muscles that produces an opposing joint torque to the agonist muscles. Antagonist muscle can slow movement down or to some degree reverse the action of the agonist muscle. Opposite of *agonist muscle.

Antagonistic Pairs

Antagonist and agonist muscles often occur in pairs. As one muscle contracts [*contraction], the other relaxes. It’s important to understand that antagonist and agonist functions are not intrinsic properties of a particular muscle or muscle group; they are the role the muscle plays during movement. Examples of antagonistic pairs: biceps and triceps for arm flexion and extension; quadriceps and hamstrings for knee extension and flexion.


Synergist muscles, also called fixators, assist the *agonist muscle for a specific action at a joint. These are not the primary muscles involved in the action, but they work in synergy with the main muscle group to make the movement more accurate and fluid.