As we know through our study of anatomy, bones cannot produce movement on their own. They are moved by muscles. In fundamental terms, the tendons at either end of a muscle are attached to adjacent bones of a joint. When the muscle contracts, the bones are pulled and the joint moves.
As its tendons have fixed points of origin and insertion, a muscle can only move a joint in one direction. It cannot ‘move’ it back in the opposite direction.
For this, it needs an opposing partner. This partner works to return the joint to its starting position.
The muscle that is contracting to produce a specific movement is the agonist, also known as the prime mover.
The opposing muscle that is relaxing or lengthening at the same time is called the antagonist.
This partnership is called an antagonistic muscle pair.
An excellent example of such a pair is the partnership between the biceps and the triceps.
During a bicep curl, the agonist (biceps) contract to raise the forearm.
At the same time, the antagonist (triceps) relax, allowing the movement.
In order to reverse this movement and lower the forearm, the triceps contract, becoming the agonist, and the biceps relax, becoming the antagonist.
Further examples of antagonistic muscle pairs:
Agonist Antagonist Movement
Sternocleidomastoid, Trapezius Neck flexion
Trapezius, Latissimus dorsi Shoulder elevation
Rhomboids Serratus anterior Scapula retraction
Erector spinae Rectus abdominus Trunk extension
Iliopsoas Gluteus maximus Hip flexion
Quadriceps Hamstrings Knee extension
Gastrocnemius Tibilais anterior Plantar flexion
Wrist extensors Wrist flexors Wrist extension
Agonistic muscles are considered the prime movers as they perform the movement. However, the antagonistic muscles play a vital role in this movement.
They maintain the body or limb in a fixed position, eg. holding the arms out to the side.
They control rapid limb movement, such as when a boxer practises shadow boxing, throwing fast punches repeatedly, without hitting anything or anyone.
So, both agonist and antagonist muscles are needed for efficient, fluid movement.
And finally, is it possible for a muscle to be antagonistic to itself?
The deltoid muscle is an antagonist to itself as its posterior fibres laterally rotate the shoulder and its anterior fibres medially rotate the shoulder.
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