Muscles cannot work alone because a muscle can only pull. Because of this, they work in pairs or groups so that as one muscle contracts, the other relaxes to support a movement. The muscle that contracts to bring about a movement is called the prime mover or agonist muscle.
The antagonist muscle is the muscle that relaxes to allow movement in a joint when the agonist muscle contracts. There are also the synergist muscles, which assist indirectly in a movement by stabilising a certain joint or body part.
For example, during a biceps curl, the biceps is the agonist muscle, which contracts to cause the flexion of the elbow. The triceps is the antagonist muscle. The deltoid acts as one of the synergist muscles, which contract to stabilise the shoulder joint.
The following are some examples of agonist and antagonist muscle pairs in action.
Isotonic Contractions
An isotonic contraction causes a muscle to change in length as it contracts to move a body part. There are two types of isotonic contractions:
Concentric Contraction:
The muscle shortens as it contracts.
An example is the biceps muscle shortens as it contracts during the upward phase of the bicep curl.
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An example is the biceps muscle lengthens as it contracts during the downward phase of the biceps curl.
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Isometric Contractions
An isometric contraction causes a muscle to contract without lengthening or shortening it, so there is no movement. For example, when a person carries an object in front of him/her, the weight of the object pulls the arms down, but the muscles stay contracted to hold the object at the same level. Another example is the muscles along the vertebral column that maintain the body in an upright posture while standing.
The amount of force a muscle is able to produce during an isometric contraction depends on the length of the muscle at the point of contraction. Each muscle has an optimum length at which the maximum isometric force can be produced.
The two types of isotonic contractions can work together in an exercise to develop a specific group of muscles. For example, when executing an inclined chin-up, the latissimus dorsi and biceps muscles are developed when they contract concentrically to raise the body towards the bar. When the body is lowered in a controlled manner, the latissimus dorsi and biceps muscles are also developed when they contract eccentrically to control the downwards motion.
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