Muscles contract to move our bones by pulling on them. However, muscles can only pull; they cannot push. This is why they usually work a joint in pairs. One muscle of the pair contracts to move the body part, the other muscle in the pair then contracts to return the body part back to the original position. Muscles that work like this are called antagonistic pairs.
In an antagonistic muscle pair as one muscle contracts the other muscle relaxes or lengthens. The muscle that is contracting is called the agonist and the muscle that is relaxing or lengthening is called the antagonist. For example, when you perform a bicep curl the biceps will be the agonist as it contracts to produce the movement, while the triceps will be the antagonist as it relaxes to allow the movement to occur.
The biceps contracts and raises the forearm as the triceps relaxes. A set of antagonists called the hamstrings in the posterior compartment of the thigh are activated to slow or stop the movement. These terms are reversed for the opposite action, flexion of the leg at the knee. In this case the hamstrings would be called the agonists and the quadriceps femoris would be called the antagonists.
There are also muscles that do not pull against the skeleton for movements such as the muscles of facial expressions. This group of muscles is known as antagonistic muscles. Thus, one group of muscle, i. Accordingly, to carry out any movement, the main muscle, or the agonist muscle, contracts and shorten in length. Synergistically, antagonistic muscles work in complementary or the opposite direction, i.
Thus, when the agonist or the primer or agonist muscle contracts, the antagonistic muscle relaxes to complete the movement. In summary, the complementary action of agonist and antagonist muscles is the prerequisite for any action to be carried out efficiently. This further implies that the muscle works in pairs wherein one muscle group takes up primer or agonist roles while the other functions as an antagonist.
For the efficient working of the muscles, fixators assist by providing support and stabilize the joint and the rest of the body. The fixators that assist the agonist are known as a synergist , thus when primer muscle contracts, the synergistic muscle simultaneously contracts. Synergists or synergistic muscles are also sometimes referred to as neutralizers as these muscles help reduce the extra movement induced by the agonist muscle, thus maintaining the working plane of the agonist muscles.
For example, for flexing the elbow joint using biceps, the trapezius muscle acts as a fixator while stabilizing the whole body for the lower abdominal movement, i. The combination and coordination of four categories of the skeletal muscles, i. Take note that the antagonism of a muscle is not a fundamental or predetermined property of a muscle; it is a role undertaken by the muscle complementary to the current agonist muscle.
Imagine a player who is about to take a kick at a soccer ball. Prior to kicking the ball, the knee flexes. The hamstrings contract while the quadriceps relax or lengthen in order to carry out the movement. See Figure 2. Next, when the player has completed kicking the ball, the knee extends. This also results in quadriceps contracting while hamstrings relaxing. In this case, the quadriceps become the agonist while the hamstrings are the antagonist in this movement.
See Figure 3. Any movement in the body is the result of the coordination of action between agonist and antagonist muscles. The co-activation of these two sets of muscles is critical. Our understanding of the concept till now makes it clear that the co-activation of the antagonistic muscle provides resistance to the action of the agonist muscle. Agonist muscles produce the primary movement or series of movements through their own contractions.
To generate a movement, agonist muscles must physically be arranged so that they cross a joint by way of the tendon. Contraction will move limbs associated with that joint.
During flexing of the forearm the biceps brachii is the agonist muscle, pulling the forearm up towards the shoulder. The majority of muscles are grouped in pairs, with an antagonist to each agonist muscle. Exceptions include those muscles such as sphincter muscles that act to contract in a way that is opposite to the resting state of the muscle. Antagonist muscles act as opposing muscles to agonists, usually contracting as a means of returning the limb to its original, resting position.
During flexing of the forearm, the triceps brachii is the antagonist muscle, resisting the movement of the forearm up towards the shoulder.
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