For any exercise that you perform, there are a multitude of muscles that become activated. Some of these muscles are directly responsible for execution of the motion, some assist, some counteract and others ensure a smooth execution of the movement. Each of these muscles are grouped according to their actions during every exercise you perform.
Agonist and Antagonist
The primary muscle that contracts and produces the desired movement is called the agonist, or prime mover.
For example, the biceps is the agonist during a dumbbell curl as it mainly produces the movement. The muscle that resists the actions of the agonist and is located opposite to it is the antagonist. For example, the triceps is the antagonist to the biceps during a dumbbell curl.
To directly target a specific muscle, and thus an agonist, you must ensure that the direction of force being generated is correct. For example, if you want to target your infraspinatus (rotator cuff) muscle, you should do an external rotation movement. However, there is difference between doing this movement in the standing position using a dumbbell or a cable pulley machine.
If you’re doing a dumbbell external rotation while standing, the resistance would be perpendicular to gravity. This means that the infraspinatus is not working against gravity and instead, the biceps are
working isometrically to keep the dumbbell from moving downward. In fact, doing this movement while standing without a dumbbell would have the same effect on the infraspinatus as it would with a dumbbell.
To properly execute the external rotation with a dumbbell, you must lie down on your side. The dumbbell, and thus the resistance, would then be parallel to gravity. This would allow the infraspinatus to have to work against the resistance. In contrast, you can perform an external rotation using a cable pulley while standing to work the infraspinatus. In this case, the resistance is the weight stack of the machine. So, because your body is parallel to the weight stack, the infraspinatus is pulling against resistance.
Each time an agonist contracts, a synergist contracts at the same time. There are three types of synergists: assistant mover, stabilizer, and neutralizer. An assistant mover assists the agonist in producing the movement. Continuing with the dumbbell curl example, the brachialis and brachioradialis are the assistant movers to the biceps.
The involvement of these assisting muscles can usually be increased or decreased depending on how an exercise is performed. For example, the brachioradialis involvement can be increased with the use of a neutral grip, such as when doing a hammer curl. In this case, the brachioradialis becomes
the agonist while the biceps becomes an assistant mover along with the brachialis.
A stabilizer, the second type of synergist, is a muscle that contracts but produce little to no movement. This helps maintain posture while an exercise is being performed. During a dumbbell curl, the anterior deltoid and wrist flexors are among the muscles that help stabilize the body and eliminate undesired movement. The work of stabilizers can be increased or decreased depending on if an exercise is being performed using free weights or machines. The former increases stabilizer actions while the latter decreases it.
The third type of synergist, the neutralizer, is a slightly antagonistic muscle that contracts to oppose unwanted movement of the agonist. For example, during the dumbbell curl, the pronator teres contracts to prevent supination of the biceps.