Understanding muscle roles reveals some useful insights for training.
The three major roles are:
Many muscles have multiple roles and change them for the actions required.
An agonists, protagonists, or prime movers create force in the same direction as the action and are most responsible for the movement.
Antagonists could create force opposite to the joint action applied by the agonists. It depends on the action whether or not they do. They either contract while lowering and holding, such as when quickly ending a fast movement, or avoid contracting to permit smooth movement.
Due to slower movement during lifting, the antagonists should act passively, meaning they activate as little as possible. Reciprocal inhibition is the improvement that results from blunting the antagonists as you learn an exercise. Otherwise wastes energy, hampers movement, and harms performance.
Sometimes antagonists prevent undesired movement.
Stick out your hand with your palm facing down. Place your opposite hand on the back of the arm for the active hand. Quickly twist your twist toward a palms-up position. Notice that your triceps contract in addition to your biceps. Although only the biceps play a role in supinating or twisting the wrist from this position, the triceps must contract to prevent movement at the elbow.
A similar effect occurs when flexing your arms. You may desire to tense and show just your biceps but the triceps will contract as well. This immobilizes the joint and protects the elbow, an effect you fail to get on an isolation exercise.
Synergists assist the agonists, act as stabilizers, or perform both. They may fix limbs in place or allow good posture. Since muscles contract at all their attachments, they also may work to prevent unwanted movement from a prime mover, this meaning that more muscles are involved in even the most simple movements than we realize.
Some synergists function as emergency muscles. These assistant movers may perform the same function as prime movers but play a lesser role. They activate weakly during less forceful movements.
The brachioradialis in the forearm helps the biceps and the brachialis to bend the elbow. Without enough effort, it will not help much unless the biceps is disabled, such as with an overhand grip. This shows the importance of working hard as some muscles may not recruit intensely for lesser efforts.
Stabilizers activate with little or no movement. This allows the prime movers to operate efficiently. The upper back muscles on the bench press play this role, holding your position.
They often work in pairs, with the abs and obliques countering the erector spinae (lower back) on a squat. This prevents hyperextension of the spine. It also trains the whole core on a movement you otherwise may think would not involve it much at all.
Muscles also work best when the stabilizers work hard. Relaxing them during any heavy exercise will harm your performance and safety. Stay tight instead.
Muscle Role Conclusions
Information is not knowledge.
– Albert Einstein
Learning plays a role in developing strength.
You must work hard to involve all the possible muscles.
Less stabilization occurs on isolation.
Stay tight to perform your best.
More muscles activate during a movement than the obvious.