The musculoskeletal frame of the body is an arrangement of bones, attached to one another by a series of ligaments, at structures called joints. The muscles crossing these joints provide the force necessary for body movements. Skeletal muscles do not contract independently of one another. Rather, the movements performed about a joint involve several muscles, each having a different role.

Agonists or synergists are the muscles that work together as a team, cooperating to perform a movement.

Antagonists are the muscles that act in opposition to the agonists during a movement. In most cases, especially for skilled and experienced athletes, the antagonists are relaxed, allowing the motion to be performed with ease.

This shows that the interaction between agonist and antagonist muscle groups directly influences athletic movements. A motion that looks jerky, or is performed rigidly, might be the result of an improper interaction between the two groups. Only by concentrating on relaxing the antagonists, can one improve the flow and smoothness of a muscular contraction.

Prime movers are the muscles primarily responsible for producing a comprehensive strength movement. During a biceps curl exercise for example, the prime mover is the biceps muscle, while the triceps act as an antagonist and need to be relaxed in order to facilitate a smooth flexion.

The line of pull for strength training and bodybuilding represents an imaginary line that crosses the muscle longitudinally, connecting the two extreme heads of the muscle. The highest physiological and mechanical efficiency of a muscle contraction is achieved when performed along the line of pull. An example using the biceps muscle will clarify this point.

Elbow flexion can be performed with the palm held in several different positions. With the palm turned upward, the line of pull is direct, creating the highest efficiency. With the palm facing down, efficiency on contraction decreases because the tendon of the biceps muscle wraps around the bone radius. In this case, the line of pull is indirect, which wastes a large portion of the contractile force. If one is looking for maximum strength gains and optimal muscle efficiency, strength exercises must be performed along the line of pull.