Muscles generate force through three primary types of muscle action. Concentric movement is the action that allows a person to overcome an external load or accelerate an object. This muscle action is the most recognizable component of strength training, as it is the phase where the body actively exerts sufficient force to move a resistance against gravity or inertia.
Defining Concentric Contraction
A concentric contraction is a type of muscle action where the muscle shortens while actively producing tension. This shortening occurs because the force generated by the muscle is greater than the external resistance or load it is working against. This phase is often referred to as the “positive” portion of an exercise, as it involves actively lifting or pushing the weight.
When performing a bicep curl, the concentric phase is the action of lifting the dumbbell toward the shoulder as the biceps muscle shortens. Similarly, standing up from the bottom of a squat is the concentric action for the quadriceps and gluteal muscles, requiring them to shorten to extend the hips and knees against the resistance. For a push-up, pushing the body away from the floor until the arms are straight is the concentric movement of the chest, shoulder, and triceps muscles.
The Mechanics of Muscle Shortening
The physical shortening of the muscle during a concentric action is explained by the Sliding Filament Theory, which describes the interaction of proteins at the microscopic level. Skeletal muscle is composed of numerous muscle fibers, and within each fiber are contractile units called sarcomeres. The sarcomere is the fundamental unit of muscle contraction, structured by thick myosin filaments and thin actin filaments lying parallel to one another.
Contraction begins when a nerve signal triggers the release of calcium ions within the muscle cell. These ions bind to regulatory proteins on the actin filaments, exposing binding sites for the myosin heads. The myosin heads, fueled by adenosine triphosphate (ATP), attach to the actin filaments to form cross-bridges.
Once attached, the myosin heads perform a “power stroke,” a forceful pivot that pulls the actin filaments inward toward the center of the sarcomere. The repetitive cycle of cross-bridge formation, pulling, detachment, and reattachment causes the actin filaments to slide further past the myosin filaments. This inward sliding shortens the entire sarcomere, and the cumulative shortening of millions of sarcomeres results in the overall shortening of the muscle, generating the required force to move the external load.
Comparing Movement Types
Concentric movement is typically paired with the other two types of muscle action: eccentric and isometric.
Eccentric Contraction
Eccentric contraction occurs when the muscle lengthens while under tension, which happens when the external load is greater than the force produced by the muscle. This is the “lowering” phase of an exercise, such as slowly lowering a dumbbell back down during a bicep curl, where the muscle is actively resisting the pull of gravity and controlling the descent.
Isometric Contraction
In contrast, an isometric contraction occurs when the muscle produces force without changing its overall length. This happens when the force generated by the muscle is exactly equal to the external load. Holding a weight steady in a fixed position, such as pausing at the bottom of a squat or holding a plank position, are examples of isometric actions.
For comprehensive strength development, all three phases are utilized in a continuous motion during a typical exercise repetition. For instance, in a bench press, the concentric phase is the press upward, and the eccentric phase is the controlled lowering of the bar to the chest. While the concentric phase is effective for acceleration and lifting power, the eccentric phase is associated with greater overall strength gains and muscle hypertrophy because muscles can handle a higher load while lengthening compared to shortening.