Both located on the upper arm, the biceps and triceps move the elbow when they contract. The biceps are on the front of the upper arm. When they contract, they flex the elbow, decreasing the distance between the forearm and upper arm. The triceps are on the back of the upper arm. When they contract, they extend the elbow, increasing the distance between the forearm and upper arm.
When muscles -- such as the biceps and triceps -- are located on different sides of a limb, they often have an agonist-antagonist relationship. This means that the muscles work together, but in opposite ways, to produce movement. While one -- the agonist -- contracts, the other -- the antagonist -- must relax. At times, the antagonist contracts eccentrically to slow down the movement produced by the agonist.
Description of the Exercise
You can perform biceps curls with barbells, dumbbells or resistance bands. To begin, you stand or sit and hold the weight with your arms extended toward the floor. You bend your elbows to bring the weight toward your shoulder, stopping when your forearm is perpendicular to the floor. Then, you lower the weight to return to the starting position.
Lifting the Weight
As you lift the weight, the flexion of the elbow is produced by the biceps. The triceps must relax to allow the biceps to contract. At the top of the movement, the triceps contract eccentrically to slow down the motion.
Descending the Weight
When you lower the weight, the triceps contract to extend the elbow. The triceps are aided by gravity, which pulls the weight down to the starting position. Here, the eccentric contraction of the biceps is necessary to resist the force of gravity and slow down the descent of the arm.