The biceps and triceps muscles work in opposition to each other.
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During a biceps curl, which flexes the arm, the triceps muscle stretches as the biceps contracts. The triceps, located in the posterior compartment of the upper arm, and the biceps, in the anterior compartment, act in opposition to each other. Whether the exercise is performed with or without resistance or weights, the action remains the same.
In order for your elbow to bend during a bicep curl, your triceps muscle must relax and stretch.
Identify Agonist and Antagonist
Because it supplies the main force in flexing the arm, the biceps is referred to as the agonist muscle. The triceps, acting in opposition, is called the antagonist. Because the two muscles are located on opposite sides of the arm, the triceps must relax enough to allow the biceps to tense and contract during a curl. As it contracts, it shortens. As the triceps relaxes, it lengthens.
Break it Down
The upward phase of a biceps curl, when the biceps works hardest to overcome gravity, is called the concentric phase. The downward phase, when the biceps resists gravity, is called the eccentric phase. If you do a biceps curl with heavier weights, the biceps continue to work to slow the downward phase of the curl. Too rapid a release might result in injury to the elbow. The biceps muscle remains the agonist or prime mover against gravity, while the triceps remain the antagonist, even in the downward phase.
Balance Your Muscles
Some people make the mistake of overdeveloping one member of opposing muscle groups, at the expense of the other. If you just focus on doing biceps curls and neglect your triceps muscles, the biceps will naturally become much stronger. This leads to the two muscles becoming unbalanced. The greater the imbalance, the greater the potential for injury. Most people tend to have stronger biceps than triceps.
Strengthen Your Triceps
The biceps tend to be stronger because it gets more use in everyday life than the triceps. Flexing your arm to lift and carry loads is more common than extending the arm to lift loads overhead, which works the triceps. During a triceps extension, the triceps is the agonist and the biceps is the antagonist. If you are doing biceps curls, you should also do triceps extensions to maintain a more even balance between the muscles.