The biceps and triceps are opposing muscle groups on the front and back of your upper arm, respectively, and should be balanced for optimal performance and to minimize injuries. The biceps muscle group consists of two heads and the triceps have three. Their purpose is to flex and extend the elbow.
Tri is for Three
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It can be common to see bodybuilders and fitness enthusiasts place more attention on building the biceps, since they are front and center on the arm and viewed more often than the triceps. However, since they have more heads, the triceps can cause problems to the arms and shoulders if they are weaker than the biceps. The training ratio should be one-to-one, with a slight emphasis placed on the triceps.
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Aesthetic Appeal
Even if the look of your arms is more important to you than the function, the biceps and triceps should be trained equally. While the biceps dominate when looking at the body from a front view, side and rear views show a different picture and require developed triceps. Additionally, an arm that is fully developed from all angles will give a more aesthetically appealing look overall than one that is unbalanced.