When you swim, multiple muscles in your arms, back, core and legs work together to maintain forward momentum. Your triceps, on the back side of the upper arm, works with the biceps to straighten and bend the arm at the elbow. Bending your elbow makes your triceps lengthen and your biceps contract, while straightening your arm does the reverse. Because of this, swimming strokes that bend and straighten your arms repeatedly work out your triceps.
Swimming freestyle requires your arms to move cyclically. You lift one arm above the head, elbow bent and up high, then return it to the water with your palm downward. At the same time, your other arm pushes the water behind you. As your arms go back and forth between a bent position with a high elbow and an outstretched position directly in front of you, the biceps and triceps work in tandem to create a balance between each other. Because they work at the same time when you swim freestyle, these muscles develop evenly.
Breaststroke begins with your arms extended in front of you. Next you push the water behind you with both hands, bending the elbow. This arc continues as you propel the body forward. Keep your elbows bent until both hands meet again in the front of your chest. Your triceps contract every time your arms straighten back out to a streamlined position. Similarly, every time you bend your elbows, your triceps lengthen.
With butterfly, your arms need to stay straight out, elbows locked in position. Each time your arms come out of the water, your triceps contract so they can extend. When they re-enter the water, they push the water behind you in the same way, but straight behind you. In this case, your biceps and triceps have to work double time to lift your arms out the water and keep your elbows from dropping so you can complete a clean stroke.
Like freestyle, backstroke is a cyclical motion, with one arm reaching up into the air while the other pushes water behind. However, it has an extended glide phase. This means that there is a bit of extra rest between strokes. So while the elbow is bending under the water as it comes back up to the surface, there is a pause, which means your triceps and biceps are not worked out nearly as much as when you swim freestyle.
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