Whether you have bent or straight arms swimming depends on the part of the stroke. Over the water, i.e. your recovery, it really makes no difference as long as your movements are controlled and smooth, and your arms are going forwards.
Under the water it becomes a matter of degrees. The straighter (and therefore deeper) your arm is, the more your shoulder will take the brunt of the work.
Swimmer's shoulder: what causes it, and three exercises to treat it
The more bent the elbow, the more you can recruit the back and side muscles of your lats to do the job. Your lats are the biggest muscles in your upper body, and therefore much stronger than your shoulders. They’re why swimmers have the iconic triangular back shape – think Michael Phelps!
In an ideal world your arm will track a mostly straight path back from entry to exit, with a slight curve under the water to gain the best contact and hold on the water. The key here is the ‘high elbow’ that is often coached – ideally keeping your hand below your elbow – and then looking to press/push the water backward using not only your palm but your forearm too.
This article appeared in the December 2017 (345) issue of 220 Triathlon. You can subscribe to the magazine here