It’s okay to straighten your arms, but don’t lock them. Here’s why.
When performing upper-body moves that involve flexing and extending at the elbows — including bench presses, pushups, biceps curls, and overhead presses — it’s best to straighten your arms without locking them, says exercise physiologist Dean Somerset, CSCS. “Just getting them to a point where they’re stable and straight is perfectly fine for most people.”
Locking the elbows puts them in a hyperextended state. Adding load, whether it’s body weight while doing pushups or an external weight for presses, further increases the chance of overextending the joints, putting you at risk for injury. This is true for most people, and especially those who are extremely flexible, or hypermobile, Somerset says.
That doesn’t mean you should leave them at a 20- or 30-degree angle: If your elbows are bent too much, you won’t reap the full benefits of the exercise. For example, you won’t get nearly as much out of doing quarter reps at the top of a biceps curl as you would from full extensions. Somerset recommends using your elbows’ full range of motion without forcing them into locked hyperextension.
This advice also applies when doing yoga. Poses such as downward-facing dog and high- and side-plank are more stable if you leave a slight bend to straighten up without locking out.