3 Stop pressing your flyes
Single-joint moves like flyes and cable cross-overs are typically done at the end of a training session. On these moves, your elbows should be locked in a slightly bent position throughout. As a result, you won’t be able to use as much weight as you can with presses.
Perhaps in the name of trying to push more weight with their single-joint moves, some trainees mistakenly load up more weight than they can reasonably handle. As a result, their form breaks down. While in this case that doesn’t mean you’re at greater risk of injury, the semi-press/fly movement reduces the degree of isolation.
You see it most often with cable cross-overs. Here’s how to tell if you’re doing it: Watch the degree of bend in your elbows over the course of the range of motion. What you’re supposed to be doing is locking a slight bend in your elbows and maintaining it for the entire set. What’s happening instead is your elbows are closing up—a sign that you’re pressing the weight—and then opening toward the top.
Again, there’s nothing wrong with doing presses, but cables aren’t a good place to be pressing a weight, because your body isn’t stabilized against a bench from which to strongly push. (Besides, we assume that, at this point in your workout, you’ve already done all your presses.) So drop the weight to one that allows you to maintain your form for the duration of the set, and keep an eye on your elbows!
If you’re still having trouble holding (locking) the slight bend in your elbows as you perform flyes or cross-overs, jump on the pec-deck machine, which more effectively holds your arms in position. This makes it damned near impossible to extend your elbows during the positive contraction.