When working the chest muscles (pectoralis major), many exercises only offer a partial range-of-motion for the muscle itself. Full-contraction of the chest is achieved when the arm’s drawn forward and across the body. Indeed, some exercises where you may come forward and across the body, such as flyes, pec dec and cable crossover, still only bring your arm partially across your body and do not therefore fully-contract the chest muscles. So, how do we achieve full-contraction? We perform a crossover with one arm at a time.
Many gyms are now equipped with a Dual Adjustable Pulley or similar. Adjust the cable column height so that it is level with your outstretched arm position, just below shoulder height. This is a single-joint (shoulder) movement; therefore, slightly kink your elbow and hold this arm position stiff (not straight!). Being a single-joint movement, you will not need much weight to perform this exercise. Assume the load and stand slightly further away than arm length. Standing at 30 degrees to the cable column should allow for a good-stretch and full-contraction without the cable touching your forearm at any point. If it does, you are either standing at an incorrect angle, flexing at your elbow joint or rotating your torso. If you are making these errors, you are likely using too much weight! Remember, this is movement of only a single-joint (shoulder) and you will not need much weight. Once completing a set, swap arms and repeat. There is no need to rest.
To best engage with the exercise, I recommend holding the working pec muscle with your free hand. At the start position, you should feel the stretch toward the muscle’s insertion point at the outside of your chest. At the mid-position, you should feel the full-contraction all the way to the muscle’s insertion point at your inner chest.
Here's a video tutorial going over the key points to performing a Single-Arm Cable Crossover correctly:
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