How you breathe during an exercise is important. There’s no golden rule though. For the most part, a bodybuilder should breathe in prior to the start of a rep and breathe out just as the hardest part is reached. Never try to breathe in and out through the nose when training vigorously.
The nasal passages are just not set up for huge rushes of air intake. Inhale through pursed lips and exhale with a blast as the rep is concluded.
There are some exercises where it is best to breathe only every two or three reps, and others that lend themselves to double breathing.
If you are training with a good tempo, don’t just allow your body to recover by sitting down and panting. Load up with oxygen. Hyperventilate. Place your hands on your thighs, knees bent, and lift your chest high as you slowly take each breath through pursed lips. After taking five or ten breaths in this manner, you should be ready to begin your next set. I call this “oxygen loading.”
Your muscles definitely benefit from this oxygen loading principle. By pouring oxygen into the blood, through the lungs, you are paying back the debt created by the last set of exercises. This produces a terrific muscle pump. It also aids in maintaining exercise rhythm, which is important for maximum workout efficiency.