There is a lot of controversy regarding the best time to breathe when you’re lifting weights. Some hold strong to the theory that exhaling while you finish a movement is best, while others propose that athletes should just breathe normally during the lift.
The latter point is best because you will notice you naturally choose to hold your breathe during periods of peak exertion. As you hold your breath, you maintain the intra-abdominal pressure to keep the stress off of your lower back. However, be careful to avoid holding the breath for longer than a few seconds, so it’s best to exhale and inhale only between repetitions.