Proper breathing is often overlooked by beginners in the sport of weight training. Since our muscles require oxygen to be delivered through them, we must make sure that we breathe correctly. You will find that through proper breathing, you will actually add a few reps to each set, thus giving you a better workout! So what is the proper way of breathing? Well, that depends on the exercise that you are doing. In the most general terms, you must breathe out on the power stroke of the movement; i.e. the hardest part of the movement. Before each set, you should also take a few deep breaths. This helps you focus on the set that is to follow, and it also brings more oxygen to your muscles before the set. Exhaling during abdominal workouts (crunches, for example) also helps because it helps tighten your abs. Let out a whole breath when you are in the tuck position, and feel the difference in the tightness of your abs as opposed to trying to tighten them while holding air in your lungs - big difference!
Below is a basic chart which shows when you should breathe in (inhale) and when you should breathe out (exhale) during basic exercises. All other exercises are derived from these basics and breathing will be the same. Pay close attention to the pushing or the pulling of the weight for the different exercises.
BENCH PRESS - Inhale as you bring the weight down towards your chest. At the bottom, you should have a full breath of air, as you get a nice stretch in the pectorals. As you push the weight up, exhale that breath, finishing your exhale at the top of the movement, while you squeeze the chest muscles.
DUMBBELL FLY - Inhale as you bring the weights down, stretching the chest muscles. Exhale as you list the dumbbells towards the middle position. By the top of the movement, you should be fully exhaled and the muscles should be squeezed tight.
PUSH UP - Similar to bench presses. Inhale as your body comes towards the floor, and exhale as you go up towards the top of the movement.
PARALLEL BAR DIP - Inhale as you lower your body between the bars. Exhale as you lift your body up towards the top of the movement.
SQUAT - Inhale as you come down towards the squatting position. Exhale on the way up. Try controlling your breathing so you don't have to take multiple breaths during one repetition - this can cause dizziness.
LEG PRESS - Inhale as your knees come down towards your chest and exhale as you push out.
LEG EXTENSION - Inhale as your legs go towards the 90 degree position and exhale as you straighten your legs and squeeze your quads.
HAMSTRING CURLS - Inhale as you extend your legs and stretch your hamstrings. Exhale as you pull the weight up towards your hamstrings. Note, this is opposite of leg extensions; this can be said about any opposing muscles, biceps and triceps in particular.
BARBELL BICEPS CURL - Inhale as the barbell goes down towards your lower body. Exhale as you pull the weight up, squeezing at the top of the movement.
TRICEPS CABLE EXTENSIONS - Inhale as your arms bend and the weight rises, exhale as you push the weight down towards your lower body. Note here again, that this is the opposite of the bicep curls.
SIDE LATERAL RAISE - Inhale as you lower the weights to your sides, exhale as you lift the weights to the position parallel to your shoulders.
SEATED DUMBBELL SHOULDER PRESS - The breathing for this exercise is exactly like any other pushing exercise, like bench presses. You inhale as the dumbbells come down, and exhale as you push them up. This should help you see the similarities between exercises.
DEAD LIFTS - Inhale as you lower the weight towards the ground. At the bottom, complete your breath, then start exhaling as you lift the weight and squeeze the lower back muscles.
WIDE GRIP CHINS - Inhale as you lower your body to the hanging position. Start exhaling as you pull yourself up towards the bar, contracting the back muscles and exhaling fully at the top.
CRUNCHES - Inhale as you lower your body on the floor, and exhale as you crunch up and squeeze the abs. Breathing is extremely important in abdominal exercises, not only for oxygen delivery, but also for getting a better contraction in the muscle.
MOST AB MOVEMENTS - Every other exercise is exactly the same as a basic crunch when it comes to breathing. Since the abs only involve the same pulling motion from different angles, breathing is the same.