If you've been around fitness long enough, you've probably noticed a few different methods of breathing while exercising. If you haven't, then the thought probably hasn't crossed your mind yet that how you breathe matters. Fair enough -- it's not something you normally think about in every day life. That is, until you start reading about breathing, and then become acutely conscious of every single breath entering and leaving your body. Or is that just me right now?
There are four approaches to breathing that I'll be covering here.
The first is the casual, don't-think-about-it variety. This is best left for cardio. If you're activity is light enough for you to breathe through your nose without thinking about it, you don't need to think about it. Once cardio gets intense enough that you start breathing through your mouth...you still probably don't need to think about it, unless you have something like asthma (nasal breathing results in slower breathing due to smaller pathways; it also results in the inhaled air being warmer, cleaner and moister by the time it reaches the lungs, which is all relevant if cold, dirty, dry air is a problem). This variety of breathing tends to become more problematic when it comes to strength training.
If you have cardiovascular issues, the safe way to breathe during strength training is to inhale on the eccentric and exhale on the concentric. Due to the strenuous nature of strength training, people often need to be reminded to breathe, as momentary strenuous work and holding your breath tend to go together. The issue for people with cardiovascular issues, especially high blood pressure, is that strenuous activity increases blood pressure, and so does holding your breath. Put the two together and it might be time to say your goodbyes. Breathe fluently throughout the exercises, on the other hand, and you might live to fight another day. This is the most common method of breathing recommended in gyms (in my experience).
The Valsalva maneouvre is literally not for the faint of heart. This method of breathing makes use of the tendency to hold your breath under strenuous load. While it depends on the lift, the Valsalva maneouvre is generally performed by taking a deep breath and holding it before the lift begins, and either holding that breath until the rep is completed or exhaling only once you've passed the hardest point in the lift. This breathing pattern allows greater tightness in the trunk than breathing in on the eccentric and out on the concentric, making it ideal for maximal strength work, but it does spike blood pressure, so only use it if your health permits.
The last method I'll mention is to take the Valsalva maneouvre but hold the breath for more than one rep. This isn't just "not breathing" while you lift -- you're strategically inhaling deeply at the right time and holding it. I personally find this is best left for moderate weight, higher rep sets than max strength work, but your mileage may vary. Depending on the exercise, I'll often get 2-5 reps at a time out of a 10+ rep set while breathing this way. This is an extension of the Valsalva maneouvre, so if it's not safe for you to do Valsalva, it's not safe to do this.