Matt Roberts, personal trainer, replies: Good breathing technique is vital when lifting weights and as a general rule you should exhale on the lifting/effort phase and inhale on the lowering. Don't hold your breath as this can be dangerous as it can cause blood pressure to rise, which could, in extreme cases, cause nosebleeds and even unconsciousness.
Occasionally, particularly when lifting heavy weights, breathing can turn into grunting and sometimes screaming, although noisy exhalations are not unique to weight training (watch some women's tennis) but, to be quite honest, 99 per cent of the time it's a testosterone-fuelled habit that is simply bad gym etiquette.