When you exert yourself, it helps to stiffen your torso to provide a rigid anchor point for the muscles of your arms and legs. This works best if you hold your breath and tense your abdominal muscles, compressing the air in your lungs.
As you ease off and breathe out again, the air is forced out of your throat, still partially clamped shut, and this creates the grunting sound. A 2014 study found that grunting gives tennis players stronger serves.
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