Since any interruption in blood supply might cause us to faint, perhaps fatally, high-resistance exercise requires the heart to generate high pressures that have to be withstood, especially by the heart itself and by the aorta. For this reason, as blood pressure shoots up, we instinctively inflate our chests and briefly hold our breath. This vital reflex, known as the Valsalva maneuver, lessens stress on the heart, and it also helps rigidify the trunk and stabilize the spine.38
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