Why do we hold our breaths when we push or pull on heavy objects?
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There are three thick muscular layers of the human abdominal wall. From inside to outside, the layers are called transversus abdominis, the internal abdominal oblique, and the external abdominal oblique. These muscles attach to the rib cage and the pelvis, providing tension to move the spine, particularly the lumbar spine. The lumbar spine is very important for weight-bearing activity including heavy lifting. In addition, contraction of these muscle groups compressed the contents of the abdomen, which are then displaced upwards into the diaphragm to move the diaphragm upwards and blow air out of the lungs. When you try to blow a strong breath (E.g. blowing out birthday candles), it’s the muscles of the abdominal wall that provide most of the force.
When you lift something heavy, your lumbar spine needs a lot of support. The muscles of the abdominal wall need to be tense so the spine can be fairly rigid. Achieving this tension requires a strong contraction, but that contraction would just blow air out of the lungs if you didn’t close your vocal cords to hold your breath. By holding your breath, you keep the lungs full of air so the diaphragm can’t move, which means the muscles of the abdominal wall can contract strongly and raise the pressure inside the abdomen. The high pressure in the abdomen, as well as the tension on the spine produced by contracting muscles, keeps the lumbar spine rigid, which allows you to lift something heavy.
This behavior must be learned. A newborn infant does not know to “bear down” (Valsalva maneuver) when pushing hard with the legs. But they figure it out pretty quickly. By the time your brain is capable of forming meaningful memories, you’ve already figured out how to use the Valsalva maneuver to lift/pull things, so you don’t even think about it anymore as a child/adult.
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