We all do it whether we're conscious of it or not. We hold our breath when exerting force. Most commonly, we do this while defecating or performing a strenuous task like lifting a heavy weight. Some of us do it more frequently, and may do it when we're stressed or simply bending forward to pick something up.
Why do we do it? Most of the time we're doing it to essentially compensate for a lack, or add to, our core stability. Outside of extreme weight lifting, this is a bad plan! First of all, it's a compensation and in order to perform the activity in the most functional, pain-free, and mechanically correct way, we need to engage our core properly before and during the activity. Secondly, it has tremendous physiological impacts and cause lightheadedness, dizziness, or syncope (passing out).
The Valsalva maneuver occurs when we forcibly exhale against a closed glottis or throat (again, think defecating). This causes multiple things to happen: intra-abdominal and intrathoracic pressures increase. This excessive pressures cause compression of the vessels of chest, including the aorta, and decrease venous return and carbon monoxide levels. It will also drastically slow heart beat and increase sympathetic (flight of flight nervous system) activity by stimulating baroreceptors. To combat this and maintain perfusion and carbon monoxide levels, heart rate and blood pressure will rise. When the maneuver ends (when we finally breathe out), venous return and carbon monoxide quickly rise and intra-abdominal pressure plummets causing a significant stress on the heart. These effects can be particularly dangerous for the elderly, anyone with heart or blood pressure conditions, and pregnant women.
This can also lead to incontinence issues or pelvic pain for both men and women. When we forcibly exhale this way, our pelvic floor muscles naturally contract to combat the increase in intra-abdominal pressure and keep our organs and urine/feces inside our bodies. If/when we accidentally leak a little urine during this activity, we have had a failure of the pelvic floor and need to seek rehabilitation quickly in order to nip this problem in the bud. If this is not addressed, the incontinence can worsen or we can develop pain in our pelvic floor (especially during sex for women), hip, or lower back area.
To that point, a Valsalva should never be used while using the bathroom. In that situation, the point is to pass urine or feces through our pelvic floor and let it leave our bodies. We do not want to start that motion with a contracted pelvic floor. Not only will that make evacuation much more difficult, the increase in pressure can head to hemorrhoids or tearing of skin/muscle. It is also bad mechanical training. We always want to reinforce a pelvic floor contraction with keeping things in the body, the muscles need to relax to release things out of it. Practicing bad form while going to the bathroom confuses the muscles and can create problems.
Lastly, this causes increased stress on other areas of the body and can create a pathology. When we take that large inhale before we hold our breath, it extends our thorax on our lower back. When we hold our breath, our thorax is locked in this pattern during the duration of the exercise. This places our abdominals in a lengthened position and make it hard to properly utilize, activates our lower back extensors and forces them to do most of the work stabilizing our spine, and places the balls of our arms and hips in an improper place in the sockets. When you then try to use your shoulders to lift that laundry basket, or squat down to pick up that dropped item, the muscles surrounding that joint aren't able to be properly activated and can cause injury or pain.
How do you combat this? Simply remembering to exhale (through either your nose or mouth) when you're moving, especially when you're straining is a good start. For people who have been doing this for some time may notice that this is a particularly hard pattern to break, or that they feel much weaker if they aren't holding their breath. For those individuals, respiratory, diaphragmatic, core, and joint stabilization training need to be implemented.