Why do trainers (not that I have one) always say "breathe while doing your exercises". Well what about situps?

If you don’t exhale on the most strenuous movement, your blood pressure pops up (your cardiovascular system is closed, and pressure in a spot compresses it) At best, you will get a headache. At worst, you will spring a leak. I do not buy the notion that you can not do the exercise without holding your breath. That is simply a result of conditioning, poor coaching and/or habit. You can and should break the habit. If you feel you must strain, get a mouth guard and clench against it.

Oxygen is needed for your muscles. Without oxygen, your muscles start making lactic acid which causes muscle cramps.

I exhale up and inhale down. I agree with @josie, and think you’ve picked up a bad habit of breathing somehow.

It sounds like you’re “crunching” incorrectly. A “crunch” shouldn’t be a crunching motion. It should be more of a lift. Exhaling while lifting and inhaling while lower will help you move.

Holding your breath briefly is actually required as “best practice” for certain single bursts of strength (e.g. a single bench press for max weight).

The problem arises if you have to do continuous work (say a rapid set of 10 bench presses, or 25 sit-ups). If you hold your breath for too long on the first, you’re limiting oxygen for later work. It’s a rhythm thing. Beginners literally forget to breath during long sets. This is where “good form” comes into play. This is a low hanging fruit suggestion for trainers. The suggestion is probably queued up before you even start lifting because they have seen it so often.

Note that it’s also a technique to hold your breath for entire sets. Lots of variations on weight training. Holding your breath (or not) is an art unto itself depending on what you’re going after. Good form generally starts with controlled breathing (vs. not breathing).

OK breath holding does lots of things. I think most of them have been hit on here, but just in the way of summary. As your muscles work they produce carbon dioxide. This is eliminated by exhaling it ( the kidneys can help with this but it takes days). If too much of it builds up then your tissues have too much acid in them and perform suboptimally.

To work at top levels your tissues need oxygen. The way we deliver it to them is by keeping a rich source of oxygen in your lungs. If tissues don’t get oxygen, the resort to anaerobic metaoblism. This is much less efficient that using oxygen and it has a nasty biproduct, lactic acid. Lots of lactic acid build up in your muscles is what leaves them aching later. For most of the group of us that are irregularly irregular excercisers, we don’t want to do things that cause this cause then we are “too sore” to work out for a few days.

Lasting holding your breath is equivalent to straining. I raises the pressures inside your skull, which also increases the pressure on your brain. This isn’t the best for your squash. But for most people who are healthy is does little more than leave you with a small headache if you do it lots.

Because hemorrhoids hurt