In gym class growing up, we always did a “warm up” by holding a stretch on our muscles and then going to play volleyball or basketball or whatever we were doing. But that’s not really the best way to warm up. Mostly because it doesn’t actually “warm up” your muscles and it can possibly INCREASE your risk of injury while exercising.
The whole point of a warm up is to increase the blood flow to muscles you’re about to strenuously exercise. The best way to do that is to use the muscle. Your body increases the blood flow to areas you are actively using. That makes sense. If you do that your muscles can have the adequate resources (blood carries oxygen and nutrients) they need to work better. An increase in blood flow means your muscles can also get rid of the increase in waste products they produce when they work harder. Increased blood flow to an area makes it feel warmer, so you are literally making your muscles warmer during a warm up.
(Side note: when you eat, your body sends increased blood flow to your stomach and gut. That’s why “you’re not supposed to swim after you eat”. You don’t have enough blood to operate your digestive system and all your muscles at max power at the same time so neither will work well. These are actually two opposite systems in your body and aren’t supposed to work at the same time. But that really only applies if you’re about to have a really strenuous swim or work out in general, where you are using your muscles to the max they can perform. You’re not going to cramp up and drown if you dip your toes in the water after a meal)
If you’re stretching in a way in a way that people usually do, you hold a stretched position against the floor or chair or whatever for some period of time, say a minute. This is a “static” stretch. That means you aren’t actually moving, you’re just holding a position. If you aren’t moving, your muscles aren’t contracting and you aren’t actually using your muscles. If you aren’t moving and contracting your muscles, there’s no need to increase the blood flow to those muscles. So you’re not actually warming up at all. You will be able to increase your range of motion for a short period of time after a stretch though. But that’s actually not the best thing for you. This is actually caused by your nervous system becoming temporarily more comfortable in those ranges so it doesn’t protect you as much (your muscle doesn’t just become longer after a 1 minute stretch, that muscle tissue was always there). Your system allows your muscle to go into a further range without automatically contracting the muscle involuntarily to protect it from tearing.
The reason you don’t actually want this temporary increase in range of motion is because you aren’t trained in this new range of motion. If you aren’t trained in this new range of motion, your system will be less likely to tolerate high work loads in this new range. For example, if you stretch before going for a sprint, you’re now performing a maximal contraction of a muscle (running as fast as you can, you’ll contract your muscles as hard as you can very quickly) in a range it never uses or isn’t trained in. This makes it more likely to injure yourself or feel pain because your system can’t tolerate these new stresses in this new range.
The best way to warm up is to do a lighter version of the strenuous activity you’re about to do so that the muscles you’re about to use get worked. If you’re about to sprint, perform a light jog. If you’re about to push 200 lbs, 5 times, start by pushing 50 lbs, 20 times first. The whole idea is you want to prepare your body for the strenuous activity it’s about to do so that you can build it’s tolerance to that activity.
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