My yoga instructor always asks me not to bounce while trying to touch my toes. I'm not able to statically bend and touch my toes, without bending my knees. So I used to bounce up and down. But he asked me not to, but was not clear why. Any thoughts?
The type of stretching that involves "bouncing" or lunging in a repeated pattern is called ballistic stretching. While there are a few, very limited uses, it has been contraindicated as a stretching method for many years.
Every tendon (structure that connects muscle to bone) has what is known as a stretch reflex, that when triggered, causes the muscle connected to it to contract. One well known example is when the doctor whacks just under your knee with that rubber hammer and your leg kicks. He is hitting the patellar tendon, and causing the quads to contract quickly. Any time a tendon stretches unexpectedly and sharply, this reflex can activate.
So, when you are stretching, you are elongating the muscles. If you bounce while doing this, you can activate that reflex, and cause the muscle to try and contract at the same time you are actively lengthening it, which can cause damage. This damage can range from mild muscle tears up to either detaching the tendon from the bone or rupturing it. There are some stretching methods where you purposefully contract a muscle under stretch, but it's done in a slow, controlled manner and is considered an advanced stretching method.
Because you are risking a hamstring tear and/or lower back injury, for little or no gain. Both are very painful and take a long time (months) to recover.
First of all, as others have pointed out, you should not do ballistic stretching.
In addition, you should always keep your spine straight when stretching the hamstrings. When you reach the limits of your flexibility, your hamstring will tense. The hamstrings attach to your pelvis (the part which you sit on) and they will pull the pelvis backwards. That means your pelvis is locked and will not be able to tilt forward. At the same time, you forcefully reach for your toes and most likely bend your spine forward. This results in strong compression in the lower back, which can lead to serious injury.
Always move slowly, and stop when your hamstring starts to tense up. Always keep your spine straight in forward bends, even if that means you can not reach the toes (yet). You will arrive there earlier if you are patient and not injure yourself.
You're describing two different types if stretching: ballistic, when you push with momentum, and static, when you push slower and hold the stretch steady for 30 seconds or more. Static is safer because you are much less likely to push too far when you know you're going to have to deal with the pain for the next 30 seconds or more. With ballistic, there is more of a chance that you will bounce too far/hard and pull/strain/tear something by going past the point your body would stop you at if you were moving slower. With static stretching, you generally stretch kinda slowly, so there's no momentum that would push you past where your body would tell you to stop.
I've always been told in my health classes that ballistic stretching should be avoided unless you know what you're doing. In general, taking what is normally a static stretch and making it dynamic by bouncing, is usually not a good idea.