There are several different types of stretching, but I'm going to focus on two:
- Dynamic Stretching: This means moving a limb to the limits of its range of motion, then back (as opposed to holding a limb in the stretched position). Think leg swings, or mae keage, or yoko keage.
- Static Stretching: This means holding a limb at a point near or at the limit of its range of motion. This is the typical sitting on the floor with legs spread, leaning over one leg or the other and holding the position for some period of time.
- Movement prep: This is stretching before a workout, as part of your warmup, or before a performance of some kind. Think of the stretching you do before practicing kicks. Also known as limbering up or loosening up.
- Flexibility enhancement: Stretching you do to increase your long term flexibility.
Look, we've all been told at some point to sit on the floor and static stretch for fifteen minutes or half an hour before karate class or other exercise. We've been told that it prevents injuries and prepares the body for the workout. This is just not true. The scientific evidence behind using static stretching pre-workout for injury prevention is nil - it just doesn't work. Holding stretches for a long time, like sitting in a near - split and trying to touch your chest to the floor for 30 seconds or a minute, significantly reduces the amount of force your muscles can generate for quite a while thereafter. Do you think that's what you want to do - weaken your leg muscles right before practicing kicks? Or before fighting?
On the other hand, two to four minutes of dynamic stretching will get you to the limits of your dynamic flexibility - that is, get you to the point where you can kick as high as your body will allow you to kick. Don't believe me? Try it. Do a quick warm up - any movement that raises your core body temperature. Do some planks and some glute bridges maybe (lie on your back, put your feet flat on the floor, and thrust your pelvis at the sky. Repeat). Then do some leg swings - 10 each leg, first 10 to the front, then 10 to the side. Repeat for 2 or 3 rounds if you need to. Do these fast, but not full speed - you're trying to move your leg through its full range of motion and then back, not hold it up in the air. Total time? Maybe 5 minutes. Then practice your high kicks.
Does that mean no static stretching? Well, not exactly. There are two reasons to do static stretching. The first is that some static stretching will help you increase your flexibility, if you need it. And if you have minor muscle damage (tweaks, not full tears) stretching seems to help recovery. So if you're still working towards a full split, sitting in front of the TV every night and cranking your legs apart will probably help you get there. Do this stretching while fairly cold and preferably after some foam rolling or massage.
The other thing to do if you're trying to improve your range of motion is to increase your strength in those muscles at the stretched position. How? Well, try getting into a horse stance (kiba dachi). Widen it as far as you can - keeping the knees bent, try to get as close to a split as you can. Then squeeze the floor with your inner things - try to pull your heels towards each other. Hold, really working hard to contract those muscles, for 30 s or so.
You can do anything else that works those muscles from the stretch position. My favorite? I set up a pair of gymnastic rings. I put my feet into the rings (one in each). Then I lower myself into a split position, or as close as I can get, without using my hands (I'll usually keep a palm on the wall in front of me for balance). Then I pull myself back up - so my feet come together - using my hip adductors alone.
Do only dynamic stretching before your workout. Save the static stretching for off days or nighttime, not close to your workout.
I recommend dynamic stretching once or twice a day, every day. There are two reasons for this: the first is that it prepares you for mini workouts - like getting out of your chair and just throwing five or ten kicks with each leg. The second is a self defense issue. If you use high kicks in your karate practice and get into a fight you should be prepared to throw those kicks anytime. No mugger (or whatever) is going to jump you, then stand aside and wait ten minutes for you to warm up and stretch before continuing the beating.
If you're curious, the core of these recommendations comes from Thomas Kurz and Mac Mierzejewski. check out their stuff at stadion.com.