"How much should I warm-up before practice?"
"How much should I warm-up before my event in a meet?"
I hear these questions a lot. A "warm-up" should be just that--you need to raise the core temperature of the body, and you don't do that by doing an easy jog around the track a couple of times. Then after you do get "warmed-up" don't stand or sit around and let the body temperature go back to normal.
In the running events, it is important that you go to the line "warmed-up" and with the PR (Pulse Rate) slightly elevated. There is a lag in time between when the body starts doing work and when the cardio/vascular system catches up. Having the PR in the 100 to120 range rather in the 60 to 80 range helps to "jump start" your body. During the "warm-up" phase, the PR should be taken up to near the race PR for a brief period of time. Doing several "build-ups" a few minutes before the start of the race will accomplish this.
STRETCHING DURING WARM-UP
Most people do static stretching as part of their warm-up routine. I used to. I used to coach it. A few years ago I attended a couple of "coaching clinics" . One was on "Training Sprinters" and featured Kevin O'Donnell of Speed Dynamics. The other "Hurdling Into The 21st Century" featured Brent McFarlane, head coach of the Canadian Olympic T&F team.
Both of these experts recommended using dynamic stretching during the warm-up and then doing the static stretching after the session. The reason for eliminating the static stretching before the session is that it tends to turn off the sensors and receptors in the body. The body shuts these down when there is no activity in the area for a short period of time. That is why you don't feel the hat on your head or the watch on your wrist.
Simply put, dynamic stretching is moving the arms and legs through their full range of motion while you are moving. This gets them in motion and ready to go. The link below will take you to a page covering dynamic stretch warm-up information.
Make sure that you do the static stretch after the session, because this is the only way that you can lengthen the muscles. The other link will take you to more detailed information on static stretching.