Before weight training, do some exercises to warm the muscle groups you plan to work. If you plan to work your legs, you could bike gently or walk for five to seven minutes. If it is an upper body workout, try using a rower. After this warm-up, stretch the muscle group you are planning to work.
Muscles are like a rubber band.
If you stretch a cold rubber band, it may break. However, if you warm the band and then stretch it, it can lengthen without breaking. The same is true for muscles. If you stretch a properly warmed muscle, it can lengthen without damage. Just as a rubber band will snap back with more force if it is stretched, your muscles are able to store elastic energy and contract more forcefully after stretching.