You won't always get injured if you stretch a cold muscle, but it's not worth the risk. A proper warm-up consists of five to 10 minutes of full-body movements designed to increase blood and muscle temperature. A warm muscle stretches easier than a cold muscle, and you will feel and see the flexibility improvements. When you are unable to complete a dynamic warm-up, you can use heat applied directly to the muscles prior to stretching in the way of heat packs or moist heat from a bath or shower.
Stand underneath a hot shower for 10 to 20 minutes to warm your muscles for stretching. Set the water temperature high, but not to scalding. Let the water blast on individual muscle groups such as your lower back, hips, chest, hamstrings and calves.
Soak in a hot bath of a temperature that is comfortable, but not painfully high. Rest in the bath for 10 to 20 minutes and perform a few stretches in the tub such as a seated forward fold, which stretches your hamstrings.
Apply a heating pad, or a moist heating pad, to the muscle you wish to stretch. Keep the heating pad on the muscle for 10 to 20 minutes. Stretch, and then move the pad to the next muscle group.
Make a heating pack by using rice or buckwheat and a sock. Fill the sock halfway with rice or buckwheat. Hand sew the top of the sock closed. Place the sock in the microwave for 30 to 60 seconds. Rest the sock for 10 to 20 minutes on the muscle you wish to stretch.
Relax and stretch only to the point you feel a mild pull on the muscle. Try not to hurry or overstretch your muscles.