If you are serious about maintaining and improving your health and fitness, make a conscious effort to include a warm up and cool down to each of your training sessions.
Warm Up — BEFORE you train
The purpose of a warm up is to physically prepare your body for the demands of a strenuous workout by gradually increasing your body temperature. You should only need 5–10 minutes for a good warm up and should include movements that gradually increase blood circulation throughout the body. For example, a brisk walk or a light jog will increase your heart rate gradually and warm up your muscles. Your ligaments and tendons will become more flexible which will reduce the chance of tears or further injury to the muscle. It is also important to include dynamic stretches that take you (gently!) to the limits of your range of motion. Good examples of this include arms swings, leg swings and walking lunges.
Stretch — AFTER you train
Post-workout, stretching the major muscle groups you have used is SO important. It increases blood flow, which in turn brings a greater nutrient supply to muscles themselves, thereby reducing soreness and helping to speed up recovery. Stretching also helps to ensure correct posture by lengthening tight muscles that pull areas of the body away from their intended position. For example, stretching the muscles of the lower back, chest & shoulders can help keep the spine in better alignment and improve overall posture. Finally, the most obvious benefit of stretching is to help improve flexibility, which will assist you during your workouts and general day-to-day living. At the end of your workout, it is important to include static stretches of the major muscle groups you have worked. This involves stretches held in a challenging but comfortable position for a period of time, usually somewhere between 10 to 30 seconds. Examples of these could include a seated hamstring stretch, standing calf stretch etc.
It is essential that you include a warm up at the beginning of your workout and a stretch at the end of your workout every time you train. This is just as important if not more important that the workout itself.