I have read and heard that warm-up and cool down are important in preventing pulled muscles and preventing muscle soreness. Exactly how long and strenuous should they be?
Warm-up should last about 5 to 10 minutes and would be performed at a work rate or speed that is well below that used for the main workout. For example, a jogger might use walking as a warm-up, or a slow jog a minute or two slower per mile than the pace used in the main body of the workout. Walkers should start out easily and work their way up to a brisk pace. Weight trainers might do 5 to 10 minutes of light aerobic exercise such as described here before ever picking up the first weight. Even then, the first set of each exercise should be light, approximately half the weight to be used in the heaviest set. For example, someone who will bench press 120 pounds in the last set might use about 60 pounds in the first set.
The value of warm-up extends beyond pulled muscles. Other benefits include shifting blood flow to the muscles and heating up the muscle and connective tissues so that they are more resistant to strain, but also because they work more effectively. Warm muscle promotes the speed of chemical reactions that release energy for contraction. The addition of blood to the muscle allows for more of the energy needed for the muscle contraction to be released through oxidative chemical reactions rather than reactions not dependent on the presence of oxygen. This reduces the amount of lactate, a waste product, developed in the muscle. Lactate interferes with chemical reactions and makes one breathe faster and deeper. Together, all of these benefits make your body work more efficiently, comfortably, and safely.
One study found vigorous exercise performed without the customary warm-up to be dangerous for the heart. Most of the young, healthy subjects were found to experience abnormalities in their EKG during the vigorous exercise. This indicates that middle-aged and older exercisers in particular need to warm up thoroughly before starting to exercise.
Cool down is the light exercise that is performed after the main part of the workout. Like warm-up, it should last about 10-15 minutes. Contrary to popular opinion, cool down does not reduce muscle soreness. Instead, it is always recommended because the easy movements during cool down promote the return of blood from the veins back to the heart. If one stopped running suddenly and simply stood still, the blood in the veins tends to remain there which reduces the volume of blood the heart receives which in turn reduces the amount it can pump. With reduced blood leaving the heart, the tissues rapidly become hypoxic (low oxygen content) leading to dizziness, possible fainting, and nauseousness.
By continuing to move after exercise, albeit at a slower pace, the veins are squeezed by surrounding skeletal muscles which pushes the blood back to the brain, muscles, skin, etc. Consequently, we recover much faster and feel better sooner.
Failure to cool down properly can lead to cardiac arrhythmia (a heart that fails to contract at a constant rate) and even a heart attack. As with lack of warm-up, middle-aged and older exercisers should view cool down as a critical part of a safe and healthy exercise program.