I know, real men don't tweet, and execise warm-ups and cool-downs are just a waste of time. You're a Type-A, and your motto is "do it, get it done, and move on to the next thing." But you're treading on dangerous ground, and here's why:
The warm-up is simply a brief period of slow, rhythmic aerobic activity involving the large muscle groups of the body, like the arms, legs and back, as well as other muscle groups that will be used during the exercise session which follows.
And the warm-up does just that - it raises the temperature of the muscles which, in turn, makes them more pliable and delivers an increased oxygen- and nutrient-rich blood supply to them. The oft-used analogy is to that of a piece of taffy: sit on it for a while, or warm it in your hands for a few minutes, and you can bend it; otherwise, try bending it and it will break.
By taking a few minutes to properly prepare your muscles for the training which follows is the best way to minimize the risk for soft tissue (muscles, tendons) injury, as well as psychologically preparing you for the exercise session to follow.
The cool-down is similar to the warm-up phase in that it is performed the same way - slow, rhythmic movement of the large muscle groups.
As important as the warm-up is, the cool-down is at least as important, and perhaps even more so.
The cool-down prevents post-exercise blood pooling in the lower body. It also prevents a too sudden drop in blood pressure, which could otherwise lead to lightheadedness and even fainting.
Other important reasons for an adequate cool-down are to prevent or reduce muscle cramping and spasms, and reducing the risk of post-exercise disturbances in cardiac rhythm.
What to do
For cardio sessions, the best warm-up and cool-down is to perform the same or similiar activites, but at a much lower intensity. For instance, walking is a great warm-up and cool-down activity for jogging; low-intensity cycling is best before and after cardio-intensive cycling, etc.
For resistance training, spend five minutes before and after your workout doing low-intensity cardio (cycling, walking, etc.).
There are no exceptions for performing warm-ups and cool-downs. However, while five minutes of doing these activities before and after is appropriate for most people, some health and/or physical conditions call for longer warm-up and cool-down periods. This applies especially to pregnant women, and to people with asthma, high blood pressure, heart disease, and arthritis.
Stretching will be discussed more thoroughly in other articles under the heading of Flexibility. However, it is rightfully mentioned here as well, albeit for different reasons.
Admittedely, there is some debate within the fitness community as to the need for stretching before, after, or before and after cardio or strength training. I am definitely in favor of stretching both before and after.
Stretching the principle muscles that will be used during the exercise session acts to loosen the associated muscle tissue, enhancing the fluidity and range of motion during the exercise activity.
Pre-exercise stretching should always be done after the warm-up and before the exercise component. This way, the muscle tissue is already warm and pliable.
Post-exercise stretching should be done following the cool-down. It helps return exercised muscles to their normal length, and facilitates removal of waste products which have accumulated in the tissue during the exercise session.
On a final note
Not warming-up and cooling-down is the single biggest mistake we trainers see people make. It is an invitation to musculoskeletal injury or worse. It's like driving home drunk; you may not have an accident any one day, but do it every day and chances are good that some day you will.