There is more to exercise than jumping on a treadmill, lifting weights or playing basketball. To avoid injury and maximize gains, a warm up, stimulus (or conditioning) and cool down must be included in your routine.
Warm ups are needed to move your body from a resting to an exercise state. A good warm up gets you ready for exercise by increasing: blood flow to your muscles, body temperature and metabolic rate. It also decreases the risk of injury.
The warm up should last 5-10 minutes and be a lower intensity version of the exercise you are warming up for. If you plan on running, warm up by walking or even jogging. The intensity at the end of your warm up should be at the lower end of your actual exercise intensity. Finish your warm up with some low intensity stretching of the major muscle groups.
The stimulus (or conditioning) phase can be considered the main part of the workout. It can include endurance, resistance and flexibility training. A well rounded program will include a combination of all three of these.
This phase can last anywhere from 20-60 minutes depending on your goals. During this stage, be sure to include activities you enjoy such as basketball, swimming or running. Choose a sport in which you have a high skill level in. This will help get a higher intensity.
The cool down will help your body move from the exercise state to a resting state. It brings your heart rate and blood pressure back to normal ranges decreasing the possibility of cardiovascular complications.
A cool down returns your body to a normal temperature, gets rid of lactic acid from the muscle and decreases the risk of dizziness. This is vital step and should not be skipped by anyone just because it burns fewer calories.
These three stages shouldn't be something you have to write down and constantly think about. The transition from one stage to the next, over time, will become more automatic for you.
The Bottom Line
Skipping one or more parts will make your workout incomplete and in some cases increase injuries, but going through all of them will make your workout safer and more effective.