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I am getting ready to start an exercise program. I have allotted an hour of time to my workout. I am interested in maintaining good cardiovascular fitness and have no previous injury to my back or legs. How much time should I spend stretching before and after my workout?
Your question addresses two points of interest: 1) when should you stretch and 2) will stretching prevent injury. Stretching is a type of exercise that elongates tissues. It was typically thought that stretching should be done both before and after exercise. However, by elongating tissue, stretching also helps to relax the muscle fibers in a static position. Therefore, since you do not have an injury now, the best option for you given your limited amount of time available for your workout would be to stretch at the end. Stretching at the beginning of a workout is controversial in terms of its efficacy. Specifically, why would you want to relax tissues prior to a workout (e.g., say racquetball) when you are typically trying to get ready for maximal performance. In that case, a dynamic warm-up would be more beneficial. This could include walking lunges, squats, heel walking, toe walking.
Tight muscles can cause injury. For example, if your hamstring muscles on the back of the leg are tight it can flatten your lumbar curve and put stress on your back. In this case once you are warm, this would be the best time to stretch. The time when you are most warm is at the end of your workout. Muscles have an "elastic" component to them and if you want the biggest bang for your buck, stretching at the end will have the most benefit. Stretching for 30-90 seconds for 3 reps on the major muscle groups (that are tight) is best (quads, hamstring, rotators of the hip, groin, and calf muscles).