All five of the standard bodyweight exercises are effective for improving muscle strength and endurance, although not as productive as progressive resistance exercises performed with free-weights or weight-stack machines. The reason is that bodyweight exercises permit only one means of progression, namely, more repetitions using the same bodyweight resistance. On the other hand, weight-training exercises permit two means of progression. You first increase the number of repetitions, such as from 10 reps to 15 reps, then you increase the resistance, such as from 100 pounds to 105 pounds. Adding resistance makes the muscles adapt to a higher level of contraction tension, which greatly enhances strength development.

Nonetheless, bodyweight exercises have certain advantages from a practical perspective. For example, they can be done almost anytime and anywhere, especially squats, trunk curls and push-ups. They require no special equipment or facilities, which make them ideal for at-home training and at-hotel training. Unfortunately, a major problem with bodyweight exercises is redundancy. That is, doing seemingly endless repetitions of the same exercises in the same manner. Eventually, the muscles become so accustomed to your unchanging exercise regimen that they no longer respond to the training stimulus.

One means of addressing the problem of staleness is to change some aspect of the exercise program. Because the exercises are pre-determined and the number of repetitions performed is closely tied to your fitness level, I recommend making a change in your movement speed. You will find that doing each bodyweight exercise more slowly makes it much more challenging. Slower movement speeds reduce the role of momentum and force the muscles to work a lot harder.

Our research with strength training speed has demonstrated 50 percent better results with slow repetitions (14 seconds each) compared to standard speed repetitions (7 seconds each). With this in mind, I have designed a bodyweight exercise training program based on 15 second repetitions to maximize the strength-building benefits. Each repetition is performed in the following manner: take 10 full seconds for each upward movement, and five full seconds for each downward movement. Try your best to do the exercises according to the performance descriptions, and breathe continuously during every repetition.

Knee-Bends (Squats) Begin standing with your feet about shoulder-width apart and pointing forward. Place your hands on your hips for balance. Slowly lower your hips downwards and backwards in five seconds, keeping your knees directly above your feet. Stop when your thighs are parallel to the floor, then slowly raise to the standing position taking 10 full seconds to do so. This exercise works almost all of the muscle groups of the legs, including the front thighs (quadriceps), rear thighs (hamstrings), and hips (glusteals). Try to do three knee-bends in 45 seconds, and progress gradually to six knee-bends in 90 seconds. You may be surprised at how much more difficult it is to perform slow-speed knee-bends.

Trunk Curls Start this exercise by lying face-up on the floor with your knees comfortably bent (about 45 degrees). Place your hands loosely behind your head for support with your elbows out to the sides. Curl your head, shoulders and upper back off the floor very slowly (10 seconds), and return back to the starting position in five full seconds. You will be barely moving but your abdominal muscles (rectus abdominis) will be fully activated throughout the entire repetition. Begin with two repetitions in 30 seconds, rest for 30 seconds, and complete two more repetitions in 30 seconds. Work your way up to four repetitions in 60 seconds, rest for 30 seconds, and do four more repetitions in 60 seconds.

Push-Ups The key to properly performed push-ups is straight body position which requires substantial midsection strength to prevent sagging. Begin in the up position with your hands slightly wider than shoulder-width apart and your arms straight. Lower your torso slowly by bending your elbows, taking five full seconds to touch your chest to the floor. Push yourself up very slowly and deliberately, counting from one-thousand-one to one-thousand-ten before reaching the top position. Push-ups address several upper body muscle groups, including the chest (pectoralis major), front shoulders (anterior deltoids) and rear arms (triceps). Try to do two slow push-ups separated by a 15-second rest period. Work your way to four slow push-ups separated by 15-second rest periods. When this can be accomplished, reduce the rest periods to 10 seconds, then to five seconds, until you are capable of completing four successive slow push-ups in 60 seconds.

Bar-Dips My favorite exercise is bar-dips, which simultaneously work the chest (pectoralis major), front shoulders (anterior deltoids), rear arms (triceps), and upper back (latissimus dorsi) muscles. Unfortunately, bar dips require parallel bars or two stable kitchen chairs for proper execution. Start in the up position, with your arms straight, your body straight, and your knees bent if you are using kitchen chairs. Slowly lower your body by bending your elbows until your upper and lower arm form a 90 degree angle. The lowering phase should take five seconds. Pause momentarily, and push up slowly (10 full seconds) to the starting position. Because this is a demanding exercise, I suggest doing just one bar dip at a time with 30-second rests between repetitions. When four separate repetitions can be completed, gradually reduce the rest periods. When you can perform four slow bar-dips in 60 seconds, you have achieved a very high level of muscular fitness in your upper body pushing muscles.

Chin-Ups Without question, the most difficult of the bodyweight exercises is chin-ups. This exercise involves your upper body pulling muscles, including the upper back (latissimus dorsi), rear shoulders (posterior deltoids), front arms (biceps) and, surprisingly, the abdominals (rectus abdominis). Of course, a chinning bar, or properly positioned tree limb, is necessary to perform this exercise. Begin by grasping the bar with an underhand, shoulder-width grip, and hanging with your arms fully extended. Pull your chin above the bar slowly, taking 10 seconds for the upward movement. Once there, lower yourself to the starting position in five seconds. Try for two slow chin-ups in two-minutes, taking a 90-second rest between repetitions. Gradually, reduced the recovery period until you can complete three consecutive chin-ups in 45 seconds.

Because slow-speed bodyweight exercises are so physically demanding, take at least one rest day between training sessions. Keep careful records of your performance progress, and be sure to reward yourself when you reach your exercise goals.

Wayne L. Westcott, Ph.D., is fitness research director at the South Shore YMCA in Quincy, MA., and author of several books including the new releases Strength Training Past 50 and Strength and Power for Young Athletes.