Would you like to become stronger? Pick six to ten weight-machine exercises and do them in three sessions a week. In each exercise, try to lift the heaviest weight that you can lift comfortably ten times in a row without hurting yourself. When an exercise becomes easy, increase the weight. In five months, you should be able to increase your strength significantly and be proud of your larger muscles.
You now decide that you want to become even stronger. Would you increase your strength more by increasing the number of repetitions or by increasing the weight that you lift? For example, should you try to do three sets of ten for each exercise or stay at one set of ten, just try to lift a heavier weight once a week? Dr. Michael Pollock of the University of Florida in Gainesville divided recreational weight lifters into two groups. In one group, they tried to do three sets of 10 three times a week. In the other group, they did just one set of 10 three times a week, but tried to lift progressively heavier weights. Those who did one set of ten with heavier weights three times week were stronger than those who did three sets of ten without increasing the weight.
Exercise does not make you stronger. If it did, marathon runners would have the largest muscles of all athletes. The single stimulus to make muscles larger and stronger is to stretch them while they contract. When you try to lift a heavy weight, your muscles stretch before the weight starts to move. The greater the stretch, the greater the damage to the muscle fibers and when they heal after a few days, the greater the gain in strength. The results for this study give a clear message. You become stronger by lifting heavier weights, not by exercising more. If you do too much work, you can't lift very heavy weights and you do not become stronger. When it comes to becoming very strong, less may be more.
Note: If you are middle-age or older, or out-of-shape, read my report on Weight Lifting for Middle-Age and Beyond before you start a weight lifting program
Hass CJ et al. Med Sci Sports Exerc 2000(Jan);32(1):235-242.