The exercise resistance should be high enough to produce a high rate of strength development and low enough to pose a low risk of injury. Empirical evidence clearly indicates that using 75 percent of maximum resistance meets both of these training criteria.
Research indicates that most people can complete eight to 12 controlled repetitions with 75 percent of their maximum resistance. Generally speaking, if you cannot perform at least eight repetitions the resistance may be too heavy, and if you can complete more than 12 repetitions the resistance may be too light. Working within the eight to 12 repetition range is recommended for safe and effective muscle development.
Every strength-training program needs a protocol for progressing to heavier workloads. While it is important to increase the exercise resistance periodically, it is equally important to do so gradually. A safe and productive progression is known as the 12 by 5 rule. That is, whenever you can complete 12 repetitions of an exercise in good form, you increase the resistance by 5 percent or less. The 12 by 5 procedure adds small but frequent weightload increments to progressively stress the muscular system.
Unfortunately, there is little consensus on the best training speed for strength development. Our research indicates, however, that slow movement may be preferred over fast movement, because a slow speed produces less momentum and more muscle tension. At six seconds each, eight to 12 repetitions require about 50 to 70 seconds of continuous muscle effort, which provides an excellent anaerobic stimulus for muscle building. We have obtained consistently good results training with six-second repetitions, taking two seconds for the harder lifting movement and four seconds for the easier lowering movements.
Full-range muscle strength is best developed through full-range exercise movements. In other words, the training effect is greatest within the exercised portion of the joint movement range. Full-range strength reduces injury risk and increases performance potential. Try to perform each repetition through a full range of movement, but never to a position of discomfort.
Wayne L. Westcott, Ph.D., is fitness research director at the South Shore YMCA in Quincy, Massachusetts, and author several books on strength training, including his most recent text, Strength Training Past 50.