Intensity of Exercise: Amount of Resistance
The amount of weight (resistance) you lift in weight training exercises is equivalent to intensity in cardiorespiratory endurance training. It determines how your body will adapt to weight training and how quickly these adaptations will occur.
Choose weights based on your current level of muscular fitness and your fitness goals. Choose a weight heavy enough to fatigue your muscles but light enough for you to complete the repetitions with good form. (For tips on perfecting your form, see the box “Improving Your Technique with Video.”) To build strength rapidly, you should lift weights as heavy as 80% of your maximum capacity (1 RM). If you’re more interested in building endurance, choose a lighter weight (perhaps 40-60% of 1 RM), and do more repetitions. New research has found that you can stimulate muscle hypertrophy using only 30-50% of maximum capacity if you stress the muscles adequately.
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For example, if your maximum capacity for the leg press is 160 pounds, you might lift 130 pounds to build strength and 80 pounds in more repetitions to build endurance. For a general fitness program to develop both muscular strength and endurance, choose a weight in the middle of this range, perhaps 70% of 1 RM. Or you can create a program that includes both higher-intensity exercise (80% of 1 RM for 8-10 repetitions) and lower-intensity exercise (60% of 1 RM for 15-20 repetitions); this routine will develop both fast-twitch and slow-twitch muscle fibers.
Because it can be tedious and time-consuming to continually reassess your maximum capacity for each exercise, you might find it easier to choose a weight based on the number of repetitions of an exercise you can perform with a given resistance.
