Conventional weight training wisdom dictates that a person should choose a weight they can lift eight to 12 times in a set. While this range is a useful rule of thumb, the question of how many repetitions to perform depends upon a person's individual strength goals, age, experience and a variety of other factors. As general rule: If your goal is to gain strength and power, use more weight and do fewer repetitions. If your goal is endurance, use less weight and do more reps.
More or Less
A repetition, or rep, is a complete motion of a particular exercise. A set is a group of repetitions. Opinions vary widely as to the numbers of reps and sets one should do to build strength. But generally, if you want to get larger and gain strength, choose a heavier weight that you can lift four to six times per set. The weight should be heavy enough so that the last rep requires a great deal of struggle. If you are using heavier weights, allow yourself more time to rest between sets.
If your goal in weight training is tone and endurance, choose a lighter weight that you can lift 10 to 12 times. Again, your weight should be heavy enough so that the last repetition is very difficult to lift.
Why does using more weight and performing fewer reps promote more strength? For one, a person using heavier weights tends to work out harder. Body makeup is also key. The human body is composed of two different types of muscles: slow twitch and fast twitch.
According to Indiania University School of Medicine, slow twitch muscles contract slowly, use a lot oxygen, are heavily fueled by blood vessels and are capable of greater endurance. They are drawn upon for such endurance exercises as long-distance running and cycling.
Fast twitch muscles contract quickly, do not utilize many blood vessels and are capable of more force but less endurance. Sprinting and throwing are activities that draw heavily on these muscles. When you lift heavy weights, you engage and develop more of these fast twitch muscles, thereby developing your body's strength.
Variations
Keep yourself engaged. Work on both strength and tone by trying different sizes of weights on different days. Also, try varying sizes of weights between sets. For instance, instead of simply performing three to five sets of the same weight and number of repetitions, try the "pyramid" system, which entails increasing the load and reducing the repetitions with every set. You might, for instance, lift 100 pounds 10 times in the first set, 120 pounds six times in the second set, and 140 pounds four times in the third set. In the "reverse pyramid" approach, you would do the opposite of this, decreasing the load and increasing the repetitions between sets.
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