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Q. What’s the difference between lifting heavy weights for low reps (8 to 10) or light weights for higher reps (15 to 20)?
A. You’ve probably heard that lifting heavy weights for a low number of reps builds muscle, while lifting lighter weights more times tones them. But that isn’t the case. People commonly use the term muscle tone to describe how firm or tight a muscle looks. Your muscle has the same tone whether you are in shape or not or if there is fat covering it or not. Its tone does not change with exercise. The appearance of the muscle is determined by the amount of fat covering it and the elasticity of the skin. Lifting light weights for high reps will not tone your muscle.
There is no inherent difference between using heavy weight and light weight. The difference is a matter of degree and is relative to the lifter’s ability to lift the weight.
Lifting weights presents two basic challenges to muscle tissue: supporting the physical load being placed upon it, and generating energy rapidly enough to support intense muscle contraction. These adaptations are rapid when the stimulus is new or unfamiliar, but they begin to slow until they eventually stop, which occurs when your muscle can meet the demands of your workout. At that point, the only way to cause further adaptations is to increase the demand. Hence the principle of “progressive resistance.” The effectiveness of any amount of weight (and the number of reps it allows to be performed) is dependent on the condition of the muscle at the time it is lifted. Light weight will be effective at causing desirable muscle changes in the beginning, but its effectiveness will eventually wane, so you will need to increase the weight. Use a weight that allows 15 reps in the beginning and increase the weight until 6-8 reps is your max.