Performing a high number of repetitions, or reps, with light weights is commonly associated with toning muscles, according to exercise physiologist Jessica Matthews. A study by the "Journal of Strength and Conditioning Research" found that comparable results can be achieved whether completing an exercise set with a high number of repetitions or a low number of repetitions. Matthews cites research that shows that exercising to achieve muscle fatigue within 90 seconds should be the goal to increase muscle strength and endurance.
Light or Heavy Weights
One of the main benefits of using light weights is the diminished risk of injury. A light weight also allows you to perform a higher number of repetitions. According to Matthews, a light weight is acceptable as long as the muscle fatigues in less than 90 seconds. If your current choice of weight is light enough to allow you to perform an exercise for two minutes or longer without fatigue, you should switch to a heavier weight that tires your muscle in 90 seconds or less. Personal trainer Joe Dowdell states that lifting heavy weights can help you gain a leaner body faster.
High Number of Repetitions
Bodybuilding conditioning specialist Josh Henkin writes that an individual performing a higher number of reps has a greater chance of gaining muscle mass because gains in muscle mass are usually related to the overall amount of exercise performed. WebMD recommends, for beginners, eight to 12 reps for exercises working the body's major muscle groups -- chest, back, shoulders, arms, stomach and legs.
Effect on Muscle Strength
In the "Journal of Strength and Conditioning Research," men performing six sets of four reps until they experienced muscle failure achieved similar results to those performing three sets of 10 repetitions. Exercise physiologist Anoop T. Balachandran states that bodybuilders tend to use lighter weights and high number of reps, around eight to 12. Balachandran says that a high number of reps with low weights works best for muscle growth due in part to less muscle damage and nervous system fatigue. By contrast, Matthews says that genetic factors are largely responsible for a muscle's response to resistance training.
Effect on Muscle Endurance
Personal trainer Joe Dowdell's typical female client works out using weights at 50 to 60 percent of her maximum capability and performs 15 to 20 or more reps per set. This type of combination will develop the muscle's endurance capabilities. He further states that building muscle or maintaining your current muscle tone is important for your metabolism because muscle tissue uses up more calories than fat tissue and allows you to maintain a leaner body.