"Performing lighter weight with more repetitions (15-20 reps, 20-30 reps, or 20-50 reps) does not burn more fat or tone (simultaneous decrease of fat and increase muscle) better than a heaver weight with moderate repetitions (8-12 reps). Weight training utilizes carbohydrates after the initial ATP and CP stores have been exhausted after the first few seconds of intense muscular contraction. Typically a set's duration is 20 to 30 seconds. For the average fit person, it requires 20 to 30 minutes of continuous aerobic activity with large muscle groups (eg. Gluteus Maximus and Quadriceps) to burn even 50% fat; fat requires oxygen to burn. Performing a few extra repetitions on a weight training exercise is not significant enough to burn extra fat and may in effect burn less fat. If intensity is compromised, less fat may be burned when light weight is used with high repetitions. The burning sensation associated with high repetition training seems to be the primary deterrent for achieving higher intensities.
For individuals attempting to achieve fat loss for aesthetics, the intensity of weight training can be a double edge sword. When beginning an exercise program, muscle mass increases may out pace fat losses, resulting in a small initial weight gain. Significant fat loss requires a certain intensity, duration, and frequency that novice exercisers may not be able to achieve until they develop greater tolerance to exercise. If an exercise and nutrition program is not adequate for significant fat loss, a lighter weight with higher repetitions may be recommended to minimize any bulking effects, although less fat may be utilized hours later. If an aerobic exercise and nutrition program is sufficient enough to lose fat, a moderate repetition range with a progressively heavier weight will accelerate fat loss with a toning effect. If a muscle group ever out paces fat loss, the bulking effect is only temporary. For a toning effect, fat can be lost later when aerobic exercise can be significantly increased or the weight training exercise(s) for that particular muscle can be ceased altogether. The muscle will atrophy to a pre-exercise girth within months. Higher repetitions training may be later implemented and assessed.
Higher volume weight training (ie 3 sets versus 1 set of each exercise) with short rest periods of approximately 1 minutes can stimulate a greater acute growth hormone realease (Kraemer 1991, 1993; Mulligan 1996). Growth hormone is lipolytic in adults. It is hypothesized that maximal effort is necessary for optimizing exercise induced secretion of growth hormone. Growth hormone release is related to the magnitude of exertion (Pyka 1992) and is attenuated with greater lactic acidosis (Gordon 1994).
Intense weight training utilizing multiple large muscles with longer rest between sets may also accentuate body lipid deficit by increasing post training epinephrine. Intramuscular triacylgycerol it thought to be an important energy substrate following repeated 30 second maximal exercise with 4 minute recovery intervals (McCartney 1996, Tremblay 1994). Rest periods lasting approximately 4 minutes between maximal exercise exercise of very short duration is required for almost complete creatine phosphate recovery required for repeated maximal bouts (McCartney 1986).
It still may be recommended to perform high repetitions (eg 20-30) for abdominal and oblique training. It has been theorized muscular endurance may be more benifitial for low back health than muscular strength. Furthermore, moderate repetitions with a greater resistance can increase muscular girth under the subcutaneous fat, particularly in men, who have greater potiential for muscular hypertrophy. Increasing the thickness around the waist with existing abdominal fat may further increase bulk, particularly in men who typically have greater intra-abdominal and subcutanuous fat in this area. The abdominal muscularture is composed of relatively small musclemass as compared to the glutes, quadriceps, hamstrings, chest, and upper back. Performing high reps with a lighter resistance should not compromise metabolism or muscle increases as would performing high reps with light resistance on larger muscle groups. See Spot Reduction Myth above.
It is plausible that the high repetition myth was originated and later propagated by bodybuilders that used calorie restrictive diets to shed fat before a contest. Because of their weakened state from dieting, they were unable to use their usual heavier weights. When inquired about their use of lighter weights, they explained they were "cutting up" for a contest. This is merely a theory, but it is easy to see how it may have been misunderstood that the lighter weight was used to reduce fat instead of actually being a result of their dietary regime.
Typically with weight training alone, fat loss is similar to muscle gain, give or take a few pounds. Certain dietary modification can have much greater impact on fat loss than weight training alone. The ideal program for fat loss would include the combination of proper diet, weight training, and cardio exercise."