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Each year, I deliver approximately 25, 3-hour workshops on exercise for busy professionals; generally executives in medium size corporations. To ascertain the audience’s present exercise knowledge, I begin each workshop by having everyone write down their answers to 6 fundamental questions about exercise. Essentially, I’m trying to assess what myths the audience is continually exposed to. 
Unequivocally, the one myth that prevails is:
Heavy weights with fewer reps build strength.
Lighter weights and more reps build endurance.
To be clear, this message (and its kin, heavy weights build large muscles and lighter weights tone muscles) is likely the single most pervasive myth in the field of exercise science. It is so frequently repeated that it is almost woven into the fabric of fitness folklore. But as the adage goes, no matter how often a myth is repeated, and no matter how many people believe it, it is still a myth. And unfortunately, this particular mythical axiom has sent well-intended exercisers in pursuit of a strength training dichotomy that does not exist.  A close examination of the scientific research reveals that muscle strength and muscle endurance are not separate trainable entities, but instead, are inextricably linked. When you improve your muscle strength (your ability to lift a heavier weight for fewer repetitions) your muscle endurance (your ability to lift a lighter weight for more repetitions) improves in almost a linear manner, and vice versa.
In a 2004 review published in the Journal of Exercise Physiology, researchers Smith and Bruce-Low concluded that, “The weight of scientific evidence, therefore, does not support the idea that different numbers of repetitions have differential effects on muscular strength and endurance.” In a 2004 review published in the Journal of Exercise Physiology, researchers Carpinelli, Otto, and Winett concluded that “All these studies strongly suggest that within a reasonable range of repetitions, approximately 3 to 20, there does not appear to be a specific number of repetitions (e.g., 4-6, 7-10, 12-15, etc.) that will elicit more favorable gains in muscular strength, power, or hypertrophy.” In a 2008 review published in the Journal of Exercise Science and Fitness, Carpinelli corroborated this finding stating, “The preponderance of studies strongly suggest that effective resistance training simply requires the selection of a desired range of repetitions, which is based on a personal preference rather than a specific goal, and a progression of the resistance to stay within the desired range of repetitions.”  Stated otherwise, it does not matter how many reps you perform, as long as you continue to increase the weight (for the next workout) when the target rep is achieved.  Thus, it matters not if your target rep goal is 6 reps or 15 reps. The focus should be on performing a set of exercise to the point of momentary muscle failure. It does not matter if “failure” is reached on your 5th rep or your 19th rep. As long as you get to the point where you cannot possibly perform another rep, you have recruited all of your available muscle fibers and stimulated a cascade of positive physiological changes.
A few practical applications and corollaries of this principle:
  • A marathon runner need not focus on muscle endurance by exclusively performing a higher number of reps. She may perform 6 reps or 15 reps and her results will be the same.
  • A football player need not focus on muscle strength by performing 3-5 repetitions with an extremely heavy weight. He may perform 3-5 repetitions but he will produce the same results with 12-15 repetitions.
  • Strength training as a whole becomes much less complicated because the number of reps and the amount of weight used is of much less importance than previously assumed.
  • Strength training becomes, perhaps, more enjoyable because we can introduce more variety by changing the repetition ranges of our sets.
  • The focus of each exercise should be on reaching the point of momentary muscle fatigue, not a predetermined number of repetitions.
Take home message:
Lifting a heavier weight for fewer reps does not preferentially improve muscle strength. 
Lifting a lighter weight for more reps does not selectively improve muscle endurance. 
Muscle strength and muscle endurance will both improve regardless of the number of reps performed and these two entities do not need to and should not be trained separately.

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