In one of our most interesting research studies (7) we investigated how many repetitions people could perform with 75% of their maximum resistance (1RM weightload) in a standard chest exercise (10 degree chest machine). The majority of our 141 subjects completed between 8 and 12 repetitions with 75 percent of their 1RM weightload, the average being 10 repetitions.
This is representative of most men and women, who have a fairly even mix of fast-twitch and slow-twitch muscle fibers in their major muscle groups (e.g., quadriceps, pectoralis major, etc.). That is, most people possess moderate endurance muscles that lose about 2.5 percent of their starting strength every repetition during a challenging set of resistance exercise. In other words, when you can no longer lift 75 percent of your maximum resistance you have reduced your starting strength by 25 percent, and if that occurs in 10 repetitions your strength loss is 2.5 percent per rep (25 % divided by 10 reps = 2.5 percent/per rep).
However, you will note that a couple subjects could compete only 5 repetitions with the same relative resistance (75% of 1RM). These were power athletes (sprinter and thrower) who have inherited a relatively high percentage of fast-twitch muscle fibers and low muscle endurance. Although typically very strong they fatigue quickly, losing about 5 percent of their starting strength every repetition (25% divided by 5 reps =5 percent/rep). These individuals respond better to fewer repetitions per set to match their strength training protocol to their muscle physiology.
You will also observe that one subject performed almost 25 perfect repetitions with the same relative weightload (75% of 1RM). This young lady was an outstanding endurance athlete (winner of the Iron Man Triathlon) who inherited an extremely high percentage of slow-twitch muscle fibers and high muscle endurance. Remarkably, she reduced her starting strength by only 1 percent each repetition (25% divided by 25 reps = 1 percent/rep).
Generally speaking, the number of repetitions you can perform with 75 percent of your maximum resistance is genetically determined by your percentage of fast-twitch (low endurance) and slow-twitch (high endurance) muscle fibers. Most of us have moderate endurance muscles that respond productively to strength training protocols between 5 and 15 repetitions per set. Power athletes have low endurance muscles that respond better to strength training protocols with fewer repetitions per set (e.g., 3 to 7 reps). Conversely, endurance athletes have high endurance muscles that respond better to strength training protocols with more repetitions per set (e.g., 13 to 17 reps).
Specific Repetition Ranges
Although a very small percentage of people possess predominantly fast-twitch or slow-twitch muscle fibers, most of us have moderate endurance muscles that can be effectively trained through a range of repetitions that produce muscle fatigue within the anaerobic energy system. For practical purposes, let’s designate the anaerobic energy system as 20 to 90 seconds of high-effort muscle exercise. At controlled movement speeds of about 6 seconds per repetition (2 seconds lifting and 4 seconds lowering) 3 repetitions take about 20 seconds and 15 repetitions require about 90 seconds.
When you consider that the recommended time frame for aerobic activity is 20 to 60 minutes (1), the 70-second range for anaerobic exercise is extremely brief. Nonetheless, we frequently hear that 3 to 5 repetitions (18 to 30 seconds) are best for developing muscle power, 6 to 8 repetitions (36 to 48 seconds) are best for improving muscle strength, 9 to 11 repetitions (54 to 66 seconds) are best for increasing muscle size, and 12 to 15 repetitions (72 to 90 seconds) are preferred for enhancing muscle endurance. Although these apparently arbitrary anaerobic exercise classifications have long been considered general knowledge and standard procedures, there is little research to support their application.
On the other hand, several studies (2,3,5,6,8) have demonstrated no differences in strength development when training with low or high repetitions, provided that each exercise set fatigues the target muscles within the anaerobic energy system (approximately 20 to 90 seconds). Consider two of these studies completed in the past two years.
Chestnut and Docherty (3) divided 19 previously untrained men (mean age 24 years) into two exercise groups. One group performed 6 sets of 4 repetitions each and the other group completed 3 sets of 10 repetitions each, 3 days a week for a period of 10 weeks. The training exercises were the triceps bench press, triceps pressdown, barbell curl and dumbbell curl. All of these subjects were assessed for changes in their muscle strength (1RM) and muscle size (cross-sectional area). The researchers reported that both the 4-repetition training and the 10-repetition training elicited statistically significant and equal increases in muscle strength and muscle size. These findings suggest that shorter and longer bouts of high-effort resistance exercise produce similar results when muscle fatigue occurs within the anaerobic energy system.
The second study (8) was conducted at the South Shore YMCA, and placed 44 previously untrained men and women (mean age 53 years) into two exercise groups. One group performed each resistance exercise for 1 set of 6 to 8 repetitions and the other group performed each resistance exercise for 1 set of 13 to 15 repetitions. The training exercises were the leg extension, leg curl, chest cross, chest press, pullover, lateral raise, biceps curl, triceps extension, low back extension, abdominal curl, neck flexion and neck extension. After 10 weeks of training, the 22 subjects who did about 7 repetitions per set averaged a 14.4–pound strength gain, and the 22 subjects who did about 14 repetitions per set averaged a 15.0-pound strength gain. Both training groups experienced statistically significant and essentially equal increases in muscle strength. Like Chestnut and Docherty’s research, our study showed no difference in the physiological adaptations associated with lower and higher repetition protocols when training to momentary muscle fatigue within the anaerobic energy system parameters.
Based on the results of these (3,8) and other studies (2,5,6) it would appear that you may receive similar strength building stimulus from various resistance training protocols that produce muscle fatigue within the anaerobic energy system. While some individuals (e.g., those with mostly fast-twitch or slow-twitch muscle fibers) may respond better to lower or higher repetition training, most of us should attain about the same strength gains training with a range of repetitions (e.g., 5 to 15 reps per set).
Repetition Ranges For Children
Because children are not simply small adults, we also studied their physiological response to lower and higher repetition resistance training (4). This study involved 43 boys and girls (mean age 8 years) who trained with one set of 6 to 8 repetitions or one set of 13 to 15 repetitions, or served as controls (non-training subjects). The strength training groups performed one set of each resistance exercise, twice a week for a period of 8 weeks, using the following youth-sized weightstack machines: leg extension, leg curl, leg press, hip abduction, pullover, chest press, seated row, abdominal curl and pulldown. Unlike our adult studies, the youth who trained with moderate weightloads and more repetitions (13 to 15 reps) gained 28 percent more strength whereas those who trained with heavier weightloads and fewer repetitions (6 to 8 reps) gained 18 percent more strength.
We therefore recommend that preadolescents typically train with higher repetition ranges (e.g., 10 to 15 reps/set), as this protocol appears to be more productive for strength development and more conservative with respect to injury prevention.
Genetics generally determines whether you have low muscle endurance (more fast-twitch fibers), high muscle endurance (more slow-twitch fibers), or moderate endurance muscles (even mix of fast-twitch and slow-twitch fibers). Moderate endurance muscles respond well to a fairly wide range of repetitions (e.g., 5 to 15 reps), low endurance muscles respond better to fewer repetitions (e.g., 3 to 7 reps), and high endurance muscles respond better to higher repetitions (e.g., 13 to 17 reps).
Most adults have moderate endurance muscles that increase strength equally well from 5 to 15 repetition training, as long as they experience temporary muscle fatigue within the anaerobic energy system (approximately 20 to 90 seconds). One study found no differences in strength development from 4-repetition training and 10-repetition training, and another study showed similar strength gains from 7-repetition training and 14-repetition training.
Research indicates that children, unlike adults, respond more favorably to higher repetition training (13 to 15 reps) than to lower repetition training (6 to 8 reps).
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