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.