How to Determine Muscle Fatigue Characteristics

How many repetitions in a set of exercise for best results? The long-time wisdom of 10 was standard for years and worked for most; but has recently been challenged by speed of movement advocates. A fast set of 10 repetitions might last 10 seconds, where a slow set, properly performed, might last several minutes. The results from such "different" sets of 10 might be less than ideal.

MedX inventor Arthur Jones developed tools in the mid-1980's that measured muscle strength, and eventually, fatigue rates - using the terms "time under load" and "inroad" in his writings. "Time under load" meant the amount of time a muscle remained exposed to a resistance during exercise - the total time to perform an exercise. "Inroad" referred to the percentage of strength lost from the time an exercise started (fresh muscle strength) to the time it ended (strength in a state of fatigue). He believed muscles were best stimulated (for strength) when the "inroad" fell between 15-20% - that is, when a muscle lost 15-20% of its strength as a result of an exercise. He also found that the average muscle lost approximately 2% strength per repetition (10 reps led to a loss of 20%). He defined "average" as a muscle with a relatively equal distribution of slow-twitch (slow-fatigue, endurance) fibers and fast-twitch (fast-fatigue, powerful) fibers.

Some muscles he tested, however, were not average: They fatigued more quickly and responded better to fewer repetitions, less time under load. Others were laden with endurance fibers, fatigued more slowly (if at all) and responded best to more repetitions, greater time under load. The different repetition schemes required of such unique muscle fiber-types would more likely leave a fatigued muscle in the 'ideal' inroad range.

How can we use this information?

The results of Jones' experiments and a method to determine the ideal number of repetitions for single-joint muscle systems were reported by Ellington Darden, PhD in his recent book, "The New Bodybuilding for Old-School Results." The method:

A. Find out how much weight you can lift for one repetition.

B. Rest at least 5 minutes.

C. Using 80% of that weight (A), perform as many repetitions as possible (to fatigue).

D. Multiply that number by .15. Add the result (rounded to the nearest number) to the number of repetitions performed in C to determine the upper limit of 'ideal' repetitions. Subtract the calculated result (rounded to the nearest number) from the number of repetitions performed in C to determine the lower limit of 'ideal' repetitions.

Example: A one-repetition lift of 140 pounds would result in an attempt to lift 112 pounds for repetitions. A performance of 7 repetitions would result in an ideal repetition scheme of (7 x .15 = 1.05 plus 7 = 8.05 or 8 as the upper limit; and (7 x .15 = 1.05 subtracted from 7 = 5.95 or 6 as the ideal lower limit. Six to eight repetitions (6-8), ideal for that muscle group.

If the repetition scheme in the above calculation is less than 8-12, the muscle has a tendency to fatigue more quickly than normal (fast-twitch). If the scheme is greater than 8-12, the muscle is more endurance oriented (slow-twitch).

From a time perspective, fast-fatigue muscles respond best to exercise that lasts between 30-40 seconds. Slow-fatigue muscles respond better to an exercise duration of 80-90 seconds. Muscles with an average mix of fibers respond best if exercise lasts approximately 60 seconds. According to Jones, 70-80% of muscles fall in the "average" category, responding best to 8-12 repetitions.

Time it, rep it or both - now you can train for best results.