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.

The Fast Twitch Boys of Summer

In my 2005 book, In Arthur's Shadow, I predicted that once the fitness craze took over, professional golf would fall into the same traps that bodybuilding has over the years - more is better, two-a-day workouts, split training systems and special routines for golfers, etc. Add this to the list.

A low-handicap golfer approached me the other day reporting that a mutual friend (a physical therapist who works with PGA Tour players) mentioned that many pro golfers were working on their "twitch" muscles. Once I explained the concept to him, I thought, "Sounds like a knee-jerk response to the latest trend in sports-performance training - training muscle fibers to respond to specific needs." All I can say is "Good luck!"

Muscles have a mix of fiber types. That is, some fibers are strong and powerful when activated; others are endurance oriented. The strong ones are called "fast twitch" muscle fibers and, like sprinters, they don't last long. The weaker ones are called "slow twitch" fibers and, like marathon runners, they can last a long time. The buzz among athletes and trainers is that different training methods can DEVELOP certain fibers. For example, that moving quickly or explosively with weights, that using heavy resistance and few repetitions can PROMOTE the use of fast-twitch fibers. And the contrary, that moving slowly against resistance while performing higher repetition schemes promotes or develops slow-twitch fibers. Not so.

Muscle fibers are activated by intensity of effort during exercise. When intensity is low (during the first few repetitions), the brain perceives little need to call upon the big boys, so it sends in the lower order of fibers (at least lower on the strength/power scale), the "slow-twitch" fibers. Toward the end of a difficult set, the brain senses "Help" and recruits higher order "fast-twitch" fibers. If the intensity falls much short of a full effort, the fast-twitch fibers will stay home - no need to make an appearance. As far as converting muscle fiber-type to the needs of the athlete, high repetitions for endurance does not work; and low reps, heavy weights for strength does not work. And varying movement speed during exercise has nothing to do with fiber-type recruitment. If you think otherwise, show me the research . . . and good luck.

Professional golf's attempt at recruiting or developing fast-twitch muscle fibers to produce a more powerful golf swing is doomed. The quality and quantity of muscle fiber is dictated by genetics - you have it or you don't - and what you get is NOT SUBJECT TO CHANGE.

Golfers don't know much about exercise, but trainers are to blame - dumb, follow-the-crowd trainers. Let's hope they don't send too many athletes to the hospital in the process.