May 8, 2002

An Experiment in Muscle Fiber Recruitment

"The one constant through all the years, Ray, has been baseball. America has rolled by like an army of steamrollers. Its been erased like a blackboard, rebuilt, and erased again. But, baseball has marked the time. This field, this game, is a part of our past, Ray. It reminds us of all that once was good, and could be again." -James Earl Jones, "Field of Dreams"

Recently, Tom Kelso, University of Illinois-Chicago, conducted an experiment with his athletes, to help determine the relationship between muscle fiber recruitment and lifting speed. This was in response to the overwhelming myth that lifting quickly is superior to slow controlled lifting.

Coach Kelso's findings follow below in RED.

We actually conducted an informal experiment with some of our athletes to determine the effects on motor unit recruitment and speed of exercise movement.

We obtained a 1-rm on the bench press, then used 60% the 1-rm and performed a "fast" test and "slow" test on separate days to determine the extent of motor unit recruitment during each.

The first day the athletes were instructed to move the 60% resistance as fast as possible -- both concentrically and eccentrically-- and for as many reps possible. Naturally, we do not recommend this speed of lifting, however for the sake of this test we did deviate from our norm. We did make sure the athletes minimized 1) any bouncing of the weight off the chest, and 2) any resting in the locked-out position.

The number of reps obtained and the total time it took to reach muscular fatigue were recorded.

Following a 3:00 rest, 90% of their 1-rm was then used for another maximum rep test. In most cases, a person can perform anywhere from 3 to 5 reps with 90% of a 1-rm -- depending on the exercise -- when "fresh." The point of the 90% test was to determine the extent of fatigue from the 60%/fast test to muscular fatigue. We hypothesized that using 60% -- still a significant amount of resistance -- and working to muscular fatigue even though moving relatively "fast" would still a lot of muscle -- even type ii motor units -- and then would have an adverse effect on the number of reps possible with 90% of the 1-rm.

Results of the 60% "fast" day with the 90% test:

  • Average number of reps performed with 60% of the 1-rm = 23.25.

  • Average total time of the 60% test to muscular fatigue = 33.24 Seconds.

  • Average rep time = 1.43 Seconds.

  • Average number of reps performed with 90% of the 1-rm following the 3:00 recovery = 1.86.

On the second test day the athletes were instructed to lower the 60% resistance slowly (on a 4-count, counted aloud by a staff member), then raise it forcefully, but under control, without resting in the locked-out position --again, for as many reps as possible. The hypothesis here was the 60% resistance would again recruit and overload a lot of muscle -- eventually "getting to" the type ii motor units, thus adversely effecting the results of the forthcoming 90% test-- even though it was consciously moved slower and under control.

The number of reps obtained and the total time it took to reach muscular fatigue were again recorded.

Following a 3:00 rest, the 90% of the 1-rm was again used for another maximum rep test.

Results of the 60% "slow" day with the 90% test:

  • Average number of reps performed with 60% of the 1-rm = 11.6.

  • Average total time of the 60% test to muscular fatigue = 52.5 Seconds.

  • Average rep time = 4.66 Seconds.

  • Average number of reps performed with 90% of the 1-rm following the 3:00 recovery = 1.66.

An interesting comparison of the results in both test days. In all tests, motor units were activated in accordance to the initial tension/work demand sensed by the brain, henneman's principle governed further recruitment of motor units, fatigue took its toll, and overload eventually occurred as a result of working to muscular fatigue. The athletes could gain from these bouts of exercise provided recovery factors were addressed prior to the next workout. However, the fast speed of exercise movement was unnecessary for two reasons: 1) it created excess stress on the muscles and joints and 2) it did not create overload any better than the safer, slower movement speed as evident by the number of reps performed in the 90% type ii motor unit-dependent test (i.E. "Fast" effect test = 1.86 Reps w/90% and "slow" effect test = 1.66 Reps w/90%).


1. Momentum (or lack of it) played a large role in both the fast and slow tests. Increased momentum made more reps possible in the fast test due to less tension throughout both the concentric and eccentric ranges of motion. Decreased momentum in the slow test lowered the number of potential reps but muscular tension was heightened improving the quality of the exercise.

2. The number of reps is really not the issue here...It is the time of exercise, or time under load/tension, that is significant: 33.24 Seconds for the fast (more joint/muscle stress) test and 52.5 Seconds for the slow (safer) test.

3. The rationale for the fast test reaching fatigue sooner is simply because of their conscious effort to push really hard, they recruited the type ii, faster-to-fatigue motor units sooner in the set, thus they "ran out of gas" earlier (but at a greater risk of joint/muscle trauma).

4. The slow test did not rely on a high percentage of the higher threshold type ii motor units initially, however, they were recruited later in the set as fatigue took its toll and the demand for continued force occurred (henneman's principle).

5. Obviously, the higher threshold type ii motor units were recruited and overloaded in the slow test as evident by the results of the 90% test (only 1.66 Reps obtained following the slow test, and 1.86 Reps obtained following the fast test...Not statistically significant, but nothing gained from moving fast).
Bottom line: moving the resistance faster did not recruit and overload the muscles any better than the controlled, safer lifting cadence.

It is hoped that coaches can understand that "quick lifts" are not the only way to go.
Coach kelso reminds us, "understand that many do not want to give up what they have been taught/have been doing their entire careers. No one wants to admit they may be wrong or there might be a better way. Much of it is ego and the inability to humble themselves. They'll hang on to even the smallest shred of evidence and milk it to the end, even though it can be disputed with legitimate, scientific proof. Pure conjecture, blind-faith, and/or anecdotal evidence alone is a weak leg to stand on."

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