Home | Articles | Mission | FAQ | About Stronger Athletes | Links

Stronger Athletes

Safe, Effective Strength Training for Athletes

April 21 "The sole advantage of power is that you can do more good." –Baltasar Gracian

Muscle Fiber Recruitment-Again

First we would like to apologize to our regular readers for not having posted a new article in two weeks. Thanks for coming back.

We are very thankful for the help we received from a several of coaches across the country who helped us put this article together. Namely, Coach Tom Kelso and his staff at University of Illinois-Chicago. Some did not wish to receive credit for their help as they do not want to receive the "ranting" e-mail that come with taking a stand for safe, productive and efficient training. The topic of Muscle-Fiber Recruitment is an important element of why we train our athletes the way we do. However, this topic, like others in the strength training world can be confusing and misleading for many coaches. With the help of these coaches, who are considered experts in the field, we will make a further attempt to clear up this issue.

Coaches, please take the time to educate yourself on issues of strength development. If strength training is important enough to make your athletes do, then it is important enough for you to learn as much as you can about it. We know the time commitment coaches are under, we are coaches ourselves. This website serves to be an informal, yet informative resource for the busy strength coach. We are by no means experts! However, we rely on what we can read ourselves, and from coaches whom we have grown to respect who are on the front lines of athletic strength development.

If you are truly interested in learning more about safe, productive, and efficient strength training we recommend two books in particular: "Maximize Your Workout" which has various essays written by strength coaches from around the country who work with developing athletes in a variety of sports and "A Practical Approach to Strength Training" by Matt Brzycki. We are not trying to make a buck off of this endorsement, go check them out from the library for all we care. However, we feel that these books gave us a greater understanding of the topics discussed on this website and can help you as well.

Listed below are several responses to our question about muscle-fiber recruitment. They are listed in no particular order but are meant to clarify various issues concerning this topic. We begin with opening remarks from two collegiate strength coaches.

Many high school programs (and some smaller college programs) are sub-par (poor supervision, lack of qualified strength coaches, poor facilities, etc.) The information on StrongerAthletes.com is to provide practical suggestions to coaches who may have substandard "resources" to work with. Aside from this point, its difficult to argue against safe and time-efficient programs as they are proven to work at both the collegiate and professional levels, especially due to 1) limited time we all face and 2) the potential of legal issues in our increasingly lawsuit-happy society.

From the outset, please understand this fact: one does NOT have to do Olympic lifts and/or variations of them in order to win championships or improve the physical qualities of athletes that will help them achieve in sport. Numerous teams/individuals have proven this – especially on the professional and collegiate levels. You are NOT at a disadvantage if you do not do them provided you are doing a progressive, total body-emphasized strength program, coupled with out-of-the-weight-room sport-related speed and skill training. End of that issue. -Tom Kelso

In my opinion, you must ask yourself three questions before you engage in a philosophical conversation regarding strength training.

  • 1. What is strength?
  • 2. What is strength training?
  • 3. Why strength train?

In my opinion, a major reason for the dissension within the profession is nebulous terminology. Forget HIT vs. periodization or one set vs. multiple sets, ask ten individuals their definition of strength and get ten different opinions. If you are going to develop something, it should be understood what it is that you are attempting to develop, in our profession, strength. -Coach B [This coach who, with the help of one of his university's faculty, were happy to help contribute to this article did not want us to use their name or school. They understand the problem associated with taking a stand for safe, productive and efficient training: angry e-mail's! While they support these issues they do not wish to spend their time receiving any e-mail from every power cleaning coach in the country.]



In regards to muscle fibers being recruited in an orderly fashion, some believe that by training fast in the weight room one can develop fast twitch fibers. If this were the case one would skip over the small motor units, A.K.A. slow twitch fibers, and begin to work the larger units or fast twitch fibers. Some coaches maintain that the muscle recruitment pattern is not the same from set to set.

Yes, each set would be "different" if 1) different loads were used, i.e. 70% would initially recruit "x" number of fibers and 85% would recruit "x" + "x" more, and/or 2) in consideration of fatigue, the fatigued fibers in set one would create a slightly different situation in set two due to some fibers being less than 100% fresh. However, a similar demand pattern is naturally elicited for each situation: clearly, the greater the demand, the greater number of fibers twitching and/or being recruited as opposed to a lesser demand where a lesser number of fibers are twitching and/or being recruited, independent of a person's genetic make up (i.e., fiber type, number and distribution through the body). That is why if you had ten people lift a 100 lb. barbell as many times possible you'd get varied results, but each would be governed by the aforementioned force demand/recruitment phenomenon. This is the essence of the Henneman's Principle of motor unit/muscle fiber recruitment.

Henneman's Principle is the generally accepted recruitment process within the neuromuscular system. It is based on both science and common sense. Lower demand activities don't require a lot of force (relatively speaking), so the lower threshold (type I) fibers are called upon first. If more force is needed, 1) the working fibers (type I or even the higher threshold type II fibers, depending on how low the demand is) are stimulated more frequently then 2) more fibers are recruited to assist. Newly recruited fibers are then stimulated more frequently if more force is needed, then further recruitment of higher threshold fibers occurs to keep the activity going. This is the basis of the aforementioned scenario regardless of one's genetic make up (i.e., again, fiber type, number and distribution throughout the body).

Apply this to any situation you've encountered, athletic or not. For example, if I squat down to lift a two-pound rock, it requires very little fiber involvement. If I squat down to pick up a two hundred pound rock, I have to recruit a heck of a lot more fibers. I could rise and descend many times when repetitively lifting and lowering a two-pound rock before becoming fatigued, but the greater initial effort required to lift the two hundred pound rock would be more taxing when it is lifted repetitively. Consequently, I would have to halt the endeavor or take longer rest intervals between a series of lifts, long enough to replenish ATP-PC stores and remove accumulated lactic acid in order to continue. -Tom Kelso

Through personal research and in conversation with colleagues, the general principle of muscle fiber recruitment doesn't change per exercise. Those fibers that attach to lower electrical threshold motor units are activated first regardless of the activity. Granted, a second set of the same exercise may involve different fibers due to a change in repetition speed, angle of movement, etc. but that doesn't alter the general principle of muscle fiber recruitment. -Coach B



Some coaches believe that it is not possible to determine how muscle fibers are recruited during training.

It is true that increasing frequency of activation (twitching) and increasing the number of fibers being recruited lead to increased force production. However, the point that ".. it is not possible to determine how muscle fibers are recruited during training" contradicts the previous statement referring to increased force production by: 1) increasing frequency of activation (twitching) and 2) increasing the number of fibers being recruited. That IS how it is done: some combination of increasing firing rate and further recruitment of fibers. Specifically how it happens is not important, but what is important is we know it happens as the demand for force increases. This is simple common sense. -Tom Kelso



In an earlier article we wrote about muscle fiber recruitment we used an example of how one might go about tapping into their fast twitch muscle fibers. We labeled the various levels of motor unit groups into Type I (slow twitch), Type IIa (intermediate slow), Type IIb (intermediate fast), and Type II (fast twitch fibers). This example was a simplified breakdown of the various levels of motor units used. We have been criticized for limiting the body to these 4-types while there is a whole range of muscle types ranging from slow to fast twitch.

There are a number of classification systems, but for sake of simplicity, it is pragmatic to use the 4-class system as it represents a reasonable consensus on motor-unit (fiber) classification. The bottom line is that there are obviously different fiber types for different situations. How else could you explain the short-nature of lifting a heavy resistance for only a few number of repetitions, or the longer duration potential when lifting a lighter resistance? Likewise, muscle fiber characteristics do play a key role in running a marathon as opposed to sprinting 100 meters. I think we'd all agree to that. -Tom Kelso

I would agree that there is a continuum of muscle fiber types with Type 1 at one end and Type 2B at the opposite end. Along this continuum, there are intermediate fibers displaying characteristics that "bridge the gap" as the essay states.

To simplify the obvious, approximately 10-180 slow twitch fibers attach per motor neuron. Approximately 300-800 fast twitch fibers attach per motor neuron. This explains why fast twitch can be defined as "strong" (capable of producing a large amount of force) and slow twitch are considered "weak". In regards to contraction speed, fast twitch fibers reach tetanus in approximately 10-50 milliseconds and slow twitch fibers can contract within 100-110 milliseconds. Therefore, the contractile difference is only 60-90 milliseconds. This fact becomes lost because of the term "slow" twitch fiber. "Slow" carries a negative connotation within the strength training field. -Coach B



Critiquing the same muscle fiber recruitment article some coaches argue that our example is misleading. We state that during the first 2 reps one trains the Type I fibers, the 3-4 reps train the Type IIa fibers, 5-6 train the Type IIab fibers and the 7-8 reps train the Type IIb fibers. That example can be misleading but the point we were making is that if a person were to reach the point on momentary muscular fatigue at the 8th rep they would be recruiting all of their muscle fibers, including the Type IIb. As the athlete was getting to rep 8 he would be depleting the slower twitch fibers and gradually acquiring the need for more motor units.

Tom Kelso gives another explanation: Maybe a better way to explain the schematic would be to say that on repetition number 1 "x' number of fibers are working. For sake of example, this could be 500 of one type, 200 of another and 125 of another. As each repetition is performed and fatigue begins to set in, the "x' fibers are stimulated more frequently (some of them being rendered useless due to fatigue) and more fibers ("y") are then recruited to assist. For example, one could then progressively recruit 150, 75 and 100 additional fibers of each of the previous types. As further repetitions are performed, the newly recruited "y" fibers are stimulated more frequently and the process continues (some combination of increasing frequency of stimulation and recruitment of more fibers) until it is impossible to due to fatigue.

This is a hypothetical and simplistic overview of what happens, but a very good description of what is actually "going on" in muscle tissue/the neuromuscular system. Understand that there are numerous scenarios based on the force demand and time components of the situation. For example, a 2-RM resistance exercise would recruit a large number of fibers due to its nature, but would be a very short-duration endeavor due to the recruitment of the faster-to-fatigue, higher threshold type II fibers (B and C, for sake of example). When they are fatigued, the event is done as it is then impossible to continue on with the other "weaker" but un-fatigued fibers. A 20-RM resistance would actually work a greater "pool" of fibers due 1) a more prolonged frequency of stimulation and 2) the progressive recruitment of more and more fibers over a longer time period – that is, a greater percentage of overall fibers are recruited and a longer time they are under tension). This is why one feels more fatigued (lactic acid accumulation) following a higher repetition set as oppose to a lower repetition set (and one reason you can get "more bang for the buck" with sets of this nature). -Tom Kelso



Some coaches feel that Henneman's Size Principle does not apply to all situations, that there is a way to go around the slow twitch muscle fibers. They maintain that this can be done by using high speed movements that do not allow the slower type units to make their connections, or create tension.

Regarding "selective recruitment" (disregarding Henneman's Principle by suggesting lower threshold fibers are bypassed to go directly to higher threshold fibers), it depends on how you view this. Simply stated, if a huge demand is required (i.e., heavy resistance), then the dependence shifts to the higher threshold type II fibers (B and C, for sake of example). No question about this. However, it does not mean the lower threshold fibers are not recruited. They are. They would have to be, but their contribution is overshadowed by the higher threshold fibers. I state they "would have to be" because if the generally accepted Henneman's Principle is relevant, it would have to apply to ALL situations as it can't indiscriminately be applied. This would defy scientific law, similar to the law of gravity that applies everywhere and every time (on Earth). So, a high-demand activity does involve type I fibers, but the critical ones are the higher threshold type II fibers. This is analogous to going to war where a large-scale battle requires a large number of troops and progressively increased firepower. The first line of troops may be equipped with rifles, which gets part of the job done. As more power is needed, here come the tanks and Bradley Fighting Vehicles. To totally fortify the attack – building upon the continued effort and needed contribution of the rifles, tanks and BFV's – F-18 bombers are brought in to significantly impact the effort, making the greatest contribution of the four. -Tom Kelso

Is the fiber recruitment pattern set in stone? I would agree that it isn't. However, is it orderly? Yes. Is it possible for a movement to bypass slow twitch fibers. Yes. It is possible for a movement to bypass all fibers. Most biomechanics textbooks that I have read suggests that muscles respond to tension. Therefore, if no tension is placed upon a muscle there is no need for the cross bridges to attach because the muscle isn't performing mechanical work.

Place a subject on a force plate connected to an oscilloscope and have the subject perform the jerk or push press. A sixty-pound barbell would exert as much as a few hundred pounds of force and as little as zero. With zero force, no mechanical interaction is present, because the muscles are not under tension (i.e. no need to perform work).

In regards to skipping over the slow twitch and developing the fast twitch fibers, one potential problem is how fast is fast enough to bypass the slow twitch fibers? At the speed needed to bypass the slow twitch fibers, do the forces exceed the structural integrity of the joints and connective tissue. If so, then doesn't an injury occur? As a health care professional, I'm under the Hippocratic oath, which states first, do no harm. -Coach B



Some coaches overly credit the use of quick lifts as the reason for their athletes' success on the field. These coaches down play the role genetics plays in determining fiber type make-ups. A reader recently wrote to us comparing weight lifters (athletes training for traditional sports) with bodybuilders noting the higher level of fast twitch to slow twitch fibers present in the weight lifter as compared with the body builder.

World-class weightlifters are undoubtedly genetically gifted. The amount of resistance they are lifting cannot solely be attributed to their work ethic and training program. They are blessed with an abundance of Type II fibers, good nervous systems and advantageous body leverages. Otherwise, you could take anyone (read: a "slow-twitcher" with lousy leverages) and put him/her on the same program as a world-class lifter and they'd lift similar poundage. It won't happen as the "raw material" (the aforementioned genetic advantages) must first be present.

It has been proven that intermediate fibers (for example type IIA) can be influenced to "take on the look" of either type IIB or type I fibers if exposed to training that stresses them. In other words, if one does a lot of aerobic/low-intensity work, the intermediate fibers will adapt to that form of stress. Likewise, if high intensity exercise is undertaken on a regular basis, they will adapt to that. Therefore, if lifting very heavy resistance on a regular basis (i.e., weightli