Muscle Fiber Recruitment-Again

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.

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.

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.

The point here is simply as long as one is trying to be explosive against a resistance when strength training (significant tension-producing resistance), it does not have to move at the speed exhibited during an actual sport skill. In fact, it is impossible to exactly replicate the exact speed of ANY skill, even when only a few pounds of resistance are used. So, lifting meaningful resistance (i.e., 60%+ of a 1-RM) will naturally move "slow" but involve a greater amount of muscle fibers (force/demand relationship). The heavier it is the slower it becomes but with greater fiber involvement, all other factors equal. So, improving muscle force potential in the weight room (read: getting stronger!) will then better one's ability to exert force in an athletic situation, all other factors equal. This is really simple to understand, but many have hard time grasping it for some reason (ego? financial gain? Ignorance?). -Tom Kelso

Ultamitly, the disagreement over this issue comes down to this. Some coaches will maintain that by moving a lighter load quickly an athlete can develop more power that moving a heavy load slowly.

This is a great point, and is essentially the crux of the matter: Part I

Remember, depending on our individual genetics, we can express force in three primary ways as in the previous examples: 1) the quick burst (power) force application, 2) the maximal bearing-down (strength) application and 3) the sub-maximal extended (muscular endurance) force application. ALL OF THESE CONSCIOUS AND SELF-REGULATED EXPRESSIONS OF FORCE ARE GOVERNED BY ONE'S STRENGTH LEVEL AND INHERENT NERVOUS SYSTEM THEY ARE BORN WITH (AGAIN, FIBER TYPES, NUMBER AND DISTRIBUTION THROUGHOUT THE BODY). -Tom Kelso

Some coaches maintain that both moving a lighter load quickly and moving a heavy load slowly will both develop fast twitch muscle fibers.

The crux of the matter: PART 2.

But a more significant point is this: if moving both slow and fast work fast twitch (type II) fibers, THERE OBVIOUSLY IS NO SUPERIOR ADVANTAGE IN MOVING FASTER (INJURY) AS COMPARED TO MOVING SLOWER. Likewise, there is no superior advantage in moving slower as compared to faster, other than less chance of injury (a no-brainer!) Rational conclusion: make it safer by slowing it down. You CANNOT argue that point when you are dealing with the health and well being of athletes. It would be quite awkward explaining to Mr. and Mrs. Smith that their son, athlete Johnny, fractured his wrist while performing an exercise and/or using a speed of movement that could have been avoided. -Tom Kelso

The term explosive is deceiving as you know. You can train explosively without a great deal of speed being produced on the bar.

We have a student-athlete that participates in the sport of football. This student-athlete is non-traditional and was an alternate on the US Olympic weightlifting team for a few years. According to the preceding paragraph, this young man's percentage of fast twitch fiber to slow twitch fiber should be off the chart. It probably is. The bottom line is this athlete doesn't play in the fall due to limited football skills. All the fast twitch fiber in the world doesn't ensure success in specific sport skills. -Coach B

Many coaches are frustrated with our down play of using movements such as the power clean.

Regarding the power clean, I've heard many reasons why to do it:

• 1) "It's a football lift" (so I assume basketball players have a "basketball lift"?).
• 2) It develops power.
• 3) It develops "explosion."
• 4) It replicates jumping, which then would purportedly augment rebounding in basketball, spiking a volleyball, high jumping or blah, blah, blah).
• 5) It develops overall coordination.
• 6) It enhances "Hip Roll" (to purportedly augment football tackling).
• 7) "It's a total body lift."
• 8) It makes one mentally tough.

Heavier resistance do not and cannot move fast, but the conscious effort to try to move them fast is the key. If moving fast in the weight room was important for improving RFD, then the lightest possible resistance would be used as it would offer the fastest speed. However, a light resistance requires less overall muscle fiber involvement, rendering them ineffective. Outside the weight room in practice is where one can augment RFD by performing sport skills as fast as will be needed in competition. They can train the nervous system to activate specific muscles, do so in the proper sequence(s)/joint angle(s) and recruit individual motor units/muscle fibers to express the force requirements needed to complete the task(s) (i.e., maximal, sub-maximal or quick burst actions). -Tom Kelso

Some coaches disagree with our use of the term "Intensity". We use the term intensity to mean the increasing difficulty to move a resistance through a working set. For example while the first few reps of a set are performed with somewhat ease the intensity is low. However, as the set continues and the resistance becomes more difficult to move the intensity increases. Finally when the set reaches the point just before momentary muscular fatigue, the intensity it at its highest. Some quick lifting coaches follow the Russian definition of intensity which is measured by a percentage of an athlete's 1-RM.

Strength training properly done takes great effort. It would be very difficult to find anyone who became larger and stronger without working hard at some point along the way (unless they were genetically gifted and/or utilized an effective drug program, yet even then they would have to exude some level of "above average" effort to create an overload on their muscles). It makes perfect sense then to define intensity as it relates to strength training in terms of level of effort/exertion (as intensity is defined in any dictionary).

The Russian definition if intensity is applicable to a point, but doesn't take into account the trainees' level of effort exerted, which makes it somewhat of a misnomer and thus confusing (also, any mention of the former Soviet methods should include their performance enhancing drug program that undoubtedly was a factor in their overall success in international competition). They suggest that the heavier the resistance, the more intense it is. Therefore, lifting 90% of a 1-RM is more intense than lifting 70%. This is true if only a few repetitions were performed. Three to four repetitions with 90% would result in, or come very close to, muscular fatigue. The same number of repetitions with 70% of 1-RM would not, thus it would not be intense relative to those who ascribe to intensity being related to level of effort/exertion exuded. On the other hand, it's ridiculous to label 70% of a 1-RM as "low intensity" (by Russian standards) if it is lifted to the point of muscular fatigue. Because muscular fatigue is an objective of most safe and efficient programs, the level of effort required to take 70% of a 1-RM to the point of fatigue would be very high, thus making it intense. In fact, 70% worked to muscular fatigue involves a greater "corridor" (as Zatsiorsky calls it) of motor units/muscle fibers as compared to 90% worked to muscular fatigue. -Tom Kelso

Some coaches feel that by not preparing an athlete for the demands of sports, such as using momentum to ones advantage and absorbing momentum we are doing our athletes a disservice.

Now we are getting into a totally different topic: sport-skill training. To better prepare athletes for the demands of competition the practice of competition-specific skills/situations must occur. Through repetitive practice of sports/sport skills, it does prepare athletes to play with the forces of momentum. If I am an offensive tackle, to better myself I'm surely going to practice coming out of my stance, foot work and pass blocking against a live opponent. Same for a wrestler or tennis player: practice wrestling maneuvers and hit/return tennis balls against a live opponent, replicating competition-specific situations.

In the above examples, they will use and absorb momentum through the nature of the sport, which could lead to an injury even when practicing. As a result, having stronger muscles throughout the body will minimize this risk in both practice and competition as stronger muscles can contract to better support/stabilize joints and absorb forces during skill performance and when the body is placed in compromised positions (i.e., landing or falling awkwardly). Lifting weights fast to develop supposed similar momentum encountered in competition is borderline ludicrous as it 1) does not replicate exact sport skills in the first place, 2) could lead to an immediate injury due to increased strain/force placed on muscles and joints and 3) increases the number of "exposures" to high-momentum situations which over time could lead to an over-use, wear-and-tear injury. -Tom Kelso

Some readers we hear from down play the role of training to fatigue use the example of distance runners, who train for long periods of time but obviously do not make use of their fast twitch fibers.

This is a questionable and somewhat irrelevant point. Distance runners do fatigue fibers, but primarily the Type I fibers as the intensity of distance running is far from the intensity of work that recruits the higher threshold fibers (i.e., sprinting, weightlifting). And fatigue DOES play a role in muscle fiber recruitment (Henneman's Principle, fatigue/continued force output options = firing rate, then recruitment of new fibers as FATIGUE sets in). Low intensity training (marathoners) = call upon predominately type I fibers; High intensity training (sprinters) = call upon predominately type II fibers. -Tom Kelso

First we must define intensity and duration of exercise. You may work out at a high intensity or a long duration but not both. For example, is a 100 meter dash an all out activity or do you run a 100 meter dash at 90% capacity? If the former, how long could you maintain the pace? 300 meters? 400 meters? I don't know that distance running is a good example of what I would consider a high intensity activity. -Coach B

Some coaches question our use of training to failure as athletes, specifically, football players use short bursts of energy. Train in the weight room in the manner in which you play is their argument.

How much is enough to prepare an athlete for competition? Ken Mannie has a great quotation regarding this line of reasoning. "Using potentially dangerous movements in the weight room to prepare for potentially dangerous activities is like banging your head against the wall to prepare for a concussion." -Coach B

We are talking about two different things here: 1) strength training and 2) sport-related conditioning. In the weight room (strength training), "…stopping an exercise short of fatigue…" will undoubtedly mean some fibers were not recruited and overloaded. The closer to muscular fatigue one gets, the more productive the exercise is. If one stops an exercise at 10 repetitions when 15 could have been performed, those fibers that would allow for the performance of the additional 5 repetitions were not worked as hard as they could have been. If 14 repetitions were performed, it would be a better situation as those extra 4 repetitions worked more fibers, thus more were overloaded. (duh!)

Bottom line: the weight room is for getting stronger to 1) improve the force needed for conscious expressions of strength, power and muscular endurance and 2) minimize the risk of injury. Running/conditioning workouts predominantly prepare the athlete for the energy demands of their sport to prolong their ability to execute skills properly (i.e., delay the onset of fatigue that diminishes functional ability). -Tom Kelso

Do you need to train to failure for a training adaptation to occur? Research has not been able to prove the minimum level of intensity to stimulate muscular growth. Is it 85%? If so, how do you determine 85% intensity? Do not confuse intensity with a percentage of a 1 RM. On the intensity continuum, only two levels of effort can be accurately identified. Zero percent or total inactivity and 100%, characterized by the inability to perform another repetition, an all out effort. -Coach B

Some coaches refuse to see the logic in finding the most efficient manner in which to train athletes. By efficient we mean best return in terms of the time spent in the weight room.

Here is just one example of why we believe in efficiency: Here is the reality of strength training. We currently have 13 men's basketball players training in off-season workouts. Twice a week for one hour. Four of the 13 are from Puerto Rico and lack basic English speaking skills. We have enough problems communicating coaching points of basic compound and isolation movements without over complicating the issue. In fact, our staff would be better served to enroll in some Espanol classes as opposed to attending a strength seminar. These realities are not understood or swept under the rug during academic discussions regarding strength training. Another reality, you set up a time to train a student-athlete (i.e. 2:00). The student-athlete shows 20 minutes late because of a meeting with an advisor. You have 40 minutes to train this athlete because a team lift is schedule at 3:00. Do you spend time teaching the skill of certain exercises or do you prescribe a protocol that you know will provide musculature overload with minimal coaching points? For me, a no brainer, option two. -Coach B