Muscles for Athletes - Part 1

The Science of High-Threshold Motor Recruitment for Hypertrophy

This article series is written first and foremost for athletes who want to add muscle mass to their frame while improving their performance on the field of play. Too many people believe that any time an athlete adds muscle mass, he'll become a better athlete. After all, isn't muscle what produces strength and power?

Well, this isn't necessarily so. While in theory a bigger muscle is a stronger and more powerful muscle, in reality it doesn't always work that way. Muscle mass can in fact impair athletic performance in some regards.

For example, added muscle bulk isn't always associated with a proportional increase in strength. When higher rep, lower weight methods are used, it's not out of the ordinary to see mass gains far surpass strength gains. In that case, you're basically adding a lot of weight to the body of your car without actually making the engine powerful enough to compensate for the added weight.

The result is a decrease in performance. This is what's called "non-functional hypertrophy" – an increase in muscle mass that's not accompanied by a performance improvement.

It's also possible for a muscle to become so big that it interferes with some athletic movements. I'm not talking about being "musclebound" (lack of flexibility), but rather about a muscle interfering with a certain movement because of its size. For example, if the biceps are too big, they can restrict the range of motion during an arm flexion movement, especially if the individual has short forearms. Granted, this problem is quite rare.

Athletic Muscle-Building

Athletes need muscle like a car needs an engine. Without muscle there's no movement possible. Since athletes require strength, power, and speed in their movements, they also need the type of engine that has a lot of horsepower!

The size of a muscle determines its strength and power potential. I say "potential" because without the proper neural adaptations a big muscle won't be a super strong muscle. Similarly, a super efficient nervous system without the proper engine (muscle mass) won't be very powerful either.

So athletes do need to increase their muscle mass, but they must do so in a functional or rational manner: the size gains must come with an increase in physical performance that leads to an improvement on the field of play. To accomplish this, we must stimulate hypertrophy via means that also lead to improvements in physical capacities required by the athlete's sport of choice.

The athlete needs:

  1. To focus on stimulating his high-threshold motor units (HTMUs)/fast-twitch fibers.
  2. To select methods that will increase strength and efficiency in all types of muscle actions (concentric, isometric, and eccentric).
  3. To improve the three main force production factors: muscular, reflexive, and elastic.

Let's take a closer look at each:

1 – Focus on stimulating high-threshold motor units (HTMUs)/fast-twitch fibers

Success in strength and power dominated events (powerlifting, Olympic lifting, gymnastics, sprints, throws, football, etc.) and in individual actions requiring a high degree of either strength and/or power (sprinting, jumping, changes of direction, throwing, hitting, etc.) is highly correlated with the fiber distribution within the involved muscles – high proportions of fast-twitch fibers being correlated with a higher power and strength production.

Sadly, for the most part at least, our proportion of different types of muscle fibers is genetically predetermined. Yes, it's possible to stimulate some subtype changes over time, but not to a very significant extent. Plus, it's generally easier to go from a "fast profile" to a "slow profile" than vice versa.

So basically, if you're not born with a high proportion of fast-twitch fibers you're not likely to become fast-twitch dominant in this lifetime, at least when in comes to the number of fibers. However, there is hope for you!

According to Hungarian sport scientist Jozef Tihanyi (1997), compensatory hypertrophy (selective hypertrophy of the fast-twitch fibers) can make up for a genetic slow-twitch dominance. According to professor Tihanyi, someone who has a 30% proportion of fast-twitch fibers can attain the same rate of force development and power production potential as someone who has 70% of fast-twitch fibers if there's a selective hypertrophy of the fast-twitch fibers.

This basically means that even if a muscle is slow-twitch dominant in fiber distribution, it can become fast-twitch dominant in its properties by increasing exclusively (or almost) the size of the existing fast-twitch fibers. The result is that even if you have less FT fibers, the area covered by these is larger than that covered by the ST fibers.

So if you catch my drift, utilizing methods that predominately stimulate the growth of the FT fibers can help an athlete overcome a mediocre fiber makeup when it comes to power, speed, and strength sport. This is one case where hypertrophy will increase not only athletic performance, but also athletic potential.

2 – Select methods that will increase strength and efficiency in all types of muscle actions (concentric, isometric, and eccentric)

Most athletes, at least those who are using strength training to improve their performance, have an acceptable level of concentric (lifting) strength. This is due to the fact that this type of muscle action is emphasized in regular lifting exercises. However, rarely have I seen an athlete with sufficient, much less optimal, eccentric and isometric strength.

This is somewhat illogical because, if anything, eccentric (lowering or absorbing) and isometric (holding, static) strength are more important than concentric strength. Before an athlete can overcome, project, or move a source of resistance, he must first be able to absorb its force and stop it.

Any sprinting, jumping, or change of direction where the athlete must absorb and stop the weight of his body, requires a lot of eccentric and isometric strength. Without sufficient levels of both, the transfer from the eccentric to the explosive projection phase will be slower, leading to an inferior force production (less speed, slower changes of direction, shorter jumps).

Any action where you must stop an opponent also requires a lot of both of these types of strength: you must first be able to stop the opponent before being able to overcome him! Thus you need to be efficient at force absorption. Without sufficient eccentric and isometric strength, you'll always have trouble stopping, and thus overcoming, an opponent.

Isometric strength is also important for several other types of athletic actions. For example, every movement that requires the athlete to hold a pre-determined body position (e.g. alpine skiing's bent knee position, the iron cross in gymnastics) requires great isometric strength. Actions where there's a rapid switch from eccentric to concentric (running, changes of direction, etc.) also need isometric strength since, as we saw earlier, before the switch can occur the resistance must be stopped, and that requires both eccentric and isometric strength.

So in that regard, athletes should devote at least 30% of their training volume to eccentric and isometric training (20% eccentric and 10% isometric is generally recommended). Personally I use a greater percentage with athletes lagging in eccentric and isometric strength, sometimes up to 70-80% of the training volume of a training phase!

These methods can obviously develop strength and size. And because of the nature of these exercises, which favor an important reliance on the high-threshold/fast-twitch motor units, the hypertrophy stimulated via these methods will be highly beneficial to the athlete.

In most individuals, more HTMUs are recruited during a maximal isometric action than during a regular lifting movement. This is especially true in beginners. In that regard, isometric exercises can be used to develop the nervous system's capacity to recruit these HTMUs.

As your CNS becomes more efficient at recruiting HTMUs during isometric actions, its overall capacity to tap into these powerful fibers will also increase. As a result, you'll eventually become more efficient at recruiting HTMUs in regular lifting movements. More HTMUs recruited equals more muscle growth and greater strength gains.

As for eccentric training, there's some evidence that maximal eccentric actions will preferably recruit fast-twitch muscle fibers (high threshold motor units), which are more responsive to muscle growth and strengthening (Nardone et al. 1989, Howell et al. 1995, Hortobagyi et al. 1996). In fact, eccentric training may stimulate an evolution toward a faster contractile profile (Martin et al. 1995).

Furthermore, there's a higher level of stress per motor unit during eccentric work. Less motor units are recruited during the eccentric portion of a movement, thus each of the recruited motor units receives much more stimulation (Grabiner and Owings 2002, Linnamo et al. 2002).

Since the nervous system seems to favor the HTMUs during an eccentric strength exercise, and there's a greater amount of stress per recruited motor unit during that type of training, it stands to reason that the strengthening effect on the HTMUs will be a lot more important during eccentric than during concentric work.

3 – Improve the three main force production factors: muscular, elastic, and reflexive

To maximize HTMU recruitment and stimulation, you must produce as much force as you can in training. Three main factors can contribute to force production:

1 – The Muscular Factor

This is the amount of force produced by the contraction of the muscle itself. It's in direct relation to the amount of tension that can be produced by each muscle: the harder a muscle contracts, the more tension is produced and thus the higher the force output will be.

To recruit the greatest amount of motor units possible, you must generate as much force as possible at any given moment of a set. Some people will be quick to mention that since force production is the key to motor unit recruitment, we should always lift maximal weights (in the 90-100% range). This isn't the case, and it shows a lack of understanding of the definition of force. In biomechanics (and physics) force is defined at:

"F = ma"

Force (F) equals mass (m) times acceleration (a). So an increase in the generation of force can be accomplished either by increasing the acceleration with a certain load or by using more weight. Maximum recruitment is generated when the intended force production is at its greatest. For that reason, we should try to reach maximum acceleration with any given weight and any given fatigue level.

Obviously, when the weight used is very heavy, or when we're tired at the end of a set, the actual movement of the bar to be lifted will be slow. However, the actual intent to accelerate as much as possible has the same training effect on the nervous system (including MU recruitment, high firing rate, and rapid rate of force development) as if the bar was actually moving fast.

This is what led to the compensatory acceleration technique (CAT). CAT means that you compensate for a non-maximal weight by accelerating it as much as you can. A non-maximal weight lifted without the intent to create as much acceleration as possible won't lead to the recruitment of the high threshold motor units until you reach a level of fatigue that requires your nervous system to finally tap into these strong fibers.

So if you were using a moderate load lifted without CAT, you'd only have the last 2-3 reps of a set that would actually recruit the HTMUs. And according to Dr. Vladimir Zatsiorsky, a motor unit that isn't fatigued isn't being trained. As a result, if you're not able to thoroughly fatigue those HTMUs with the last two to three reps of a set (if muscle failure occurs due to an accumulation of metabolites for example), that set was wasted, at least when it pertains to stimulating maximal muscle growth.

The fact that you can eventually tap into your HTMU pool as fatigue sets in has led to the saying, "Those last few reps are the most effective for growth." While with regular bodybuilding training this is probably true, using CAT will make each single repetition effective at recruiting the HTMUs. So the HTMUs will get stimulated with every rep of every set, instead of only 2-3. As a result, you're more likely to fatigue and stimulate the HTMUs, which is what we want with athletes.

2 & 3 – Elastic and Reflexive Factors

I'm including both of these in the same category since they seem to go hand-in-hand for the purpose of maximizing force production.

The most important reflex to use is the myotatic stretch reflex, which occurs (as its name implies) when the muscle is forcefully stretched. When this occurs (muscle is stretched under load) there will be a reflexive action that facilitates muscle shortening. This facilitation can greatly contribute to force production during the concentric action following the loaded stretching of the muscle.

The elastic component of a muscle and its tendon can also contribute to force production. Muscular and tendinous tissues are elastic in nature and, when stretched, this elastic property will also enhance contractile strength.

So it should be obvious that by preceding the concentric phase of a movement (the lifting portion in our case) with a loaded stretch can significantly increase the amount of force produced. This is due to:

  1. The potentiating effect of the myotatic stretch-reflex. When a musculotendinous structure (a muscle and its tendons) is forcefully stretched, there's the onset of a "stretch reflex" governed by the activation of the muscle spindles. Muscle spindles are small fibers that run parallel to your muscle fibers. When they're stretched beyond a certain point they initiate the myotatic stretch reflex that helps the body to shorten. This is a protective mechanism designed to protect the musculotendinous structures against tears caused by excessive stretching.
  2. The elastic component of the musculotendinous structure. The muscles, fascias, and tendons are elastic by nature (more or less depending on the structure) and just like a rubber band, if they're stretched they'll tend to shorten powerfully. This characteristic of the musculotendinous structures can also contribute to an increase in force production.
  3. The increase in motor-units activation . Walshe et al. (1998) have stated that pre-stretching a muscle prior to a concentric phase promotes a higher active muscle state. They also found the forceful stretch could potentiate the capacity of the contractile elements of the muscle.
  4. The evolution toward a fast-twitch muscle fiber dominance over the long run. Paddon-Jones et al. (2001) have demonstrated that rapid eccentric actions (the forceful and rapid stretch at the end of the eccentric phase) lead to an increase in fast-twitch fibers/motor-units over the long run (using a ten week protocol in the study).

Fast eccentric movements decreased type I fibers from an average of 53.8% to an average of 39.1%, while type IIb fiber percentage increased from an average of 5.8% to an average of 12.9% (thus, there must have been a significant increase in IIa fibers too, but this wasn't measured). In the long term, this type of training effect could greatly improve an individual's capacity to stimulate hypertrophy as well as strength and power gains.

Not only can a pre-stretch enhance force production, it can also increase force production at any given training velocity. Normally, muscle contractions respond to the inverse force-velocity curve proposed by Hill (illustrated below).

inverse force-velocity curve

However, this curve was developed from non-SSC movements. The force-velocity curve during movements involving the SSC is different. The following graphic adapted from Komi et al. (1996) illustrates the difference between the theoretical force-velocity curve and the one observed with SSC movements.

theoretical force-velocity curve

So the take-home message is that to focus on HTMU stimulation you must maximize force production on every repetition of every set. To do so you must always try to generate as much force (F = ma) as possible on each rep and that each contraction should be preceded by a loaded stretching of the muscle.

This means selecting exercises in which the targeted muscle is placed in a stretched position at the end of the eccentric/lowering phase, preceding the concentric/lifting phase.

So Where Does That Take Me?

Okay, I'll be the first one to say that the preceding was pretty dense and dry. However, I've always been one to believe in understanding the reason why something works. But to summarize what's been said so far:

  1. High-threshold motor units have the greatest growth potential. A specific hypertrophy of these fibers can increase athletic performance and potential.
  2. HTMUs are brought into play when the demand for force production is high.
  3. HTMUs can also be brought into play when the fatigue in the intermediate threshold fibers leads to an insufficient force production; then the HTMUs must be recruited. This is why during "normal" bodybuilding-type sets, the most effective reps are the last ones.
  4. You can "compensate" for the lack of fatigue in the first reps of a set by always trying to create as much acceleration as possible on each rep of every set. This way you make each rep effective at stimulating growth.
  5. The intent to (trying to) accelerate is responsible for the recruitment of the HTMUs. Even if the bar doesn't move rapidly, if you're really trying to push it as hard as you can, it'll have the same effect as actually lifting with great speed. The key is to reach maximum acceleration at a given weight and fatigue level.
  6. Accentuating the eccentric stress, or adding eccentric-based exercises during a session, will lead to more strength gains. The reasons are related to structural as well as neural adaptations.
  7. The eccentric portion of a movement is the main stimulus for muscle growth as it's the cause of most of the micro-trauma inflicted on the muscles, which acts as the signal to kick the muscle building process into overdrive. It also predominantly recruit HTMUs.
  8. A loaded pre-stretch of the muscle before the concentric portion enhances force production via reflex, elastic, and contraction factors.
  9. The pre-stretch increases the body's capacity to maximize force production, even at a high velocity of movement.
  10. As soon as you reach the pre-stretch position, initiate the lifting action as fast as you can. Remember to try to generate as much acceleration as you can on each rep. As fatigue sets in, the actual speed of the reps will decrease, but the intent to accelerate has the same training effect as the actual speed of the movement.
  11. Isometric work can help you improve the capacity to recruit HTMUs over time, especially in beginners and individuals with an inefficient CNS.
  12. Isometric exercises are characterised by a high level of force production which can be used to stimulate HTMUs into growth and strengthening.
  13. You can use isometric movements to potentiate (make more effective) regular lifting exercises by performing a maximal isometric contraction lasting 5-10 seconds, 2-3 minutes prior to your regular lifting set.
  14. If you have a specific weak point in a certain lift, you can rely on isometric exercises performed at that sticking point to correct the problem.
  15. Isometric work is much less energy-costly than regular lifting. It won't cause much muscle damage either.
  16. Several athletic actions require isometric strength. It's especially important for individuals participating in sports where a fixed body position is used or where frequent changes of direction are required.


In the second part of this article I'll be detailing the best training methods athletes can use to increase their muscle size while also improving their performance level. In the third and final instalment of this series, I'll detail how to design a training plan taking full advantage of all of these methods! Stay tuned!

Christian Thibaudeau specializes in building bodies that perform as well as they look. He is one of the most sought-after coaches by the world's top athletes and bodybuilders. Check out the Christian Thibaudeau Coaching Forum.