September 14, 2014

Excitation Thresholds - Why A Muscle Won't Grow, And What You Can Do About It

The 2 Primary Training Related Reasons Why A Muscle Won’t Grow

1.    One has difficulty recruiting a specific muscle group (likely because it’s the weak link in the chain, and stronger muscles take over during movements that are ‘supposed’ to target a specific muscle)

2.    One has difficulty recruiting the high threshold motor units/fast-twitch fibers of the targeted muscle (which have the greatest potential for growth and are only recruited only when the tension is high enough – which is maximized with the use of heavy loads, explosive movements, or at the end of a set taken to failure with lighter loads)

Excitation Thresholds – What You Know About That?

The greatest factor that is missed, or misunderstood, by most experts is that of excitation thresholds – motor units and recruitment patterns. Literature suggests that the nervous system has a marked ability to selectively preferentially recruit segments of a muscle over the targeted intention. This would indicate that the joint angle, and angle of contraction, play a larger role when it comes to recruiting the desired segments of a muscle group than the amount of weight used, thus making these variables (angle of contraction and exercise order) of greater significance when it comes to building muscle.

Motor units (muscle fibers and the motor neuron that innervates them) with lower excitation thresholds are always preferentially recruited during a given movement, regardless of the intent. This is why it’s not uncommon to have a seemingly unresponsive bodypart located right next to a highly responsive bodypart. What’s happening is the more neurologically dominant and responsive muscles, those with lower excitation thresholds, act first and basically take over exercises that are ‘designed’ to target other muscle groups. This goes beyond what most experts address when explaining why muscles are unresponsive and/or weak, and is why certain exercises can be especially effective at stimulating growth for one individual, while having little to no effect for another, and why exercise prescription must be personalized, irrespective of the goal.

The ability to not only recruit selective segments of a muscle, but to recruit the fast-twitch muscle fibers of that muscle, is very often a limiting factor as it relates to the capacity to build muscle. If ones nervous system fails, or is unable, to efficiently recruit these muscle fibers within the targeted muscle, then growth of that particular muscle is limited. Therefore, as it relates to training, maximal growth potential can be defined as the sum of one’s ability to stimulate maximum muscle fibers.

Insanity – Doing The Same Thing Over And Over, And Expecting A Different Outcome

When a muscle is seemingly resistant to grow, the common solution is to do more work. This is done by performing more frequent workouts for the targeted muscle group, or adding sets and reps to what is already being done. While this approach may seem logical, in reality it is extremely flawed considering that doing more of the same will not make up for the underlying problem regarding innervation, in fact, it will only make it worse.

A muscle with suboptimal innervation can never fully be under maximal loading, which is necessary for the fast-twitch fibers are to be recruited, as other highly responsive nearby muscles will interfere. Increasing the volume in these situations only ends up creating a greater reliance on the highly responsive nearby muscle groups, or slow twitch muscle fibers of the targeted muscle. As a result, the fast-twitch fibers of the targeted muscle group remain untrained, and over time the nervous system becomes better, and better, at relying on the highly responsive nearby muscle groups, or slow twitch muscle fibers of the targeted muscle group, and this cycle takes on a life of its own and feeds itself.

Break The Cycle

Only once the nervous system is able to recruit the targeted muscle, including the fast twitch muscle fibers of that muscle, is increasing the training volume an effective solution. At that point in time is when the volume can be increased without ill effect. Until then, training smarter is needed to break the cycle, not harder.

Training Smarter

Some effective and practical strategies in regards to taking an unresponsive, high excitation threshold muscle group, and reducing its excitation threshold so that it is more responsive to training, are:

·         Pre-exhaust the dominant/responsive muscles at the onset of the workout to avoid having them take over throughout the rest of the workout. This is self-explanatory - simply fatigue the low excitation threshold, highly responsive muscle groups, so that their contribution during the rest of the workout is minimal, and by default the high excitation threshold, low responsive muscle(s) are left with bearing the load.

·         Pre-exhaust the unresponsive muscle at the onset of the workout to ensure it is maximally stimulated throughout the rest of the workout. By isolating and exhausting the targeted muscle(s) early on, they become the limiting factor which ensures the highly responsive, low excitation threshold muscle groups are essentially undertrained by comparison, which lessens the discrepancy between the relatively weak, and relatively strong muscle groups.

·         Chain together compound and isolation exercises as part of a superset to enhance the mind-muscle connection which facilitates the recruitment of the high-threshold motor units of the targeted muscle(s). The innervation threshold of the targeted muscle should determine where in sequence the isolation movement is placed. If recruiting the targeted muscle is problematic, and nearby muscle groups are overpowering during exercises designated for the targeted muscle, then performing the isolated movement first in sequence is preferred (to pre-exhaust). If however, stimulating the fast-twitch fibers is problematic, then performing the isolated movement second in sequence is preferred (to post-exhaust).


If recruiting the targeted muscle group is problematic, the pre-exhaustion method offers two distinct benefits:

·         It can improve the capacity to recruit the targeted muscle during other movements, which allow for the use of greater loads, by enhancing the mind-muscle connection. The localized ‘pump’ and fatigue created by pre-exhausting a muscle allows the working muscle to be felt to a much greater degree than under normal conditions, and this carries over to other movements, thus increasing their overall value in a training program, all of which contributes to reducing the excitation threshold of the targeted muscle, thus improving its responsiveness to training.

·         Fatiguing the targeted muscle ensures that it is fully stimulated during movements in which nearby, low excitation threshold, highly responsive muscles, are involved. This is especially true for larger muscle groups which do not get fully stimulated as a result of the smaller, albeit more responsive muscle groups, being the limiting factor.

To reiterate, the goal with the pre-exhaust superset is to learn to integrate the targeted muscle into other movements in which other muscles generally take over.


Before being able to fully stimulate a muscle to grow, one must be able to recruit it – that’s where the pre-exhaustion method is of greater value than the post-exhaustion method. When innervation is optimal, the post-exhaustion method offers one primary benefit:

·         All things being equal in terms of innervation thresholds, a greater load is indicative of greater recruitment, and since a major contributing muscle is not pre-fatigued, greater loads can be handled during the compound movement.

To maximize recruitment during the compound movement, select movements that place the targeted muscle in a relatively stretched position, as muscles that are stretched the most, are recruited the most, and always aim to lift with as much acceleration as possible as fast-twitch fiber recruitment is proportionate to force production. The greater the muscle is stretched, the more fast-twitch fibers that are recruited. The more fast-twitch fibers that are recruited, the more force that is produced. The more force that is produced, the faster the weight is lifted. As you can see, the process is cyclical, as all of these factors build upon one another, and contribute to fast-twitch fiber stimulation.

To reiterate, the goal with the post-exhaust superset is to recruit the targeted muscle with heavy weights, and further fatigue it with the use of isolation movements.

Complex-Contrast Training

Advanced lifters, those who have no problem recruiting a muscle at will, can get away with using complex-contrast training to stimulate newfound growth, without the added risk of injury that usually accompanies highly intensive techniques. Generally complex-contrast training is reserved for those looking to increase performance with the goal of increasing their strength levels, but the same principles can be tailored to those looking to build muscle.

Complex-contrast training consists of first performing lifts which allow for supramaximal loads to be used, to recruit as many muscle fibers as possible, before performing a traditional movement through a full range of motion with a slightly greater load than would otherwise be possible. By contrast the weight feels lighter after lifting the supramaximal load, and therefore it is sometimes possible to perform more reps with the same amount of weight, or lift slightly more weight for the same amount of reps.

Up to 10% more muscle fibers can be recruited during a maximal isometric contraction compared to a concentric contraction, and muscles are typically capable of lowering 30% more weight eccentrically, than concentrically, as the fast-twitch fibers are preferentially recruited during eccentric contractions, and are therefore under greater stress. This opens the door for a few different combinations:

*A power rack is required for the following techniques

Overloading a muscle eccentrically is done by putting more weight on the bar than would be possible to lift concentrically, and lowered as slow as possible (goal is 4-6 seconds) until the bar rests on the pins in the power rack.

Overloading a muscle isometrically is done by either pressing/pulling a bar into the pins in the power rack (which are strategically placed based on the movement being performed), or holding a bar at the joint angle in which you are strongest, loaded up with more weight that would be possible to lift through a full range of motion concentrically, for roughly 6 seconds.

Immediately after performing either of the options above, the same movement is performed through a full range of motion (with a load that allows for 3-5 or 4-6 reps, resting as long as needed in between, 2 minutes being ideal for most, and repeated for 4-5 complex-contrast sets).

If you have any questions about excitation thresholds and how they affect building muscle, or how to stimulate newfound growth, feel free to contact me at I'm available for online consulting and personalized program design, as well as one on one training if you are located in the Greater Toronto Area (GTA).

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