Can muscle fibers grow without being activated?
After publishing my last short article, I received some feedback from friends and colleagues notifying me that one of the reasons that many people are struggling with the use of the principle of neuromechanical matching for governing exercise selection during strength training is that they have been persuaded that muscle fibers do not need to be activated in order to experience hypertrophy. So let’s look at this question today.
What causes a muscle to grow?
Muscles grow when muscle fibers increase in size (either in diameter or length). Muscle fibers grow when their internal mechanoreceptors detect a mechanical tension stimulus. Let me emphasize that mechanoreceptors are located INSIDE of individual muscle fibers. For example, the costamere complexes bind the insides of a muscle fiber together with its surrounding endomysium, while titin runs longitudinally down inside the muscle fiber, adjacent to the actins and myosins of the myofibrils. When these receptors detect mechanical tension, they send a signal to the muscle fiber in which they are located to adapt. They cannot and do not ever send signals to other muscle fibers in the same muscle (even though some fitness influencers have allowed their imaginations to run riot and have speculated that this can indeed occur, with zero evidence for the idea). Therefore, each muscle fiber must exert and then experience a mechanical tension stimulus in order for it to then display a hypertrophic response. That is why we refer to muscle fiber hypertrophy as an intrinsic process.
What causes a muscle fiber to generate mechanical tension?
Muscle fibers can only experience two types of mechanical tension: [1] active mechanical tension, and [2] passive mechanical tension. Active mechanical tension is generated when the muscle fiber is activated by motor unit recruitment, leading to excitation-contraction coupling, which then causes myosin heads to bind with actins, forming crossbridges that pull the sarcomeres of the muscle fiber to shorter lengths. Evidently, this process cannot happen without the muscle fiber being activated first. This means that if an exercise cannot activate all the muscle fibers in a muscle (because that muscle has poor leverage and so is not switched on by the brain in accordance with the principle of neuromechanical matching), then that exercise will never maximize the growth of that muscle by means of active mechanical tension. No matter how many other clever-sounding reasons your favorite fitness influencer gives you, and no matter how much word salad they write about changes in muscle length, an exercise cannot maximize the hypertrophy of a muscle if it does not first activate all of its muscle fibers.
Passive mechanical tension is generated when the titin molecules inside of the sarcomeres of the muscle fiber are elongated beyond a certain critical threshold. If the muscle fiber is not activated, then this threshold occurs at a really, really long muscle length. Typically, we do not reach this critical muscle length unless we perform an intense static stretch. While we refer to certain strength training exercises as “stretched-position” exercises, they are not stretching the muscle to anywhere near this length. For example, squats do not stretch the quadriceps or the gluteus maximus much more than simply sitting on a chair and yet they are called “stretched-position” exercises. They are not stretching the sarcomeres of the muscle fibers to the point where they can produce passive mechanical tension while in a deactivated state. Otherwise, we could grow muscle while sitting down all day, which is certainly not true.
When muscle fibers are activated, the threshold of sarcomere elongation for passive mechanical tension to reach the required level is reduced. This is because titin responds to muscle activation, shortening its effective length and producing a larger amount of passive mechanical tension at a shorter sarcomere length. In other words, we do not need to stretch a muscle fiber as far when it is activated in order to stimulate hypertrophy. This is why stretched-position exercises can cause stretch-mediated hypertrophy despite not reaching as long a muscle length as static stretching. Yet, the only muscle fibers that produce and experience this passive mechanical tension are the ones that have been activated. Once again, this means that if an exercise cannot activate all the muscle fibers in a muscle (because that muscle has poor leverage and so is not switched on by the brain in accordance with the principle of neuromechanical matching), then that exercise will never maximize the growth of that muscle by means of passive mechanical tension. No matter how many other clever-sounding reasons your favorite fitness influencer gives you, and no matter how much word salad they write about changes in muscle length, an exercise cannot maximize the hypertrophy of a muscle if it does not first activate all of its muscle fibers.
N.B. Before asking me about whether “loaded stretching” is better than any other type of stretching, please establish for yourself exactly how loaded stretches differ physiologically from a stretch done yourself or with someone else helping you. Physiologically, the only variable that determines the amount of passive tension is the amount of elongation. So whether you apply that by holding the limb in place yourself, by asking a colleague to assist you, or by applying a weight is totally irrelevant. Loaded stretches are not more intense than any other kind of stretch. It is the range of motion that matters. Now, if you start activating the muscle in order to resist the stretch (which you can do in any static stretch), then you will see a different effect in the activated muscle fibers, but the presence of load does not guarantee muscle activation. OK?
So can muscle fibers grow without being activated?
Can muscle fibers grow without being activated if you do intense static stretching? Yes, there is now lots of evidence that static stretching can and does produce muscle growth, and it happens by means of the passive mechanical tension produced by the stretching of the sarcomeres past the critical threshold. Can muscle fibers grow without being activated during strength training exercises? No, it is not possible to cause a muscle fiber to grow during a strength training exercise with either active mechanical tension or passive mechanical tension unless it has been activated first. The length of the muscle is simply not long enough to produce enough passive mechanical tension without muscle fiber activation being present. Whether a muscle fiber is activated in an exercise is determined by the principle of neuromechanical matching, so that’s where we start when talking about exercise selection. There is literally no alternative theory that makes any sense whatsoever.
What is the conclusions?
The maximum hypertrophy of a given muscle will be achieved by those programs of exercises that activate the largest number of its muscle fibers. In contrast, less hypertrophy will be produced by programs of exercises that activate fewer of its muscle fibers but which use stretched positions because they are trying to target greater passive mechanical tension. In the context of strength training exercises, passive mechanical tension can only be applied to activated muscle fibers. So, if a muscle has very poor leverage in the lengthened position, it makes more sense to train that muscle at the joint angle where it has best leverage rather than claim that “everything grows better at longer lengths” when that is physiologically impossible.
