Hypertrophy guide: My insights...Volume 2 - How is muscle built?

Volume 2 - How is muscle built? Review of motor unit recruitment and mechanical tension

After listening to Jake Doleshal and Chris Beardsley discuss the mechanisms behind muscle growth on their “Hypertrophy Past and Present” podcast, I think it is safe to say that many people do not truly understand how muscle is built in the first place. It is a common idea that “micro-tears” will accumulate within a muscle and that through high degrees of metabolic stress and a crazy pump, you can get jacked quickly. However, this cannot be further from the truth. In fact, we have no scientific evidence that these “micro-tears” even exist, and are likely using this term to describe the muscle damage that accumulates post workout. Fortunately, we now know that muscle damage and hypertrophy are inversely correlated, as in order for us to grow muscle we need to have high degrees of mechanical tension and motor unit recruitment. If muscle damage was truly the driver of hypertrophy, we would see marathon runners becoming bodybuilders, as they accumulate high degrees of fatigue and muscle damage in their lower bodies during their sessions.

Now that we know the two true drivers of hypertrophy are mechanical tension and motor unit recruitment, let's talk about what each means and how we can maximize them. Beginning with mechanical tension, this is simply the force experienced by a muscle fiber when it produces or resists a movement. This tension, especially when muscle experiences a stretch under a heavy load, triggers anabolic processes such as sarcomerogenesis and myofibrillar protein synthesis. There are two forms that exist: active and passive mechanical tension. Active tension happens during the contraction, where the muscle is able to produce the most force. On the other hand, passive tension occurs when the muscle is stretched under load, and proteins within your muscles called titin resist the lengthening of sarcomeres. These both matter, since active tension will drive myofibrillar protein addition, while passive tension will lead to stretch mediated hypertrophy. These overlap best when training a muscle in its lengthened position, where both high active force and passive stretch exist. However, it is important to understand that training a muscle at longer lengths comes with greater degrees of calcium ion related fatigue, meaning that the extra stimulus may not always be worth it. Factors such as volume, frequency, and intensity will need to be set before determining whether lengthened position exercises are worth it in a program.

Next, there is motor unit recruitment, which determines how many and which fibers are activated during a lift. Muscles are made up of thousands of motor units, and only the high threshold ones, or type 2 muscle fibers, have the greatest potential for growth. These units only get recruited when the speed of contraction is fast against a heavy load. This is why simply moving weight is not enough to grow. You can have high degrees of mechanical tension with low motor unit recruitment, like doing a heavy leg press but stopping ten reps before failure. On the other hand, you can have high recruitment but low tension, such as performing fast reps with light weights far from failure. In order for hypertrophy to occur, both are necessary. This can be achieved through lifting moderate to heavy loads close or to muscular failure, ensuring that you are pushing contraction speeds as fast as your muscles will allow in a standardized range of motion.

A technique that Jake and Chris mention that can help boost motor unit recruitment during a set is by using post-activation potential ion, or PAP, before a set. This is where you would do a near-maximal contraction, like a single rep warm up, at 85-95% of your working weight. This “primes” the high threshold motor units before your working set, allowing them to be fully recruited once you do your set. Another misunderstood idea that I find interesting is range of motion. Most people preach a full range of motion, but don’t understand that range of motion is almost always arbitrary. What really matters is that you are training the muscle at a position where it can produce the greatest leverage in that given range of motion. Every muscle has a specific position where it has greater relative leverage over other muscles in a certain plane. For example, in the frontal plane the side delta have best leverage for shoulder abduction from zero to ninety degrees. However, from 100 degrees and up, the front delts take over in terms of relative leverage. We can use these markers to determine how to best train muscles, as working them in the range of motion where they have the best leverage will be ideal to achieve high degrees of mechanical tension and motor unit recruitment.

Understanding this reveals why isometrics can be just as effective as isotonic exercises when performed properly. Holding an isometric contraction at a position where a muscle has peak mechanical leverage, such as a bicep curl yielding isometric at 40 degrees of elbow flexion, provides high degrees of mechanical tension and motor unit recruitment without a full range of motion. It is important to understand that on a muscle-fiber level, your muscles are always contracting—regardless of whether you use a full range of motion or not.

At the end of the day, your muscles don’t understand ranges of motion, but instead understand mechanical tension and motor unit recruitment. However, I would still recommend standardizing a range of motion over isometrics since this allows for a greater progressive runway and an ability to track strength on lifts. After all, a stronger muscle is a bigger muscle, and by tracking progressive overload and strength increases, we can best determine if we are growing muscle.