Metabolic stress does some of the work and mechanical tension does a lot.
Beyond being in a state of caloric surplus, rested, and some more generalized debates over volume/intensity/progressive overload, the proposed processes which stimulate muscle growth come down to metabolic stress and mechanical tension.
The old theory was muscle damage. “Break it down to build it up,” was a common bro-science aphorism. Not only isn’t it true, but it actually cannot be. No expert in the field affirms this anymore, because of the many counterexamples and obvious impossible paradox therein. Elongating muscles creates the greatest muscular damage, and training which emphasizes forced stretches and hyper-mobility does NOT improve muscular size or efficiency. The contrast of gymnasts versus yogis is perhaps the clearest example of how fixation on lengthening or over-lengthening muscle tissue does not result in a size, speed, or strength gain. Are there benefits to improving control of greater range of motion? Of course. But the very act of “relaxing into a stretch” instead of developing control in it and power over it means limiting mechanical tension, maximizing damage, thereby cutting the strength and size stimuli.
Metabolic stress is real but harder to nail down. Contraction of skeletal muscle does produce myokines, alters energy uptake through changes in glucose transporters, and there are some real measurable reactions in hormone responses on site and throughout the person. The degree to which this contributes to overall health and fitness is significant. The precise muscle size impact from any one cascade is unclear, however. There are plenty of athletes who contract muscles millions of times at low tensions and have zero muscular development despite the metabolic stress contribution. See olympic and elite marathoners and endurance cyclists.
This leaves us with mechanical tension. I have noticed a trend with upstarts in the past five years or so where they are obsessed with mechanical tension as the be-all end-all. And I’d agree that across decades of professional experience I MOSTLY see muscle size gains in response to mechanical tension. That is, as people truly encounter muscular performance limits, and barely eke out a last or genuinely failed rep, that is where we tend to see size gains most dramatic. The journey requires progressive overload, obviously. But the individual mechanism within a workout is the enhancing of mechanical tension.
To be more clear, when a relatively rested person encounters a strength exercise in a stable position and performs 5-20 reps but only ends the set because the velocity of the movement grinds to zero, THAT is that person’s maximum mechanical tension for the intended muscle group of that exercise.
However, I want to caution against a cult-like insistence on ONE way and only one way. The first reason for this is that we don’t KNOW for a fact that getting to absolute muscle failure is better than getting very close. And we don’t KNOW how much better (or if) very-close is than close. The second reason I caution people is that to be able to effectively get near muscle failure, you have to first develop expertise. And to become advanced and skillful (and retain said skill) you will have to train sub-maximally some of the time. Moreover, if people do not include some power, speed, and greater range of motion training into a program at some point, the participant is more likely to get hurt or discontinue, such that he or she can no longer gain the reward of mechanical tension.
At this point, most people might be asking, “What does this translate into for my workouts?” Or, “how do I apply this?”
Let’s take a machine chest press as an example. Imagine you are able to press about 200lbs on this exercise for around 8 reps. Let’s say we start this workout with this exercise, such that you are fully rested once getting to this piece of equipment. Potentially, you’d perform a set of 100lbs for 5 reps, very strictly and intent on creating tension and mentally connecting with the pecs, triceps and deltoids. Afterward, you might wait 3 minutes, and perform another sub-maximal set, 3 reps with 150lbs, perhaps. Wait another 3 minutes. Now, it’s time for business. You set up with all of your normal posture, cues, settings and mentally commit to getting 9-12 clean and toilsome reps with the 200lbs. The first 7 reps may be harder than you anticipate. 8 is grueling. At the 9th rep, it’s a serious question whether you can complete it or not, but, through gritting teeth and eating the pain, you get it. Now, you fight headlong into the 10th rep and the weight is grinding to a halt; but you continue pouring your every fiber of will into budging it another millimeter. And possibly it doesn’t go. You still go to war in your heart and drive your body in the back pad as the bones in your arms seem to bend against the impossibility of the handles. THAT is mechanical tension. THAT is one working set. And THAT alone is sufficient stimulus to get stronger and gain muscle size.
Afterward, you might desire doing another few exercises for the same or similar muscle groups. There’s nothing wrong with that. Potentially, you could muster almost the same wherewithal for a legitimate working set like that on the same muscle group a few times in a whole workout sessions. If you include a separate muscle group, certainly you could. However, any degree of workout far in excess of that is not supported by evidence for the purposes of this simple discussion. Certainly, you could add many more exercises and sets and reps and alternate modalities, if you want to generate some more muscular endurance or train a variety of additional skills. But for muscle size, there isn’t a need to add lots of additional sets or exercises.
In time, that chest press must be 210, 220, and 12 reps or more, BUT in the same manner, the same grind, the same intentionality, the same degree of battle.
THAT is how muscle is actually built.