Your body adapts to a task to be more efficient at that task; while muscle costs energy to maintain, being able to do a task with less effort can lower the energy expended.
There’s a fun Top Gear clip that illustrates how this can work for motors (https://youtu.be/F04MXepYiBs?t=92); a Prius is driven flat out while a faster but less efficient car effortlessly keeps pace with it. Because the Prius’ engine is being pushed, but the other car isn’t, the other car ends up being more efficient (for that ‘test’).
Your body faces a similar paradigm with your muscles. Building more muscle means more energy expended just sitting around (which is why the muscle will be discarded if you *aren’t* using it), but if it takes your workout from 90% effort to 50% effort, the energy savings in your workout outweigh the ‘maintenence cost’ (especially since heavily stressing your system with a “90% workout” requires some ‘repair work’.
Your body is adapting to your workout (or even just regular labor) by becoming more efficient, reducing the stress that the work puts on your body. Because you’re more efficient when not putting in near-100% effort, this adaption is normally building or refining muscle. It wants to be the BMW in the TG clip, not the Prius.
The reason you ‘plateau’ is that you *need* that stress to trigger growth. Once you get to the point where an exercise is only getting you to 50% or so, you aren’t stressing the muscles enough to trigger growth. You *are* the BMW now, so there’s no impetus to get faster.
The simplest method of progression from there is to just add more weight so you stay in that ‘high stress’ area where you continue to trigger growth (to continue to torture my analogy, you’re making yourself the Prius again).
The types of plateaus where you end up needing to change your routine, then (rather than just adding more weight), are caused when some other factor is preventing you from pushing the weight up to the next level, but your current workout isn’t stressing you enough. This usually means there is an imbalance or weakness somewhere; a stabilizer or secondary muscle isn’t getting worked enough by the exercise to develop, but is also limiting your ability to push the ‘main’ muscles to 90% to trigger growth (note that these percentages are picked for illustration, not necessarily accurate).
For example, your deadlift might be being limited by your ability to grip the bar. Your legs and core are strong enough that your current lift is ‘too easy’ to trigger growth, but you can’t add more weight because then your *hands* fail the lift for you. And conversely, because you’re deadlifting heavy, but aren’t doing other grip strength exercises, you aren’t “targeting” grip strength, so it’s not growing. You’re stuck.
In this case, if you change your routine and end up with one that has more stuff that exercises grip strength, your grip strength will go up and you’ll be able to deadlift heavier, getting back into the ‘growth zone’ for your legs and core.
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