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Proxying Stimulus

This is an excerpt from Bodybuilding Anatomy by Michael Israetel,Jared Feather & Christle Guevarra.

There are at least four stimulus proxies: the mind–muscle connection, pump, perturbation, and disruption.

Mind–Muscle Connection

The mind–muscle connection (MMC) is often used interchangeably with kinesthetic awareness, but these are not the same concept. If you feel your pecs squeezing at the top of a cable flye, you are in fact kinesthetically aware of the pecs in that exercise. However, this awareness only indicates that a muscle is being used during the movement—it does not indicate that the muscle is being used in a way that will drive a high muscle growth stimulus. You’ll need more than just awareness of muscle contraction to define the MMC. You’ll need the detection of high tension or burn in the target muscle to make sense of MMC. If you have some measure of both, you have some measure of MMC; the more you have of either, the higher your MMC will be in a given training situation. Let’s look at both tension and burn in turn.

Target Muscle Tension

Tension in MMC involves the experience of feeling like your target muscles are producing a huge amount of force relative to their abilities; it also involves feeling like the target muscles are being stretched nearly to their breaking point, especially on the eccentric part of the exercise. If you feel your biceps seemingly tearing (in the good way) during cable biceps curls, it’s likely the exercise is highly stimulative of growth—more so than if you feel it mostly in your forearms and the biceps don’t feel as tense. Tension is especially likely to be a good proxy for stimulus with lower repetition range exercises, such as sets of five to 15 reps.

Target Muscle Burn

Especially in higher rep exercises (with sets of 15 to 30 or more reps), accumulating metabolites that trigger muscle growth also irritate the nerve endings around your target muscle(s) and cause the burning sensation commonly associated with hard training in the gym. If your target muscle gets a profound burn with higher sets of reps (especially toward the last five or so repetitions), this is a better sign that more growth is being driven than if muscles other than the target muscle(s) are experiencing the burn.

Pump

Because cell swelling is mechanistically and empirically linked to growth, exercises that (for any given number of sets) cause the biggest pumps will more likely stimulate higher levels of growth than exercises that cause less impressive pumps. For example, if leg presses swell your quads to balloon animal status, but hack squats for the same rep range and set number leave your quads mostly deflated, then leg presses are likely (at least on the pump proxy alone) to cause more muscle growth for you at that given time.

Perturbation

Perturbation is a catchall concept. Basically, it means that stimulative muscle contractions that fatigue target muscles cause more growth than contractions that do not cause as much fatigue. Perturbation is composed of three functions: local strength loss, local coordination loss, and increased cramping.

Local Strength Loss

If you can typically curl 100 pounds for 10 reps, you may find that five sets of dumbbell curls later, you can curl 100 pounds for only eight reps. Instead if you did five sets of cable curls and could curl only 100 pounds for four reps, then we can say that cable curls locally fatigue the biceps much more than dumbbell curls (and likely stimulate more growth). This is especially likely if you feel that the biceps are “out of juice” while your shoulders, back, and overall systemic drive feel good and are not the limiting factors on your post-curl test with 100 pounds. You won’t often have to test your strength loss, but it will be apparent from simple daily tasks. For example, if you trained your quads for three sets on the leg press machine and then walked down the stairs with no trouble afterward, it is unlikely that such a session was as stimulative of growth as a session of three sets with another leg press machine—three sets that leave you having to hold the hand rails walking down the stairs because your quads have difficulty producing enough force to prevent you from falling!

Local Coordination Loss

Perturbed (and thus likely stimulated muscles) can experience both the fatigue of their local peripheral nerves and excitation-contraction uncoupling at the neuromuscular junction. In other words, perturbed muscles have trouble receiving communication from the nervous system. This is often perceived by the lifter as a reduction in the usually smooth coordination of muscles that target muscle to produce movements of daily life. For example, if your shoulders are so perturbed that picking up your water bottle feels strange and you look clumsy doing it, it’s likely that a bigger growth stimulus was delivered than if you did another exercise that left you as capable as ever in coordinating smooth movements after the training session. If you’ve ever tried to drive after a tough leg or triceps workout, you’ll recognize this phenomenon, as it can make pressing the pedals and steering the wheel more challenging than it usually is!

Increased Cramping

In more extreme cases, cramping can result in the severe perturbation of target muscles. While it doesn’t occur reliably enough to be used as a dependable stimulus proxy, if your muscles are so perturbed that they are cramping after the workout, there is a higher chance they were stimulated sufficiently, and perhaps even too much. Put another way, if you trained your triceps hard enough to make them cramp after training, it’s likely that you stimulated them enough. If one exercise makes your triceps more likely to cramp after training than another exercise, then the first exercise is likely more stimulative.

Disruption

While perturbation can be detected right after the exercise session is over, disruption indicates that the muscle was considerably damaged from the training. If only because high tension and metabolite production cause the most damage, exercises that tend to cause high degrees of damage usually also cause the highest degrees of muscle growth. Proxying damage is indirect (because most muscle damage is imperceptible), but there are at least two ways to tell if a muscle has been notably damaged from training and is, therefore, likely significantly stimulated to grow.

Muscle Tightness

Some muscles don’t really get sore no matter how much you train them, but all muscles will feel tight and less flexible than usual for at least some hours after training, if not days. If you’re comparing two exercises and, for the same number of sets, one exercise leaves your target muscles feeling tight for some hours or days after training but the other doesn’t leave your muscles as tight or they aren’t tight at all, then the first exercise is likely stimulating more growth. If the exercises cause a similar level of tightness, but one exercise’s tightness lasts for longer, that exercise is probably also the more stimulative one.

Muscle Soreness

While all muscles get tight after training, some also get sore—superficially in a delayed onset fashion, where soreness begins to set in after a few hours and peaks in a day or two. This delayed onset muscle soreness (DOMS) is an indicator that so much damage occurred that the immune system needed to come in to help heal it. Such massive damage is indicative of the muscle having been exposed to a high degree of tension or metabolites, which is predictive of growth. It is important to note that excessive damage, indicated by DOMS that lasts longer than about half a week, can compete with growth resources for mere recovery and lead to less total muscle gain. Thus, we’re not looking for the most DOMS from an exercise. We’re looking for the most DOMS for the fewest number of sets; you should do no more sets than cause moderate DOMS in most cases and thus stimulate the highest levels of muscle growth. For example, if you have one exercise that causes three days of DOMS after two sets, and another exercise that causes a day or so of tightness after four sets, it’s likely that the former exercise is more growth stimulative than the latter.

In the next section, we’ll talk about how to use these stimulus proxies in an organized fashion to help determine the total stimulus magnitude of a given exercise as compared to others. Before we do that, let’s examine the fatigue proxies.

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