Why the Stimulus-to-Fatigue Ratio Matters if You Want to Build Muscle

Don’t confuse fatigue with stimulus….

Could fatigue be negatively impacting on your muscle and strength gains?

If you’re consistently smashing your body in the gym, fatigue will inevitably catch up to you.

Fatigue reduces our performance. Reduced performance means you will not increase your strength or build new muscle. In fact, you could even end up losing some of your hard-earned gains.

The worst possible outcome!

In this article, we discuss how you can use the stimulus-to-fatigue ratio to identify how effective an exercise might be for helping you reach your fitness goals in the gym and build muscle.

What is stimulus?

In the context of exercise, the stimulus is how effective one is for stimulating an adaptive hypertrophic response.

In layman’s terms, how good an exercise is for building muscle and/or increasing strength.

The more stimulation and exercise provide for muscle growth, the less total volume (sets) we require for the same outcome.

Three crucial components of an exercise create a stimulus for growth: range of motion, muscle tension, and muscle groups activated.

Range of motion

As they say in the lifting community, “not a single rep was achieved that day” after watching someone quarter-repping squats instead of going below parallel for a full-depth squat (Yes, some people mechanically cannot squat below parallel — but that’s not my point).

An exercise taking a muscle through a full range of motion creates a more stimulative response than an exercise with a partial range of motion.

Muscle tension

The second component of stimulating a hypertrophic response is the tension placed on the muscle by an exercise.

It’s more beneficial for growth to have significant tension on our target muscle(s) throughout the complete range of motion rather than doing an exercise with varying tension levels. This is called the resistance or strength curve.

For example, cable chest flies keep tension throughout the movement, but dumbbell flies lose tension at the top half.

Muscle groups activated

How compounded is the exercise? Are multiple muscle groups activated, or is just one muscle targeted?

For example, a barbell bench press for targeting the chest versus a pec deck chest fly.

The pec dec isolates the chest, whereas a bench press will also activate the triceps to a substantial extent and the front deltoid (shoulder) muscles.

The more muscle groups activated during an exercise, the higher the fatigue.

What is fatigue?

In terms of exercise, fatigue is weakness or tiredness, causing a reduction in maximal performance.

Simply, fatigue means you cannot squat as heavy, jump as high, or run as fast as usual. Fatigue reduces our ability to perform to our maximal potential with force production and technique execution.

Fatigue results from two key areas: training stimuli and life. Life factors that can increase our fatigue include work, family or relationship stress, and poor sleep and nutrition.

We are looking at training-induced fatigue in this article.

All resistance training exercises will induce some level of hypertrophy whilst generating fatigue.

Training-induced fatigue is compounding. Fatigue can affect just a single session, or when consistently training with high intensity and volume, culminated fatigue can negatively impact our long-term performance and results.

If your muscles consistently feel sore and you feel drained, you have accumulated too much fatigue and you need to back off. This is where people usually deload.

Three key components generate fatigue — the load lifted, stability demands and cardiovascular demands.

The Load

How heavy are you lifting?

Strength training is typically in the lower rep range with heavy loads.

For example, we might do three sets of three reps at 80% of our one-rep maximum.

This kind of training, given we do compound lifts like deadlifting and squats that use multiple muscles, is very taxing on our bodies. Because we must exert so much effort in these exercises, they generate a lot of fatigue.

Typically, the heavier the load, the more fatigue it will generate.

For example, barbell back squats with 100+ kg on the bar for reps are far more fatiguing than lunges holding 20kg dumbbells.

Stability demands

The second component of an exercise that generates fatigue is its stability demands. The exercises requiring accessory muscles to help stabilise multiple joints will be highly fatiguing.

For example, a barbell squat will activate several stabilising muscles. But on the other hand, a hack squat requires less, and the leg press requires no stabilisation.

Any exercise where we lift/hold a weight above our torso requires a lot of stabilisation. Our whole core is engaged, with multiple small muscle groups working together, generating a lot of fatigue.

If you have ever been tramping over uneven terrain like rocks, think about how sore your legs were afterwards compared to a normal. It’s all those stabilising muscles working.

Cardiovascular demands

Exercises with high cardiovascular demands typically induce more physical and mental fatigue than exercises requiring less cardiovascular output.

For example, a set of barbell squats for twelve reps requires far more cardiovascular fitness than twenty reps on the leg extension machine.

If you are not huffing and puffing after a set of squats, you’re not training anywhere heavy enough.

Why the stimulus-to-fatigue ratio matters

We do not have an infinite ability to train at peak intensity in the gym.

Our bodies are only good for so much punishment until our performance diminishes. Thus, when programming your exercise routine, consider the stimulus-to-fatigue relationship.

To maximise our exercise results, we want to generate as much stimulus for growth as possible whilst minimising the fatigue we must recover from.

Loading our workout with heavy compound exercises like squats, deadlifts, and military presses might generate a lot of stimuli for growth, but it also generates a lot of fatigue.

We cannot train at the required intensity to progressively overload and make muscle or strength gains when we’re highly fatigued.

Compound exercises that use barbells, such as conventional deadlifts and rack pulls, tend to have a poor stimulus-to-fatigue ratio. They’ll stimulate many muscles but will leave you feeling wrecked.

They use lots of weight, require long warm-ups, and drain your body’s resources quickly while providing an unfavourable return on hypertrophy.

Some people can squat four times a week without any recovery issues and grow huge legs. Others might feel like they can’t walk for a couple of days after squatting once a week.

It is crucial to listen to your body and track your results in the gym (e.g., how many reps at a certain weight you did this week vs a month ago).

“You have a certain sum total of fatigue that you can accumulate, beyond which is over your maximum recoverable volume, and you can no longer sustain a program. So, you have a fatigue allotment so to speak.” — Dr. Mike Israetel

Maximum recoverable volume (MRV) refers to the highest total amount of work in the gym benefiting growth. This ideal volume will vary depending on muscle groups, genetics, training experience, and technique.

If your performance in the gym is lacklustre and you’re sore during and after workouts, you are probably doing more than your MRV. You could reduce your volume (the total sets) or swap some exercises for those with lower recovery demands.

Like using machines instead of free weights.

Exercise choice to limit fatigue and maximise stimulus

According to well-respected YouTuber and exercise scientist Doctor Mike Israetel, “the ultimate program has as much stimulus as possible with as little fatigue as possible.”

Compare the components of stimulus: range of motion, muscle tension, and muscle groups activated; to those of fatigue: the load lifted, stability demands and cardiovascular demands.

We want to focus most of the tension on one target muscle to limit fatigue and maximise the potential for muscle growth. Strength is another conversation — in that instance, we want multiple muscles working in unison to make a lift as efficient as possible.

In terms of building muscle, using machines to target specific muscles provides an excellent stimulus for growth.

Many exercises we perform with free weights also have a machine alternative for that basic movement.

Like a hack squat or pendulum squat.

To optimise your workout for maximal hypertrophic stimulus and minimal systemic fatigue, choose exercises that do the following:

• Progressively overload.
• Create a mind-muscle connection to get a noticeable pump in the target muscles.
• Don’t mentally drain you to complete multiple sets.
• You can recover from and perform at a high intensity every time you’re in the gym.

On the other hand, don’t neglect the big compound lifts.

As a part of a structured training programme, compound movements are beneficial because they stimulate numerous muscle groups at once. Thus, one compound exercise could replace doing 2–3 isolation exercises targeting different muscles.

Concluding Thoughts

As we’ve discussed, the stimulus-to-fatigue ratio is the hypertrophic stimulus of an exercise versus how much fatigue it generates.

We’re not talking about feeling a little tired during a single workout after some heavy squats — we’re talking about ongoing systematic fatigue.

“Systemic residual fatigue carries on to overtraining, defined as a state of overstress or failure to adapt to an exercise load and/or to a drop in performance level. In extreme cases, when not compensated it leads to the overtraining syndrome.” — Zając et al., 2015

Fatigue culminates if we don’t rest enough to recover sufficiently. And if we’re fatigued, we can’t train hard enough to generate our desired results.

Exercises demanding elevated levels of both physical and mental effort are especially fatiguing.

I hate to keep bringing up my favourite exercise — the barbell squat, but people don’t often talk about how much mental exertion is required for these significant movements.

Making progress with strength training can be just as psychological as physical. That’s part of why I love the squat — progress is challenging. I must psych myself up and “dial in” to the movement before every set.

But, if our primary goal is to build muscle, that fatigue might not be worth it, and it’s more beneficial to substitute an alternative exercise.

Stacking your workout with high-fatigue exercises eventually becomes unsustainable. Progress stalls and then injuries become inevitable.

Before I sign off, I’ll go back to the premise of the article title, “don’t confuse fatigue for stimulus”.

We gym junkies can tend to believe we must do “more” to maximise our exercise results. Trust me — I’ve been there!

But more volume doesn’t mean more improvement.

Muscle soreness might feel like progress — but chances are it is fatigue. And fatigue doesn’t stimulate results. It might even have the opposite results… And who wants that?

Thank you for reading.

If you enjoyed the content, you might be interested in the below article about why overtraining could be negatively impacting on your exercise results.

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