What is the Stimulus to Fatigue Ratio?
Stimulus to fatigue is a term popularized by Dr. Mike Israetel, a bodybuilder with a Ph.D. in Sport Physiology. It refers to the stimulative effect of an exercise that drives adaptations compared to the amount of fatigue it generates. The idea is that high stimulus with low fatigue is ideal for optimizing training progress while minimizing the risk of recovery issues.
A Deeper Look at Stimulus and Fatigue
Stimulus drives adaptations (strength gain, muscle growth, endurance, and others) and is crucial for long-term results. Fatigue is the physical stress we place on ourselves, which we must recover from to see positive results.
For example, when a person does a highly stimulative push workout, they generate local fatigue in the muscles they’ve trained (e.g., the chest, shoulders, and triceps) and general fatigue that affects other parts of the body and the central nervous system.
As the body overcomes that, it adapts to the specific demand to better handle it in the future.
So, the more stimulative a workout can be relative to the fatigue it generates, the more quickly trainees can recover and return to the gym with full force.
But here’s the thing. Fatigue is part of stimulus; the two don’t exist independently in a vacuum.
It’s impossible to have a stimulative workout without also generating fatigue. After all, fatigue signals the body that it needs to adapt to handle the same physical stress with less effort in the future.
This is what develops cardiovascular endurance, strength, and explosiveness, and leads to muscle gain.
Plus, as the body adapts to training and we become physically fit, fatigue goes down. The same amount and type of work is easier to recover from and no longer feels as challenging.
For instance, when someone does four sets of heavy deadlifts for the first time, they could call it a day because that would be some grueling work to handle and recover from.
However, talk to that same trainee after six to eight weeks of consistent deadlifting. That amount of work no longer feels as challenging because they’ve improved their work capacity and adapted to the specific stress imposed on their body.
So, How Does the Stimulus to Fatigue Ratio Make Sense?
The idea behind this ratio is not that we must avoid fatigue at all costs but that we must evaluate the fatigue every exercise generates and determine whether it’s worth it, given our training goals and situation.
Let’s take the low-bar back squat as an example. It’s a must-do exercise for powerlifters looking to compete and put up the biggest possible total on competition day. However, the movement also highly taxes the posterior chain and spine.
This fatigue undoubtedly leads to whole-body strength and muscle development, but that doesn’t mean every trainee must endure it.
For example, if I’m coaching a 40-something-year-old who doesn’t care about strength and is purely after muscle gain, I would prescribe less fatiguing exercises they can do to build up the lower body––leg presses, leg extensions, machine hack squats, and even Bulgarian split squats, to name a few.
The idea is to more efficiently separate the training stimulus we want from the fatigue we don’t need and reach our goals.
As a coach writing workout plans for clients and yourself, consider the fatigue cost of exercises and determine whether the stimulus provided is worth it while keeping your client’s goals, training preferences, injury history, and free time to train in mind.
Optimizing Stimulus to Fatigue For Your Clients
Any half-decent coach can put together a training plan that generates a ton of fatigue, but it takes a great trainer to build a plan that minimizes fatigue while maximizing the training stimulus. Here are some tips:
1. Pick Exercises Carefully
When picking movements for your clients’ workout plans, consider the systemic and local fatigue they generate versus the stimulus provided and whether that makes sense.
For example, if a client wants to build muscle, would the big three be the best movements? Perhaps substitutes would bring the same results without being as fatiguing.
Here are some options:
- Bench press ⇒ Machine chest press, push-up variations, and dips
- Back squat ⇒ Leg press, hack squat, and lunges
- Deadlift ⇒ Hamstring curls, glute kickbacks, hyperextensions, and lat pulldowns
Also, consider the time and effort it takes to set up for specific exercises, as that also plays a role in the fatigue part of the equation.
2. Pick the Correct Weights
As with most aspects of training, there is a Goldilocks zone when picking weights. Going heavy is good for strength development, but it’s also more challenging to recover from and takes longer to complete each workout.
Going too light might seem better, but it leads to many unnecessary reps that only generate fatigue without providing much stimulus (i.e., its junk volume).
Training in the 8-30 rep range is ideal for most people in most scenarios.
3. Start With Less Volume
High-volume training is the gold standard for muscle growth. However, many trainees, especially those with less experience, can see great results from a more moderate approach.
As a coach, start your clients on a more moderate volume (around ten weekly sets for the larger muscles and 5-6 for the smaller ones) and assess their performance and recovery. Add volume if clients feel fresh and recovered but aren’t progressing steadily.
4. Limit the Number of Sets Taken to Failure
Research comparing training to failure versus training close to failure doesn’t find a significant difference in strength or hypertrophy outcomes. However, data does find that taking sets to failure generates more fatigue.
Given the similar outcomes, it would make sense to choose the option that generates less fatigue. This means limiting the number of sets taken to failure and mostly training to an RPE of 7-8.
FAQ
1. What is the stimulus to fatigue ratio (SFR)?
It refers to a specific exercise’s stimulative effect to drive adaptations (e.g., muscle growth) versus the fatigue it generates.
2. Should you pay attention to the stimulus to fatigue ratio?
Paying attention to SFR as a coach or trainee is vital because it allows you to build more efficient workout plans that provide the same stimulus and drive the same progress without generating as much fatigue.
