Fitness-Fatigue Model: implications for flexible program design

This was originally posted as part of the series “Pliable Programminig” on the De Novo Elements platform.


  • Performance is the difference between strength capacity and accrued fatigue at any given time.
  • There is a large degree of volatility that can occur on both sides of the performance “equation”, especially regarding the magnitude/source of stress inputs and associated fatigue cost.
  • Training strategies should aim to improve fitness capacity while mitigating fatigue.
  • There are many programming modalities that can accomplish the above criteria.

Strength athletes love to argue over the internet. Sometimes I wonder if there are similar underworlds for other hobbies where the participants hold such adamant positions. Are there bowling arguments on Facebook? Surely there are….right??

While at times it’s unclear who is seeking truth vs validation, one thing is certain: we are ALL motivated to get stronger. As motivated athletes, we like feeling as if we are in full control of our destiny when it comes to our performance in the gym or on the platform.  However, it’s not uncommon for well-intentioned trainees to view their training and progress in a vacuum and perhaps place too much stock in subtle program manipulations. We envision ourselves as the mad chemist formulating the ultimate solution for progress. Unfortunately, biology isn’t entirely predictable.

This article serves as an introduction to a multi part series. The ultimate goal of this series is to illustrate that there are a number of modalities to increasing strength, and being steadfast or married to one single approach/methodology may be showing some ignorance to the overall landscape impacting its expression. If we are stronger by the end of it all, the strategy worked.  Is it optimal? There is no way of knowing, but we have clues…

The foundation of this discussion is built upon the Fitness and Fatigue Model. In this context, think of fitness as our maximum ceiling for strength capacity at any given time (this is different from capability). Long term progressive overload raises this ceiling over time. Fatigue is the recovery cost associated with any effective training stimulus and external stressors. What we observe as the expression of performance/capability in sport is based on relationship between the two 1.


Strength Capacity – Accrued Fatigue = DISPLAYED PERFORMANCE/CAPABILITY


While the equation and graph above may help conceptualize this model, many people are simply looking for quantified facts with regards to organizing their training. Unfortunately, few facts regarding optimization exist with the wide degree of volatility on both sides of the “equation”.

The energy balance equation (Intake-Expenditure) serves as a somewhat parallel model to help explain the point I’m trying to make. While its feasible to manage intake down to the calorie, the expenditure side of the equation can be susceptible to a wide range of variability (the details of which extend beyond the scope of this article). Outside of the planned expenditure through training/cardio, our total daily energy expenditure has an intimate relationship to our actual intake as well as the environment in which we live our day to day lives.

Our performance holding a barbell is similar in many ways. We have some significant control over the stress input side of the “equation” with regards to imposed TRAINING stress. We do not, however, have full control over the magnitude of external stress inputs. Relationships, work, parenthood, etc. can all impose additional layers of stress. Therefore, the fatigue side of the performance equation is largely dependent on factors which are outside of our control and NOT directly related to our neatly packed “fitness inputs”. We must cope with external stressors that manifest as accrued fatigue and compound the fatigue already present from training. Furthermore, fatigue can’t be quantified to begin with, as there is no unit for fatigue like there is for energy intake and expenditure. Therefore, the waters are a bit murkier when it comes to optimizing and predicting performance. A great performance in the gym or on the platform has a degree of “best guess” as to the influence of each impactful variable.

So where does that leave us? Similar to energy balance, we control what we can while respecting the fact that micromanaging training inputs can sometimes be an exercise in futility. Any push towards optimization NEEDS to be pliable and work with the unknowns.

That being said, the importance of well thought out, empirically driven programming strategies should not be downplayed. Programming is one of the few variables we do have full control over and is extremely important. It’s simply a reminder to the reader that there are a lot of moving parts when it comes to both acute and downstream strength performance. Isolating specific training variables as the heroes or villains for the failure or success of a strategy can be careless.

For example, if your bench press sees a robust increase across a six week period, it may not be that extra 1500 lbs of volume per week done at 81.67%, or the implementation of floor press in place of close grip as an accessory. It COULD mostly be a result of your newborn sleeping through the night. Hell, it’s even possible (but most certainly impossible to quantify) that your programming adjustments were hindering you with regards to increasing fitness capacity.  Net fatigue may have just decreased to a greater degree than your capacity for strength. The net result would still be a performance improvement. Kind of weird to think about, huh?

In a quest for optimal, what is optimal when it comes to formulating a strategy to express maximal strength?

Rather than looking for the optimal program, let’s instead aim to adopt an optimal framework for thinking. We want robust training effects while mitigating fatigue cost in and outside of training. When we think along those lines, you can see how many different strategies/methods can have significant utility when taking everything into context.

The future segments of this series will go into more detail by first breaking down some of the primary variables that contribute to both fitness and fatigue and then discussing actual strategies for effectively managing both sides based on what we can control. In the meantime, keep an open mind!



  1. Chiu, L. Z. F. & Barnes, J. L. The Fitness-Fatigue Model Revisited: Implications for Planning Short-and Long-Term Training. Natl. Strength Cond. Assoc. 25, 42–51 (2003).
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