The Fitness-Fatigue model: the relationship between fitness adaption, fatigue and the impact on your sports performance 

Even the most advanced training programs are based around simple models. 

You work out. You recover. You get fitter. Easy, right?

The Fitness-Fatigue model (or ‘dual factor theory’) explains the relationship between fitness adaption, fatigue and how they effect sports performance.

Helping coaches plan and execute specific and focused training. Encouraging athletes to develop the right skills at the right time.  

Read on to learn more about the science behind the fitness-fatigue model and the effect it has on successful (and unsuccessful) training.

Key points:

  • Several models of stress physiology have sought to explain the dynamic effects of exercise performance. Including the general adaptation syndrome and the stimulus-fatigue-recovery-adaptation model
  • The fitness-fatigue model (also known as the ‘dual factor theory’ or ‘impulse-response’ model) was originally developed by human performance researcher Eric Banister. Building on established principles of previous theories
  • Adaptation to training begins almost immediately after a workout but is outweighed by the fatigue effect. It is only when this fatigue dissipates that improved performance can be seen
  • Cumulative training can lead to greater fatigue, but also a larger supercompensation effect. This is referred to as ‘functional overreaching’

A history of stress and sports performance

Human performance has been ingrained in our culture for as far back as history books go.

Ancient philosophical thinkers such as Galen were extoling the virtues of ‘motion and rest’ almost 2000 years ago. In his treatise Preservation of health, Galen talks about using logic and reasoning to improve strength and speed work, by sequencing the order of exercise.   

Similarly, Philostratus proposed a system of exercise that helped the athlete manage the most grueling physical demands - and ‘peak’ for athletic events.

Of course, sport science has moved on since then. But their fundamental ideas still drive what we think about fitness, performance and stress today.

More recently, coaches from Soviet Russia developed a system of ‘periodization’. A systematic training regime targeting specific events, designed to make USSR athletes peak at the Olympic Games.

Periodization and the ‘general adaptation syndrome’

These groundbreaking Soviet training models training were underpinned by science - in particular the general adaptation syndrome (GAS). This model provided the biological basis for early periodized programming.

GAS was developed by scientist Hans Selye back in the 1950s and is one of the foundational theories of stress physiology. The story goes that it was identified almost by accident whilst Selye was experimenting on rats. But the resulting model now helps us understand the human response to a ‘stressor’.

The GAS model explains that no matter what the stressor is, the body reacts in the same physiological manner - based around three distinct stages:

  • Alarm phase: This is the initial reaction to a stressor prompting the ‘fight or flight’ mechanism. Typically, your adrenal glands throw out epinephrine into your bloodstream. This will speed up your breathing and heart rate, pumping more blood into your muscles – so you’re ready to fight off the stressor or run away from it.
  • Resistance: Once the alarm phase has kicked in, you should (in theory) have escaped the stressor - either by fighting it off or running away from it. At this point your body will restock energy levels, return to homeostatic resting point and adapt. But if the stressor continues, more and more energy is broken down and the homeostatic disturbance occurs over an extended period. Eventually, your body will either adapt to the stressor or begin to fight back.
  • Exhaustion: If the original stressor becomes chronic and unresolved, your body will push back. Struggling with a physical or psychological bout of stress for too long leads to ‘exhaustion’. And while the term is a bit of a misnomer, the results are fatigue, burnout and anxiety. In the context of exercise, this can lead to overreaching and overtraining.

In Selye’s mind, this model explained the body’s reaction to stress, no matter what the cause. So regardless of whether the stressor was the threat of being caught by an axe-wielding maniac or worrying about paying your next mortgage bill on time, the body would react the same.

Although it’s outside the scope of this article, evolution teaches us that the ‘fight or flight’ response was developed to deal with short-term physiological stress in life threatening situations.

But over time, humans have developed a way of ‘stressing about stress’. That is, things that might never happen – like a job interview going wrong, or messing up a keynote speech, or other unforeseen mishaps.

An exercise-specific model: The stimulus-fatigue-recovery-adaptation theory

GAS is a great theory and model of how the body triggers its ‘fight or flight’ mechanisms to help you deal with stress. However, GAS was never meant to be a model to explain exercise. But a theory around the generalized response to stress, whether physical and psychological.

So while there are several generalized responses to exercise stress, the GAS model doesn’t explain specific adaptations based on different exercise stimuli. Or how fatigue and adaptations to exercise work for and against each other.

But it’s okay, because The stimulus-fatigue-recovery-adaption (SFRA) model does.

The SFRA model of stress physiology shows how after putting your body through a tough workout, the initial response is a decline in performance due to fatigue.

If you’ve ever attempted a 1-RM test after a hard session on the weights, you’ll understand this completely.

And while your body begins to initiate the adaptive response to the workouts pretty much right away, performance is masked by fatigue in the hours – or perhaps days* - afterwards.

*Note: The duration and magnitude of fatigue is correlated to the intensity and duration of the workout. Brutal training sessions will result in a longer period of recovery and restoration.

As fatigue begins to wear off and fitness adaptations are ‘realized’ – as the Soviet periodization partisans used to say - performance increases. If at that point, you train again, the same process occurs until you notice significant gains in both fitness and performance.

BUT… if you don’t train again for a period of time, those improvements will soon disappear. And ‘involution’ or detraining can occur.

If you perform another workout before fatigue has fully disappeared, improvement accumulates. And subsequently take even longer to dissipate.

This is no bad thing. Research shows that short-term periods of intensified training - known as functional overreaching – can result in significant performance improvements after a deloading (reduced training) period.

However, when used incorrectly this can also result in ‘non-functional overreaching’ or overtraining.  

And of course, the SFRA is an important model when planning a successful periodization plan.

What Is The Fitness-Fatigue Model?

Okay, we’ve dipped our toe in the history of periodization. Now let’s dive into a more modern interpretation of stimulus-recovery and adaptation. It’s time to look in more detail at the fitness-fatigue model.

Originally proposed by Eric Banister in the early 1980s, the fitness-fatigue model set out to explain the interaction between fitness and fatigue using a ‘two-factor’ theory.

Previously, a more ‘single factor’ approach focused purely on what happened to fitness. But Banister showed that after a workout, fatigue will cause an initial reduction in performance. Then over time a super-compensatory effect will result in an improvement in performance.  

The interaction of fitness and fatigue is what this model is built on.

  • Fitness: Multiple, specific adaptations caused by the stimulus of exercise - some of which last longer than others
  • Fatigue: Transitory reductions in performance caused by homeostatic disturbance

Fatigue is multi-dimensional and - much like specific fitness adaptation - is specific to the exercise.

For example, some activities result in central nervous system fatigue. Others result in a more peripheral fatigue. And others will cause greater amounts of muscle damage.

Each will affect the magnitude and time-length of fatigue. And recovery, of course.

The fitness effect + the fatigue effect = performance

Similar to the SFRA approach, the fitness-fatigue model clearly shows that fitness doesn’t take weeks to develop.

Research shows that lifting heavy weights leads to central nervous system adaptations in the hours following a workout. And that bodybuilding results in increased muscle protein synthesis in less than 24 hours after high-intensity training.

Chui et al, circa 2003:

The fitness after-effect is a positive physiological response, whereas the fatigue after-effect is a negative physiological response. The interaction between these two after-effects results in the change in performance following the stimulus

The fitness-fatigue model helps us understand the specific processes that occur after training, and the time-schedule (and route) to performance gain. It also helps coaches and athletes plan specific training to maximize ‘preparedness’ too.

This will help optimize performance via the cumulative effect of periodized training. 

There are several factors that can influence the gradient and time-course of the fitness-fatigue model. For example, weight training at low volume will result in minimal fatigue - that will return to baseline within hours. Whereas weight training at high-volume can take days to recover from.

But the biggest factor of fatigue and performance is multiple vs. single bouts of exercise. As you can see from the diagram above, cumulative fatigue takes much longer to recover from - but also leads to a larger increase in fitness. 

This justifies systematic, periodized training. Where workouts are intended to invoke a significant stimulus - resulting in a sharp drop in performance during exercise, but a measurable improvement upon completion.

Takeaway…

Since the very earliest research into human performance, physiologists have sought to define the relationship between stress and adaption.

Banister’s fitness-fatigue model illustrates the immediate post-training effects of fatigue and how recovery helps to realize improved performance. This in turn helps the creation and management of periodization planning. For targeted and optimized performance.

Ultimately, hard training is key to performing at your best. But quality recovery is crucial to making that happen.