What is the fitness-fatigue model?

Chris Beardsley
Mar 15, 2018 · 6 min read

When we use strength training to help an athlete improve performance in a sporting movement, such as sprinting or vertical jumping, we can use the principle of specificity to determine which exercises, types of loading, and other factors will transfer best to that sporting movement.

This will tell us what to put into a workout.

Even so, this is not enough information to build a strength training program, since programs are made up of sequences of workouts, and the distribution of the workouts is just as important as the content.

Since the principle of specificity cannot help, we need (at least one) other framework, and the fitness-fatigue model is a useful one.


What is the fitness-fatigue model?

The traditional fitness-fatigue model starts with the observation that performance tends to reduce immediately after a workout, and stays reduced for up a few days in some cases. Yet, after the initial reduction, there is a rebound, and performance then improves.

The fitness-fatigue model explains this curve by proposing that it is the sum of two curves, one representing the fatigue effect, and the other representing the fitness improvement. Only once the fatigue effect has dissipated is it possible to see the fitness effect, even though fitness has actually been improving from immediately after the end of the workout.

Without the fitness-fatigue model, we could easily fall into the trap of believing that the fitness adaptations to a workout only occur after a couple of days, because of the reduction in performance. In fact, adaptations probably occur very soon after the workout itself.

For example, in strength-trained lifters, a large proportion of the adaptations in the central nervous system after a strength training workout occur on the same day as the workout. Also, muscle growth is stimulated by a transitory increase in the rate of muscle protein synthesis that is already falling before 24 hours have passed.


What is “fitness” in the traditional model?

In the traditional fitness-fatigue model, the improvement in fitness represents the underlying adaptations that lead to increases in performance in the long-term. The term “fitness” refers to the adaptations that permit improved performance, and not the actual change in performance itself.

While the model can be applied to any type of physical training, after strength training, the improvement in “fitness” refers to those positive adaptations that improve strength.

Strength gains occur when we experience changes in the central nervous system and inside the muscle-tendon unit that improve our ability to produce force. Although increases in voluntary activation and muscle size are the most widely discussed, there are actually dozens of changes that occur that can affect force production, and their role differs based on the way in which force is expressed.

Exactly *which* changes occur in the central nervous system and in the muscle-tendon unit depends on the type of workout, and this variability is what underpins the principle of specificity in strength training.


What is “fitness” in an updated model?

The traditional fitness-fatigue model does not take into account newer research showing that strength can be positively affected by transient changes in the ability to produce force, as well as by long-lasting adaptations.

Long-lasting changes in strength involve adaptations inside the central nervous system or muscle-tendon unit, such as increases in voluntary activation, or increases in muscle size.

Transient improvements in the ability to produce force occur because of potentiation, which may also involve changes in either the central nervous system or inside the muscle.

Potentiation has most often been observed to occur immediately after a strength training workout, where it is referred to as the “post-activation potentiation” effect. Even so, potentiation can also be recorded several hours or even days after a workout. It is typically only seen when fatigue is minimal, such as after training with light loads and fast bar speeds.

Thus, in an updated fitness-fatigue model, we should refer to multiple fitness effects, some of which reflect long-lasting adaptations, and some of which reflect short-term potentiation.


What is “fatigue” ?

In the traditional fitness-fatigue model, the temporary increase in fatigue that causes transitory reductions in performance was not well-defined, because of a lack of research in the area. Yet, the fatigue and recovery literature has developed substantially since the fitness-fatigue model was first suggested.

Research into recovery has shown that there are three factors that cause transient reductions in strength after a workout: (1) peripheral fatigue, (2) central fatigue, and (3) muscle damage.

Peripheral fatigue after a strength training workout is largely caused by the accumulation of metabolites, and its effects are dissipated within hours. Similarly, central fatigue is extremely transitory, and is rarely seen beyond an hour after a strength training workout (except when it occurs subsequent to severe muscle damage).

Muscle damage can produce reductions in strength that last for up to weeks in some cases, where the damage to the muscle is severe. More commonly, however, the reductions in strength because of muscle damage last only a couple of days.

Thus, in an updated fitness-fatigue model, we should refer to multiple fatigue effects. Also, we should be clear that peripheral and central fatigue usually reflect effects lasting only a few hours, while muscle damage reflects a longer-lasting effect, and is likely the primary determinant of losses in strength on the day or days after a hard workout.


What does this mean for strength training?

When analyzing strength training, the most important lesson we can learn from the traditional fitness-fatigue model is that strength may be temporarily reduced through fatigue, even though the underlying adaptations that allow us to produce force are being improved, or have already improved.

The updated fitness-fatigue model initially adds another level of complexity. Fortunately, this complexity quickly falls away in practice.


#1. Types of fatigue

While there are multiple types of fatigue (peripheral fatigue, central fatigue, and muscle damage), only muscle damage can contribute to reductions in strength on the days after a workout.

Workouts that have the potential to cause a high level of muscle damage should be programmed in the knowledge that strength will be suppressed for a few days afterwards, and knowing that loading the muscle too soon after this training session may cause further damage, as muscle damage is almost certainly cumulative. (Excessive muscle damage causes muscle loss, and may increase the risk of strain injury).

In practice, doing another workout for the same muscle group before strength has (mostly) recovered is not recommended, since it means loading the muscle before it has repaired the muscle damage from the preceding training session, which risks accumulating more damage.


#2. Types of fitness

While there are multiple types of fitness (short-term potentiation and long-lasting adaptation), only high-velocity strength training or very low volume heavy strength training are likely to produce a short-term potentiation effect without also producing fatigue that stops that short-term potentiation effect from translating into improved performance.

In practice, it makes sense to do workouts that have the ability to cause a potentiating effect on days before competitions or before higher volume workouts, in the knowledge that little muscle damage has occurred and strength will actually be transiently improved.


What about the general adaptation syndrome?

The general adaptation syndrome (GAS) is another common framework for understanding the effects of strength training workouts. It is based on the work of the stress researcher, Hans Selye.

Selye identified that when animals experienced a stressor, they displayed the same set of physiological responses, including activation of the hypothalamic-pituitary-adrenal (HPA) axis, which we often measure by reference to cortisol levels.

The GAS is useful for analyzing the effects of a high-volume strength training program that involves excessive muscle damage without sufficient recovery and thereby becomes a stressor, and risks taking an athlete to the point of overtraining. It is therefore helpful for modeling functional and non-functional overreaching, and overtraining.

Yet, the GAS is not directly applicable to lower volume strength training programs under normal circumstances, since neither heavy strength training nor power training workouts elevate cortisol levels, suggesting that the body does not perceive them to be a stressor in the strict sense.


What is the takeaway?

The fitness-fatigue model provides a framework for understanding the immediate effects of a strength training workout, which can help us build a sequence of workouts into a strength training program.

While the traditional fitness-fatigue model only refers to single fitness and fatigue effects, we can use modern research to update this to refer to multiple fitness (short-term potentiation and long-lasting adaptation) and fatigue (peripheral fatigue, central fatigue, and muscle damage) effects.

The updated fitness-fatigue model underscores the importance of making sure that athletes have recovered their strength after the previous workout, before loading the same muscle group again, and also reveals the value of using low-volume, potentiating training sessions to increase performance either in competition or in later workouts.

Chris Beardsley

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