A Stress Fatigue Model

Beat yourself into a puddle of sweat on the ground, if you’re not training hard, you’re not training. Well, I’m older so I should do low-impact things like water aerobics or maybe yoga. No, lightweight high reps, that’s the way, right? I heard high reps are how you get big though and I don’t want to get bulky. If you want to get bulky you should do drop sets. You don’t need drop sets you just need to have a good finisher.

Finisher? Are we training or are we playing Mortal Combat…

Do these statements about the right way to train hold any value at all? They are all just as likely to cause the desired effect as they are not to. It depends on a host of variables, like how someone would define any of those things, how they would go about applying them, what else they are doing, what they want to achieve, and what resources they have to do that, which itself is made up of a bunch of variables.

So, is any of that a useful way of thinking about training stress? The short answer, no. The longer answer is below.

Two things are required to drive progress in training, they can appear in many variations, but they must appear. They are intensity and volume. These are our stresses, and they will come at a fatigue cost. The balance of intensity and volume can make training productive, a frustrating slog, or less likely to occur than our lizard overlords telling us that the world is indeed flat. Let’s take a quick look at each.

Intensity

Intensity refers to load, stress or stimulus that is applied to us. Moving weight plates with a barbell is applying a stimulus. Holding them in place is a stimulus. Lifting a stone is a stimulus. So is running, walking, sitting on or standing up from a chair. Everything is a certain amount of intensity to everyone and, as such, can be used as a training stimulus. The control of a given training stimulus typically comes from subjective ratings such as rate of perceived exertion, or RPE [1]. This is what we refer to as autoregulation. Autoregulation is a method of load management used with the intent of applying an appropriate stimulus for a given individual on a given day. Our goal with training is to apply an appropriate stimulus to drive adaptation. Not so little that we don’t encourage change, and not so much that we outstrip our resources to do so.

Volume

Volume refers to the total sum of reps and sets. So, 2 sets of 8 reps is a total volume of 16 reps. Volume can be calculated by whatever metric you choose. For example, a 10-minute walk would be a total volume of 10 minutes. Four lots of 2 minutes each is a total volume of 8 minutes. Again, applying an appropriate amount of volume to a given individual to drive adaptation is our goal. Not so little that we don’t encourage change, and not so much that we outstrip our resources to do so.

Stress Fatigue Model

Now let’s put these two things together and look at fatigue cost. Let’s say we have a 100-metre track, and we decide to run the length of it. Running the 100-metres is a given amount of intensity to a given individual. So, at the 100-metre mark, you will have a feeling of what it was like to run that distance. You have applied a stimulus, and that stimulus came at a fatigue cost[2]. You may feel that was easy, a low fatigue cost. Or you may get to the 100-metre mark and rate that as more challenging, with a higher fatigue cost.

Now let’s add some volume. We will run the 100-metre track twice, and let’s say we want each attempt to feel roughly about the same. We go on our run, cross the 100-metre mark, and again have a feeling of how challenging that was to complete. You may decide that wasn’t too hard and think you could turn around and do it again, straight away, with the same pace, and have a similar outcome. Alternatively, you may cross the 100-metre mark and think that you need to take a break, or ‘rest period’ before repeating the run, at the same pace, to have a similar outcome. In our thought experiment, we have now considered how challenging the stimulus was and made a choice based on that feeling. We have used a subjective rating to regulate the stimulus we are applying; we have used autoregulation.

Now let’s say we run our 100-metre track multiple times. At the end of each lap, we would have our feeling of how challenging it was, we will have accrued a fatigue cost for each stimulus we apply. In our second scenario, we took a break long enough to repeat the action with a similar outcome. Well, what if we don’t do that? What if we made our rest periods shorter, short enough that each stimulus we applied felt noticeably harder than the last? Eventually, we would run out of resources to keep going. How else could we do that? We could keep the rest periods the same as in the second scenario but run faster. Well, now we would achieve the same outcome of not being able to continue as we would again outstrip our resources. What if we kept the rest periods the same, and our pace the same, but extend the track to 150 metres? Once again, we find ourselves unable to continue. We would have to adjust the other variables, i.e., the stimulus we are applying, by choosing a slower pace or longer rest periods to allow for the fatigue cost. If the task we set ourselves was too difficult to continue we could adjust our variables to make it achievable. Equally, if the stimulus was not challenging enough, we could again adjust our variables to make it more challenging. So, we are adjusting the stimulus we apply, thereby managing the fatigue cost, to make exercise sustainable. Sustainable, it’s the keyword. There is no one training session that will gain all the extra muscle or put all the extra weight on your lift. It will take a series of training sessions done over time. We can use our concept of adjusting variables to make something sustainable and apply those over days and weeks. This can be done in multiple ways, adjusting intensities, volume, frequency, exercise selection, and formulation of training sessions (how the exercises are ordered in a day and between days). In the above scenario we aimed to have the same output over the working sets, this is what we would refer to as sets across. That is just one method of dosing stress, it is however a useful one to help us to grasp this concept of managing fatigue.

So, what’s the practical takeaway here? If you are finding yourself too beaten up to train consistently, adjust the variables. Instead of looking for that trick or tip that will push you through something that is currently out of reach, adjust the variables to make the unachievable achievable. The more compound the movement (multi-joint), the higher the intensity (closer to failure), the more novel (different), the higher the fatigue cost will likely be. That by no means makes those things bad but it does mean they need to be taken into account. Equally, not enough stimulus can be just as likely to stifle progress as too much.

Balancing stress with fatigue to create sustained training, like most things, is nuanced, and can also involve variables outside of training like diet and life stressors. So, if you would like to discuss this in greater detail feel free to make a time to come in and talk about your training.

1. Zourdos, M.C., RPE and RIR: The Complete Guide. MASS, 2019. 3(9).

2. Vieira, J.G., et al., Effects of Resistance Training to Muscle Failure on Acute Fatigue: A Systematic Review and Meta-Analysis, in SpringerLink. 2021.