It’s totally random. Play it safe and measure your fatigue.
The holy grail of coaching and exercise science is finding the perfect amount of training to optimize athletic performance.
At the physiological level, exercise training is a process that causes a disturbance in your body’s regular functions, which is restored and improved during recovery. As you chronically overload your body with exercise training, it will respond and adapt. But the relationship between training load and performance is a messy one.
Believe it or not, there is very little research on the influence of different training loads on performance. One of the few studies that’s examined this question found absolutely no relationship between training load and changes in performance. These findings further emphasize the complexities of athletic performance. There are just too many variables – some fitness related, some not; some quantifiable, some not – that determine performance.
That being said, the relationship between training load and performance has been modeled. Many scientists agree with the training impulse equation that at any given time, Performance = Fitness – Fatigue. However, time is really the main determinant of your fitness and fatigue.
Your “gains” in fatigue happen much faster than your improvements in fitness. That’s obvious: immediately after a training bout, you feel fatigued. It takes several hours, days, or weeks for your body to recover from and adapt to that training bout. At the same time, your loss of fatigue happens faster than your loss of fitness. This allows you to improve performance over time, as you reduce your fatigue and increase your fitness.
But as I’ve said before, humans are not math equations. The rates of change in your fitness affect the rates of change in your fatigue, and vice-versa. Those rates are relative to each other, and to each individual athlete.
So how do you balance your gains and losses of fatigue and fitness? My advice is always to find the simplest way possible. And that doesn’t involve fancy predictions.
If we took all the exercise science literature and broke the data down into fitness and fatigue, we would find that the gains and losses of fitness are largely predictable. But changes in fatigue are highly variable, and highly individualized.
In other words, fatigue is really the limiting factor in your performance, not the amount of training.
So perhaps we’ve been asking the wrong question about training for performance. Instead of “How much training do I need to improve performance?” we should ask, “How much fatigue will inhibit my performance?”
That means that in order to understand exactly how much training will improve your performance, it is absolutely critical that you measure your fatigue. And your perceived rating of exertion may be the best overall metric of your fatigue.
The rates of change in your training load should be dependent on your fatigue, not the other way around.