When I reach my tenth press up, I start to complain. I am fatigued. Kieran, my trainer, ignores my complaints and tells me to press on. Is he just being a motivational trainer or does he suspect that my fatigue is in my mind? We assume that when we burn physical or mental energy, we will get tired. Recent research suggests that fatigue may be a phantom of our mind not body.

How long does it take for fatigue to start?

A series of studies have looked at when fatigue starts and when it peaks. They have looked at self-reports, at the view of independent observers and even taken physiological measures.

The ballet position shown in the figure is called “Developpe a la seconde”. It involves holding the leg fully extended at ninety degrees or higher. It is technically very difficult. It requires many years of training to execute. It burns lots of energy and causes fatigue. Suppose that a professional dancer is asked to hold the position for 70 seconds. It does seem like a long time but even professional dancers with many years of training found it difficult. How long is it before they feel fatigue set in. How long before the feel that the fatigue has peaked. Suppose now we ask a second dancer to hold the position for 80 seconds, only a few seconds more. Holding the position requires the same amount of energy. We would expect them to start to feel fatigued after the same number of seconds. Their peak sensation would be at the same point if fatigue is about energy consumption.

A study of many dancers in a professional German company found that reality is very different. The first dancer feels the onset of fatigue about halfway through the 70 seconds. Their peak fatigue is at about two thirds the way through. The second dancer follows the same ratio. Onset starts at the same halfway point, but of the longer time.At around 40 secs rather than 35 secs. Fatigue is a mental state not a physical one. It depends on how long you have been asked to do something. Videos of dancers performing this experiment were shown to experts. They were asked when a dancer started to tire and when they looked at peak fatigue. The experts agreed with the dancers.

How tiring is your drive?

Researchers surveyed students driving back to college. They asked them at what point did they started to feel tired and at what point they felt the most fatigue. The results were the same. The fatigue did not depend upon the consumption of energy. Instead, it depended on the length of the journey. No matter how long the journey is, tiredness sets in about halfway through the anticipated journey time. Peak fatigue was at two thirds the way through the journey. The only logical explanation is that fatigue is a psychological not physical phenomenon. At least it is for these settings.

More work should tire you out sooner.

Another study varied the intensity of work. In this case they were looking at mental work. Respondents were give 200 numbers between 1 and 80. They were asked to mark any number that could be divided by 3. A simple task that requires concentration and computation. They were given fifteen minutes to complete the work. Again, they were asked about fatigue onset and peak. Their brains were connected to electrodes to assess activity.

There were two other groups. One was given 400 integers to assess and the third group 800. All three groups were given fifteen minutes for the job. Clearly the amount of mental energy required should have increased. There were more integers to be assessed in the fixed time. Using the “energy use causes fatigue” hypothesis the amount of time to get to the start and peak of fatigue should have gone down as the number of integers went up. In fact, all groups had the same results. Self reported fatigue was driven by the time not by the effort. Fatigue, measured by brain activity, started halfway through the time irrespective of the effort. It confirmed the self reported view.

The Power of Targets

In each of these studies the target regulates when we feel tired. What happens if there is no target? In this study male respondents were given a hand grip test. They squeezed a hand grip until it was fully closed. They were then asked to hold it for targeted times of 120s, 180s, and 240s. A fourth group were given no target. They were asked to hold the grip for “as long as possible”.

About a third of all respondents could not hold the grip closed for the targeted time. Interestingly they gave up 58% of the way through the allotted time. This was the same point at which those who completed the task said they had reached peak fatigue. For all those with a target the results followed the pattern of the other studies. Fatigue was driven by the target not effort.

Those who were given no target said that they started to tire earlier than any other group. They also said that they reached their perceived maximum tiredness much earlier than any group that had been given a target. An open-ended task gives the body no chance to regulate energy consumption. It seems that when Kieran tells me to do 15 squats, he is postponing my illusion of tiredness.

Ageing and Fatigue

One of the common complaints of ageing is “lack of energy”. People claim that they cannot do as much and need to rest more often. Their metabolism has slowed down. They need their afternoon nap (Newsletter 194 New Year Naps). At the same time there are 80- and 90-year-olds who seem to have boundless energy. Is this a metabolic effect or a psychological effect?

The implications for Kieran and all other trainers is clear. Set high targets. This should slow the onset of fatigue and reduce the client moaning. Of course, it will only work if the client has not read (or written) this Newsletter.