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Marcora, SM., Staiano, W., Manning, V. Journal of Applied Physiology.2009; 106: 857-864
Background
The effects of mental fatigue on mental tasks and skilled performance have been well investigated but the effects of mental fatigue on physical performance are still largely unknown.
Methodology
Sixteen subjects (10 male, 6 female) were selected for the study. The average age of the subjects was 26 years and all were involved in regular aerobic exercise. Each subject undertook 3 trials, using a single-blind, randomized design.
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‘Blind' Study Designs
When an aspect of a study is described as blind, it means that certain information is withheld from the subject/researcher in an attempt to avoid a placebo effect or bias. This study is referred to as single-blind, i.e. the participants are not told about the aim or hypotheses of the study. Below you will read that encouragement was given by a ‘blind' researcher - to stop the researcher influencing the participants via their interaction.
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First trial was an incremental test to determine VO2peak and peak power output (PPO) using an indoor cycle. Subjects then performed two randomized trials, consisting of an experimental treatment followed by a time to exhaustion trial at 80% PPO (in other words roughly 10mile TT pace). The two experimental treatments were 90 minutes of the AX-continuous performance test (AX-CPT) or 90 minutes of watching emotionally neutral documentaries (Control). The AX-CPT is a computer program that displays different letters of the alphabet at 4 second intervals. Users are required to press a response key every time an A or an X flashes up and subjects are scored on the number of correct responses. Although simple, prolonged exposure has been shown to induce high levels of mental fatigue. Heart rate was monitored throughout the treatments and the Brunel Mood Scale (Brums) questionnaire was administered pre and post treatment to assess for mental fatigue and vigour in particular. During the time to exhaustion trials rating of perceived exertion (RPE) was assessed using a 15 point Borg scale and blood lactate was monitored at 5 minute intervals, and oxygen uptake and minute ventilation were measured breath by breath.
To try and limit the effect of subject motivation a £50 prize was offered for the best performance in the AX-PCT trial and also for the best performance in the cycling trial after each treatment. Verbal encouragement was given by a research assistant blind to the treatment previously given.
Results
Average heart rate during the AX-CPT treatment was 3bpm higher than during the Control treatment and the Brums questionnaire showed that mental fatigue was also higher. The percentage of correct responses during the AX-PCT trial also decreased during the treatment so it is clear that the AX-PCT treatment had the desired effect in inducing mental fatigue compared to the Control.
Mean time to exhaustion following the AX-PCT was 640s compared to 754s after the Control treatment - a decrease of 114s. Individual time to exhaustion was shorter following the AX-PCT treatment in 13 out of 16 subjects. There was no significant effect on the order that the two treatments were administered. Average cadence during the two trials was not significantly different but blood lactate and HR were higher at the end of the Control trial. This was probably due to the longer exercise duration. None of the physiological variables measured were significantly different at the corresponding time between each trial. RPE increased to maximum in both trials but in the mentally fatigued condition RPE was consistently higher and so subjects reached their own ‘limit' in a shorter time, at which point they terminated the trial.
What does this mean?
Fatigue is a hot topic in the sport science community at the moment. Tapering training in the lead up to a competition by reducing volume is a widely used and accepted method of shedding physical fatigue. It is hard to imagine an athlete choosing to race in a physically fatigued state but this study is one of the first to provide evidence of the detrimental effect of mental fatigue on physical performance. The key to understanding the decreased cycling performance seems to hinge on the subjects RPE. Ina mentally fatigued state the RPE was consistently higher and so the subjects reached their ‘limit' in a shorter time. In other words exercising/racing at a given power output will feel much more difficult when mentally fatigued. A 114s or 15% decrease in performance cannot be ignored (although this study uses non-cyclists - the time gaps may be different in a trained population), so this points to the importance or making mental preparation part of your pre-race routine. |
