Yawning in the middle of the day is completely normal for people with hectic schedules. How do you know whether it’s a sign to grab a cup of coffee or a red flag of an underlying condition? Why do people with depression, multiple sclerosis, and other neurological conditions often feel exhausted throughout the day? Researchers believe they finally have the tools to answer these questions thanks to a newly developed method for measuring feelings of fatigue.
In a research paper published in Nature Communications, professor Vikram Chib, from the Johns Hopkins University School of Medicine and colleagues conducted a study to objectively quantify how tired people feel.
“We know the physiologic processes involved in fatigue, such as lactic acid build-up in muscles, but we know far less about how feelings of fatigue are processed in the brain and how our brain decides how much and what kind of effort to make to overcome fatigue,” said Chib.
After training to recognize an effort scale, a cohort of 20 adult participants was asked to hold and apply pressure to a sensor in repeated trials until they became fatigued. They then were given two choices: a coin flip which randomly assigned an effort level for the next round, or a set effort level. The uncertainty associated with the “risky” choice allowed the researchers to understand the decision-making process behind how much effort the subjects were willing to make.
“Unsurprisingly, we found that people tend to be more risk-averse—to avoid—effort,” said Chib, with most of the fatigued participants less willing to take the chance of having to squeeze the sensor with more effort than they could handle.
Chib and colleagues also used functional magnetic resonance imaging (fMRI) scanning to follow the path of blood flow through the brain during the exercise, showing which regions of the brain showed the highest activity during decision making whilst fatigued.
Interestingly, the motor cortex was found to be silenced at the moment when the participants had to make a choice regarding the effort level, meaning that fewer brain signals were being transmitted to the muscles. Their findings suggest that during fatigue, the body follows cues from the motor cortex to not exert excessive effort. If the brain didn’t send these signals to slow down, a buildup of lactic acid in the muscles would contribute to even more fatigue.
According to Chib, these findings could help in the creation of diagnostics and therapies for conditions associated with fatigue and may also help to boost performance levels in healthy individuals.