Chapter 1: The Nature of Fatigue

Experiencing midday drowsiness is a common occurrence for those juggling busy schedules. However, distinguishing between a mere need for caffeine and a potential warning sign of a deeper issue can be challenging. Why do individuals with conditions such as depression or multiple sclerosis frequently experience persistent tiredness? Recent advancements in research may provide the answers, as scientists have developed innovative methods to quantify feelings of fatigue.

In a notable study published in Nature Communications, Professor Vikram Chib from the Johns Hopkins University School of Medicine, along with his team, aimed to objectively measure how fatigued individuals feel.

“We understand some physiological factors that contribute to fatigue, like the accumulation of lactic acid in muscles, but our knowledge about how fatigue is experienced in the brain remains limited,” Chib explained.

The researchers trained a group of 20 adult participants to identify various levels of effort. These individuals were then asked to exert pressure on a sensor in repeated trials until fatigue set in. Following this, they faced two options: a coin flip that randomly determined their effort level for the next trial, or a pre-defined effort level. The element of uncertainty in the "risky" choice offered insights into the decision-making processes regarding effort exertion.

“Not surprisingly, our findings indicated that individuals tend to be more risk-averse when it comes to avoiding effort,” Chib noted, as most fatigued participants were hesitant to risk squeezing the sensor with an effort beyond their capacity.

Additionally, the team employed functional magnetic resonance imaging (fMRI) to monitor cerebral blood flow during the exercise, identifying which brain regions were most active during decision-making under fatigue.

Interestingly, the motor cortex was observed to be less active at the moment participants had to decide on their effort level, suggesting that fewer signals were being sent to the muscles. This indicates that when fatigued, the brain signals the body to limit excessive effort. Without these signals, the accumulation of lactic acid could exacerbate fatigue.

Chib suggests that these findings may pave the way for new diagnostic tools and therapeutic approaches for fatigue-related conditions, while also enhancing performance in healthy individuals.

Sources: Nature Communications, Johns Hopkins University.