Exercise makes us tired. A new study helps to elucidate why, and also suggests that while it is possible to push through fatigue to reach new levels of physical performance, it is not necessarily wise. Scientists have long been puzzled about just how muscles know that they’re about to run out of steam and need to convey that message to the brain, which has the job of telling the body that now would be a good time to drop off the pace and seek out a bench.
So, a few years ago, scientists at the University of Utah in Salt Lake City began studying nerve cells isolated from mouse muscle tissue. Other research had established that contracting muscles release a number of substances, including lactate, certain acids and adenosine triphosphate, or ATP, a chemical involved in the creation of energy. The levels of each of those substances were shown to rise substantially when muscles were working hard.
To determine whether and how these substances contributed to muscular fatigue, the Utah scientists began adding the substances one at a time to the isolated mouse nerve cells. Nothing happened when the scientists added the substances individually. But when they exposed the cells to a combination of all three substances, many of the nerve cells responded.
Since rodent nerve cells are not people, however, the scientists next decided to repeat and expand the experiment in humans. For a study published in February in Experimental Physiology, they recruited the thumbs of 10 adult men and women.
So, asking each volunteer not to move his or her hand, the researchers injected lactate, ATP or the various acids just beneath the tissue covering one of the muscles in the thumb. After the discomfort from the injection had faded, they asked the volunteers if they felt anything. None did.
They then injected volunteers’ thumbs with the three substances combined and at a level comparable to the amounts produced naturally during moderate exercise. After a few minutes, the volunteers began to report sensations similar to fatigue, describing their thumbs as feeling heavy, tired, puffy, swollen and, in one case, “effervescent”.
In a subsequent injection, the researchers increased the amount of the combined substances until they approximated those produced during strenuous exercise. The volunteers reported intensified sensations of muscular fatigue and also some glimmerings of aching and pain.
Finally, the researchers upped the levels until they were similar to what is seen during exhausting