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 muscular contractions. After this injection, the volunteers reported considerable soreness in their thumbs, as if the muscles had been completing a gruelling workout.
What the studys findings indicate, said Alan R Light, a professor at the University of Utah and senior author of the study, is that the feeling of fatigue in our muscles during exercise probably begins when these substances start to build up. These feelings bear only a slight relationship to the remaining fuel and energy in our muscles. Each subsequent increase in the levels amplifies the sense of fatigue, Dr Light said, until the substances become so concentrated that they apparently activate a different set of neurons, related to feelings of pain. At that point, the exercise starts to hurt and most of us sensibly will quit.
Of course, improvements in physical performance sometimes demand that we continue through fatigue and on to achiness. There is some truth to the adage about no pain, no gain, Dr Light said. But disregarding all the signals from your muscles can be misguided, he said.
In recent experiments at his lab, cyclists who were given mild opiates that block the flow of nerve messages from the muscles to the brain and vice versa could ride faster than they ever had before, with a sense of unfettered physical ease - until, without warning, their leg muscles buckled and, limp and nearly paralyzed, they had to be helped from their bikes. Ignoring fatigue and pain is not a good, long-term competitive strategy, Dr Light said.
- GRETCHEN REYNOLDS