It's a familiar sensation to just about everyone who has exercised strenuously, exerted themselves on a long run, or pushed themselves hard on a cycling trip: that feeling of your muscles giving out, of providing less power less often. It's the feeling of hitting "the wall" that so many athletes fear and yet know so well, the feeling that your muscles have given everything they could and you'd better find a place to collapse safely.
Could it be that this sensation is not an accurate reflection of the actual energy and ability left in the muscles but instead your brain deciding that enough is enough? Could a little inspiration, a little reward, or a little trick convince your brain and thereby your muscles that relief is around the corner and therefore squeeze a bit more out of those abused muscles?
English researchers have discovered that the brain may play far more of a role in muscle endurance than previously thought. In a study published in the February online edition of the Journal of Physiology, scientists had eight well-trained cyclists perform a rigorous, all-out trial on exercise bikes while being monitored by cardiovascular and power-output measuring equipment. During the ride, the subjects were allowed to swish various carbohydrate-rich liquids in their mouths but were not allowed to swallow. Control subjects were allowed to swish flavoured, sugar-free water in their mouths but were also not allowed to swallow.
Those cyclists who were given carbohydrate drinks to rinse with were able to finish the trial significantly faster than those who had been given the sugar-free water even though they did not swallow the liquids. The carbohydrate group also has higher higher rates and power outputs than the water group. Both groups, however, reported similar levels of effort required to complete the trial when asked to rate the difficulty of the ride.
The experiment also included the carrying out of a functional M.R.I. on subjects who had just swished a carbohydrate-rich drink. It found that the reward, motivation and emotion centres of the brains of these subjects were activated by the swishing of carbohydrate drink even though the subjects had not swallowed the liquid. Apparently, the brains of the riders had been fooled into believing they were going to get a dose of carbohydrates and therefore allowed the muscles to perform harder and faster.
How much of a role the brain plays in deciding the muscles' exertion level and in triggering fatigue is a contentious issue. It was thought by most researchers until recently that muscle failure (i.e. fatigue) was almost exclusively due to biochemical reactions in the muscles themselves: the lack of oxygen getting to them, the build up of too much calcium or lactic acid, etc. Muscle cramps were seen as having an almost entirely physical cause.
As Ross Tucker, a researcher for the Sports Science Institute of South Africa, points out, "We know that people speed up at the end of exercise." Tucker, who has done extensive studies on fatigue, adds, "If calcium or other biochemical changes in the muscles caused muscle failure, speeding up exercise would be impossible at the end, when these changes are at their greatest level."
Even though it is still a subject for debate, there is a large and growing number of physiologists who believe that exhaustion is a process that involves the brain, not just between the muscles and the brain but between various parts of the brain. "There's an interplay of central processing and muscular exertion," says Tucker, "the brain asks for and gets constant feedback from the muscles and other systems especially about body temperature." When someone exercise long and hard, parts of the brain can become "de-aroused" in a similar manner to depression, which obviously plays on the subject's motivation. The subject begins to question the reasons for pushing so hard and starts to slow down.
Although the mechanisms aren't fully understood, the brain keeps track of the fuel levels in the muscles and calibrates the body's core temperature and metabolism accordingly. If the fuel drops below a certain level and the temperature rises excessively, the brain senses that it is approaching a danger zone. It begins to reduce "the firing frequency of motor neurons to the exercising muscle, leading to a loss of force production,” noted Ed Chambers, one of the researchers at the School of Sport and Exercise Sciences of the University of Birmingham in England. Chambers was also one of the authors of the carbohydrate-drinks study.
If the brain can recognise that you're not going to damage it by pushing too hard, ignoring its fuel needs, or forgetting to provide enough hydration, then it's quite possible to push the limits of exertion beyond that which the body is accustomed to. This in essence is one of the major components of long-term physical training and conditioning: acclimating your mind to the new reality of physical performance.
