From jog to slog - why you get so tired it hurts

Thursday, 02 June 2011 07:37

From jog to slog - why you get so tired it hurts

Gina Kolata
May 26, 2011

Feel the burn ... there’s a fine line between feeling fatigue and pain.
Do we run because we actually like the pain?

Avid runner and sports columnist Peter Sagal thinks so.

''The pain is sort of the point,'' he says. ''It's good to push yourself to the point of breaking … the pain isn't an obstacle to achievement so much as part of the achievement. We want to suffer.''

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Others beg to differ. We don't want to suffer. We run, among other reasons, for the euphoria and for the challenge. We run in competitions to see how well we can do.

Scientists have addressed the question of pain in exercise with new research that explains why one person's idea of pain is not the same as another's. The problem, it seems, is that we have only one word, ''pain'', for something that should have many descriptors.

What actually stops or slows most people during exercise is fatigue, not pain, Markus Amann, a muscle researcher at the University of Utah and the Salt Lake City Veterans Affairs Medical Centre, has found.

This is regulated by a group of nerve fibres, the so-called ergoreceptors, which respond to a mix of metabolites - calcium ions, lactate, hydrogen ions - released by muscles during exercise. In response, the brain ''decides'' to slow down, Amann says.

But, he says, evidence suggests the brain makes sure to signal us to slow down or drastically limit the exercise when the muscles still have some reserve.

In extreme circumstances, the signal to slow down can be overridden, allowing an athlete to tap into that muscle reserve and put on a burst of speed at the end of a race, only to collapse when they cross the finish line.

A second set of nerve fibres, the nociceptive nerves, signal deep muscle pain. Amann's colleague, Alan Light, reports that the nociceptive nerves look exactly like ergoreceptive nerves but respond to much higher concentrations of the muscle metabolites - levels so high, they are not normally associated with intense exercise, perhaps because a signal to slow down or stop comes on before levels get that high.

It is not known whether these nociceptive nerves are also involved in making people stop or slow down while exercising.

Light and Amann showed the effects of the two types of nerve fibres by injecting muscle metabolites into the muscle at the base of subjects' thumbs. At lower concentrations, the fatigue nerves were dominant and subjects said their thumb muscle felt heavy, exhausted. At concentrations higher than would be expected during normal exercise, the pain nerves were activated and people complained of aching and heat. So fatigue, it seems, is the body's way of telling you not to run any more.

Of course, there is a certain amount of pleasure in the fatigue - that drained, depleted feeling - after a long, hard run.

But many of us will attest that the depleted feeling pales in comparison with euphoria, which is unpredictable and unforgettable.

Euphoria being a strong motivator makes sense. In psychology, the most powerful way to induce a behaviour is to reward it only sporadically and unpredictably.

So when does fatigue pain become ''real pain'' asks Sagal. ''If you're so fatigued that every step is an effort; your legs are giving out and you're chafing yourself bloody and the heat is making you light-headed, where are you on that spectrum? Where does discomfort end and pain begin?''

Maybe there is some confusion between the pain of fatigue and the pain of an acute injury, such as the kind that comes from a broken bone or a torn tendon, which, researchers say, can make you stop immediately, since you are unable to override the sensation.

That is the kind of strong sensation I think of when I refer to running ''pain''.

Yet when Sagal refers to the pursuit of running ''pain'', he seems to be referring mainly to fatigue. That means we were assigning very different definitions to that broad term ''pain''.

Amann understands the difference. ''I used to be a cross-country skier,'' he says. ''So when people tell me their exercise is painful, I know exactly what they are talking about.'' But, he adds, ''from a scientific point of view, it is not pain''.

The New York Times

Read more: http://www.theage.com.au/executive-style/fitness/from-jog-to-slog--why-you-get-so-tired-it-hurts-20110525-1f450.html#ixzz1O6O0hBxQ