Posted: August 22, 2004

Science of Sport: Tired muscles? Don't blame lactate

Lactic acid, produced in the muscles from intense exercise, enhances performance rather than reduces it as commonly believed, according to a new study.

A team of Australian and Danish researchers publish their findings in today's issue of the journal Science.

Lactic acid has long been associated with muscle fatigue, the loss of force and power with repeated muscle contractions.

"Everybody thinks lactic acid is a bad thing and that it's deleterious to performance but what we're showing is that it's a help," said researcher Professor Graham Lamb of Melbourne's La Trobe University. "It actually reduces fatigue."

Lactic acid is produced when a muscle works so hard it is forced to convert glucose to energy without enough oxygen. Less energy is produced per molecule of glucose but it's a way of the body squeezing the last ounces of energy out of glucose despite there being enough oxygen.

Lamb said that sports commentators and trainers often said athletes needed to "warm down" after intense exercise to wash out lactic acid.

"The reason people thought lactic acid caused fatigue is that when muscles fatigue they see lactic acid increase," Lamb, a muscle physiologist, told ABC Science Online. "It turns out it's a correlation but not the cause."

Not only does lactic acid not cause fatigue, said Lamb, it improves the conditions for muscle contractions.

Contractions, contractions

Muscle contraction relies on a brief change in electrical potential across a muscle cell or fibre membrane. This occurs by the selective flow of positively and negatively charged ions.

Under strenuous exercise, potassium ions build up outside the cell causing the cell to lose its normal potential and thus the ability to contract. Muscles also have a natural "brake" on them, in the form of chloride ions, which prevent muscles contracting on their own.

Lamb's team has shown that lactic acid seems to remove this chloride ion brake on muscle contraction.

"You are taking away the inhibition of chloride and this lets the electrical impulses keep going when it would have failed."

Lamb and team looked at what happened under different levels of acidity in "skinned" rat muscle fibres bathed in a solution that simulated a hard-working muscle.

"When it's normal acidity it stops working. When it's more acid it keeps going."

Lamb was unsure whether the practice of warming down to wash out lactic acid should change as a result of the findings.

"I'm not saying it isn't useful but the reason that it's useful is not that they're getting rid of lactic acid," he said.

How about the brain?

Dr David Bishop, president of the Australian Association for Exercise and Sport Science, said the research was provocative and would stimulate a lot of debate.

"It is important to remember though, that all the cited studies are on a single muscle fibre," he told ABC Science Online. "There is something very important missing here, the brain."

Bishop, who is based at the University of Western Australia, said many researchers believed the brain interacted with lactic acid to cause fatigue.

He also cited other research that supported the role of lactic acid in fatigue, including his own, which has shown improvements in athletic performance following a drink of sodium bicarbonate to neutralise lactic acid.

Exercise physiologist and former triathlete, Dr Paul Laursen of Edith Cowan University in Perth, told ABC Science Online that physiologists were starting to appreciate that muscle fatigue was a complex phenomenon and the idea that it was all down to lactic acid was false.

But he described the new research as "very interesting".

"It was news to me. I think it's going to be news to most," said Laursen, who researches exercise fatigue.

He was also unsure whether it would change athletic practice.

From ABC Australia Online August 20, 2004