Do men and women fatigue differently?

When it comes to exercise and cycling performance, the physiological differences between men and women are small.

It's true that women tend to have less lean muscle mass and carry a bit more body fat, which makes it harder for them to achieve the same power-to-weight ratio as men. However, in terms of the actual metabolic processes that take place in the muscles, there's no evidence of any difference between males and females.

That being the case, it's natural to assume that the mechanisms of fatigue during hard exercise are the same regardless of gender. However, new research by US scientists suggests that these mechanisms differ markedly - something that could have quite significant implications for the way women and men should best train.

The science

In the study, the researchers examined the relative contributions of peripheral (ie stemming from the muscles) and central (ie stemming from the brain) fatigue in men and women during prolonged cycling. To do this, they used a technique known as ‘peripheral nerve magnetic stimulation', which gives information on the behaviour of nerves (eg motor nerves that activate muscles) when immersed in a powerful magnetic field.

Eleven men and nine women cycled for two hours at their ventilatory threshold (the point at which breathing begins to become laboured - fairly hard). During this ride, they had to perform five one-minute sprints and then immediately afterwards, they performed a flat-out 3km time trial. Before and after the cycling test, quadriceps (frontal thigh muscles) strength testing was performed pre and post-cycling.

The cyclists were asked to perform maximum power voluntary contractions of their quadriceps muscles on three occasions while undergoing peripheral nerve magnetic stimulation. Importantly, the magnetic field was adjusted during these contractions so that the relative contributions of peripheral and central fatigue could be calculated.

In a nutshell

Some of the findings were entirely to be expected. For example, when adjusted for body mass, men and women had similar strength levels. Also, the measured decline in maximum voluntary contractions were broadly similar between sexes, declining 22 per cent in men and 16 per cent in women - a difference that was not considered statistically significant.

However, when the researchers looked at the relative contributions of fatigue across the sexes, they found that while both men and women seemed equally affected by central nervous system (brain) fatigue, it was only the men that lost strength as a result of actual muscular fatigue; their levels dropped from an average of 6.3N per kilo to 5.2N per kilo. Those of the women were unchanged between pre and post-cycling.

So what?

What's interesting about this study is that it seems to show that the causes of the tiredness we feel when cycling hard are not identical in men and women.

In women, central nervous system fatigue appears to account for most of the fatigue experienced whereas in men, although central nervous system fatigue is important, muscular fatigue brought about by chemical changes in muscles also plays a significant role.

The implications of this are important because it suggests that women are likely to respond especially well to strategies aimed at reducing or managing central nervous system fatigue (music, caffeine, imagery etc).

These would also work for men, but men may be more receptive than women to muscular fatigue reduction strategies such as the use of supplements to reduce lactate accumulation, maintaining maximum high levels of muscle glycogen etc.

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