There are sex differences in every cell, which forms the basis of the physiology of an organism. Men have more muscle, larger hearts, less body fat, more haemoglobin, are 20-50% stronger, and more powerful (Reminder: strength is how much you can lift, power is how much and how fast you can lift).
The absolute difference between men and women marathon runners at the top level ranges between 9-16%. This shows that the difference between the sexes is beyond training or willpower because one can safely assume that men and women world record holders are evenly matched for resolve.
- Muscle fatigability can be defined as a reduction in the force/power of a muscle induced by exercise. Progressive fatiguing happens when you are asked to do either predictable, isometric (contract a muscle and hold it) tasks or dynamic tasks (when there is extension or contraction of a muscle). This applies for both maximal and sub-maximal intensities.
- Fatigability depends on age, sex, strength, genetics, and health status, to name a few. Dr Hunter focused on sex in her talk. From a review of 55 studies, it turns out that women are more resistant to fatigue than men for a range of muscle groups.
- For example, the time to fatigue for an elbow flexion task was twice as long for women than for men. Men were almost twice as strong, and it turned out that stronger people (primarily men) fatigued faster. They had lower endurance. This seems to make sense as you can typically train only one attribute at a time – stronger people tend to be weaker at endurance.
- Her lab found 10 men and 10 women who were matched for strength. The sex difference disappeared when these groups were tested again. However, in intermittent tasks, females fatigued much slower again.
- “Voluntary activation” could be, in very simple terms, seen as a measure of fatigue in the central nervous system. Voluntary activation could primarily drive these sex differences is the hypothesis put forward. Women don’t show a reduction in voluntary activation, but men do.
- In a task where they were asked to do 6×22’ long maximal contraction, there was no difference in the brain. This is in contrast to the earlier paragraph. This is how science works. Do you see the inconclusiveness? This means that it could be specific to the muscle group.
- Men have faster muscle contraction i.e. they are more powerful, but fatigue faster. Almost in every muscle group, women have more type 1 muscle fibers than men. Type 1 and Type 2 muscle fibers’ claim to fame is the slow twitch/fast twitch debate.
- Speaking of the brain, work-related neuromuscular disorders could be due to perceived stress, mental attentiveness, or muscle fatigue. Prof. Hunter’s lab tested subjects for physical performance in the presence of cognitive stress i.e. a difficult mental math test. With the stressor, the time to failure dropped drastically for women, but not that much for men. But, when the same groups were asked to be mentally attentive, the difference was stark again. Blood pressure (mean arterial pressure) increased for women, as did heart rate. Weak people were more susceptible to the high stressor environment. But what explains all of this? The data are not conclusive.
- ‘Brain activation’: women activate more of their brain to do the same task in general.
Dynamic tests: no big difference in elbow and knee. This shows how you define and measure fatigability really determines what you find.
Her final thoughts: As mentioned earlier, there is a 16% difference in speed at the New York Marathon between men and women during the period 1980-2011. But, there are twice as many men running the marathon than there are women. In the older age groups, there are twenty times as many men than women. Of the 16%, about 11% can be explained by physiology. What could the other 5% be due to? Dr Hunter believes that it is due to participation. How will we understand the difference if we don’t have enough participants and data? Of 78 muscle fatigue studies, only 20-25% study women. The disproportionate nature of our studies might be masking our full understanding of the underlying mechanisms.
To know a bit more about why and how all this happens neurologically and at the muscle level, watch the full video.