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Why do lighter fighters have better cardio than heavier ones?

Of course, this isn't a rule. There's the case of monsters like Cain Velasquez. But in general, it seems light fighters get tired more slowly than heavy fighters. Why is this?

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    I'm voting to close this question as off-topic because it is a about biology (and possibly fitness) and has nothing whatsoever to do with martial arts. – Sardathrion - against SE abuse Feb 2 '15 at 11:05
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Note that this answer is going to ignore elite athletes. Frequently these elite athletes are not normal people who have taken training to the ultimate limit, rather they are people with various genetic or physical traits that are suited to the specific activities they excel in. These traits might be something as simple as longer limbs, a larger than normal heart, a slower than normal accumulation rate of lactic acid, unusually strong connection points for muscles, denser layers of fat or muscle over vital points, etc. Note that while I've linked to Wikipedia articles, it was for simplicity - there are many references I could have cherry picked, and for good basic stuff like this Wikipedia isn't too bad.

Cardiovascular fitness is a measure of the efficiency of use of oxygen. A higher rate of oxygen intake and conversion (to energy along with glucose) is equated to a better cardiovascular fitness. Cardio fitness and aerobic fitness are synonymous.

According to simple laws of physics, the smaller an organism (animal) is, the less energy it takes to move, and the better its strength to weight ratio. Conversely the larger an organism is (the greater its surface area), its volume increases at a cubed rate (so the increase is exponential1, not linear). This means that the larger you are, you need to use significantly more energy to do the same action. You can best observe this over a period of time - like a fight or a race - you can clearly see the difference between two competitors; i.e. you can see the difference between a competitor who is 50kg (110lb), and one who is 100kg (220lb). The heavier competitor could have outstanding cardiovascular fitness, but it is still relative - he has to expend a lot more energy and use a lot more oxygen than the lighter competitor. This is one reason why you don't see 100kg competitors doing Olympic marathons - those little 45kg Ugandans and Kenyans will run circles round them. When it comes to shorter high intensity sports like you get in the ring, you'll find that the heavier competitors tend to rely on anaerobic fitness as much as their aerobic fitness.

1 Scaling: Why is Animal Size so Important?

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Lighter fighters have less weight to carry around so it takes longer for their muscles to tire.

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As slugster has noted, muscles require oxygen for aerobic metabolism. Consider this: if you were to gain 50% in weight in muscle do you suppose your lung surface area would increase in proportion? Also consider whether you'd be able to breathe in 50% more oxygen. Now if the lungs do not increase in capacity or efficiency you're now powering a larger amount of muscle using the same amount of fuel. Not only that, the amount of mass that must be moved has increased meaning it also takes more energy to move that mass. Now I'm no biologist, but it seems reasonable to believe this setup can only result in decreased aerobic efficiency.

One of my favorite essays on the topic of size: On Being the Right Size.

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  • You should read a source that is newer than 1928. Oxygen utilization in the body has many adaptive mechanisms (Such as neovascularization) to compensate for increased demand. If you gained 50% more muscle mass your body would adapt to compensate if you kept up the same cardio demands. – JohnP Feb 10 '15 at 17:12

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