I am primarily limited by heat dissipation.

If I'm outside working in the yard at 17C in the sun, I have about an 85% duty cycle, where 15% of the time I have to get into the shade and wait for the body heat to dissipate. By 23C my duty cycle drops to about 50%.

This is partially due to being overweight, but at 100kg and nearly 6ft I'm not morbidly overweight. It is partially due to the seasons changing and my body not yet having adapted to working in hot weather (These figures were measured over the last 3 days).

But there is a third reason I find more interesting. My blood vessels are barely on the surface. You can see them on the backs of my hands and the tops of my feet and that is about it. Phlebotomists have a hard time finding my blood vessels to draw blood. This is a genetic adaptation to my ancestors living in very cold weather, keeping the blood warm by having the blood vessels run deeper. The downside is that I can't dissipate heat very well when I need to, but apparently my ancestor's problems were primarily freezing to death, not so much laboring under the summer sun.

Unfortunately when working with others, people don't understand this and think I'm lazy, always wanting to take a break. But put me in 8C weather and I can go at max VO2 for many hours. And once in pain, give me some alcohol and I'll go for another few of hours.

As an aside, the fact that my body wants to carry more fat than modern medicine considers healthy is also probably genetically selected as a way to stay warm in the winter.

Maybe I should move somewhere colder.