I've got no idea how Selenium will help you with your riding!! Here's SOME info, though, and I hope it helps.
Selenium is an essential trace element but its main function is as an antioxidant in an enzymes (selenium-glutathione-peroxidase) and it does aid in the function of antibody stimulation in response to vaccines (like when your children get their baby vaccines, Selenium helps this process work). Also helps with protein synthesis in regards to growth and in men it has been shown to improve the production of sperm.
Most foods contain Selenium naturally, especially shellfish, chicken, red meat, grains and (bclapham was right afterall!) GARLIC. The amount of selenium in the soil vegetables grow in has a lot to do with the amount in the veg.
Dont overdo the supplements if you go with more than the amount occuring naturally in foods or a basic supplement. Excess selenium can give you trouble with your teeth, skin, hair, nails, etc.
I want to thank you for posting this question, I thought I had the answers from memory, but found I had to recall/search a few things :think: .
This was fun for me, but maybe someone with more experience in sports nutrition/medicine can answer your specific question about stamina and arm pump.