Why do you feel fatigue in your forearm rather than your fingers?

Because your forearm has muscles while your fingers don't actually have muscles. 

In your fingers you only have tendons which don't contribute to the feeling of fatigue. This is different for your forearms because there you would usually find muscles and which, when worked enough, build up lactic acid that we're feeling as fatigue. 

Sugar + oxygen > carbon  dioxide + water + energy  c6h1206 + 6o2 > 6co2 +6h2o + energy  and the numbers after the element symbols are small.