Breathing less oxygen while exercising hard makes endurance athletes tire out faster because it weakens their muscles' ability to work properly, but getting hot doesn't really change how their muscles perform.

When endurance athletes push themselves to exhaustion, their jumping ability gets worse in normal or low-oxygen air, but stays the same in hot conditions. This shows that different types of tiredness affect muscles differently depending on the environment.

When endurance athletes push themselves hard in hot conditions, their bodies produce more of one inflammation marker, but when they exercise with less oxygen, they produce less of another inflammation marker—showing different reactions to heat versus low oxygen.

When people do a special warm-up exercise, regular active folks get more tired, but top runners don't get any more tired—showing that your past exercise habits change how your muscles react.

Doing a lot of reps with lighter weights right after each other makes your legs feel way more tired right after than doing fewer reps with heavier weights.