If triathletes train too hard for three weeks and get overly tired, they don’t get better at using oxygen when they cut back to rest—unlike those who aren’t overdoing it, who do get noticeably better.

If triathletes feel tired but still perform well during intense training, they tend to bounce back stronger after a rest period—gaining about 2.6% more peak performance—compared to those who push too hard and burn out.

When male triathletes take a break from intense training, most hit their best performance in the first two weeks, even if they were really worn out before — heavy training doesn’t seem to delay their recovery during this rest period.

Triathletes who push their training too hard without enough recovery are way more likely to get sick—7 out of 10 had infection symptoms, compared to just 2 out of 10 who were tired but not overtrained, and 1 out of 10 in rested athletes.