For aerobic exercise, athletes ran on a treadmill at 85% of maximum effort to exhaustion. Weight training involved a circuit with fie exercises that the subjects performed until exhausted. Free radicals-high reactive chemicals produced naturally during metabolism were higher at the end of exercise and during recovery, compared to rest. This coincided with a decrease in markers of immune system function. Other studies have found that marathon runners are more susceptible to upper respiratory infections for up to 30 days after a race. Athletes must balance the stress of training with adequate recovery to avoid illnesses. Getting a cold or flu can stop athletes in their tracks.