The pain and cramps which accompany physical exhaustion are associated with the accumulation of excess lactate.
a) where does the lactate go after we stop exercising?
b) It's not possible to perform exercise without the build up of lactate. However -- in 2-3 sentences, hypothesize a scenario where you could exercise continuously and "drain" the lactate away so you could exercise forever without fatigue.
Note: (b) is hypothetical. But use this chapters material & present an idea in 2-3 sentences that might work.