08/09/2026
As far as one-liners go, “use it or lose it” is actually fairly on point, but only when aimed at the correct target: aerobic fitness. It might sound counterintuitive, but easy come, easy go is a helpful way to think about both cardio gains and detraining.
In short, strength adaptations are highly durable; check our last post for more info on this. But aerobic fitness is far more perishable. Even in trained endurance athletes, VO2 max (the gold standard measure of aerobic fitness) drops by around 5% in just a fortnight.
The reason comes down to blood volume. Aerobic training makes you carry more blood to deliver more oxygen to working muscles. This fluid volume is quick to build and even quicker to lose:
Your blood plasma volume actually starts to drop within 48 hours of stopping exercise.
So because you’re carrying less fluid, less oxygen-rich blood is pumped with every single heartbeat. That is exactly why, after just a fortnight away, you’ll find yourself out of breath much faster than before the break.
If you’re getting back into your routine this week, expect your strength and fitness to behave differently:
▫️Aerobic work needs patience. Because it’s “easy come, easy go,” give your legs and lungs some grace, at least initially.
▫️Lifting is still there. Your strength adaptations are durable. Trust in the foundation you’ve already built.
Remember, stronger lives are built one rep, step, and session at a time.