Even if the air has the same oxygen level, being at real high altitude makes your body respond differently to weightlifting than breathing the same air through a mask—your muscles get more oxygen, you produce more lactic acid, and your heart works harder.

Even when the air has the same amount of oxygen, breathing it through a mask at low pressure makes your body produce less lactic acid and keeps your heart rate lower during weightlifting than being at real high altitude.

Breathing air with less oxygen through a mask (simulated altitude) makes your muscles get less oxygen during exercise than being at real high altitude, even when the oxygen level in the air is the same.

When you breathe oxygen-poor air through a mask for only 5 minutes before lifting weights, your blood oxygen level drops more than when you’re at real high altitude—even if the air has the same oxygen content.