As you load your muscles, your blood vessels (including the ones in your brain) swell to carry more oxygen.
If the pressure is too great, you get an exertion headache, says strength coach Mike Donavanik.
You can drink water to relieve pressure – blood plasma is mainly H2O, so if you’re dehydrated, your blood runs thick.
Check your pee; if it’s not clear or light yellow, drink up.
Or breathe deeply with each rep.
Pushing against a closed airway on a heavy lift makes your blood work harder to carry limited oxygen.
The resulting blood pressure spike can cause pain.