Intense exercise increases free hydrogen ion concentration in muscle tissue, which causes metabolic acidosis and results in reduced muscle performance.

Study: Active and passive recovery and acid-base kinetics following multiple bouts of intense exercise to exhaustion.

Intense exercise does make muscles more acidic, as shown by lower blood pH after hard workouts. But surprisingly, when people rested or did light pedaling between workouts, their next round of exercise wasn't better or worse—even though acidity changed. So yes, acidity goes up, but it might not be what's slowing you down.

Study: The total ionic status of muscle during intense exercise.

When people exercise really hard, their muscles make more hydrogen ions, which makes the inside of the muscle more acidic — and this study saw that happen. It didn't prove that this acidity makes muscles tired, but it did confirm the acid buildup happens.

Study: Factors influencing hydrogen ion concentration in muscle after intense exercise.

When people exercise very hard, their muscles make a lot of lactic acid, which causes more hydrogen ions to build up and makes the muscle environment more acidic — this study directly measured that happening.

Study: Acute whole-body vibration as a recovery strategy did not alter the content of gluteus medius monocarboxylate-transporters, lactatemia, and acidosis induced by intense exercise in horses

When horses ran really hard, their muscles got more acid, just like the claim says. The study didn't find anything that changed that — so it supports the idea that intense exercise makes muscles more acidic.