Caffeine increases perceived exertion and workout motivation without directly causing muscle hypertrophy when mechanical load is not increased.

Study: ACE gene polymorphisms (rs4340) II and DI are more responsive to the ergogenic effect of caffeine than DD on aerobic power, heart rate, and perceived exertion in a homogeneous Brazilian group of adolescent athletes

Caffeine made these young athletes feel like they were working harder and helped them run farther, but it didn’t make their muscles bigger — that still needs lifting weights or similar effort.

Study: Acute caffeine intake improves muscular strength, power, and endurance performance, reversing the time-of-day effect regardless of muscle activation level in resistance-trained males: a randomized controlled trial

Caffeine helped people lift heavier and do more reps without their muscles working harder, meaning it made them feel stronger and more motivated, but didn’t directly make their muscles grow.

Study: Effects of acute caffeine ingestion on muscle strength, muscular endurance, rating of perceived exertion, and pain perception during strength exercise until the failure

Caffeine helps people lift more reps and feel less tired during workouts, but it doesn’t make muscles grow on its own — it just lets you work harder, which then helps muscles grow over time.

Study: Six weeks of caffeine supplementation enhances muscle thickness without augmenting strength gains—a randomized controlled trial

This study found that people who took caffeine for six weeks ended up with thicker muscles, even though they didn’t get stronger. That means caffeine might help muscles grow on its own—not just by making people feel more motivated to work out.