Taking caffeine before lifting heavy weights can increase the power generated during exercises like the back squat and bench press, with the largest gains occurring when lifting at 75–90% of maximum capacity.

Taking 6 milligrams of caffeine per kilogram of body weight before a workout can increase the number of repetitions a person can perform to failure during bench press and squat exercises at 70% of their maximum strength, based on measured effect sizes in young men who rarely consume caffeine.

Taking 4 mg of caffeine per kilogram of body weight, whether in gum or pill form, increases the maximum strength and power output during bench press and squat exercises in men who regularly train with weights, compared to taking a placebo.

In recreationally active men, caffeine may help increase muscle thickness slightly during the first 6 weeks of strength training, but it does not lead to greater muscle growth or improved performance over the long term.

Taking a dose of 6 mg of caffeine per kilogram of body weight has almost no meaningful impact on how much weight a young man who rarely consumes caffeine can lift in a single repetition of bench press or squat, even if the change is statistically detectable.

Taking caffeine while doing resistance training does not lead to meaningful changes in body fat or muscle mass over six weeks in active men, even though some muscles may appear thicker.