In young males, whether someone is regularly strength-trained or just casually active does not change how much acute caffeine improves muscular endurance, jump height, or one-rep max strength.

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 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 caffeine supplements for six weeks while doing strength training does not lead to measurable improvements in maximum strength, endurance, or body fat and muscle mass in men who exercise regularly, even though their muscles may become slightly thicker.

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.

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.