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.

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.

In recreationally active men, taking a specific dose of caffeine before upper-body workouts three times a week for six weeks is linked to a larger increase in biceps muscle thickness compared to a placebo, but does not lead to greater gains in strength or changes in body composition.

In sedentary overweight and obese women, taking a caffeine dose of 3 mg per kilogram of body weight 60 minutes before three weekly sprint training sessions for 12 weeks is associated with larger decreases in body fat, greater gains in leg strength, and improved aerobic fitness compared to taking a placebo.