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.

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 caffeine supplements during upper-body weight training may lead to greater increases in the size of the biceps and triceps muscles compared to other muscles, without changing overall body adaptations in men who exercise regularly.

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.