The fat-free mass index, known as FFMI, indicates the body’s muscle development. It can therefore be used to monitor the success of strength training or of a change in diet. For ambitious or professional strength athletes, the FFMI is used as a replacement for the body mass index (BMI), which is no longer appropriate for muscular people in particular. BMI is designed more for untrained people and only produces an initial comparison. The fat-free mass index, by contrast, is more suitable for trained athletes and provides them with a significantly more accurate assessment of their physical condition.