Fat Free Mass Index as an Important Performance and Health Indicator for Athletes 

Body composition in sport can be a controversial topic. In recent years, reports of unhealthy practices, pressures to maintain low body weight, and toxic cultures of body shaming at collegiate and professional levels have received widespread media attention (1, 2). This has led some programs to prohibit mandatory testing and reporting of data to coaches (3). Unfortunately, these types of experiences extend to all sports and competition levels, and are far too common, particularly for female athletes. However, when used appropriately and considered within the broader context of the overall health and performance goals of the athlete, body composition can be a valuable metric.

Beyond Body Fat:

Depending on the demands of the sport and position, certain body composition ranges favor peak performance. Recommendations regarding target body composition goals are typically based on weight and/or percent body fat, which then set a basis for nutrition and training recommendations. Body composition, however, also includes measures of fat-free/lean mass, bone density, body water, body fat distribution, and limb composition, all of which are highly relevant for sport performance. Fat-free mass (FFM) in particular, has direct implications for strength, power, speed, and general athletic performance, in addition to overall athlete health. Greater FFM is associated with higher bone mineral density and improved injury risk outcomes (4). For weight-sensitive sports, maintenance of FFM becomes especially important for maintaining athlete performance and health. In regards to nutrition and training, recommendations based on FFM are not only advantageous, but also open the door for a more holistic conversation shifting away from calorie restriction and food phobia to a focus on food as fuel, nutrient timing, optimal food selection, and healthy body weight.

Fat-Free Mass Index:

Establishing FFM recommendations for athletes can be difficult due to the influence of height on absolute amounts of FFM and the range of body types that can excel in a specific sport or position. Fat-free mass index (FFMI; kg/m2) is a height-adjusted assessment of FFM, which helps normalize values and improve generalizability for recommendations across sports. FFMI can be useful for tracking athlete development. For strength and power athletes, upper limits of FFMI can be used to evaluate potential for FFM gain (5). For weight-sensitive sports, lower limits (6) for FFMI can be used as a meaningful screening tool for injury risk or complications related to relative energy deficiency in sport (RED-S) (7). Importantly, FFMI is not significantly related to percent of body fat, making it a valuable outcome that can be discussed separately from body fat.

How to measure FFMI:

FFMI is calculated by dividing FFM (kg) by height (m2). Most body composition devices used in athletic settings today either provide an estimate of FFM or it can be easily calculated (FFM (kg) = Body mass (kg) – [%Body Fat × Body mass (kg)]). Measurement accuracy and what is quantified as FFM varies by device and method, so it is important to know your measurement device and its limitations. Discussion of different body composition devices and definitions of FFM can be found here (8–10).

Read more about the interpretation of FFMI, practical points & sample reference data
Author

Katie Hirsch, PhD, EP-C, CISSN is an Assistant Professor in the Department of Exercise Science at the University of South Carolina. Her research focuses on the effects of exercise and nutrition on body composition, muscle and protein metabolism, cardiometabolic health, and performance, with a focus on sex differences and female physiology across the lifespan. Dr. Hirsch completed postdoctoral research training in the Center for Translational Research in Aging & Longevity at the University of Arkansas for Medical Sciences. She completed her PhD in Human Movement Science and MA in Exercise Physiology at the University of North Carolina at Chapel Hill and BS in Exercise Science at Truman State University. She is a Certified Exercise Physiologist with the American College of Sports Medicine and Certified Sports Nutritionist with the International Society of Sports Nutrition.

References:
  1. Chavez, C. Inside the Toxic Culture of the Nike Oregon Project ‘Cult’. Sports Illustrated (2019).
  2. Goe, K. Women athletes allege body shaming within Oregon Ducks track and field program. The Oregonian/OregonLive (2021).
  3. Goe, K. Oregon Ducks athletic programs no longer can monitor athletes’ weight, body fat percentage. The Oregonian/OregonLive (2021).
  4. Roelofs, E. J. et al. Muscle Size, Quality, and Body Composition: Characteristics of Division I Cross-Country Runners. J Strength Cond Res 29, 290–296 (2015).
  5. Trexler, E. T. et al. Fat-Free Mass Index in NCAA Division I and II Collegiate American Football Players. J Strength Cond Res 31, 2719–2727 (2017).
  6. Harty, P. S. et al. Upper and lower thresholds of fat-free mass index in a large cohort of female collegiate athletes. J Sports Sci 37, 2381–2388 (2019).
  7. Zabriskie, H. A. et al. Energy Status and Body Composition Across a Collegiate Women’s Lacrosse Season. Nutrients 11, 470 (2019).
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  9. Blue, M. N. M., Tinsley, G. M., Ryan, E. D. & Smith-Ryan, A. E. Validity of Body-Composition Methods across Racial and Ethnic Populations. Advances in Nutrition 12, 1854–1862 (2021).
  10. Heyward, V. & Wagner, D. Applied Body Composition Assessment – 2nd. (Human Kinetics, 2004).
    Currier, B. S. et al. Fat-Free Mass Index in a Diverse Sample of Male Collegiate Athletes. The Journal of Strength & Conditioning Research 33, 1474–1479 (2019).