Associations Between Fat Free Mass Percentage and Relative Force Production in Two Strength Tests

Abstract: Background: Knowledge of the relationship between fat free mass (FFM) and strength capacity is not only useful in athlete coaching, but may also be important in developing clinical strategies for evaluating health in general populations. Sufficient skeletal muscle mass and strength are important factors for health and physical function. Bioelectrical impedance analysis (BIA) is a popular and clinically useful tool for assessing body composition. However, being able to simultaneously estimate whole body muscle strength without having to perform additional tests may prove useful in evaluating an individual’s health and physical function. Aim: The primary purpose of this study was to quantify the association between total body fat free mass percentage (TBFFM%) and relative strength in the isometric midthigh pull test (IMTP) and grip strength (GS) test in a healthy general young Swedish population. A secondary purpose was to investigate if there are any sex differences.  Methods: The design for this study was cross-sectional. The studied sample represented a general Swedish population of healthy young adults. Participants were asked to participate in testing on one occasion. Three tests were used for collecting data: BIA for data on FFM and both IMTP and hand dynamometer grip strength test for data on maximal force production. All tests were performed in the movement lab on Halmstad University campus. Statistical analyses were done using Pearson’s correlation and linear regression analysis. Results: Participants for this study consisted of healthy females (n=18) and males (n=17) aged 19-39. The primary findings of this study are that there are strong correlations between TBFFM% and relative strength (r = 0.621) in the IMTP test as well as relative GS (r = 0.705) on a whole-group level. The correlation between TBFFM% and relative GS was slightly stronger. Categorized by sex, we found strong positive correlations between TBFFM% and relative IMTP (r = 0.551) and relative GS (r = 0.596) for the female group and moderately strong correlations for the male group (relative IMTP r = 0.411; relative GS  r = 0.422).On a whole-group level, every unit increase in TBFFM% increases relative strength in the IMTP by 0.538N/kgBW (β = 0.538) and relative GS by 0.015 kg/kgBW (β = 0.015). Conclusion: TBFFM% was strongly associated with both relative IMTP strength and relative GS where the association was stronger for females than for males. TBFFM% values may be indicative of an individual's strength, which could be of value in a research setting.