Magnetic resonance imaging-measured skeletal muscle mass to fat-free mass ratio increases with increasing levels of fat-free mass.

Background: To investigate the skeletal muscle mass to fat-free mass (SM-FFM) ratio in female and male athletes, as well as to examine the relationship between ultrasound predicted SM and magnetic resonance imaging (MRI)-measured SM.

Methods: Seven female track and field athletes (female), 8 male collegiate swimmers (male-G1) and 8 male collegiate Olympic weightlifters (male-G2) volunteered. Whole-body SM volume was measured using MRI images obtained from the first cervical vertebra to the ankle joints. The volume of SM tissue was calculated and the SM volume was converted into mass units by an assumed skeletal muscle density. Muscle thickness was measured using ultrasound at nine sites and SM was estimated using an ultrasound-derived prediction equation.

Results: Percent body fat was similar among the groups. FFM, MRI-measured SM and SM-FFM ratio were greater in Males-G2 compared to the other two groups and those variables of Male-G1 were higher than the Female group. There was an excellent correlation (r=0.976) between MRI-measured and ultrasound-predicted SM (total error=1.52 kg). No significant difference was observed between MRI-measured and ultrasound-predicted SM in the overall sample or within each group. The SM-FFM ratio was positively correlated (r=0.708) with FFM in female and male athletes.

Conclusions: We provide evidence for how the MRI-measured SM-FFM ratio changes with increasing levels of FFM and provide data that the ultrasound may be useful in estimating SM in athletes. Given the size limitations with MRI, both of these findings may be useful for future research investigating large sized athletes.