Muscle Thickness and Passive Muscle Stiffness in Elite Athletes: Implications of the Effect of Long-Term Daily Training on Skeletal Muscle

To examine the effect of long-term daily training on athletes' skeletal muscle, this study determined the relation between their muscle thickness and passive muscle stiffness, and compared the muscle thickness and muscle stiffness between athletes and non-athletes. Participants were elite Japanese athletes (278 men, 200 women) from various sports and non-athletes (35 men, 35 women). Rectus femoris (RF) muscle thickness was measured using B-mode ultrasonography and was normalized to the total body mass (muscle thickness/body mass(1/3)). RF passive muscle stiffness (shear modulus) was assessed by ultrasound shear-wave elastography. There was a negligibly significant correlation between muscle thickness and muscle stiffness in male athletes (p=0.003; r=-0.18) but not in female athletes (p=0.764; r=0.02). Among men, muscle thickness was significantly greater in athletes than non-athletes (p<0.001), whereas muscle stiffness was significantly less in athletes than non-athletes (p=0.020). Among women, muscle thickness was significantly greater in athletes than non-athletes (p <0.001), whereas muscle stiffness did not differ significantly between athletes and non-athletes (p =0.412). These results suggest that the effect of long-term daily training performed by athletes on muscle stiffness is more complicated than that on muscle thickness.