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 mass1/3). 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.
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