Time spent in moderate‑ to vigorous‑intensity physical activity is associated with lower limb muscle thickness in collegiate healthy young women without regular exercise habits

The present study aimed to investigate the association of physical activity and with or without regular exercise habits with lower limb muscle thickness, intramuscular adipose tissue (IntraMAT) and muscle strength in healthy young collegiate women, focusing on muscle thickness and IntraMAT using ultrasound tomography. The study participants included 20 healthy young collegiate women (age, 20.7±1.0 years). Physical activity was measured using a uniaxial accelerometer, and lower limb muscle thickness and IntraMAT were measured using ultrasound tomography. Muscle strength was measured as the maximum voluntary contraction force during isometric knee extension. The present study examined the association of physical activity with lower limb muscle thickness, IntraMAT and muscle strength, depending on whether the participants followed an exercise habit or not. Single correlation analysis revealed no significant correlation between the physical activity level and muscle thickness or muscle strength in all the participants. However, in the participants without regular exercise habits, the number of step counts and physical activity levels, particularly the time spent performing moderate- to vigorous-intensity activity, were positively associated with vastus lateralis muscle thickness (P=0.018 and P=0.048), and tended to have higher echo intensity of rectus femoris (P=0.056). These results suggest that a reduction in the time spent performing moderate- to vigorous-intensity physical activity may affect lower limb muscle thickness in healthy young collegiate women without regular exercise habits. In addition, the results suggest that a lack of exercise habits may influence the increase in IntraMAT in healthy young collegiate women.