Background: Grip strength is a marker of future health conditions and is mainly generated by the extrinsic flexor muscles of the fingers. Therefore, whether or not there is a relationship between grip strength and forearm muscle size is vital in considering strategies for grip strength development during growth. Thus, this study aimed to examine the association between changes in grip strength and forearm muscle thickness in young children. Methods: Two hundred eighteen young children (104 boys and 114 girls) performed maximum voluntary grip strength and ultrasound-measured muscle thickness measurements in the right hand. Two muscle thicknesses were measured as the perpendicular distance between the adipose tissue-muscle interface and muscle-bone interface of the radius (MT-radius) and ulna (MT-ulna). All participants completed the first measurement and underwent a second measurement one year after the first one. Results: There were significant (P < 0.001) within-subject correlations between MT-ulna and grip strength [r 5 0.50 (0.40, 0.60)] and MT-radius and grip strength [r 5 0.59 (0.49, 0.67)]. There was no significant between-subject correlation between MT-ulna and grip strength [r 5 0.07 (
0.05, 0.20)], but there was a statistically significant (P < 0.001) between-subject relationship between MT-radius and grip strength [r 5 0.27 (0.14, 0.39)]. Conclusion: Although we cannot infer causation from the present study, our findings suggest that as muscle size increases within a child, so does muscle strength. Our betweensubject analysis, however, suggests that those who observed the greatest change in muscle size did not necessarily get the strongest.  Kulcsszavak: children, muscle size, handgrip strength, growth and development, preschoolers