Background Bicycle crashes are a major cause of injuries in childhood. According to injury data from the Bodygraphic Injury Surveillance System of the National Centre for Child Health and Development, bicycles are the number one product related to childhood injuries. Although it is important to keep one’s brakes adjusted to prevent bicycle-related injuries, less attention has been paid to its importance compared to a helmet use. The purpose of this study is to measure the characteristics of children’s hand growth and to reveal the relationship between a child’s hand size and a reaction time to brake.

Methods Ten boys and 10 girls from each grade (120 students in total) participated in this study. To gather school children’s anthropometric hand data, we measured the palm length, the middle finger length, and the length between the base of the thumb and the DIP of a middle finger. For the reaction time measurement, we prepared two bikes which attached the different size of width (standard or wide). Each participant rode both bikes and measured their reaction time. More specifically, the reaction time means the time from a participant notices the sign to stop to the time when the brake in the rear wheel begins to work. We considered the standard width as fit of one’s hand size.

Results The length of both the palm and the middle finger increased 1.2 times in 6 years of elementary school. The growth rates were bigger among students in 4, 5, 6 grades compared to students in 1, 2, 3 grades. When the brake lever width does not fit one’s hand, the reaction time to brake is delayed by 0.1 second. Moreover, we found the tendency that one required more reaction time to brake if one’s hand size is small.

Conclusions We revealed the relationship between a child’s hand size and a reaction time to brake and the importance of bike’s brake adjustment. Based on the results from this study, we developed an animated video for bike safety. In future studies, we planned to implement an educational program to recommend a regular bike maintenance including a bike brake in addition to a helmet use.