Association of Smartphone Use Duration with Physical Fitness among University Students: Focus on Strength and Flexibility.

High-frequency smartphone use leads to physical inactivity and may consequently cause the loss of physical fitness. Although the relationship between smartphone usage and physical fitness has been investigated, most of these studies focused on partial physical fitness, and the evaluation of the duration of smartphone use was insufficient. Therefore, this study aimed to investigate the relationship between the time period of using smart phones and physical fitness in Chinese university students. In this cross-sectional study, 8977 college students (5189 men and 3788 women) were enrolled. The assessment of smartphone usage was performed using a self-reported questionnaire. Physical fitness was measured in a sports facility, consisting of grip strength, standing long jump, and sit-and-reach test. The post adjustment relationship between smartphone use duration and physical fitness was examined by using the ANCOVA test. In the final adjusted model, an inverse association was observed between the smartphone use duration and grip strength in male and female participants (p = 0.003 and <0.001, respectively). The smartphone use duration was also negatively associated with standing long jump in both sexes (p = 0.003 for male and = 0.026 for female). In male participants, the longer the period of using smartphones, the lower their flexibility (p = 0.026). In conclusion, this study investigated the association between the duration of smartphone use and physical fitness. The results showed that longer durations of smartphone use were associated with weaker grip strength, a shorter standing long jump, and lower flexibility. Controlling smartphone-use duration might be beneficial for physical fitness among university students.