Objective:To analyze the relationship between arch index and foot kinematic parameters and their characteristics in stress fracture of lower extremity.Methods:A case-control study was performed for 108 recruits selected from a certain army unit in 2019. Before training, the recruits′ foot print images were collected by the capacitive plantar pressure measurement system to calculate their arch indices. The kinematic characteristics of the foot were analyzed by the dynamic gait posture analysis system. Spearman rank correlation analysis between arch index and foot kinematic parameters including landing elevation angle, toe-off angle, landing speed, landing varus angle, valgus amplitude and landing valgus speed were performed. Throughout the training, orthopedic physicians followed up the recruits, among whom 10 were excluded due to other types of lower extremity injuries. The arch index and foot kinematic characteristics were analyzed and compared between the remained recruits with stress fracture of lower extremity (fracture group, n=10) and those without lower extremity injury (control group, n=79). Results:(1) For the recruits, the arch index was 0.21(0.12,0.25), with landing elevation angle for (17.31±4.02)°, toe-off angle for (63.90±5.63)°, landing speed for (176.85±24.39)°/s, landing varus angle for (13.64±4.44)°, valgus amplitude for (12.16±3.42)°, and landing valgus speed for 382.50(311.05,474.80)°/s. (2) The landing varus angle ( r=0.25, P<0.01) and valgus amplitude ( r=0.14, P<0.05) were positively related to the arch index. (3) The arch index, toe-off angle and landing valgus speed were 0.20(0.07,0.24), (61.59±5.51)° and 336.00(251.02,428.67)°/s in fracture group, significantly lower than 0.23(0.17,0.26), (64.79±4.79)° and 381.20(313.63,470.92)°/s in control group ( P<0.05 or 0.01). There were no significant differences in the landing elevation angle, landing speed, landing varus angle and valgus amplitude between the two groups (all P>0.05). Conclusions:The change of the arch index can affect the landing varus angle and valgus amplitude of the foot. Recruits who suffer from stress fracture of lower extremity have the characteristics of higher arch, lower toe-off angle and lower landing valgus speed.