## 1 Introduction  
Falls in the daily living environment and workplace are particularly hazardous for the elderly and laborers [1]. The analysis of the mechanism of fall avoidance motion will contribute to preventing fall-related injuries. Tripping when walking is a common cause of falls [2], and the recovery motions performed by an individual after tripping have been investigated for many years. [PERSON] et al. identified the \"elevating\" and \"lowering\" strategies of recovering from tripping [3]. The elevating strategy is regarded as the motion where humans overcome an obstacle by lifting the tripped leg. In contrast, the lowering strategy is the motion where humans place the tripped leg behind the obstacle and move the opposite leg forward. Furthermore, a \"delayed lowering\" strategy is a mixture of the recovery strategies described above [4]. The selection of these strategies is largely determined by the gait phase in progress when tripping occurs [4, 5]. The factor that affects the performance of recovery and verification motion was investigated. [PERSON] et al. analyzed the difference between successful and failed recovery steps [6, 7]. In the first literature, the perturbations were applied among older adults by suddenly accelerating the treadmill. In the second one, an obstacle was placed on the walk lane. According to the experiment involving tripping young adults and the elderly, a short step time and a long step length may lead to successful recovery. [PERSON] et al. investigated the effect of step length on fall speed by restricting the recovery step using a rope following contact with the obstacle on the walk lane. They found that the descending speed of the body increases when the recovery step length decreases [8]. In addition, [PERSON] et al. performed the tripping experiment by placing an obstacle on the walk lane and reported the importance of the supporting leg [9]; they found that a relatively strong supporting leg push-off increases the length of the recovery step. [PERSON] et al. estimated the tripping risk from a gait experiment performed on a walk lane and concluded that fast walking with a large step length and small cadence decreased the tripping risk [10]. Furthermore, it has been reported that the length of the first and second recovery steps varies among trials and individuals, according to overground tripping experiments using an exoskeleton [11]. Therefore, previous studies have identified several key factors for successful recovery from tripping. In gait experiments, the subject generally walks at a self-selected [4; 7; 9; 11] or controlled speed [8; 12]. Previous research suggests that the self-selected speed does not significantly affect the recovery response based on the probability of the subject recovering from tripping at various self-selected speeds [13]. However, some studies suggest an increased fall risk at a higher comfortable speed [14; 15]. [PERSON] et al. examined the increase in the effect of tripping at a faster gait speed through gait simulation [16]. Based on kinematics, gait stability can be analyzed using angular momentum [17]. Fall recovery is the process of reducing the forward angular momentum converted from the walking speed by tripping [9]. Therefore, considering the fall as the kinematic phenomenon, the magnitude of walking speed should be examined. Based on dynamic analysis, the imbalance caused by tripping is expected to increase as gait speed increases owing to the concurrent increase in kinetic energy. Treadmill experiments offer the ability to control gait speed and monitor recovery motion. Although the conditions of treadmill walking are kinematically equal to those of overground walking [18], the gait motions observed on the treadmill and overground exhibit certain differences. [PERSON] et al. reported that cadence increased by 6% and stride length decreased by 4% during treadmill walking under the same gait speed [19]. [PERSON] et al. evaluated the difference between the joint angles.