## 1 Introduction  
Studies suggest that deep knee flexion before landing is important for reducing kinetic energy in the lower extremities during a soft landing [1; 2; 3]. [PERSON] and [PERSON] reported that a soft landing, defined as \(>\)90\({}^{\circ}\) knee flexion after landing from a vertical height, could absorb 19% more kinetic energy than a stiff landing [1]. In addition, hip and knee extensors could absorb more energy during a soft landing than a stiff landing [1]. Therefore, the stress exerted on the body when landing from a fall or jump can be absorbed by landing softly in flexion [1; 4]. Research also indicates that soft landings play an important role in the energy absorption capacity of the knee joint [5; 6; 7]. In addition to changes in energy absorption contribution by the knee joint, soft landings also result in a difference in energy distribution in the lower extremities, including in the hip and ankle joints, compared with stiff landings. Soft landings may therefore reduce the impact imposed on the joints of the lower extremities by increasing the energy absorption in the knee joint, and altering the energy distribution in the lower extremities when landing. Knee injuries during sporting activities typically result from large mechanical stress applied to the joint on impact with the ground during landing [8; 9]. Among the most common knee injuries in many sports (e.g., soccer and basketball) are those involving the anterior cruciate ligament (ACL) [10; 11; 12]. Recent studies indicate that this injury typically occurs in non-contact situations, such as landing from a jump, cutting, and pivoting [11; 12; 13; 14]. Furthermore, previous studies have reported that landing with a decreased knee flexion angle may increase ACL loading by increasing quadriceps muscle activations and knee extensor moments [15; 16]. This finding is supported by some cadaver studies, which revealed that quadriceps loading, with only a slight knee flexion angle, produces significant anterior tibial translation and higher tension on the ACL [17; 18; 19]. Knee compression force has been shown to be lower in soft landings than in stiff landings [1; 20], and there is a potential reduction in mechanical stress on bone and cartilage in the knee joint associated with soft landings. In terms of frontal plane motion, it has been reported that the biomechanical components of non-contact ACL injury mechanisms are lateral trunk and knee abduction motion [21]. A forceful valgus collapse with the knee close to full extension, combined with knee external or internal rotation at the instant of injury, have also been shown to be key factors in ACL injury mechanisms [22]. Some researchers have indicated that an increased knee valgus moment [23] and medial knee displacement, evaluated by the difference between the medial knee position at initial foot contact and the peak value [14], is associated with increased ACL injury risk. Landing posture at the instant of non-contact ACL injury is characterized by an alignment that includes knee valgus, combined with other biomechanical components, such as the lateral trunk position and knee alignment in the sagittal or horizontal plane [14; 22; 23]. Hence, lower extremity alignment during landing appears to be an important biomechanical factor in the prevention of non-contact ACL injuries. In addition, increased knee loading is associated with progression of cartilage changes and onset of knee osteoarthritis [24; 25]. Therefore, large mechanical stress applied to the knee on impact with the ground during landing may affect the progression of knee osteoarthritis, with cartilage changes on application of continuous and strong mechanical stress. Some researchers have employed three-dimensional motion systems to investigate characteristic biomechanics of the lower extremities in subjects with knee valgus landings during cutting maneuvers and squatting tasks [26; 27; 28].