The purpose of this study was to compare the biomechanics of single- and double-legged drop jumps (SDJ vs. DDJ) with changes in drop height. Jumping height, ground contact time, reactive strength index, ground reaction force, loading rate of ground reaction force, joint power and stiffness were measured in 12 male college students during SDJ from 20-, 30-, 40-, and 50-cm heights and DDJ from of 20- and 40-cm heights. The peak impact force was increased with the incremental drop height during SDJs. The jumping height and leg and ankle stiffness of SDJ30 were greater than those of SDJ40 and SDJ50. The knee and hip stiffnesses of SDJ30 were greater than those of SDJ50. The impact forces of SDJ30-50 were greater than those of DDJ40. The leg, ankle, knee and hip joint stiffnesses of SDJ20-30 were greater than those of DDJ20 and DDJ40. The propulsive forces of SDJ20-50 were greater than those of DDJ20 and DDJ40. The jumping height of SDJ30 was greater than that of DDJ20. Drop height of 30?cm was recommended during single-legged drop jump with the best biomechanical benefit. Single-legged drop jump from 20?30?cm could provide comparable intensity to double-legged drop jump from 40?cm.

The aim of this study was to determine whether sex differences and effect of drop heights exist in stiffness alteration of the lower extremity during a landing task with a drop height increment. Twelve male participants and twelve female participants performed drop landings at two drop heights (DL40 and DL60; in cm). The leg and joint stiffnesses were calculated using a spring-mass model, and the joint angular kinematics were calculated using motion capture. Ground reaction forces (GRFs) were recorded using a force plate. The peak vertical GRF of the females was significantly increased when the drop height was raised from 40 to 60 cm. Significantly less leg and knee stiffness was observed for DL60 in females. The ankle, knee, and hip angular displacement during landing were significantly increased with drop height increment in both sexes. The knee and hip flexion angular velocities at contact were significantly greater for the 60 cm drop height relative to the 40 cm drop height in males. These sex disparities regarding the lower extremity stiffness and kinematics alterations during drop landing with a drop height increment would predispose females to lower extremity injury.

The purpose of this study was to determine the differences in biomechanical characteristics between one- and two-legged running vertical jumps (1-LRVJ and 2-LRVJ). Ten male college volleyball players voluntarily participated in this study. Two running vertical jumps used in volleyball were randomly performed. Three trials for each type of the running vertical jump were recorded for each participant. Data were collected using six infra-red Qualisys motion-capture cameras at a 180-Hz sampling rate and two AMTI force platforms at an 1800-Hz sampling rate. Jump height in the 2-LRVJ was significantly higher than that in the 1-LRVJ (p < 0.05). In the take-off phase, knee and hip extension impulses for the 1-LRVJ were significantly greater than those for the 2-LRVJ (p < 0.05). These results suggest that the 1-LRVJ produced greater leg stiffness than the 2-LRVJ did. We found that the 1-LRVJ caused greater lower-extremity stiffness and impulse compared to the 2-LRVJ, which is beneficial in the stretch-shortening cycle, and thus the more focus on practicing 1-LRVJs is recommended for coaches and athletes.

This study aimed to explore the biomechanical differences between single and double peak ground reaction force-time curves during the countermovement jump with respect to kinematics, kinetics, and coordination of the lower extremities. Twenty-five college students were stratified into a single peak curve group and a double peak curve group. Eight infrared cameras and two force platforms were synchronized to collect the data. Independent t-tests were performed with groups for each dependent kinematic, kinetic and time of the joint extensor concentric contraction variable. Repeated one-way analysis of variance measurements were performed for the time of the ankle, knee and hip extensor concentric contraction in each group. The double peak curve was associated with larger jump height, reactive strength index modified, rate of force development, impulse, hip, knee and ankle flexion, extension angular displacement, and hip and knee moments (p<0.05). The double peak curve group revealed a better hip, knee and ankle (proximal to distal) timing of extensor concentric contractions sequence of the lower extremities during the countermovement jump (p<0.05). The double peak curve group exhibited a more effective countermovement jump movement with respect to biomechanics compared to the single peak curve group.

The purpose of this study was to compare the lower extremity inter-joint coordination of different collision forces runners during running braking phase. A dynamical system approach was used to analyse the inter-joint coordination parameters. Data were collected with six infra-red cameras and two force plates. According to the impact peak of the vertical ground reaction force, twenty habitually rearfoot-strike runners were categorised into three groups: high collision forces runners (HF group, n = 8), medium collision forces runners (MF group, n = 5), and low collision forces runners (LF group, n = 7). There were no significant differences among the three groups in the ankle and knee joint angle upon landing and in the running velocity (p > 0.05). The HF group produced significantly smaller deviation phase (DP) of the hip flexion/extension-knee flexion/extension during the braking phase compared with the MF and LF groups (p < 0.05). The DP of the hip flexion/extension-knee flexion/extension during the braking phase correlated negatively with the collision force (p < 0.05). The disparities regarding the flexibility of lower extremity inter-joint coordination were found in high collision forces runners. The efforts of the inter-joint coordination and the risk of running injuries need to be clarified further.