Abstract: BACKGROUND: Previous research has indicated that the height of a drop jump has a significant impact on the knee joint and surrounding muscles; however, investigations on the distance of the drop jump are rare and relevant mechanical simulations combined with surface electromyography studies are particularly lacking.
OBJECTIVE: To compare the mechanical features of the knee joint and the activation characteristics of the surrounding muscles during drop jumps at various heights and distances.
METHODS: Based on varied heights and distances, the motions were separated into four groups (height×distance): group A (0.45 m×0.50 m), group B (0.45 m×0.90 m), group C (0.60 m×0.50 m), and group D (0.60 m×0.90 m). Sixteen male testers with no history of lower limb injury were recruited and selected. The Qualisys motion capture system, AMTI force plate, and Delsys surface electromyography system were used to collect data on kinematic, kinetic, and electromyographic signals during the subjects’ drop jumps. OpenSim 4.3 simulation software was used to calculate knee joint kinetics and muscle activation performance.
RESULTS AND CONCLUSION: Under the 0.45 m×0.50 m condition, increases in both distance and height increased the knee flexion angle (P=0.033, P=0.024). Enhanced distance lowered the maximum muscle strength of the rectus femoris and middle femoris (P=0.010, P=0.007), but increased the maximum muscle strength of the anterior tibialis (P=0.018) at a height of 0.60 m. The activation of the hamstrings and antagonist muscles was impacted by drop jump height and distance during the pre-activation period. Increased distance raised the co-activation ratio of the knee (P=0.045) and lowered the co-activation indexes of the biceps femoris:lateralis femoris and medialis femoris:lateralis femoris (P=0.016, P=0.012). Increased height elevated the co-activation ratio of the knee joint, but no significant difference was found among groups. These findings indicate that increasing the distance within a given range reduces joint stresses on the knee joint during drop jumps while also increasing its stability and surrounding muscle activation, and reducing the peak strength of the rectus femoris and middle femoris muscles. A drop jump of 0.45 m×0.90 m in height and distance has better stability and muscle coordination and contraction than the other groups, but the height and distance should be modulated considering the sport-specific goals.

Key words: drop jump, height, distance, knee joint, simulation, kinematics, kinetics, muscle activation