Abstract: BACKGROUND: Abnormal joint movements are associated with injury. Joint coordination is commonly used to analyze joint movements and muscle activity can potentially affect inter-joint coordination. However, there are yet no sufficient studies in this field. 
OBJECTIVE: To identify the inter-segment coordination patterns and muscle activity levels of the hip and knee joints during drop jump at different heights and the correlation between them. 
METHODS: Nineteen young men were tested with 40, 60, and 80 cm drop jumps, and the electromyographic activity of the gluteus maximus, semitendinosus, rectus femoris, vastus lateralis was measured, hip and knee joint angles were calculated and coordination patterns were calculated using a vector coding technique. Changes in lower limb muscle activity and hip and knee coordination patterns were compared using one-way repeated measures of variance. Pearson and Spearman analyses were used to relate the percentage of inter-segment coordination patterns to EMG activity levels. 
RESULTS AND CONCLUSION: There were significant differences in the percentage of in-phase and knee-phase in the sagittal plane between 40 and 60 cm, 40 and 80 cm, and 60 and 80cm during the contact - take-off of drop jumps at different heights (P < 0.001). There were significant differences in the percentage of in-phase and knee-phase in the horizontal plane between 40 and 80 cm as well as between 60 and 80 cm (P < 0.001). Muscle activity of the semitendinosus was significantly different between 40 and 60 cm (P < 0.05). Muscle activity of the rectus femoris was significantly different between 40 and 80 cm (P < 0.05). Muscle activity of the semitendinosus at 40 cm was positively correlated with the percentage of in-phase in the sagittal plane (r=0.46, P=0.046). Muscle activity of the gluteus maximus at 40 cm and that of the rectus femoris at 80 cm both had a positive correlation with the percentage of in-phase in the horizontal plane (r=0.43, P=0.064; r=0.45, P=0.069). To conclude, as height increases, hip and knee in-phase coordination decreases and distal knee dominant coordination increases, with better coordination of the lower limb joints at the height of 40 cm and 60 cm, predicting a lower risk of injury when training at this gradient. Muscle activity levels can increase with height and muscle activities of the semitendinosus, gluteus maximus and rectus femoris are associated with coordinated hip and knee movements.

Key words: drop jump, hip joint, knee joint, motor coordination, muscle activity, vector coding