Vertical Stiffness in Functional Upper Limb Synergy

FUS is mainly completed in the sagittal plane and by flexion of the knee joint. The hip joint and trunk remain relatively stable. Therefore, the LLS of athletes in the take-off preparation stage and landing r stage of FUS could be expressed by vertical stiffness. The calculation formula was as follows: $Kvert=\frac{Fmax}{∆y}$ ; where Fmax is the peak value of GRF, and $∆y$ is the vertical displacement of the body’s center of gravity. The vertical displacement could be calculated by the proportion of human morphological links (David, 2009 ), height, and angle of knee flexion extension ROM in the service preparation stage and the landing buffer stage. The calculation formula was as follows: $∆y=L_{thigh}-L\times \sqrt{{L_{shank}}^2+{L_{thigh}}^2-2\times L_{shank}\times L_{thigh}\times \cos \alpha }$ , $L_{knee}=0.039\times H$ , $L_{shank}=0.285\times H$ , $L_{thigh}=0.53\times H$ .