Knee muscular moment, tendon tension force and EMG during a vigorous movement in man.

With injuries to the components of the extensor apparatus of the knee as a background, it is interesting to investigate the magnitude of forces acting on these components, i.e. m. quadriceps femoris, the quadriceps tendon, patella, lig. patellae and tuberositas tibiae, during a vigorous but physiological movement. By means of the dynamic laws of mechanics the muscular moment of force with respect to the bilateral knee axis during kicking was calculated in 6 normal subjects. It was found that the maximum extending muscular moment in the knee occurs very early in the movement, when the initial flexion changes into extension, and thus long before the ball is hit. The peak of quadriceps EMG activity coincides with maximum moment. The EMG peak of the antagonistically acting hamstrings comes later, nearer to when the ball is struck. The greatest extending muscular moment obtained during the swing phase of kicking was surprisingly high, 260 Nm, corresponding to a tension force in the patellar tendon of 5200 N or about 7 times body weight. These values are discussed in relation to tendon strength.