Knee angle as a determinant of lower leg tremor

The effects of knee joint angle (muscle length) and spring load stiffness on some mechanical parameters of lower leg tremor were studied among twelve male subjects during isometric knee extensions against three springs differing in stiffness at knee angles of 90, 110, and 130 degrees. The power spectral density function was estimated for the acceleration (tremor) signal. The results showed knee angular position to have a significant influence on the lower leg tremor parameters so that higher peak frequencies and lower tremor amplitude values were typical for the shorter knee extensor lengths. Frequency changes with knee angle were explained by the effect of changes in stiffness of a spring-mass system (knee extensor musculature) as a function of muscle length. It was suggested that tremor amplitude is influenced by motor unit recruitment and firing characteristics.