Maximal frequency, amplitude, kinetic energy and elbow joint stiffness in cyclic movements

The present work deals with those properties of the human motor system that characterize cyclic movements of maximum (expected to be maximum) intensity. It presents the results of an experiment carried out on 11 subjects and aimed at measuring the kinematic characteristics of cyclic movements of the elbow joint executed at maximal frequency under unloaded conditions. The movements of five amplitude levels ranging from 0.1 rad to approximately 1.2 rad were considered. An observable and unequivocal relation was found between the amplitude and the maximal movement frequency. The said relation is one of an inverse relationship types described by the equation of a shifted hyperbola intersecting the frequency axis at a point marking the value of maximal movement frequency f(max) whose mean value was 8.42 Hz in the group investigated. It was also established that elbow joint movements executed at maximal intensity show significant similarity to harmonic movement, which points to the "stiff" characteristic of useful driving torque. Relationships between maximal amplitudes of angular velocity, angular acceleration, kinetic energy and movement amplitude were also determined. The nature of the said relationships derives from the superposition of the two abovementioned features - the amplitude-frequency relation and the formal relationships between the values describing harmonic movement. The elbow joint stiffness manifested during cyclic movements appears to be related to both movement frequency and amplitude. Its value increases with frequency and decreases with amplitude growth ranging from approximately 15 to 130 Nm/rad. The source of the said stiffness is to be found in the properties of the tendon-muscle complex and its changes depend on the changes of muscle tension. This feature has been illustrated by the measurement of the relation between elbow joint stiffness and the static torque generated by elbow joint flexors and extensors. It has been established that the stiffness increases with muscle tension squared.