Forearm tremor during three different isometric loadings

Forearm tremor was studied during a spring (stiffness 1090 N.m-1), a rigid isometric and a "dynamic" isometric (carrying a freely hanging mass) loading at the level of 50% of maximal isometric voluntary contraction. Thirteen physical education students ranging in age between 20 and 28 years flexed their dominant forearm isometrically towards the vertical direction (90 degree elbow angle) against the three different loads on three test occasions seated on a dynamometer which measured the force at the wrist together with vertical tremor (accelerometer). A power spectrum density function was established for the tremor (acceleration) between 1.0 and 19.9 Hz. A bandwidth of 6.9-19.9 Hz was subsequently analyzed in more detail including the determination of peak power (PMAX), peak power frequency (FMAX), mean power frequency (MPF), and average power (PAVER) as well as proposition (%) of the whole spectrum occupied by the selected bandwidth. The FMAX, MPF and band percentage variables had the best reproducibility (Cronbach's Alpha 0.85-0.95), while for the PMAX and PAVER the coefficients were lower but still satisfactory (0.69-0.89). The coefficients were rather similar for all three loading conditions. In the spring loading the spectrum components inside the analyzed frequency band occupied almost 90% of the whole spectrum, FMAX was more clearly distinguished from the rest of the spectrum, and tremor amplitude was higher and tremor frequency lower than in the rigid isometric and "dynamic" loadings. The respective tremor amplitude and frequency characteristics showed statistically significant correlations between the rigid isometric and "dynamic" loading conditions.(ABSTRACT TRUNCATED AT 250 WORDS)