Inter- and intra-limb coordination in arm tremor

Inter- and intra-limb coordination in arm tremor was examined in adult subjects under vision and no vision conditions using accelerometery techniques. The accelerometer data were analyzed using standard time and frequency domain analyses and the regularity of the acceleration time series was determined using an approximate entropy (Ap En) measure. The data analysis was structured to examine the hypothesis that there is a functional compensatory relation between the motion (tremor) of the limb segments in the arm coordination postural pointing task. The results showed that the level of acceleration increased in a proximal to distal direction within a single arm and was symmetrical across homologous arm segments. The frequency analysis showed the established power spectral profiles for each limb segment in postural tremor tasks, but the finger motion included (beyond the normal 8-12 Hz and 20 Hz tremor) a third slower peak at around 2-3 Hz, due possibly to the reactive forces of the other arm links. There was no effect of vision on the level or frequency patterns of accleration in the limb segments. The coordination analysis showed that there was no linkage between the arms in either the time or frequency domain in the execution of this postural task. This result would tend to suggest that the neuronal commands underlying normal tremor are not derived from a common central oscillator within the central nervous system but are organized in a parallel fashion. The strength of the coupling of intra-limb coordination varied according to the particular adjacent limb links. There were significant correlations in the time domain and coherence in the frequency domain in the acceleration signals between upper arm and forearm, and between hand and finger. The phase lag of the arm units within each of these respective segment pairs was close to in phase or 0 deg. Significant coherence in the frequency domain was also evident between upper arm and hand motion, with the phase lag between these segments being close to 180 deg out of phase. The Ap En analysis of the acceleration signals revealed that there was more regularity to the upper arm and hand accelerometer signals than the forearm and finger signals. The findings show that the intra-limb coordination of the arm links in a two-limb postural pointing task is effected by a compensatory synergy organized about the action of the wrist and shoulder joints. This compensatory synergy reduces the coordination of the 4 within-limb degrees of freedom (arm links) to, in effect, a single degree of freedom arm control task that is not coupled in organization to the motion of the other limb or the torso. It is proposed that this coordination solution reduces the degrees of freedom independently regulated for realization of the task goal but preserves independent body segment control in critical degrees of freedom for potential adaptation to postural perturbations.