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 t echniques. The accelerometer data were analyzed using standard time an d frequency domain analyses and the regularity of the acceleration tim e 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 t he limb segments in the arm coordination postural pointing task. The r esults showed that the level of acceleration increased in a proximal t o distal direction within a single arm and was symmetrical across homo logous arm segments, The frequency analysis showed the established pow er spectral profiles for each limb segment in postural tremor tasks, b ut the finger motion included (beyond the normal 8-12 Hz and 20 Hz tre mor) a third slower peak at around 2-3 Hz, due possibly to the reactiv e forces of the other arm links. There was no effect of vision on the level or frequency patterns of acceleration in the limb segments. The coordination analysis showed that there was no linkage between the arm s in either the time or frequency domain in the execution of this post ural task. This result would tend to suggest that the neuronal command s underlying normal tremor are not derived from a common central oscil lator within the central nervous system but are organized in a paralle l fashion. The strength of the coupling of intra-limb coordination var ied according to the particular adjacent limb links. There were signif icant correlations in the time domain and coherence in the frequency d omain in the acceleration signals between upper arm and forearm, and b etween 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. Signifi cant coherence in the frequency domain was also evident between upper arm and hand motion, with the phase lag between these segments being c lose to 180 deg out of phase. The Ap En analysis of the acceleration s ignals revealed that there was more regularity to the upper arm and ha nd accelerometer signals than the forearm and finger signals. The find ings show that the intra-limb coordination of the arm links in a two-l imb postural pointing task is effected by a compensatory synergy organ ized about the action of the wrist and shoulder joints. This compensat ory synergy reduces the coordination of the 4 within-limb degrees of f reedom (arm links) to, in effect, a single degree of freedom arm contr ol task that is not coupled in organization to the motion of the other limb or the torso, It is proposed that this coordination solution red uces the degrees of freedom in dependently regulated for realization o f the task goal but preserves independent body segment control in crit ical degrees of freedom for potential adaptation to postural perturbat ions.