Timing of anticipatory postural adjustments (APAs) is altered in cerebellar ataxia

We recently proposed that APAs not only guarantee the fixation chain for the prime mover muscle and the maintenance of whole body equilibrium, but they are also essential in controlling movement trajectory and its final position, i.e. metria (Caronni and Cavallari, Exp Brain Res 194:59-66,2009). According to this hypothesis, cerebellar dysmetria, i.e. the inability to accurately reach a specific target, could result not only from an altered activation of prime mover, but also from an impairment of the associated APAs. We searched for a link between cerebellar dysmetria and the distribution/temporization of APAs in 13 patients suffering from a slowly progressive cerebellar atrophy, without cerebral cortex, pons, medulla or white matter lesions. Results were compared to those obtained in 11 adult healthy subjects. Participants performed brisk index-finger flexions while EMG was recorded in Flexor Digitorum Superficialis (FDS, prime mover) and in the postural muscles Triceps Brachii (TB), Biceps Brachii (BB) and Anterior Deltoid (AD). The pattern of postural activity was similar in the two groups (excitation in TB, inhibition in BB and AD) but apparent modifications in timing were observed in 9 patients, in which TB excitation and/or BB and AD inhibition lagged the FDS onset by 20-105ms, instead of anticipating it by 1, 22 and 44ms, respectively (averages of healthy subjects). Although the role of cerebellum in generating or transmitting APAs still remains open, these data support the idea that both prime mover onset and the temporal distribution of related APAs are set by the "cerebellar timing machine".