EMG responses to free fall in elderly subjects and akinetic rigid patients

Objectives-The EMG startle response to free fall was studied in young and o ld normal subjects, patients with absent vestibular function, and patients with akinetic-rigid syndromes. The aim was to detect any derangement in thi s early phase of the "landing response" in patient groups with a tendency t o fall. In normal subjects the characteristics of a voluntary muscle contra ction (tibilais anterior) was also compared when evoked by a non-startling sound and by the free fall startle. Methods-Subjects lay supine on a couch which was unexpectedly released into free fall. Latencies of multiple surface EMG recordings to the onset of fr ee fall, detected by a head mounted linear accelerometer, were measured. Results and conclusions-(1) EMG responses in younger normal subjects occurr ed at: sternomastoid 54 ms, abdominals 69 ms, quadriceps 78 ms, deltoid 80 ms, and tibialis anterior 85 ms. This pattern of muscle activation, which i s not a simple rostrocaudal progression, may be temporally/spatially organi sed in the startle brainstem centres. (2) Voluntary tibialis EMG activation was earlier and stronger in response to a startling stimulus (fall) than i n response to a non-startling stimulus (sound). This suggests that the star tle response can be regarded as a reticular mechanism enhancing motor respo nsiveness. (3) Elderly subjects showed similar activation sequences but del ayed by about 20 ms. This delay is more than can be accounted for by stowin g of central and peripheral motor conduction, therefore suggesting age depe ndent delay in central processing. (4) Avestibular patients had normal late ncies indicating that the free fall startle can be elicited by non-vestibul ar inputs. (5) Latencies in patients with idiopathic Parkinson's disease we re normal whereas responses were earlier in patients with multiple system a trophy (IMSA) and delayed or absent in patients with Steele-Richardson-Olsz ewski (SRO) syndrome. The findings in this patient group suggest: (1) lack of dopaminergic influence on the timing of the startle response, (2) concur rent cerebellar involvement in IMSA may cause startle disinhibition, and (3 ) extensive reticular damage in SRO severely interferes with the response t o free fall.