INTERACTIONS BETWEEN VESTIBULAR AND PROPRIOCEPTIVE INPUTS TRIGGERING AND MODULATING HUMAN BALANCE-CORRECTING RESPONSES DIFFER ACROSS MUSCLES

Interactions between proprioceptive and vestibular inputs contributing to the generation of balance corrections may vary across muscles depe nding on the availability of sensory information at centres initiating and modulating muscle synergies, and the efficacy with which the musc le action can prevent a fall. Information which is not available from one sensory system may be obtained by switching to another. Alternativ ely, interactions between sensory systems and the muscle to which this interaction is targeted may be fixed during neural development and no t switchable. To investigate these different concepts, balance correct ions with three different sets of proprioceptive trigger signals were examined under eyes-open and eyes-closed conditions in the muscles of normal subjects and compared with those of subjects with bilateral per ipheral vestibular loss. The different sets of early proprioceptive in puts were obtained by employing three combinations of support surface rotation and translation, for which ankle inputs were nulled, normal o r enhanced, the knees were either locked or in flexion, and the trunk was either in flexion or extension. Three types of proprioceptive and vestibulospinal interactions were identified in muscles responses. The se interactions were typified by the responses of triceps surae, quadr iceps, and paraspinal muscles. The amplitudes of stretch responses at 50 ms after the onset of ankle flexion in triceps surae muscles were r elated to the velocity of ankle stretch. The amplitude of balance-corr ecting responses at 100 ms corresponded more with stretch of the biart icular gastrocnemius when the knee was re-extended at 60 ms. Absent st retch reflexes at 50 ms in triceps surae with nulled ankle inputs caus ed a minor, 12-ms delay in the onset of balance-correcting responses i n triceps surae muscles. Vestibular loss caused no change in the ampli tude of balance-correcting responses, but a negligible decrease in ons et latency in triceps surae even with nulled ankle inputs. Stretch res ponses in quadriceps at 80 ms increased with the velocity of knee flex ion but were overall lower in amplitude in vestibular loss subjects. B alance-correcting responses in quadriceps had amplitudes which were re lated to the directions of initial trunk movements, were still present when knee inputs were negligible and were also altered after vestibul ar loss. Stretch and unloading responses in paraspinals at 80 ms were consistent with the direction of initial trunk flexion and extension. Subsequent balance-correcting responses in paraspinals were delayed 20 ms in onset and altered in amplitude by vestibular loss. The changes in the amplitudes of ankle (tibialis anterior), knee (quadriceps) and trunk (paraspinal) muscle responses with vestibular loss affected the amplitudes and timing of trunk angular velocities, requiring increased stabilizing tibialis anterior, paraspinal and trapezius responses pos t 240 ms as these subjects attempted to remain upright. The results su ggest that trunk inputs provide an ideal candidate for triggering bala nce corrections as these would still be present when vestibular, ankle and knee inputs are absent. The disparity between the amplitudes of s tretch reflex and automatic balance-correcting responses in triceps su rae and the insignificant alteration in the timing of balance-correcti ng responses in these muscles with nulled ankle inputs indicates that ankle inputs do not trigger balance corrections. Furthermore, modulati on of balance corrections normally performed by vestibular inputs in s ome but not all muscles is not achieved by switching to another sensor y system on vestibular loss. We postulate that a confluence of trunk a nd upper-leg proprioceptive input establishes the basic timing of auto matic, triggered balance corrections which is then preferentially weig hted by vestibular modulation in muscles that prevent falling. The org anisation of balance corrections around trunk inputs portrayed here wo uld have considerable advantage for the infant learning balance contro l, but forces balance control centres to rely on limited sensory infor mation related to this most unstable body segment, the trunk, when tri ggering balance corrections.