Directional sensitivity of "first trial" reactions in human balance control

Support-surface movements are commonly used to examine balance control. Subjects typically receive a series of identical or randomly interspersed multidirectional balance perturbations and the atypical "first trial reaction" (evoked by the first perturbation) is often excluded from further analysis. However, this procedure may obscure vital information about neurophysiological mechanisms associated with the first perturbation and, by analogy, fully unexpected falls. We studied first trial reactions, aiming to clarify their directional impact on postural control and to characterize the underlying neurophysiological substrate. We instructed 36 subjects to maintain balance following support-surface rotations in six different directions. Perturbations in each direction were delivered in blocks, consisting of 10 serial stimuli. Full body kinematics, surface reactive forces, and electromyographic (EMG) responses were recorded. Regardless of direction, for the very first rotation, displacement of the center of mass was 15% larger compared with the ensuing nine identical rotations (P < 0.0001). This first trial reaction immediately reemerged whenever a new perturbation direction was introduced. First trial reactions (and near-falls) were greatest for backward-directed rotations and smallest for laterally directed rotations. This directional dependence coincided with early changes in vertical head accelerations. First trial reactions in EMG responses involved larger amplitudes in general and earlier muscle response onsets in upper body muscles. These findings show that first trial reactions are associated with significantly increased postural instability, mainly due to increased response amplitudes. Although rapid habituation occurs following presentation of identical stimuli, subjects immediately become unstable again when the perturbation direction suddenly changes. Excessive responses due to a failure to combine proprioceptive and vestibular cues effectively may explain this instability seen with first trials, particularly when falling backward.