Protective balance and startle responses to sudden freefall in standing humans

• First trial posture responses (FTR) to external (EXT) and self (SLF) set freefall.
• Greater incidence of rapid EXT FTR neck muscle activation than SLF condition.
• Incidence and amplitude of EXT neck and arm FTRs habituated with repeated trials.
• EXT FTR freefall includes a startle incorporated with postural responses.

The aim of the present study was to investigate whether or not startle reactions contribute to the whole body postural responses following sudden freefall in standing humans. Nine healthy participants stood atop a moveable platform and received externally-triggered (EXT) and selftriggered (SLF) drop perturbations of the support surface. Electromyographic (EMG) activity was recorded bilaterally over the sternocleidomastoid (SCM), deltoid (DLT), biceps brachii (BIC), medial gastrocnemius (GAS), and tibialis anterior (TA) muscles. Whole-body kinematics were also recorded with motion analysis. Rapid phasic activation of SCM during the first trial response (FTR) was seen for all participants for EXT and for 56% of subjects for SLF. Reductions in EMG amplitude between the EXT FTR and later trial responses for SCM, DLT, and BIC and reduced arm movement acceleration indicative of habituation occurred and exceeded adaptive reductions for SLF. These findings suggested that a startle reflex contributes to the exaggerated postural FTR observed during externally-triggered whole-body free falls.