Balance control and anti-gravity muscle activity during the experience of fear at heights

Abstract

Fear of heights occurs when a visual stimulus causes the apprehension of losing balance and falling. A moderate form of visual height intolerance (vHI) affects about one third of the general population and has relevant consequences for the quality of life. A quantitative evaluation of balance mechanisms in persons susceptible to vHI during height exposure is missing. VHI-related changes in postural control were assessed by center-of-pressure displacements and electromyographic recordings of selected leg, arm, and neck muscles in 16 subjects with vHI while standing at heights on an emergency balcony versus standing in the laboratory at ground level. Characteristics of open- and closed-loop postural control were analyzed. Body sway and muscle activity parameters were correlated with the subjective estimates of fear at heights. During height exposure, (1) open-loop control was disturbed by a higher diffusion activity (P < 0.001) and (2) the sensory feedback threshold for closed-loop control was lowered (P < 0.010). Altered postural control was predominantly associated with increased co-contraction of leg muscles. Body sway and leg and neck muscle co-contraction correlated with the severity of subjective anxiety (P < 0.050). Alterations in postural control diminished if there were nearby stationary contrasts in the visual surrounding or if subjects stood with eyes closed. The performance of a cognitive dual task also improved impaired balance. Visual heights have two behavioral effects in vHI subjects: A change occurs in (1) open- and closed-loop postural control strategy and (2) co-contraction of anti-gravity leg and neck muscles, both of which depend on the severity of evoked fear at heights.

Keywords: Balance control; fear of heights; muscle activity; muscle co‐contraction; visual height intolerance.

Figure 1.

Experimental setup. Participants were asked to stand on a force plate on a 15‐m‐high balcony to assess center‐of‐pressure displacements. Electromyographic data of three muscle pairs were recorded with a mobile EMG device fixed around the waist: (1) tibialis anterior and soleus for the leg (2), biceps brachii and triceps brachii for the arm, and (3) sternocleidomastoid and semispinalis capitis for the neck. The stance protocol included the following conditions: (1) eyes open and normal head tilt, (2) eyes open and head flexion, (3) eyes open and head extension, (4) eyes closed, (5) eyes open while performing a cognitive dual task, and (6) eyes closed while performing a cognitive dual task. The complete stance protocol was repeated in the laboratory at ground level.

Figure 2.

Sample stabilogram diffusion plot. Representative stabilogram diffusion plots (dotted lines) and fitted regressions (solid lines) of CoP displacement in the anterior–posterior direction of one individual susceptible to visual height intolerance, while quietly standing on the balcony (black lines) and while quietly standing in the laboratory at ground level (gray lines). The short‐term diffusion coefficient D_s, the long‐term diffusion coefficient D_l, and the critical point CP are shown for both stance conditions. During height exposure, the short‐term range shows an increased diffusion activity, indicating abnormal open‐loop control. In addition, the critical time interval CP, at which short‐term behavior changes into long‐term behavior, is shortened, indicating a precipitate intervening of closed‐loop control into the postural control scheme.

Figure 3.

Stabilogram diffusion analysis parameters. (A) the short‐term (open‐loop) diffusion coefficient D_s, (B) the long‐term (closed‐loop) diffusion coefficient D_l, (C) the critical time interval ∆t_c, and (D) the critical mean squared displacement <∆r²>_c. Standing on the balcony (black bars), standing in the laboratory (gray bars). Stance conditions: eyes open and normal head tilt (EO), eyes open and head flexion (EOHF), eyes open and head extension (EOHE), eyes closed (EC), eyes open while performing a cognitive dual task (EODT), and eyes closed while performing a cognitive dual task (ECDT). During height exposure under the conditions EO and EOHE postural control is altered in (1) increased short‐term diffusion activity, indicating abnormal open‐loop control, and (2) shortened critical time intervals, indicating that the primary sensory feedback threshold of the postural control system is lowered.

Figure 4.

Co‐contraction muscle activity. Co‐contraction index of (A) the leg muscles (i.e., tibialis anterior and soleus muscle), (B) the arm muscles (i.e., biceps brachii and triceps brachii muscle), and (C) the neck muscles (i.e., sternocleidomastoid and semispinalis capitis muscle). Stance conditions: eyes open and normal head tilt (EO), eyes open and head flexion (EOHF), eyes open and head extension (EOHE), eyes closed (EC), eyes open while performing a cognitive dual task (EODT), and eyes closed while performing a cognitive dual task (ECDT). During height exposure co‐contraction increases in leg muscles under all stance conditions except EOHF.