• The effect of postural threat on the scaling of anticipatory postural adjustments in healthy young adults

      Phanthanourak, Angel; Applied Health Sciences Program
      Earlier and larger anticipatory postural adjustments (APAs) are generated with increasingly destabilizing movements, such as pulling more forcefully onto a handle, to prevent a loss of balance. The purpose of this study was to determine the influence of postural threat on the ability to scale the magnitude and timing of APAs to increasing amounts of force exertion. Nineteen participants (7 F, 24 ± 2 y, 69.6 ± 9.9 kg, 1.7 ± 0.1 m) pulled on a handle while standing on a surface that was either stationary (No Threat) or that could translate in the medio-lateral direction (Threat). For both conditions, participants completed 36 handle pulls that ranged between 50% and 100% of the participant’s maximal force exertion. For each handle pull trial, APAs were quantified from center of pressure (COP) recordings and electromyographic (EMG) activity of the posterior leg muscles. Results indicated that participants were more physiologically aroused (p=0.013), anxious (p<0.001), and fearful of falling (p<0.001) during the Threat compared to No Threat condition. This threat response was associated with a reduced ability for participants to scale the magnitude of APAs to the amount of force exertion. This was evidenced by 22% shallower regression lines between COP displacement at pulling onset and force exertion during the Threat compared to the No Threat condition (p=0.019). The scaling of APA timing was affected by threat to a lesser extent, as only the regression lines between medial gastrocnemius EMG onset and force exertion were shallower (37%) during the Threat compared to the No Threat condition (p=0.049). Regression lines for COP onset and all other posterior leg EMG amplitudes and onsets to force exertion were not different between conditions. These findings suggest that increased anxiety and fear of falling may contribute to the declines in APA scaling demonstrated by individuals at an increased fall risk (e.g., older adults).