Postural Threat Increases Corticospinal Excitability in the Trunk Flexor Muscles in the Upright Stance

It has been well established that fear increases corticospinal excitability in the hand muscles. Postural threat, a kind of fear, may have a unique effect on corticospinal excitability in the trunk flexor muscles during the upright stance on high ground. We investigated whether postural threat modulates corticospinal excitability in the trunk flexor muscle in the upright stance. Seven healthy humans maintained an upright stance on a 60-cm-high platform under two support surface conditions; one was a support surface with a support base that had a large margin, and the other was a support surface with a support base that had a small margin. Transcranial magnetic stimulation was delivered to the right motor cortex, and motor evoked potentials (MEPs) were recorded from the internal oblique muscles. The perceived fear when maintaining the upright stance on the platform with a small margin of support base was significantly higher than that when maintaining the upright stance on the platform with a large margin of support base. The MEP area when maintaining the upright stance on the platform with the small base support margin was significantly larger than that when maintaining the upright stance on the platform with the large support base margin. The background electromyographic activity and the distribution of weight loading between the forefoot and the hindfoot, reflecting postural sway along the anteroposterior axis, were not significantly different between the support surface conditions. These findings indicate that postural threat increases the excitability of the crossed and uncrossed corticospinal pathways innervating the trunk flexor muscles in the upright stance. The effect is not necessarily mediated by postural sway or stiffness strategy, but it may be mediated by cognitive processes.