School of Sport, Exercise and Rehabilitation Sciences, University of Birmingham, Birmingham, UK.
Vision and Eye Research Unit, Postgraduate Medical Institute, Anglia Ruskin University, Cambridge, UK.
J Physiol. 2017 Nov 1;595(21):6771-6782. doi: 10.1113/JP274272. Epub 2017 Sep 22.
When standing and holding an earth-fixed object, galvanic vestibular stimulation (GVS) can evoke upper limb responses to maintain balance. In the present study, we determined how these responses are affected by grip context (no contact, light grip and firm grip), as well as how they are co-ordinated with the lower limbs to maintain balance. When GVS was applied during firm grip, hand and ground reaction forces were generated. The directions of these force vectors were co-ordinated such that the overall body sway response was always aligned with the inter-aural axis (i.e. craniocentric). When GVS was applied during light grip (< 1 N), hand forces were secondary to body movement, suggesting that the arm performed a mostly passive role. These results demonstrate that a minimum level of grip is required before the upper limb becomes active in balance control and also that the upper and lower limbs co-ordinate for an appropriate whole-body sway response.
Vestibular stimulation can evoke responses in the arm when it is used for balance. In the present study, we determined how these responses are affected by grip context, as well as how they are co-ordinated with the rest of the body. Galvanic vestibular stimulation (GVS) was used to evoke balance responses under three conditions of manual contact with an earth-fixed object: no contact, light grip (< 1 N) (LG) and firm grip (FG). As grip progressed along this continuum, we observed an increase in GVS-evoked hand force, with a simultaneous reduction in ground reaction force (GRF) through the feet. During LG, hand force was secondary to the GVS-evoked body sway response, indicating that the arm performed a mostly passive role. By contrast, during FG, the arm became actively involved in driving body sway, as revealed by an early force impulse in the opposite direction to that seen in LG. We then examined how the direction of this active hand vector was co-ordinated with the lower limbs. Consistent with previous findings on sway anisotropy, FG skewed the direction of the GVS-evoked GRF vector towards the axis of baseline postural instability. However, this was effectively cancelled by the hand force vector, such that the whole-body sway response remained aligned with the inter-aural axis, maintaining the craniocentric principle. These results show that a minimum level of grip is necessary before the upper limb plays an active role in vestibular-evoked balance responses. Furthermore, they demonstrate that upper and lower-limb forces are co-ordinated to produce an appropriate whole-body sway response.
当站立并握住一个固定于地面的物体时，电刺激前庭（GVS）可以引起上肢反应以维持平衡。在本研究中，我们确定了这些反应如何受到握持方式（无接触、轻握和紧握）的影响，以及它们如何与下肢协调以维持平衡。当 GVS 在紧握时施加时，会产生手部和地面反作用力。这些力矢量的方向协调一致，使得整个身体的摆动响应始终与两耳轴（即头心轴）对齐。当 GVS 在轻握（<1N）时施加时，手部力次于身体运动，这表明手臂主要起被动作用。这些结果表明，在上肢在平衡控制中变得活跃之前，需要一定程度的握持力，并且上肢和下肢协调以产生适当的整个身体摆动响应。
前庭刺激可以在手臂用于平衡时引起手臂反应。在本研究中，我们确定了这些反应如何受到握持方式的影响，以及它们如何与身体其他部位协调。使用电刺激前庭（GVS）在三种与固定于地面的物体接触程度下诱发平衡反应：无接触、轻握（<1N）（LG）和紧握（FG）。随着握持力沿着这个连续体的进展，我们观察到 GVS 诱发的手部力增加，同时通过脚部的地面反作用力（GRF）减少。在 LG 时，手部力次于 GVS 诱发的身体摆动响应，表明手臂主要起被动作用。相比之下，在 FG 时，手臂通过与 LG 相反的方向的早期力脉冲而积极参与驱动身体摆动，手臂成为主动参与驱动身体摆动的角色。然后，我们研究了这种主动手部向量的方向如何与下肢协调。与先前关于摆动各向异性的发现一致，FG 将 GVS 诱发的 GRF 向量的方向倾斜到基线姿势不稳定的轴上。然而，这通过手部力向量有效地抵消，使得整个身体的摆动响应保持与两耳轴对齐，维持了头心轴原理。这些结果表明，在上肢在前庭诱发的平衡反应中发挥主动作用之前，需要一定程度的握持力。此外，它们表明上肢和下肢力协调产生适当的整个身体摆动响应。
