Evidence for an Intricate Relationship Between Express Visuomotor Responses, Postural Control and Rapid Step Initiation in the Lower Limbs

Recent work has described express visuomotor responses (EVRs) on the upper limb. EVRs are directionally-tuned bursts of muscle activity that occur within 100 ms of visual stimulus appearance, facilitating rapid reaching. Rapid stepping responses are also important in daily life, and while there is evidence of EVR expression on lower limbs, it is unknown whether lower-limb EVRs are influenced by increased postural demands. Here, we investigate the interaction between stepping-related EVRs and anticipatory postural adjustments (APAs) that typically precede step initiation. 16 healthy young subjects rapidly stepped towards visual targets presented in front of the left or right foot. We recorded bilateral surface EMG of gluteus medius (GM), a muscle involved in both APAs and stepping, and bilateral ground reaction forces. Two conditions were introduced: a lateral or medial stepping condition with reduced or increased postural demands, respectively. In the lateral stepping condition, EVRs were robustly and strongly present in stance-side GM, and ground reaction forces revealed the absence of APAs. Larger EVRs preceded shorter RTs, consistent with EVRs facilitating step initiation. In contrast, in the medial stepping condition, EVRs were largely absent, and ground reaction forces revealed the consistent presence of APAs. When occasionally present, EVRs in the medial stepping condition preceded larger APAs and longer RTs. Thus, while EVRs in lower limbs can facilitate rapid stepping, their expression is normally suppressed when postural stability is low. Failing to appropriately suppress EVRs in such situations disrupts postural stability, necessitating larger compensatory APAs and leading to longer stepping RTs. Key Points Express visuomotor responses (EVRs) are directionally tuned bursts of muscle activity that aid the rapid initiation of a goal-directed movement. They are thought to be relayed to the motor periphery along a rapid subcortical pathway involving the superior colliculus. While EVRs have predominantly been studied in reaching, it is unclear whether EVRs extend to the lower extremities and if so, whether increasing the postural demands of a stepping task interfere with lower-limb EVR expression. We found that when postural demands were low, strong EVRs in the hip abductor muscle gluteus medius facilitated a rapid stepping response. Conversely, when postural demands were high, EVRs hindered a fast stepping response, as they necessitated larger, compensatory postural adjustments prior to step onset. These results help us better understand the interaction between ultra-rapid visuomotor transformations in the EVR network, the postural demands of a given stepping task, and subsequent step initiation.