Fig. 1
Experimental design and recruitment curve. (A) Study overview. Twenty-six able-bodied participants were recruited for this study (AIH group N = 13; SHAM group N = 13). We assessed corticospinal excitability (CSE) to the tibialis anterior (TA) using transcranial magnetic stimulation (TMS) before intervention (‘Pre-TMS’) and after five consecutive days of AIH or SHAM (‘Post-TMS’). To investigate correlations between changes in CSE and motor adaptation, participants in the AIH group performed a split-belt walking task. Retrospective comparisons included previously collected data (AIH group N = 15, Bogard et al., 2023) to evaluate the influence of perturbation size (1:2 vs. 1:1.5 belt speed ratios) on motor adaptation. (B) TMS methodology. We applied single-pulse TMS over the scalp to target the lower limb motor area of the primary motor cortex. A neuronavigational system ensured trial-to-trial consistency in the stimulation vector. The hotspot corresponded to the location that elicited the lowest threshold and shortest latency motor evoked potential (MEP) amplitude. (C) AIH group recruitment curve. MEPs were recorded using surface electromyography (EMG) placed over the tibialis anterior muscle belly. We utilized the peak-to-peak MEP amplitude as an index to evaluate changes in corticospinal excitability, where the onset was identified as the first frame to exceed three standard deviations of the pre-stimulus background EMG. (C) The average recruitment curve for the AIH group (N = 13) was generated by plotting the peak-to-peak MEP amplitude versus the normalized stimulus intensity based on 90% to 140% of the participants’ resting motor threshold (RMT) pre-AIH (grey) and post-AIH (blue). The dashed line represents the Boltzmann slope, the shaded area depicts the area under the curve, and the bars represent standard error. The significance levels of an ANOVA comparing pre-AIH and post-AIH at the stimulus intensities of 100% RMT, 120% RMT, and 140% RMT are denoted as *p < 0.05 and +p = 0.05.
