Figure 3.

Responses of upper-limb muscles during TSS delivered along the rostrocaudal axis

(A) Recruitment curves of left BIC, TRIC, FCR, ECR, APB, and ADM during stimulation delivered at EL03, EL07, EL11, and EL15. Responses were normalized for each muscle using the maximum value (MaxR) across all amplitudes and sites.

(B) Heatmap of motor thresholds (MT) of the UL muscles. The MT values for each muscle were normalized to the MT of FCR at EL07.

(C) Averaged data of the MT intensity at rostrocaudal stimulation sites (see also Table S3).

(D) Parallel coordinates plots between rostrocaudal stimulation sites to the upper limb muscles derived from maximum response. Individual muscles are grouped along the right of the plot. Thickness of connections denotes the amplitude of the maximum motor response.

(E) Averaged data of the MaxR at rostrocaudal stimulation sites.

(F) Averaged data of the R2/R1 ratio at rostrocaudal stimulation sites. Box range was set as percentage 25%–75%, bold vertical red lines in boxplots present the median values, and whiskers indicate the 95% confidence interval. A post-hoc Holm-Bonferroni correction was carried out for multiple comparison within figures. Significant differences are indicated with vertical lines. Dotted lines: p < 0.05; dashed lines: p < 0.01; and solid lines: p < 0.001. TSS: transcutaneous spinal stimulation, BIC: biceps brachii; TRIC: triceps brachii; FCR: flexor carpi radialis; ECR: extensor carpi radialis; APB: abductor pollicis brevis; ADM: abductor digiti minimi.