Methods, and primary and secondary outcomes for a randomized, double-blind, crossover, sham-controlled trial of non-invasive vagal stimulation in healthy volunteers. tVNS, transcutaneous vagus nerve stimulation; CPET, cardiopulmonary exercise test; RNaseq, RNA sequencing; IL-1β mRNA, interleukin-1 beta mRNA; JAK–STAT, Janus kinase signal transducer and activator of transcription; NF-kappa B, nuclear factor kappa-light-chain-enhancer of activated B cells; TNF, tumour necrosis factor²⁴