Experimental protocol
Each participant underwent one active and one sham tVNS stimulation in a randomized order on two different days, with a minimum interval of 24 h in between. On each day, electrocardiographic and blood pressure recordings were performed continuously in a relaxed and seated position for the duration of the experiment using the Task Force Monitor (CNSystems, Graz, Austria). The study protocol was separated into three different tasks (forearm stimulation/resting period, rhythmic breathing at 0.2 Hz, and rhythmic breathing at 0.1 Hz). These tasks were repeated three times, before (baseline), during and immediately after the active or sham tVNS (post-stimulation) (Fig. Experimental protocol of the randomized crossover study. Study participants received the application of sham and active tVNS on two different days. Following pre-experimental blood collection, younger participants were subjected to electrical nerve stimulation on the forearm, whereas older participants rested for 20 min. Afterwards, two rhythmic breathing tasks at 0.2 Hz and 0.1 Hz were performed for three minutes each. The consecutive sequence from 1 to 3 was repeated three times. The experimental day ended with the second blood collection.