Figure 1.

Study design and trial representation. The experiment consisted of one session to calibrate the level of force and pain for each participant, and three subsequent phases. During the calibration phase, we assessed the levels of pain necessary to induce a sensation of low, medium, and high pain for each individual. We also assessed the maximum voluntary contraction (MVC) of the dominant arm for each participant. During the natural history phase, we delivered a medium level of pain for a total of 10 trials. In half of the trials, participants performed the isotonic movement tailed to their individual MVC. During the acquisition phase, the participants learned to associate three visual color cues (red, yellow, green) with three distinct levels of pain (high, medium and low pain intensities) for a total of 12 trials. Participants performed the isotonic movement in half of the trials. During the test phase, we set the intensity of the painful stimulations at the same medium level for all the three cues (presented 6 times for each visual color cue) and the isotonic task was introduced in half of the trials. (A) During each trial of the acquisition and test phase, after the color cue presentation, participants were asked to rate the level of expectation about the upcoming stimulation by means of a 0–100 VAS ranging from no expected pain to maximum expected pain. Then, the painful stimulation was delivered for 10 seconds. After two seconds of rest, participants were asked to rate the level of perceived pain by means of the 0–100 VAS ranging from 0 = no pain and 100 = maximum tolerable pain. Participants were alerted whether or not to perform the isotonic movement by a sign (e.g. arm performing the movement) displayed on the monitor. An inter-trials interval with variable timing was introduced to avoid habituation (B).