The patient will have to walk on a treadmill at their preferential walking speed for 10 minutes while visualizing neutral images (landscapes, monuments...)

The patient will have to walk on a treadmill at their preferential walking speed for 10 minutes while visualizing images interpreted as nocive (cutting, jumping, contact...)

Before the start of the experimental condition, the patient will be asked to think about their willingness to perform the first movement visualised in the video, while a researcher places a jumping platform inside the laboratory.

During the test, the patient will have to walk on a treadmill at their preferred speed for 10 minutes, while watching a video showing a drop jump followed by images interpreted as nocive for the rest of the time.

The patient will have to walk on a treadmill at their preferential walking speed for 10 minutes while visualizing neutral images (landscapes, monuments...)

The fractal scaling exponent quantifies the complexity of a physiological signal and detects the presence of statistical persistence in a given time series. This parameter is calculated using Detrended Fluctuation Analysis (DFA) from each time series (stride time and stride length).

The R-R intervals will be derived from beat-to-beat blood pressure pulse intervals using finger plethysmography (Finapres® Nova, Amsterdam, Netherlands), at rest and during the gait trials. Finger plethysmography-derived peak-to-peak intervals are highly correlated with ECG R-R intervals, with similar variability. The upstroke is determined using the pressure signal with a resolution of 2 ms, and the interval between the two consecutive upstrokes is measured. In the frequency domain, the two primary components are low-frequency (LF; 0.04-0.15 Hz) and high-frequency (HF; 0.15-0.40 Hz) spectra. Heart rate variability measures provide information primarily on vagal modulation with the LF power spectrum reflecting both sympathetic and parasympathetic modulation and HF acting as a surrogate marker of parasympathetic modulation of the R-R intervals. The LF/HF ratio is used to indicate sympathovagal dominance.

The skin conductance response (SCR) of the patients will be measured while they are exposed to the stimuli of the three experimental conditions. The SCR is an indicator of physiological arousal and is measured by detecting changes in the electrical conductance of the skin, which varies with the activity of the sweat glands. This assessment aims to understand the response of the patient's autonomic nervous system to the experimental conditions.

Systolic and diastolic blood pressure values will be derived from beat-to-beat blood pressure recordings using finger plethysmography during all gait trials. Finger plethysmography-derived systolic and diastolic blood pressure signals are recorded with high precision and beat-to-beat variability in blood pressure is calculated with a resolution of 2 ms.

The variability is assessed by measuring the intervals between consecutive systolic upstrokes in the blood pressure waveform. BPV is analyzed by decomposing the signal into two primary components: low-frequency (LF; 0.04-0.15 Hz) and high-frequency (HF; 0.15-0.40 Hz) spectra. Blood pressure variability in the LF range reflects sympathetic modulation of vascular tone and arterial baroreflex sensitivity, while HF variability is often associated with respiratory influences and parasympathetic activity. The LF/HF ratio is used as an indicator of the balance between sympathetic and parasympathetic contributions to blood pressure regulation.

The KOOS is a self-reported measure that allows assessing a broad spectrum of subjects with knee injuries and OA, concerning pain and other symptoms, daily life function, sport and recreation, and knee-related quality of life. It consists of five sub-scales which are scored separately: pain (nine items); symptoms (seven items); function in ADL (17 items); Function in Sport and Recreation (five items); and Quality of Life (four items). The items are scored on a 5-point Likert scale ranging from 0 (no problems) to 4 (extreme problems). We will use the "Function in Sport and Leisure" subscale of the Portuguese version of the KOOS, for which total scores can range between 0 and 25 points, where zero represents extreme knee problems and 25 represents absence of knee problems.

Participants will be invited to rate their agreement with the statement "Somehow I felt that the visual aspects involved me". The statement was adapted from item 9 of the Portuguese version of the Presence Questionnaire. This questionnaire is normally used in virtual reality studies to characterize the magnitude of the sensation of presence. The answer is scored on a 5-ponint Likert scale ranging from 0 (totally disagree) to 5 (completely agree).