Effectiveness and Safety of a Tele-Rehabilitation Program on Fatigue and Related Variables in Chronic Fatigue Syndrome and Post COVID Syndrome

Fatigue will be measured through the Spanish version of the Chadler Fatigue Scale. It is a 14-item psychometric questionnaire designed to assess the subjective perception of physical and mental fatigue over the past few weeks. Items are rated on a 4-point Likert scale (0 = never, 1 = sometimes, 2 = often, 3 = always). The total score ranges from 0 to 42, with higher scores indicating greater levels of fatigue.

Welltory Mobile Application (v.4.16.100) is a mobile application that uses photoplethysmography through the phone's camera to measure resting heart rate. Based on these data, it estimates heart rate variability (HRV) parameters, including autonomic balance indicators such as RMSSD and SSDN. The tool has been validated in previous studies for assessing resting HRV, showing a significant correlation with reference medical devices such as electrocardiograms and heart rate monitors (intraclass correlation coefficient > 0.90).

Perceived pain will be assessed through the Spanish version of the Brief Pain Inventory (BPI), which is a self-administered instrument composed of two sections: pain intensity and the degree of interference in daily activities (such as sleep, mood, work, among others). It consists of 9 items, each rated on a numerical scale from 0 to 10, where higher scores indicate greater pain intensity and/or interference.

Perceived quality of life will be assessed through the completion of the Spanish version of the SF-36 Health Questionary. It is a widely used tool for measuring health-related quality of life. This instrument consists of 36 items grouped into eight dimensions: physical functioning, role limitations due to physical health, bodily pain, general health, vitality, social functioning, role limitations due to emotional problems, and mental health. Scores are transformed to a scale from 0 to 100 for each dimension, with higher scores indicating a better perceived health status in that domain.

Physical function will be quantified through the Sit-To-Stand Test. This test is a functional test that measures lower limb muscular endurance. It involves recording the number of repetitions of standing up from and sitting down on a standard armless chair over a 60-second period. The participant must keep their arms crossed over the chest and perform the movement as quickly as possible while maintaining proper technique. This test is a reliable indicator of functional strength and muscular endurance, widely used in both healthy populations and individuals with chronic conditions.

Interoceptive awareness will be quantified through the Spanish version of the Multidimensional Assessment of Interoceptive Awareness. This is a self-administered 32-item questionnaire that measures interoceptive awareness across eight dimensions, such as attention to internal bodily sensations, emotional regulation through bodily signals, and interoceptive confidence. Each item is rated on a Likert scale from 0 (never) to 5 (always), with higher scores indicating greater interoceptive awareness.

Anxiety and depression levels will be measured with the Hospital Anxiety and Depression Scale (HADS) which is a brief questionnaire composed of 14 items divided into two subscales: anxiety (7 items) and depression (7 items). Each item is rated on a scale from 0 to 3, with total scores ranging from 0 to 21 for each subscale. A cutoff score of ≥8 is considered indicative of clinically significant levels of anxiety or depression.

Perceived sleep quality will be measured with the Pittsburg Sleep Quality Index (PSQI). This is a self-administered questionnaire that assesses sleep quality over the past month. It consists of 19 items grouped into seven components: subjective sleep quality, sleep latency, sleep duration, habitual sleep efficiency, sleep disturbances, use of sleep medication, and daytime dysfunction. Each component is scored from 0 to 3, and the total score ranges from 0 to 21. Scores greater than 5 indicate significant sleep problems.

Fear of movement will be measured through the Tampa Scale for Kinesiophobia. The Tampa Scale for Kinesiophobia assesses fear of movement related to pain through 17 items rated on a 4-point Likert scale (1 = strongly disagree, 4 = strongly agree). Total scores range from 17 to 68, with higher scores indicating greater levels of kinesiophobia.

Somatic vigilance will be evaluated using the Anxiety Sensitivity Index (ASI; Spanish version). The ASI consists on 16 items (total score 0-64) assessing fear of anxiety-related bodily sensations, with higher scores denoting greater sensitivity.

Treatment feasibility will be measured through quantification of adherence to treatment and adverse events. An individual digital diary will be used to record this data.