The Human Activity Profile (HAP) consist of 94 questions addressing different domains of daily activity and participation, with three possible answers for each item: 'still doing,' 'stopped doing,' or 'never did.' For each item, the participant will have to choose one of these responses. Based on each answer, two scores will be calculated: the Maximal Activity Score (MAS), which corresponds to the activity with the highest effort demand that the volunteer 'still performs'; and the Adjusted Activity Score (AAS), which will be calculated by subtracting the number of items the subject 'has stopped doing' below the MAS value from the MAS itself. The AAS will provide the value of activity that the subject has ceased to perform compared to what they were doing before. Unit of Measure: Absolute number.

Screening for Sarcopenia risk will be performed using the "Strength, Assistance with walking, Rise from a chair, Climb stairs, and Falls" (SARC-F) questionnaire, which consists of five self-reported questions based on the subjects' perception of their limitations regarding strength, walking ability, rising from a chair, climbing stairs, and their history and number of falls. Each item will be scored from 0 to 2, where 0 = no difficulty, 1 = some difficulty, and 2 = great difficulty, or if the subject uses devices or cannot perform the task without personal assistance. The total SARC-F score can range from 0 to 10, and subjects with a score ≥ 4 points will be considered at risk of sarcopenia, in accordance with the recommendations of the European consensus on sarcopenia. Unit of Measure: Absolute number.

Frailty will be assessed using the five questions (fatigue, climbing stairs, walking 100 m, comorbidity, and weight loss) that comprise the "Fatigue, Resistance, Ambulation, Illnesses,Loss of Weight" (FRAIL) questionnaire. A score of 1 or higher suggests frailty. Unit of Measure: Absolute Number

Frailty will be assessed using the five components (weight loss, handgrip weakness, fatigue, low physical activity, and gait speed) of the Fried Frailty Phenotype. A score of 1 or higher suggests frailty. Unit of Measure: Absolute number

The 4-metre gait speed test will be performed preferably before the second HD session of the week, in a corridor of the unit. Patients will be asked to walk between two marks at their usual pace, using walking aids if necessary. A practice trial will be provided, and the times of two trials will be recorded in seconds and converted to m/s. The best of the two trials will be used for analysis. Unit of Measure: Absolute number

Health-related quality of life (HRQoL) will be assessed using the Spanish version of the Medical Outcomes Study Short Form-36 (SF-36) questionnaire . Scores range from 0 to 100, with higher scores indicating better HRQoL. Results are reported across eight subscales (physical functioning, vitality, role physical, general health, social functioning, mental health, role emotional, and bodily pain) and two summary components (physical and mental components). Unit of Measure: Absolute number

The Sit-to-Stand-to-Sit- 10 (STS-10) and Sit-to-Stand-to-Sit-60 (STS-60) tests will be performed preferably before the second or third HD session. Participants will be seated in a chair with their arms crossed over the chest, feet on the floor, and back supported against the backrest whenever possible. The chair must be stable and placed against a wall to minimize the risk of falls. Participants will be asked to keep their arms crossed to avoid using them for assistance and, with their feet on the ground, to stand up and sit down 10 consecutive times as quickly as possible. The time required to complete the test will be recorded in seconds.

After a rest period following the STS-10, the same procedure will be performed for the STS-60, in which the number of repetitions completed in 60 seconds will be recorded. Both the number of repetitions and the Rating of Perceived Exertion (RPE) will be documented.

Unit of measure: absolute number

Muscle quantity and quality will be evaluated using ultrasound measurement of the cross-sectional area of the rectus femoris muscle, performed 30 minutes after the start of HD, on a different day from the muscle strength assessments. The assessment will be conducted on the dominant lower limb, with the participant in the supine position, the limb fully extended, and the muscles relaxed. It will be measured muscle thickness, (unit of measure: millimeter).

Muscle quantity and quality will be evaluated using ultrasound measurement of the cross-sectional area of the rectus femoris muscle, performed 30 minutes after the start of HD, on a different day from the muscle strength assessments. The assessment will be conducted on the dominant lower limb, with the participant in the supine position, the limb fully extended, and the muscles relaxed. It will be measured muscle cross-sectional area ( unit of measure: millimeter squared).

Muscle quantity and quality will be evaluated using ultrasound measurement of the cross-sectional area of the rectus femoris muscle, performed 30 minutes after the start of HD, on a different day from the muscle strength assessments. The assessment will be conducted on the dominant lower limb, with the participant in the supine position, the limb fully extended, and the muscles relaxed. It will be measured muscle subcutaneous fat thickness, (unit of measure: millimeter).