Randomized controlled trial. Patients will be enrolled at the moment of the pre-surgery assessment after verifying the inclusion/exclusion criteria and signing written informed consent. After surgical treatment (THR or TKR) and subsequent rehabilitation treatment (which usually lasts 3-4 months) patients will be contacted and the persistence of inclusion/exclusion criteria verified again. For each type of surgical procedure (i.e. THR or TKR) those, with confirmed inclusion criteria will be randomly assigned to the IG or the CG. The IG will participate in a 6-month exercise program based on the PAIR exercise protocol and will receive educational sessions and material on the importance of maintaining an active life style after THR or TKR. The CG will receive only educational sessions and material on the importance of maintaining an active life style after THR or TKR. Participants of both IG and CG will be assessed at the moment of randomization (post-surgery baseline) and, subsequently, after 3 and 6 months.

The present study is carried out within the project "Physical ActIvity after hip and knee Replacement" (PAIR) and funded within the Erasmus Plus Sport program (Grant Agreement 613008-EPP-1-2019-1-IT-SPO-SCP). It is aimed at evaluating the efficacy and safety of an exercise program specifically designed for improving physical function and QoL in people who had undergone THR or TKR for primary osteoarthritis after completing the surgical and rehabilitative treatment.

The primary objective of the study is to compare modifications of QoL measured with the Short-Form Health Survey (SF-36) induced in the intervention groups (IG) by the PAIR exercise program versus a control groups (CG) who will undergo usual care. SF-36 is one of the most widely used questionnaire to measure the health related QoL (HRQoL) in total hip and total knee arthroplasty patients.

The secondary objectives of the study are to investigate differences between IG and CG on domains which are considered to be determinants of QoL:

Hip- and knee-related impairments and functional limitations

• Harris Hip Score (HHS), American Knee Society scoring (KSS),
• Visual analog scale (VAS) to evaluate pain, High-Activity Arthroplasty Score (HAAS), ---Western Ontario and McMasters Universities Osteoarthritis
• Index (WOMAC),
• Knee Injury and Osteoarthritis Outcome Score (KOOS)
• Hip Disability and Osteoarthritis Score (HOOS).

Hip- and knee-related functional status

• Time Up and GO (TUG),
• Single Stance test,
• The 30-second Chair-Stand Test (30s-CST),
• Hand Grip test, Maximal strength of the lower limb,
• Hip and knee joints mobility (by goniometry).

Life style The ultimate goal of the PAIR exercise program is to increase physical activity in a sedentary population. The exercise program, through its beneficial effects on health, is expected to generate a virtuous circle promoting a more active life-style. For this domain 2 instruments will be used: Recent Physical Activity Questionnaire (RPAQ) and PAIR questionnaire for patients' attitudes toward physical (instrument generated within the PAIR project).

Safety: Adverse clinical events (ACEs) occurred to the participants both inside and outside gym classes.

Adherence: adherence of each subjects to the exercise program will be monitored in the IG. The adherence will be measured as percent of exercise sessions actually performed/total number of scheduled exercise sessions. Other factors considered to influence the adherence to an exercise program will be also recorded (age, gender, comorbidity, BMI, educational level, marital status, working activity, and home gym-distance).

Participants' satisfaction Satisfaction of PAIR exercise program is a very important domain since influence adherence. It will be verified by asking few questions with structured responses based on a 7-point Likert scale, to get participants' opinion about the PAIR exercise program.

Inclusion Criteria:

Pre-surgery criteria

• Signed informed consent
• Age: 50-80 years
• Indications: Patient with unilateral advanced Osteo-Arthrosis (OA) requiring primary THR or TKR;
• General: American Society of Anesthesiologists (ASA) class 1 or 2
• Resident in Bologna metropolitan area

Post-surgery criteria

• Functional performance: Able to stand and walk >500 meters independently;
• Pain: score ≤ 4 in VAS

Exclusion Criteria:

• Unable/unwilling to sign the informed consent form of the study and/or willing to comply with the study requests
• Poor knowledge of Italian language which prevents understanding of the content of the consent form and/or of instructions for assessment and/or training;
• Severe functional limitations of other lower extremity joints besides that for which surgery is planned;
• Impairment of communicative and/or sensory functions so severe to make impossible understanding or executing trainer's instructions (dementia, aphasia, blindness, deafness);
• Heart failure (NYHA class >2);
• Unstable angina;
• Pulmonary disease requiring oxygen therapy;
• Symptomatic peripheral arteriopathy;
• Recent myocardial infarction or hospital admission for any other reason in the previous 6 months;
• Symptomatic orthostatic hypotension;
• Hypertension in poor pharmacologic control (diastolic >95 mmHg, systolic >160 mmHg);
• Relevant neurological condition impairing motor or cognitive function;
• Any other condition that the medical doctor (MD) considers to contraindicate the participation in an exercise program of moderate intensity;
• Severe depression.