This multicentre two-phased RCT aims to evaluate implementation potential, cost-effectiveness, effectiveness, and the role of exercise intensity of a home-based exercise and physical activity intervention to improve de novo kidney transplant recipients' physical fitness, cardiovascular health, gut microbiome characteristics, and health-related quality of life. The first phase of this study comprehends a six-month exercise training intervention. Patients will be randomized into (i) a sham intervention consisting of low-intensity balance and stretching exercises (LIT), (ii) a moderate-intensity aerobic and strength training intervention (MIT), or (iii) a moderate- and high-intensity aerobic and strength training intervention (MHIT). The second phase of this study comprehends a physical activity maintenance intervention provided to MIT and MHIT but not LIT. A total of 147 de novo kidney transplant recipients will be recruited from two independent Belgian transplant centres i.e. UZ Leuven and UZ Ghent.

Advances in the field of kidney transplantation have led to improved postoperative survival rates, but age-standardized mortality nonetheless remains two to sevenfold higher in kidney transplant recipients - with cardiovascular disease representing the leading cause of death in recipients with a functioning graft. Poor physical fitness, not completely recovering after transplantation, adds to the heightened cardiovascular risk of hypertension, diabetes, dyslipidaemia, and obesity. So does the posttransplant continuation of gut microbial dysbiosis, which recently emerged as a modulator of muscular, metabolic, and cardiovascular health. Exercise-based rehabilitation and physical activity interventions may prove pivotal in the care of kidney transplant recipients to address aforementioned outcomes.

This multicentre two-phased RCT comprehends a six-month exercise intervention at different exercise intensities, followed by an implementation strategy bundle promoting a physically active lifestyle. A total of 147 adult de novo kidney transplant recipients from two independent Belgian transplant centres will be randomly allocated to either 6 months of home-based exercise training consisting of muscle flexibility and postural stability exercises (low intensity training, LIT, n=49), moderate-intensity aerobic and strength training (moderate intensity training, MIT, n=49), or moderate- and high-intensity aerobic and strength training (moderate- and high-intensity training, MHIT, n=49). MIT will participate in moderate-intensity aerobic training on a stationary bicycle thrice a week and moderate-intensity strength training sessions twice a week. During the initial three months of the exercise intervention MHIT will participate in the same training program as MIT. In the final three months of the training intervention, MHIT will participate in high-intensity aerobic training on a stationary bicycle thrice a week and moderate-intensity strength training twice a week.

Following the six-month exercise training intervention, MIT and MHIT, but not LIT, will receive an individualized activity intervention aiming for long-term physical activity maintenance; using motivational interviewing techniques, co-creation of an action plan adapted to the patients' preferences, goal-setting, gradually decreasing follow-up prompts over time, and self-monitoring of physical activity behaviour. The training and physical activity interventions are initiated 3 and 9 months after transplantation, respectively. Study participants will be followed-up till 2 years after transplantation.

The investigators hypothesize that the study intervention will improve the primary outcome cardiorespiratory fitness, assessed as peak oxygen uptake, at 9 months posttransplant. Secondary outcomes include muscle fitness, motor fitness, body composition, cardiovascular health, gut microbiome characteristics, health-related quality of life, safety, cost-effectiveness, and implementation outcomes at six, nine, 12 and 24 months posttransplant. The role of training intensity and the role of baseline gut microbiome characteristics as predictor of individuals' training response will be explored.

Inclusion Criteria:

De novo adult KTRs

Exclusion Criteria:

• Underlying heart disease, defined as aberrant CPET, unstable angina, non-revascularized lesions or life-threatening arrhythmias
• Uncontrolled hypertension
• Uncontrolled diabetes, defined as HbA1c ≥ 9%
• Musculoskeletal disorders not allowing physical training on a cycle ergometer, or any other medical reasons by the physician considered to be a contraindication for moderate or high-intensity physical exercise
• Multi-organ transplantation
• Ongoing treatment for malignancies
• Unable to understand Dutch
• No access to smartphone and/or computer with internet access
• Severe pulmonary disease defined as either forced vital capacity (FVC) <50%, one-second value (FEV1) <50%, or a diffusing capacity for carbon monoxide (DLCO) <40% that excludes all serious underlying respiratory disease (pulmonary fibrosis, COPD GOLD II-IV, PAH).

Outcome assessors are blinded to the group allocation of the patients. Due to the nature of the intervention, blinding of the patients is not possible. However, a sham training intervention is provided to the control group.