1. This pilot randomized controlled trial aims at investigating the effects of different intensities of aerobic walking exercise to alleviate depression in older adults with major depressive disorder. Both baseline and post-intervention measurements will be conducted at the Exercise Physiology Laboratory, Division of Kinesiology, School of Public Health, The University of Hong Kong, while the exercise intervention will be conducted outdoors in a small group setting (3-5 participants).
2. Three-time-per-week moderate-intensity (~3.5 METs) or vigorous-intensity (~7 METs) walking exercise will be prescribed to participants in two exercise groups, while the participants in the waitlist group will receive no intervention. The intervention duration is 12 weeks.
3. We will recruit participants from the community in HK. Interested participants will be invited for a semi-structured interview including an assessment on the Beck Depression Inventory and medical history record to confirm eligibility.

Inclusion Criteria:

1. Older adults aged equal or over 50 years;
2. Ethnic Chinese;
3. Beck Depression Inventory score over 9 points;
4. Diagnosed with MDD and currently on pharmacological treatment for MDD.

Exclusion Criteria:

1. Incapable of participating in physical exercise with major confounding conditions which are known to affect mobility;
2. Cannot walk without assistive device;
3. Regular exercise habit (defined as exercise > 3 times per week and each time > 50 minutes)
4. Any serious somatic condition that prevents walking exercise participation (such as limb loss)
5. History of major diseases e.g. cancer, cardio-/cerebrovascular, neurodegenerative and renal diseases;
6. Diagnosis of dementia/Alzheimer's disease, or currently using antidementia medication;

The data of individual participants that underlie the results reported in this trial, after de-identification including text, tables, figures, and appendices, as well as study protocol and statistical analysis plan, will be shared after 3 months of study publication. Data will be shared with researchers who provide a methodologically sound proposal for academic purposes. Proposals should be directed to pmsiu@hku.hk to gain access and for data request, a data-access agreement needs to be signed.

Beginning 3 months and ending 3 years following the publication of the article

Data will be shared with researchers who provide a methodologically sound proposal for academic purposes. Proposals should be directed to pmsiu@hku.hk to gain access and for data request, a data-access agreement needs to be signed.

Outcomes assessor will be blinded from the group allocation and the participants are instructed not to reveal what kind of intervention he/she received.