Effect of yoga on musculoskeletal complaints in women during endocrine treatment for breast cancer: protocol of the randomised controlled COBRA trial

INTRODUCTION: Approximately 40% of women stop endocrine therapy for hormone-receptor-positive breast cancer within the first 5 years of prescribed treatment because of side effects. Musculoskeletal complaints are among the most frequently reported side effects. The Cancer Of the BReast Asanas (COBRA) study examines the effect of an 18-week yoga programme on endocrine therapy-associated musculoskeletal complaints in women with breast cancer.

METHODS AND ANALYSIS: In total, 140 women will be randomised in a 1:1 ratio to the intervention or waitlist control group. The intervention programme consists of two times a week 1-hour supervised Hatha or (easy) Vinyasa yoga classes at a yoga or sports centre for 18 weeks and once per week a half-hour at home using videos. The waitlist control group is asked to maintain their habitual lifestyle during the first 18 weeks and will participate in a similar yoga programme to the intervention group for the following 18 weeks. The control group yoga programme is offered live-remote. The primary outcome (musculoskeletal complaints) is assessed with the Brief Pain Inventory questionnaire at baseline and 18 weeks (primary comparison) and additionally at 36 weeks. Secondary outcomes include lower and upper extremity joint complaints, menopausal symptoms, fatigue, sleep, quality of life, anxiety and depression, cognitive complaints and habitual physical activity (all patient-reported), vital signs and anthropometrics, physical fitness, blood biomarkers, medication use, safety data and patient and teacher experiences. At baseline and 18 weeks, cognitive complaints are also assessed with an online neuropsychological test battery.

ETHICS AND DISSEMINATION: The COBRA study was approved by the Medical Ethical Committee of the University Medical Center Utrecht. The study started on 8 October 2024, and 65 participants have been included (20 January 2026). Results will be submitted to an international peer-reviewed journal.

TRIAL REGISTRATION NUMBER: NCT06480513.