Rationale: Women with hormone-receptor positive breast cancer are usually prescribed endocrine therapy for a period of 5-10 years. This treatment reduces the risk of recurrence and improves overall survival in these women. Musculoskeletal complaints are a common (~50%) negative consequence of endocrine treatment, which affects daily functioning and quality of life. These symptoms frequently result in early treatment discontinuation, which is associated with shorter disease-free survival. Musculoskeletal complaints are often pharmacologically treated with limited effect and accompanied by side-effects. Therefore, interventions to counteract musculoskeletal complaints are urgently needed in this population. A potential non-pharmacological option is yoga. In patients with osteoarthritis, there is emerging evidence that yoga is effective to reduce pain and stiffness and improve function. Yoga as treatment for musculoskeletal complaints that are associated with endocrine treatment is rarely investigated and mainly in small studies.

Objective: The objective of the proposed study is to assess the effectiveness of a 4-month yoga program compared to a waiting list control group on musculoskeletal complaints in women with hormone-positive stage I-III breast cancer receiving endocrine treatment who report musculoskeletal complaints.

Study design: The COBRA study is a randomized controlled trial with two study arms: a yoga- and a waiting list control group.

Study population: For this study, 140 women with oestrogen-receptor positive stage I-III breast cancer on endocrine treatment (aromatase inhibitors or Tamoxifen; >4 months) will be recruited. These women experience musculoskeletal complaints (>3 months) which started or exacerbated after initiation of endocrine treatment. Furthermore, eligible women did not practice yoga in the last six months and are not planning to start yoga and are not highly physically active (i.e., >150 minutes per week moderate-vigorous exercise).

Intervention: The intervention consists of two one-hour sessions/week supervised yoga and once a week 30-minute yoga exercises at home. The 4-month yoga program will be an active form of yoga, such as Easy Vinyasa yoga, which is characterized by continuous slow movements linked with breathing. The waiting list control patients will be offered an online yoga program after the 4-month study period.

Inclusion Criteria:

• Be diagnosed with oestrogen-receptor positive stage I-III breast cancer,
• Use of aromatase inhibitors or Tamoxifen (>4 months and will continue using it for at least six months),
• Have finished primary treatment (chemotherapy, radiotherapy, surgery) for at least 12 weeks,
• Experience musculoskeletal complaints (>3 months, which are at least mild in severity (i.e., score of ≥ 3 for worst pain item of a modified version of the Brief Pain Inventory [BPI](16, 42))., which started or exacerbated after initiation of endocrine treatment,
• Be stabilized on menopausal symptom medication or antidepressants for at least three months and three weeks, respectively, if applicable, and
• Be able to read, speak and understand Dutch or English.

Exclusion Criteria:

• Too physically active (i.e., >150 minutes/week of self-reported moderate-to-vigorous or leisure and sports activities)
• Following (during the last 6 months), or planned to follow yoga classes on a structural base
• Following, or planned to follow, a structured psychological intervention during the intervention period, i.e., cognitive behavioral therapy, or unstable on psychotropic medication
• Participated in the intervention group of an exercise study during breast cancer treatment
• Any circumstances that would impede ability to give informed consent or adherence to study requirements as determined by the study team
• More than 3 weeks not able to attend training sessions during the intervention period
• A body mass index (BMI) of >35 kg/m2