The dyadic coping inventory assesses perceived stress communication and dyadic coping. The 37-item dyadic coping inventory includes several subscales not relevant to this study. As such, to alleviate participant burden only the following subscales will be considered;

• Supportive dyadic coping of the partner (SDCP: items 5, 6, 8, 9, and 13)
• Delegated dyadic coping of the partner (DDCP: items 12 and 14)
• Common dyadic coping (CDC: items 31, 32, 33, 34, and 35)
• Evaluation of dyadic coping (EDC: items 36) Participants rate each of these 13 items on a 5-point Likert scale (1 = very rarely to 5 = very often). Mean scores for individual subscales will be used in the analysis.

Specific breast cancer-related symptoms will be assessed with the validated 23-item European Organization for Research and Treatment of Cancer (EORTC) Breast Cancer-Specific Quality of Life Questionnaire (QLQ-BR23). All of the scales and single-item measures range in score from 0 to 100. A high scale score represents a higher response level.

In concordance with the International Consortium for Health Outcomes Measurement, quality of life will be assessed with the validated 30-item self-assessment questionnaire of the European Organization for Research and Treatment of Cancer (EORTC QLQ-C30). All of the scales and single-item measures range in score from 0 to 100. A high scale score represents a higher response level.

Satisfaction with breast(s) will be assessed via BREAST-Q-Satisfaction with Breasts domain. There is no overall or total score, only scores for each independent scale. All scales are transformed into scores ranging from 0-100. A higher score means greater satisfaction or better QOL

Patient reported disability will be measured using the WHO Disability Assessment Schedule V.2.0 instrument. The scores assigned to each of the items are "none" (0), "mild" (1) "moderate" (2), "severe" (3) and "extreme" (4). These scores are summed for each item and are converted to a summary score ranging from 0 to 100 (where 0 = no disability; 100 = full disability).

Shoulder pain and disability will be measured using the 13 item SPADI assessing pain level and extent of difficulty with ADLs requiring the use of the upper extremities. The pain subscale has 5-items and the Disability subscale has 8-items. The pain scale is summed up to a total of 50 while the disability scale sums up to 80. The total SPADI score is expressed as a percentage where 0% indicates best and 100% indicates worst (i.e. a higher score shows more disability)

Musculoskeletal joint, back, neck, bone and muscle symptoms such as aches, pains and/or stiffness will be assessed using the 14 item MSK-HQ. A summary score between 0-56 is computed; with a higher score indicating a better MSK-HQ status. There is also a question which asks patients to indicate, in the past week, how many days they have engaged in physical activity that has raised their heart rate for 30 minutes or more (0 to 7 days).

To assess functional capacity, the investigators will record distance travelled, RPE, heart rate, estimated VO2max and oxygen saturation during a remotely supervised 6-minute walk test (6MWT), in accordance with ATS guidelines. The distance walked will be measured using manual tape measurement and validated using a cipher skin biosleeve

To assess lower extremity strength and balance a remotely supervised 30 second chair to stand (30-CST) test will be performed. The number of times a patient comes to a full standing position from a chair will be recorded (the same chair will be used for all within-participant assessments).

Hand Grip dynamometry will be used as a measure of upper body muscular strength. The investigators will follow the Southampton protocol.

To assess shoulder range of motion (RoM) the following movements will be performed under remote supervision; shoulder flexion, abduction, internal rotation, and external rotation. The RoM of each movement will be recorded using a cipher skin biosleeve and a single composite score, defined as the total of these four motions (sumRoM), calculated.

Bilateral measurements will be made via a flexible tape with patients seated and their arm supported on a table with the shoulder in approximately 30° of forward flexion and 45° of elbow flexion. The investigators will use the landmarks method, where measurements will be taken at the wrist, elbow, and axilla plus two additional points either 10 cm distal to and proximal to the lateral epicondyle (Landmarks) or halfway between the wrist and elbow or elbow and axilla (Landmarks, Midpoint) (NLN position paper). Volumes will be then calculated by the sum of truncated cones (frustum) model for both Landmark and Landmark, Midpoint methods

The investigators will track patient's habitual physical activity (i.e. outside of the prescribed exercise sessions) and sedentary behaviour via a Withings smartwatch (Steel HR) worn on patients left wrist (3 finger widths above the wrist bone). The Withings Steel HR is a high precision MEMS 3-axis accelerometer with heart rate infrared sense that provides estimates of time spent as 'active minutes'. The calculation of these minutes is based on Metabolic Equivalent of Task (METs)

Dietary intake will be assessed at each assessment point using a 4-day food diary.

Treatment-related adverse events (AEs) will be assessed via CTCAE and patient-reported outcomes CTCAE (PRO-CTCAE). Clinicians will report AEs at every consultation from T2 to T3. From the PRO-CTCAE, the investigators selected 12 core items including pain, fatigue, physical activity, bowel function, sleep, temperature, chill, sore mouth, and appetite (amscolme). The investigators will use an online reporting form that will ask patients to complete weekly for the 13 weeks between T2 and T3.

For those patients who receive chemotherapy, chemotherapy completion rate will be assessed via relative dose intensity

All participants will be invited to take part in a virtual focus group after completing the postoperative exercise intervention and at 3-months follow up to obtain qualitative data regarding topics such as patient preferences, intervention acceptability, and ideas for improving the intervention.