Assessment of shoulder complex muscle strength in womenwith breast cancer during adjuvant hypofractionated radiotherapy and its support with progressive resistance training

Radiotherapy is an effective treatment modality for breast cancer; however, it can lead to various side effects that negatively impact patients' quality of life. In this context, exercise is considered a complementary intervention to mitigate these side effects and support functional recovery during the treatment process. The aim of this study is to investigate the effects of resistance exercises performed during the hypofractionated radiotherapy (HFRT) process following breast cancer surgery on shoulder muscle function and muscle activation patterns, and to evaluate the contribution of these exercises to the reduction of treatment-related side effects. A total of 31 breast cancer patients were randomly assigned to either an exercise group (EG, n=15) or a control group (CG, n=16). Participants performed isokinetic shoulder internal and external rotation movements at angular velocities of 1.047 rad/s and 2.094 rad/s using both the operated and contralateral limbs. During these movements, electromyography (EMG) and isokinetic torque measurements were recorded. Side effects associated with radiotherapy were assessed using the Common Terminology Criteria for Adverse Events (CTCAE). Pre- and post-radiotherapy torque and EMG data were analyzed between groups using linear mixed models. The results demonstrated that torque production during internal and external shoulder rotation at all angular velocities significantly improved in the exercise group compared to the control group (p < 0.0001). Group-by-time interaction was statistically significant for both mean Functional Control Ratio (FCR) and peak EMG values (p < 0.0001 for both). However, there was no statistically significant difference between groups in CTCAE scores (p > 0.05). In conclusion, progressive resistance training combined with adjuvant hypofractionated radiotherapy is effective in enhancing shoulder muscle strength and functional control in women with breast cancer. Such exercise protocols may contribute to maintaining and improving functional capacity during the course of treatment. Keywords: Breast cancer, Hypofractional radiotherapy, Resistance training, Electromyography.