BACKGROUND We assessed the subjective and objective functional loss induced by detachment of the pectoralis major muscle and whether synergistic function of adjacent muscles compensates for such loss. METHODS A preoperative to postoperative comparison of the Disability of the Arm, Shoulder and Hand score, 3-dimensional video-analysis of the range of motion, and dynamometric force assessment with electromyography registration were prospectively conducted in 22 women who underwent bilateral preventive breast surgery. RESULTS Although the statistically significant postoperative increase of the Disability of the Arm, Shoulder and Hand score (P = < 0.01) was indicative of more subjective disability, this difference is irrelevant for activities of daily life. This is in line with the observed postoperative lack of restrictions of range of motion of the shoulder. The mean postoperative maximum force measured in 4 directions reflective of the pectoralis major muscle function was 7.13 N, or 3.8%, lower than the preoperative force (P = 0.079). The mean postoperative maximum force in the 2 contramovement directions was 15.5 N, or 14%, higher than the preoperative force (P = 0.04). Electromyographically, both parts of the deltoid muscle preoperatively and postoperatively showed the same peaks in the 6 directions. Contrastingly, the clavicular part of the pectoralis major muscle postoperatively showed a statistically significant increase of peak of muscle activity in 3 of 4 directions reflective of pectoralis major function and in both contramovement directions, indicating compensatory activity. CONCLUSIONS Daily life consequences of partial detachment of the pectoralis major muscle were acceptable in our patients.