Background: There is sparse literature studying the functional morbidity of subpectoral implant- based breast reconstruction. We aimed to prospectively investigate this technique's impact on objective upper extremity function and patient-reported outcomes., Methods: Women undergoing mastectomy and immediate subpectoral tissue expander insertion with ADM sling were enrolled from November 2014 to August 2016. Preoperative evaluation of shoulder range of motion, pectoralis major strength, and Quick Disabilities of the Arm, Shoulder, and Hand (QuickDASH) and Breast-Q surveys were conducted before surgery and at 1 month and 6 months postoperatively, or until return to baseline pectoralis major strength., Results: Eighteen women (mean age, 51 years, SD 9.6, range 35-72 years) comprising 26 breast reconstructions completed postoperative follow-up. The average follow-up length was 9 months (range, 3 -18 months; SD, 144 days). At 1-month follow-up, there was a statistically significant decrease in lower and non dominant upper fiber pectoralis strength from preoperative baseline (p < 0.05). At final postoperative follow-up, 24 reconstructions (92.3%) recovered to at least 80% of preoperative strength in upper and lower fibers. From preoperative to final postoperative follow-up, QuickDASH scores showed a statistically significant (p = 0.008) increase from 4.1 (range 0-20.5, SD 6.1) to 18.7 (range 0-45.5, SD 13.4). Physical well-being: The chest was the only Breast-Q domain in which the average score significantly decreased (p = 0.02) between preoperative assessment and final follow-up., Conclusions: After implant-based breast reconstruction, patients achieve the return of objective upper extremity function, but patient-reported outcomes do not return to baseline as shown by increased QuickDASH scores. Thus, pectoralis-sparing reconstructive strategies such as prepectoral implant insertion should be pursued., Competing Interests: Declaration of Competing Interest None., (Copyright © 2020 Elsevier Ltd. All rights reserved.)