Objective: Clinical evidence has shown that extremity dysfunction, such as muscle weakness or inhibition, is associated with spinal disorders. Spinal manual therapy is a common therapeutic approach used to address extremity muscle weakness. The purpose of the study was to assess changes in the maximal muscle strength of the shoulder external rotators immediately and at 10, 20, and 30 minutes after cervical joint mobilization at the C5-6 segment. Methods: Eighteen participants with existing or a history of neck pain were screened by 2 investigators independently for muscle weakness of shoulder external rotators. Fifteen qualified participants underwent shoulder external rotator strength testing with a handheld dynamometer. Each participant was tested 6 times, twice before, immediately after, and at 10, 20, and 30 minutes after a C5-6 joint mobilization on the involved side. The 2 strength data collected before the mobilization were used to determine intratester reliability. Results: The intratester reliability of the shoulder external rotator strength was excellent (intraclass correlation coefficient(3,2) = 0.985). One-way analysis of variance with repeated measures showed a statistical significance in strength data (P = .002). Post hoc tests revealed a significant increase between prejoint mobilization and immediately postjoint mobilization (P = .003) and between pre joint mobilization and 10-minute post joint mobilization (P < .001). Conclusions: The results of the study suggest that C5-6 joint mobilization increases muscle strength of the shoulder external rotators immediately and its effect carries over for 10 minutes but not after 20 minutes.