Context: Manual isometric muscle testing is a common clinical technique used to assess muscle strength. To provide the most accurate data for the test, the muscle being assessed should be at a length in which it produces maximum force. However, there is tremendous variability in the recommended positions and joint angles used to conduct these tests, with few apparent objective data used to position the joint such that muscle-force production is greatest. Objective: To use validated anatomically and biomechanically based musculoskeletal models to identify the optimal joint positions in which to perform manual isometric testing. Design: In silico analysis. Main outcome measure: The joint position which produces maximum muscle force for 49 major limb and trunk muscles. Results: The optimal joint position for performing a manual isometric test was determined. Conclusion: Using objective anatomical models that take into account the force-length properties of muscles, the authors identified joint positions in which net muscle-force production was predicted to be maximal. This data can help health care providers to better assess muscle function when manual isometric strength tests are performed.