For a complex list of the variety of variables that can affect muscular strength see this link.
In addition to the medical causes listed above, we can add medications, structural abnormalities, injuries, contractile history, and the unidentified mechanisms that are likely to exist to the list. The multitude of possibilities for decreased strength eliminate the possibility of an exact science and make resistance strategies imparative. Increased strength can affect joint dysfunction but the path is not linear and will require constant reconsideration. Having more medical information is helpful which makes partnering with physicians ideal. Occasionally, we see that a client is not responding to tactful attempts to increase strength with no obvious stressor. In those cases we advise our clients to speak with their physician.