Olds et al 2011 wanted to determine how stiffness and strength are affected by an anteriorly displaced shoulder. Recruiting 16 males with unilateral anterior shoulder instability (at least 2 traumatic instances), they used standardized outcome measures- Western Ontario Shoulder Index (discussed here), ASESSSAF, and the Single Alpha Numeric Evaluation.
The tell-tale sign of shoulder instability became isometric horizontal adduction strength; p=.01. Stiffness was also measured (using an angled dynamometer and resistance) and it was found that capsular and ligament stiffness seem to occur and sub maximal laods (20% and 50%) but not at heavier loads (70%).
This is where things get interesting. No correlation between strength, stiffness, the shoulder tests and quality of life could be found. While all effected shoulders demonstrated some weakness and stiffness, there was no clear trend under which one could establish a rule or even generalized association. This is puzzling but not surprising given the complexity and variations of the ball and socket design of the gleno-humeral joint. Clearly, enough variability exists even within shoulders that are chronically unstable that at this time, we can not create basic relationships between them.