Predicting shoulder strength using allometry: implications for shoulder outcome assessments

Shoulder strength is an integral component of shoulder function. In assessing shoulder function many functional scales rely on direct or indirect measures of strength. Strength, and thus these scales, is often highly correlated with age. We propose an allometric method for assessing shoulder strength. Allometric modeling has provided accurate predictions of biologic growth and physiologic function in both human and animal studies. Allometry utilizes the relationship between the two homologous structures on the left and right sides of the body, providing in effect an internal control and thus eliminating many confounding effects, e.g. age and level of activity.

Twenty patients with unilateral shoulder dysfunction underwent strength testing of their affected and unaffected shoulder. Strength testing of the bilateral shoulders was also assessed in twenty people without shoulder pain in order to delineate the effect of hand dominance on strength in those without shoulder pain. Absolute and predicted strength deficits of the involved shoulder were calculated via regression analysis. Pearson's correlation coefficients were computed between age and strength.

Utilizing the contralateral extremity as an internal control eliminates age as a confounding variable in predicting strength outcomes (r = 0.093).

Allometric concepts, applied to shoulder strength assessment, eliminates age as a confounding variable. Strength testing of both shoulders should be considered in the development of future shoulder outcome scales in order to eliminate the strong influence of age.