Reverse total shoulder - strength and activity - correlation, determination and variation

Isokinetic shoulder strength correlates with level of sports participation and functional activity after reverse total shoulder arthroplasty

These authors surveyed 51 patients (mean age 74 yrs) at a mean of 29.5 months (range, 12-60 months) after reverse total shoulder arthroplasty (RTSA) investigating the relationship between isokinetic shoulder strength and activity level.

High demand activities involved repetitive stress, loads > 20 pounds, and regular overhead movements. Moderate-demand activities involved occasional repetitive stress, lifting loads up to 20 pounds, and occasional overhead activities. Low-demand activities are defined as not imposing repetitive stress on the shoulder, not involving heavy lifting, and most shoulder movements being below shoulder height.

Reported activity was high demand in 35% and moderate demand in 43%. There was a large variation in shoulder isokinetic strength parameters especially for internal and external rotation.

Comment: While it is intuitive that shoulders that do more are stronger, the interesting message here is the variability in the relationship.

Recall that the Pearson product-moment correlation coefficient, also known as r, R, or Pearson's r, is a measure of the strength and direction of the linear relationship between the two variables (e.g. strength and activity) calculated as the covariance of the variables divided by the product of their standard deviations.

Squaring "R" gives us the coefficient of determination, the percentage variation in one variable (activity) that is explained by all the other variable (strength). As can be seen in the numbers in the tables above, the coefficients of determination (square of the "R" values) are low for these associations, meaning that variables other than strength are likely to be more influential in determining activity levels.

As an old professor once said, "the mean is meaningless, the meaning is in the variation"

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