Reliability Testing of ISOP, a Wrist Isometric Power Device for Evaluation of Wrist Function

Aim and Hypothesis: The aim of this study was to evaluate the reliability of ISOP, a new portable isometric power device (Medical Control Group Inc., Malmö, Sweden), with regard to evaluating isometric muscle force in the three planes of wrist motion. The hypothesis was as follows: Is there a significant difference in isometric power when repeated measurements are conducted at two different time intervals (so-called test-retest)?

Method: A test-retest protocol was performed in 32 volunteers (16 women/16 men, age range 28–48 years, average 38.6), with no prior hand or wrist injury (power 0.80 with estimated moderate Pearson's r = 0.4). Eight measurements were done in six different directions of the wrist: flexion, extension, radial deviation, ulnar deviation, pronation and supination of the dominant hand, as well as flexion and extension of the nondominant hand. Each measurement consisted of three sequential maneuvers using maximum force, and the results were calculated as an average of these (test 1). The exact same protocol was repeated after a week (test 2). As an internal control, the Jamar Hand Dynamometer was used, as it has previously been proven reliable.

Results: The relationship between test 1 and test 2 were analyzed using Pearson's correlation coefficient (r) and Intraclass Correlation Coefficient (ICC) with a p-value set at 0.001. The result of Pearson's r showed a very strong correlation between 9 of 10 pairs tested, with values between 0.84–0.98, and a strong correlation for the tenth pair with a value of 0.77. The strength of the correlations was confirmed by the ICC, which showed 0.88–0.99 for all pairs. A dependent t-test was additionally performed to test for possible disparities between the tests, but no significant differences were found (p > 0.05), which additionally strengthens the relationship of the tests.

Summary Points

• Evaluation of the isometric power-generating capacity of the wrist is of interest in evaluating wrist function following trauma/surgery.

• No portable device exists to date that allows for easy evaluation of wrist function in all three planes of motion.

• The new isometric power device, ISOP, has a very strong to strong correlation in a test-retest situation.

• The reliability of the ISOP is at a level akin to that of the Jamar Hand Dynamometer, and no significant differences were found between the two (p > 0.05).

• ISOP may thus be considered a reliable device for use in evaluation of wrist function and rehabilitation.