Reliability and Validity of the ActivForce Digital Dynamometer in Assessing Shoulder Muscle Force across Different User Experience Levels.

Background: Currently available hand-held dynamometers (HHD) offer a more objective and reliable assessment of muscle force production as compared to a manual muscle test (MMT). Yet, their clinical utility is limited due to high cost. The ActivForce (AF) digital dynamometer is a new low-cost HHD with unknown psychometric properties, and its utilization may benefit clinical practice. Hypothesis/Purpose: This study aimed to determine the AF intra- and inter-tester reliabilities, standard error of measurement (SEM), minimal detectable change (MDC), and criterion validity for assessing shoulder isometric force as compared to the microFET2 (MF2) across testers with different experiences. Design: Descriptive observational study. Methods: A convenience sample of 29 healthy adults were assessed twice by each of three testers (two experienced clinicians and a novice PT student) on shoulder external rotation (ER), internal rotation (IR), and forward elevation (FE) using both the AF and MF2 devices. Tester, HHD, and shoulder motion assignment orders were randomized. All testing was performed in a standardized seated position. ER and IR were tested with the shoulder fully adducted. FE was tested at 45° at the scapular plane. All testing and rest periods between testers and tested motions were standardized and monitored via a stopwatch. Results: Both devices had high intra- [ ER (.95-.98), IR (.97 - .99), FE (.96 - .99)] and inter-tester [ ER (.85-.96), IR (.95 - .97), FE (.88 - .95)] intraclass correlation coefficient (ICC) with comparable intra- (1.68-1.80) and inter-tester (2.36-2.98) SEM, and intra- (4.64-4.97) and inter-tester (6.50-8.24) MDC values across all motions. Tester experience did not affect these values. High (.89-.93) statistically significant Pearson correlations were found between HHDs for all shoulder motions. Conclusion: Both the AF and MF2 HHDs were found to have high reliability levels across all shoulder motions regardless of tester clinical experience. The AF was also found to be valid for measuring shoulder isometric force production compared to the criterion standard device, the MF2. Its low-cost and electronic accessibility features may promote better compliance for clinicians using dynamometry to objectively assess and store muscle force data in a cost-effective manner. Level of Evidence: 3.