Reliability and agreement between 2 strength devices used in the newly modified and standardized Constant score

HYPOTHESIS: The new and standardized test protocol for the Constant score (CS) provides new methodology, but different devices are still used for shoulder strength testing. It was hypothesized that strength measurements using the IsoForceControl (IFC) dynamometer (MDS Medical Device Solutions, Oberburg, Switzerland) would provide results comparable with the IDO isometer (Innovative Design Orthopaedics, Redditch, UK).

MATERIALS AND METHODS: Sixty healthy subjects, aged 19 to 83 years, were studied, with 5 men and 5 women in each of 6 ten-year age groups. The IFC and IDO were used in randomized order with an 8-minute interval between testing. Subjects performed 3 successive trials with strong verbal encouragement, with 1 minute between trials. The best strength performance was used in the analysis. The rater and subjects were blinded to all results.

RESULTS: The IFC produced 0.28-kg (0.62-lb) higher strength values on average than the IDO (P = .002). The intraclass correlation coefficient (ICC2,1) was 0.97 (95% confidence interval, 0.95-0.98), whereas the standard error of measurement and smallest real difference were 0.43 kg (0.95 lb) and 1.2 kg (2.63 lb), respectively. The total CS and strength reached mean values of 92.4 points (SD, 6.2 points) and 8.2 kg (SD, 2.6 kg) (18.0 lb [SD, 5.8 lb]), respectively, and were negatively associated with age (r > -0.407, P ≤ .001). The strength values decreased (P ≤ .001) by 1.3 CS points per decade, and women had strength values that were 8 CS points lower on average than those of men of the same age.

CONCLUSIONS: The relative (intraclass correlation coefficient) and absolute (standard error of measurement) reliability between the IFC and IDO is excellent, indicating that performances reported from settings using the IDO are comparable with those recorded with the IFC in other settings.