This cross-sectional study aimed to analyze the intra- and interrater reliability of a new isometric force measuring technique for the knee extensor and flexor muscles using a fixed HHD.
Twenty-five healthy participants (9 females, 16 males, mean age 30.4±5.9 years) were assessed in upright sitting position on an instrumented table with a HHD attached. Peak force of knee extensor and flexor were measured three times with the HHD and once with a force device of the university movement laboratory. Intraclass correlation coefficients (ICC), standard error of measurement (SEM) and minimal detectable change (MDC) were used to assess intra- and inter-rater reliability. Validity calculation included Bland-Altman analysis and Pearson’s product-moment correlation.
Intra-rater reliability was excellent for knee extensors (ICC = 0.95, 95% confidence interval (CI) 0.90-0.98; SEM = 42.5N; MDC = 117.7N) and good for knee flexors (ICC = 0.88, 95% CI: 0.67-0.95; SEM = 26.9N; MDC = 74.5N). Inter-rater reliability was also excellent for knee extensors (ICC = 0.95, 95% CI 0.90-0.97; SEM = 43.3N; MDC = 119.9N) and good for knee flexors (ICC = 0.85, 95% CI 0.63-0.93; SEM = 27.9N; MDC = 77.3N). The Bland-Altman analysis demonstrated a mean bias of -334.95 Newton (N) with a lower limit of agreement (LoA) of -501.0N and an upper LoA of -167.9N for measurements of knee extensors. For the measurement of knee flexors, Bland-Altman analysis showed a mean bias of 116.3N (lower LoA = 32.6N, upper LoA = 200.1N). The correlation between the two measurement techniques was very strong with a value of r = 0.90 (95% CI 0.78 - 0.95, p 0.0001) for knee extensors, and moderate with a value of r = 0.66 (95% CI 0.36 - 0.83, p 0.001) for knee flexors.
Measuring isometric force of the knee flexor and extensor muscles with the fixed HHD is a reliable tool. However, when comparing the new method to the force device of the movement laboratory, there was a significant bias between the two techniques: a correlation was detectable but not a significant agreement or validity. Further investigations should therefore compare the new method to the gold standard, to isokinetic dynamometry, to proof validity. In addition, despite an upright sitting position was instructed, participants often leaned forward during the measurement with the HHD in the present study. Consequently, the participant’s position should be better controlled and corrected during the measurements, if necessary.
In a clinical setting, the new measurement method using a fixed HHD is a reliable alternative for assessing isometric strength of knee flexors and extensors, and may be used to evaluate progress in force during rehabilitation. In addition, measuring isometric strength as tested is easy to replicate and implement in daily physiotherapy practice.
handheld dynamometer
knee
