Inter-rater and intra-rater reliability of hand-held dynamometry of the hip and knee in a military cohort

Introduction: Hand-held dynamometers (HHDs) are widely used to measure isometric muscle force in clinical practice. However, reliability may be affected when there is a difference in muscle force generation between examiner and patient, raising concerns about accuracy in physically trained populations, such as military personnel. This study aimed to examine the inter-rater and intrarater reliability of HHD for lower limb strength assessment in a healthy military population. Secondary aims were to evaluate the validity of HHD for knee extensor strength compared with fixed dynamometry and explore the impact of assessor and participant characteristics on reliability. 

Methods: A cross-sectional study with test–retest reliability and concurrent validity design was conducted with 30 healthy military personnel (mean age: 32±6 years; 24 males; 6 females). Peak isometric lower limb force was measured across six positions using HHDs (Lafayette, USA) by three assessors across two sessions. Knee extensor strength was also assessed using an isokinetic dynamometer (Biodex, USA). Intrarater and inter-rater relative reliability were assessed using intraclass correlation coefficients (ICC), and absolute reliability with SEM and minimal detectable change (MDC). Concurrent validity was determined using Pearson’s correlation, and systematic bias was analysed through limits of agreement. 

Results: Intra-rater reliability was good-to-excellent (ICC>0.75; SEM% 2.1%–6.3%; MDC% 5.2%–17.6%) across all positions except one assessor in hip extension (ICC=0.70). Inter-rater reliability was also good-to-excellent (ICC>0.75; SEM% 2.2%–5.5%; MDC% 6.1%–15.4%). HHD demonstrated strong validity compared with isokinetic dynamometry (r=0.80–0.819, p<0.001) despite a systematic bias with lower values. No correlation was found between peak force and inter-rater variability. Conclusions HHD is a reliable tool for lower limb strength assessment in military personnel. Although absolute values are lower than fixed dynamometry, relative comparisons over time remain valid.