Abstract
Introduction
Precise maximal strength determination is of paramount importance for practical derivations, e.g. in injury prevention (Martín-Miguel et al. 2025). Although already implemented into diagnostics, the VALDPerformance ForceFrame is insufficiently evaluated: Existing literature lacks sufficient measurement error reporting and fails to differentiate between positions for joint-specific strength tests (Arp et al. 2024; Kadlec et al. 2021), at most considering angle specificity (Ferguson et al. 2024; O' Connor et al. 2023). This study was designed to evaluate measurement errors in a test-retest design stratified for the difference of positions.
Methods
Twenty-eight participants (m: n=13, 22.7±3.5years, 80.8±8.6kg, 179±7.5cm; f: n=15, 22.8±2.0years, 62.1±8.5kg, 166.6±7.32cm) performed testing protocols applied for knee extension and plantar flexion strength on two days under strict adherence to manufacturer’s recommendations (VALD a, b). Muscle group and position order were randomized per participant. To determine position-specific effects knee extension at 90° was performed seated, supine and standing. Plantar flexion at 90° was performed seated and supine. Intraclass correlation coefficients (ICC) 3,1 and random errors via mean absolute percentage error (MAPE) were calculated together with 95% bootstrap confidence intervals (CI) based on 2000 resamples. Systematic effects were tested via a two-way ANOVA (exercise-by-testing- day-interaction). Bonferroni correction was used for post-hoc testing. Bland-Altman-analyses were conducted for severity of bias and corresponding limits of agreements (LoA).
Results
The position significantly influenced force maximum: highest knee extension forces were obtained standing (M:375N±96N) compared to seated (M:248N±111N, p<0.001) and supine (306N±96N, p=0.002) with the latter showing no significant difference (p=0.09). Plantar flexion strength was more than twofold higher seated (M:1035N±338N) than supine (M:394N±73N, p<0.001). No systematic effects for testing day were found (p=0.60-0.95). However, the systematic bias still indicated small position differences: Narrowest LoA of the knee positions were achieved standing (-2.9%±27.3%), compared to supine (-0.4%±45.4%) and seated (8.4%±63.9%). In plantarflexion supine and seated showed slightly deviating LoA (0.7%±22.9% and -1.5%±28.6%). A similar pattern was observed for MAPE: Knee extension showed similar values for supine and seated with 20.61% (CI: 12.9%-31.6%) and 20.54% (CI:13.6%-28.7%) compared to 9.6%(CI:6.4%-13.0%) for standing. Ankle showed errors of 8.38% (CI:5.7%-12.0%) for supine and 12.53% (CI: 9.6%-15.6%) for seated. In contrast, ICC indicated poor to good reliability for knee extension: Supine 0.72(CI:0,50-0,86), seated 0.65(CI:0.41-0.86) and standing 0.85(CI:0.75-0.93). For plantarflexion the ICC showed poor to excellent reliability: Supine 0.79(CI:0.48-0.93) and seated 0.90(CI:0.81-0.95).
Discussion
Reliability and test outcome differed depending on the measurement position. In terms of reliability for knee extension the standing position should be preferred, for plantarflexion the supine one. Overall measurement errors of at least 5.7%-31.6%, according to the confidence intervals, could overrule possible intervention induced changes. Such high random errors might be the result of standardization limitations, as the body and extremity position must be freely reconstructed every session.
Conclusion
The partial discrepancy between ICC and MAPE highlights the dependence of ICC on sample heterogeneity, potentially leading to an overestimation of reliability. Existing literature lacking measurement error reporting must therefore be interpreted with care. A gold standard comparison to evaluate device validity is urgently needed.
References
Arp, Kamilla; Frydendal, Thomas; Kjeldsen, Troels; Dalgas, Ulrik; Timm, Signe; Viberg, Bjarke et al. (2024): Validity, Agreement and Reliability of the ForceFrame Dynamometer in Patients with Anterior Cruciate Ligament Injury. In: International journal of sports physical therapy 19 (9), S. 1068–1079. DOI: 10.26603/001c.122486.
Ferguson, Jack; Gibson, Neil V.; Weston, Matthew; McCunn, Robert (2024): Reliability of Measures of Lower-Body Strength and Speed in Academy Male Adolescent Soccer Players. In: Journal of strength and conditioning research 38 (3), e96-e103. DOI:10.1519/JSC.0000000000004639.
Kadlec, Daniel; Jordan, Matthew J.; Snyder, Leanne; Alderson, Jacqueline; Nimphius, Sophia (2021): Test Re-test Reliability of Single and Multijoint Strength Properties in Female Australian Footballers. In: Sports medicine - open 7 (1), S. 5. DOI:10.1186/s40798-020-00292-5.
Martín-Miguel, Iván; Torrontegi-Ronco, Elaia; Martínez-Ortega, Juan de Dios; Moreno-Martín, Adrián; Montalvo-Zenarruzabeitia, Zigor (2025): Relative Values of Maximal Isometric Strength in Lower Limbs Across Different Sports Disciplines in High-Performance Athletes. In: Journal of strength and conditioning research 39 (8), e982-e990. DOI: 10.1519/JSC.0000000000005126.
O' Connor, Ciaran; McIntyre, Martin; Delahunt, Eamonn; Thorborg, Kristian (2023): Reliability and validity of common hip adduction strength measures: The ForceFrame strength testing system versus the sphygmomanometer. In: Physical therapy in sport : official journal of the Association of Chartered Physiotherapists in Sports Medicine 59, S.162–167. DOI: 10.1016/j.ptsp.2022.12.010.
VALD a): ForceFrame Test Protocol – Ankle Plantar Flexion. Online verfügbar unter https://support.vald.com/hc/en-au/articles/12960659718809-ForceFrame-Test-Protocol-Ankle-Plantar-Flexion.
VALD b): ForceFrame Test Protocol – Knee Extension. Online verfügbar unter https://support.vald.com/hc/en-au/articles/12962592988953-ForceFrame-Test-Protocol-Knee-Extension.