Validity and Reliability of a Low-Cost Load Cell for Isometric Knee Extension and Flexion in Professional Soccer Players

Abstract

Purpose: Muscle strength is essential for sport performance, but gold-standard assessment tools are costly. We evaluated the validity and reliability of a very low-cost load cell for measuring isometric knee extension and flexion strength compared with an isokinetic dynamometer (ID). Methods: Thirty-one professional male soccer players (17.8 ± 1.3 y; 71.2 ± 6.3 kg; 177 ± 0.1 cm) completed two testing sessions. Peak isometric force at of knee flexion was measured with both the LC and ID. Reliability was assessed using intraclass correlation coefficients (ICC), coefficients of variation (CV), and Bland–Altman plots. Validity was examined by comparing peak force between devices using repeated-measures ANOVA. Results: Both devices showed excellent test–retest reliability for knee extension (LC ICC = 0.91; ID ICC = 0.94) and knee flexion (LC ICC = 0.93; ID ICC = 0.99). CVs ≤ 7.2% indicated acceptable to high reliability. The LC consistently overestimated peak force relative to the ID in both movements (p < 0.001). Trial-to-trial differences were also found for knee flexion with the LC (p = 0.010). Conclusion: The low-cost LC demonstrated excellent reliability but poor validity versus the ID. It may be useful in applied settings, but systematic overestimation warrants caution.