Intra-Session Reliability and Criterion Validity of Belt-Fixated Crane Scale Knee Strength Measures at Different Knee Flexion Angles

This study aimed to assess the intra-session reliability of knee extension, flexion, and the flexion-to-extension isometric strength ratio using a belt-fixated crane scale protocol at 60° and 90° knee flexion angle positions. Validity was examined by comparison with a gold-standard rigid-chair isometric knee dynamometer. Twenty-six physical education students completed the testing. Both protocols showed good-to-excellent reliability for maximal strength (peak torque in Nm/kg) and flexor-to-extensor ratios at both angles (ICC2.1 ≥ 0.84; CV% ≤ 8.2). Strength values were lower at 90° compared to 60° (p < .05). The crane scale overestimated knee flexion strength (6–9%) and flexor-to-extensor ratios (51–54% vs. 40–44%), but underestimated extension strength (16–18%). Strength measures correlated significantly between devices and angles (r from 0.52 to 0.86), but no significant correlations were found for ratios. Findings suggest that testing device and knee flexion angle substantially affect outcomes and should not be used interchangeably.