This study aimed to investigate the within- and between-session reliability, variability, and minimal detectable change (MDC) of vertical ground reaction force (vGRF) during bilateral and unilateral lower extremity maximal isometric force tests. Eighteen participants (men: n = 9, age: 27.9 ± 6.3 y, height: 1.82 ± 0.06 m, mass: 82.4 ± 10.4 kg, strength training experience: 10.4 ± 7.7 y; women: n = 9, age: 29.3 ± 8.6 y, height: 1.68 ± 0.01 m, mass: 58.0 ± 5.8 kg, strength training experience: 5.5 ± 3.6 y) attended two data collection sessions separated by 48 h. The absolute, net, and relative vGRF were calculated across bilateral and unilateral variations of the squat, standing plantarflexion, and seated plantarflexion positions. All measures of vGRF demonstrated excellent reliability and low variability within (intraclass correlation coefficients (ICC): 0.92-0.99; coefficient of variation (CV): 2.9-6.5%) and between sessions (ICC: 0.95-1.00; CV: 2.0- 6.0%), across all positions. The MDC ranged between 135-276 N (5.1-14.5%), with the seated plantarflexion positions demonstrating the highest values as a percentage of the group mean (13.3-14.5%). Maximal isometric force testing during bilateral and unilateral variations of the squat, standing plantarflexion, and seated plantarflexion positions provides reliable measurement of vGRF in men and women.
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