Differences in Ground Reaction Force During Foot Strength Testing in Sitting and Standing Positions

Foot strength is important for gait, balance, and function, but it is challenging to measure. Evidence explaining why strength differences exist between sitting and standing test positions is needed to improve clinical value. Observing changes in ground reaction force (Δ GRF) during strength testing may help explain these differences. Our purpose was to determine if Δ GRF differed between test positions, and if Δ GRF was associated with strength. Twenty healthy adults (x¯ = 24.7 y) were randomly assigned to a first test position, sitting or standing. With one foot on a force plate, subjects pressed down and pulled with their toes on a towel connected to a scale, for each test position. Paired t tests and Pearson correlation coefficients were used in analyses of strength and 3D Δ GRF outcomes. Mean peak foot strength and Δ GRFx (anterior-posterior direction) were greater in standing (P < .001). Correlations were strong between sitting and standing peak strength (r = .703) and between peak strength and Δ GRFx (r = .738) and Δ GRFz (superior-inferior) (r = .595) in standing. Greater strength production in standing, which was directly correlated to Δ GRF, was congruent with task demands and may reflect the importance of rearfoot stability during foot strength testing. From a clinical perspective, the standing test position is favored over sitting.