Reliability of ground reaction force measurements during dynamic transitions from bipedal to single-limb stance in healthy adults.

The consistency of ground reaction force (GRF) variables underlying dynamic transitions from bipedal to single-limb stance during a single-leg flexion movement was examined in 18 healthy adult subjects aged 21 to 47 years (mean = 31.1, SD = 8.6). Force platforms were used to measure the GRFs during fast and natural speeds of movement. Separate intraclass correlation coefficients (ICCs) were calculated for three temporal variables (ie, onset of the propulsive and braking phases of the lateral horizontal GRF component and time to unload the flexing limb) and for two magnitude variables (ie, propulsive impulse [PROP] and braking impulse [BRAK]). The ICCs for both PROP and BRAK were > or = .73 for fast movements and > or = .88 at the natural speed. The ICCs for the temporal variables were > or = .66 at fast speeds and > or = .37 at the natural speed. We concluded that measurements of PROP and BRAK are reliable across a range of speeds during transitions in stance support and that these variables may be tightly regulated by the movement control system. Temporal variables, particularly at the natural speed, exhibited lower reliability estimates, suggesting that measurements of these events have greater variability. Reliability of measurements of GRF variables provides useful information for the clinician regarding underlying control processes governing dynamic transitions in stance support.