Ambulatory measurement of ground reaction forces.

The measurement of ground reaction forces is important in the biomechanical analysis of gait and other motor activities. Many applications require full ambulatory measurement of these forces, but this is not supported by current measurement systems. We propose the use of two six-degrees-of-freedom force and moment sensors under each shoe, which enables the ambulatory measurement of ground reaction forces and centers of pressure (CoP). The feasibility of this method is illustrated by experimental results in a healthy subject, using a force plate as a reference. The ground reaction forces and CoP recordings show good correspondence when they are evaluated for forces above 40 N and when it is simply assumed that the sensors are flat on the ground when they are loaded. The root mean square (rms) difference of the magnitude of the ground reaction force over 12 gait trials was 15 +/- 2 N, corresponding to 1.9 +/- 0.3% of the maximum ground reaction force magnitude. The rms difference of the horizontal component of the ground reaction force was 3 +/- 2 N, corresponding to 0.4 +/- 0.2% of the maximum ground reaction force magnitude and to 2 +/- 1% of the maximum of the horizontal component of the ground reaction force. The rms distance between both CoP recordings is 2.9 +/- 0.4 mm, corresponding to 1.1 +/- 0.2% of the length of the shoe, when the trajectories are optimally aligned.