Lower extremity angle measurement with accelerometers-error and sensitivity analysis

The use of accelerometers for angle assessment of the lower extremities is investigated. This method is evaluated by an error-and-sensitivity analysis using healthy subject data. Of three potential error sources (the reference system, the accelerometers, and the model assumptions) the last is found to be the most important. Model calculations based on data obtained by the Elite video motion analysis system showed the rigid-body assumption error to be dominant for high frequencies (>10 Hz), with vibrations in the order of 1 mm resulting in errors of one radial or more. For low frequencies (<5 Hz), the imperfect fixation of the accelerometers combined with a nonhinge-type knee joint gave an error contribution of +or-0.03 rad. The walking pattern was assumed to be two-dimensional, which was shown to result in an error of +or-0.04 rad. Accelerations due to rotations of the segments could be neglected. The total error computed for low frequencies (+or-0.07 rad) is comparable to the experimental difference between the current and the reference system.