A new method for evaluating motor control in gait under real-life environmental conditions. Part 1: The instrument.

OBJECTIVE: To devise an instrument to measure linear acceleration of the trunk during gait under real-life environmental conditions. DESIGN: A mathematical algorithm is suggested to transform data to a horizontal-vertical coordinate system. Calibration procedures are described. BACKGROUND: A portable, low mass device to measure linear acceleration of the trunk in a horizontal-vertical coordinate system during prolonged walking is of clinical interest and has not been described previously. METHOD: Linear acceleration is registered by a triaxial, piezoresitant accelerometer, sampled at 512 Hz and collected by exchangeable 20 Mb memory cards of a body-mounted data-logger. Data are transformed to a horizontal-vertical coordinate system, utilising the accelerometer's capacity as an inclinometer. Calibration testing was done in a servohydraulic jig by measuring acceleration of 5 Hz, 16 mm vertical displacements, first along each of the sensing axis, then by a tilted accelerometer. RESULTS: Precision testing of each axis and transformed data from all axes showed low variability for acceleration root mean square of sequences of 1000 samples (CV <0.001). Accuracy testing by double integration of transformed data from the tilted accelerometer demonstrated a mean absolute difference of 0.04 mm (SD = 0.03 mm) from the programmed input. CONCLUSIONS: Data from the arbitrarily tilted accelerometer transformed to a horizontal-vertical coordinate system gave precise and accurate measurements of linear accelerations when tested in a servohydraulic jig. Procedures for applying the device in gait analysis are described in a companion article. RELEVANCE: Control of upper body and body centre of mass are important aspects of balance, but gait analysis has largely focused on the movements of the lower limbs. This device can be used to register acceleration of a reference point on the upper body during walking, and may thus give new information about balance in gait.