Evaluation of a portable markerless finger position capture device: accuracy of the Leap Motion controller in healthy adults

Although motion analysis is frequently employed in upper limb motor assessment (e.g. visually-guided reaching), they are resource-intensive and limited to laboratory settings. This study evaluated the reliability and accuracy of a new markerless motion capture device, the Leap Motion controller, to measure finger position. Testing conditions that influence reliability and agreement between the Leap and a research-grade motion capture system were examined. Nine healthy young adults pointed to 15 targets on a computer screen under two conditions: (1) touching the target (touch) and (2) 4 cm away from the target (no-touch). Leap data was compared to an Optotrak marker attached to the index finger. Across all trials, root mean square (RMS) error of the Leap system was 17.30 +/- 9.56 mm (mean +/- SD), sampled at 65.47 +/- 21.53 Hz. The % viable trials and mean sampling rate were significantly lower in the touch condition (44% versus 64%, p < 0.001; 52.02 +/- 2.93 versus 73.98 +/- 4.48 Hz, p = 0.003). While linear correlations were high (horizontal: r(2) = 0.995, vertical r(2) = 0.945), the limits of agreement were large (horizontal: -22.02 to +26.80 mm, vertical: -29.41 to +30.14 mm). While not as precise as more sophisticated optical motion capture systems, the Leap Motion controller is sufficiently reliable for measuring motor performance in pointing tasks that do not require high positional accuracy (e.g. reaction time, Fitt's, trails, bimanual coordination).