Target Achievement Control Test: evaluating real-time myoelectric pattern-recognition control of multifunctional upper-limb prostheses

Abstract

Despite high classification accuracies (~95%) of myoelectric control systems based on pattern recognition, how well offline measures translate to real-time closed-loop control is unclear. Recently, a real-time virtual test analyzed how well subjects completed arm motions using a multiple-degree of freedom (DOF) classifier. Although this test provided real-time performance metrics, the required task was oversimplified: motion speeds were normalized and unintended movements were ignored. We included these considerations in a new, more challenging virtual test called the Target Achievement Control Test (TAC Test). Five subjects with transradial amputation attempted to move a virtual arm into a target posture using myoelectric pattern recognition, performing the test with various classifier (1- vs 3-DOF) and task complexities (one vs three required motions per posture). We found no significant difference in classification accuracy between the 1- and 3-DOF classifiers (97.2% +/- 2.0% and 94.1% +/- 3.1%, respectively; p = 0.14). Subjects completed 31% fewer trials in significantly more time using the 3-DOF classifier and took 3.6 +/- 0.8 times longer to reach a three-motion posture compared with a one-motion posture. These results highlight the need for closed-loop performance measures and demonstrate that the TAC Test is a useful and more challenging tool to test real-time pattern-recognition performance.

Figure 1

Target Achievement Control (TAC) Test. Subjects moved a multifunctional virtual prosthesis into a target posture. The virtual hand turned green when the target was reached within the acceptable tolerances (± 5 degrees for each degree of freedom). Pictures illustrate starting and ending positions for successful trials. (A) Example trial from Conditions 1 and 2 requiring one motion to reach the target posture (e.g. wrist flexion). (B) Example trial from Condition 3 requiring three motions to reach the target posture (e.g. wrist flexion, wrist supination, and hand close).

Figure 2

Average completion rate curves for all three conditions. Solid line indicates performance during trials that required only one motion per posture using a one–degree of freedom classifier (Condition 1). Dashed line indicates performance during trials that required only one motion per posture using a three–degree of freedom classifier (Condition 2). Dotted line indicates performance during trials that required three motions per posture using a three–degree of freedom classifier (Condition 3). Shaded regions represent ± 1 standard error.

Figure 3

Position and decision history during an example TAC Test trial requiring one motion to reach a Condition 2 target posture. A three–degree of freedom classifier is used. The virtual prosthesis began in 75 degrees of wrist extension, 0 degrees wrist rotation, and the hand 50% closed. The user had to flex the wrist to reach the target posture (0 degrees flexion/extension, 0 degrees wrist rotation, and the hand 50% closed). Gray bars indicate the target position for each degree of freedom. Since the TAC Test required all degrees of freedom to match the target position, the subject had to correct for any misclassifications (e.g. wrist pronation). The virtual arm reached the target position at 5.3 s (indicated by T^*). The trial ended at 7.3 s after the subject was able to remain in the target posture for 2 s.

Figure 4

Position and decision history during an example TAC Test trial requiring three motions to reach a Condition 3 target posture. The virtual prosthesis began in 75 degrees of wrist flexion, 75 degrees of wrist supination, and the hand 25% closed. The user had to extend and pronate the wrist and close the hand to reach the target posture (0 degrees flexion/extension, 0 degrees wrist rotation, and 75 degrees hand open/close). Gray bars indicate the target position for each degree of freedom. The virtual arm reached the target position at 18.2 s (indicated by T^*). The trial ended at 20.2 s after the subject was able to remain in the target posture for 2 s.