A comparison of the real-time controllability of pattern recognition to conventional myoelectric control for discrete and simultaneous movements

Abstract

Myoelectric control has been used for decades to control powered upper limb prostheses. Conventional, amplitude-based control has been employed to control a single prosthesis degree of freedom (DOF) such as closing and opening of the hand. Within the last decade, new and advanced arm and hand prostheses have been constructed that are capable of actuating numerous DOFs. Pattern recognition control has been proposed to control a greater number of DOFs than conventional control, but has traditionally been limited to sequentially controlling DOFs one at a time. However, able-bodied individuals use multiple DOFs simultaneously, and it may be beneficial to provide amputees the ability to perform simultaneous movements. In this study, four amputees who had undergone targeted motor reinnervation (TMR) surgery with previous training using myoelectric prostheses were configured to use three control strategies: 1) conventional amplitude-based myoelectric control, 2) sequential (one-DOF) pattern recognition control, 3) simultaneous pattern recognition control. Simultaneous pattern recognition was enabled by having amputees train each simultaneous movement as a separate motion class. For tasks that required control over just one DOF, sequential pattern recognition based control performed the best with the lowest average completion times, completion rates and length error. For tasks that required control over 2 DOFs, the simultaneous pattern recognition controller performed the best with the lowest average completion times, completion rates and length error compared to the other control strategies. In the two strategies in which users could employ simultaneous movements (conventional and simultaneous pattern recognition), amputees chose to use simultaneous movements 78% of the time with simultaneous pattern recognition and 64% of the time with conventional control for tasks that required two DOF motions to reach the target. These results suggest that when amputees are given the ability to control multiple DOFs simultaneously, they choose to perform tasks that utilize multiple DOFs with simultaneous movements. Additionally, they were able to perform these tasks with higher performance (faster speed, lower length error and higher completion rates) without losing substantial performance in 1 DOF tasks.

Figure 1

Block diagram of the three control strategies. Conventional myoelectric control (left) uses two separate decision nodes, each receiving two channels to determine a motion output based on EMG amplitude. Sequential pattern recognition control receives all EMG channels as input and can output any of the four discrete motions (Elbow Flexion/Extension or Hand Open/Close) or no movement. Simultaneous pattern recognition control also receives all EMG channels as input, outputs any of the four discrete motions, no movement, and additionally, any combination of elbow and hand movements.

Figure 2

Target Achievement Control (TAC) test. Start (top) and end (bottom) of a discrete movement of closing the hand (left) and combined movement of elbow flexion and hand close (right).

Figure 3

TAC test completion rate verse completion time for the three control strategies. These figures show the completion rate as a function of completion time across all four subjects for each of the three control strategies. The shaded regions indicate +/- 1 SEM for each of the lines. Figure 3a is for tasks with 1 DOF complexity, while Figure 3b is for tasks with 2 DOF complexity.

Figure 4

TAC test length error for each of the three control strategies. These figures show the % deviation from the perfect path averaged across all four subjects for each of the three control strategies. Figure 4a is for tasks with 1 DOF complexity, while Figure 4b is for tasks with 2 DOF complexity.

Figure 5

Percent of time in which zero, one, or two DOFs were moving for each control strategy. Figure 5a shows the average percent of time across the four subjects in which zero, one or two DOFs were moving for tasks of 1 DOF complexity for each of the three control strategies, and Figure 5b is similar except for 2 DOF complexity.