Activation of individual extrinsic thumb muscles and compartments of extrinsic finger muscles

Abstract

Mechanical and neurological couplings exist between musculotendon units of the human hand and digits. Studies have begun to understand how these muscles interact when accomplishing everyday tasks, but there are still unanswered questions regarding the control limitations of individual muscles. Using intramuscular electromyographic (EMG) electrodes, this study examined subjects' ability to individually initiate and sustain three levels of normalized muscular activity in the index and middle finger muscle compartments of extensor digitorum communis (EDC), flexor digitorum profundus (FDP), and flexor digitorum superficialis (FDS), as well as the extrinsic thumb muscles abductor pollicis longus (APL), extensor pollicis brevis (EPB), extensor pollicis longus (EPL), and flexor pollicis longus (FPL). The index and middle finger compartments each sustained activations with significantly different levels of coactivity from the other finger muscle compartments. The middle finger compartment of EDC was the exception. Only two extrinsic thumb muscles, EPL and FPL, were capable of sustaining individual activations from the other thumb muscles, at all tested activity levels. Activation of APL was achieved at 20 and 30% MVC activity levels with significantly different levels of coactivity. Activation of EPB elicited coactivity levels from EPL and APL that were not significantly different. These results suggest that most finger muscle compartments receive unique motor commands, but of the four thumb muscles, only EPL and FPL were capable of individually activating. This work is encouraging for the neural control of prosthetic limbs because these muscles and compartments may potentially serve as additional user inputs to command prostheses.

Keywords: extrinsic muscles; finger control; intramuscular EMG; prosthetics.

Fig. 1.

Columns represent an example trial showing electromyographic (EMG) signals from each tested finger muscle. Rows represent EMG from a particular muscle. Black traces represent the activity of the intended muscle during that trial, and gray traces represent EMG activity from nonintended muscles.

Fig. 2.

A: when extensor digitorum communis of finger 1 was the intended muscle, the EDC1 muscle compartment (red bars) demonstrated muscle activation for the 3 target activity levels [10, 20, and 30% maximum voluntary contraction (MVC)] with minimal coactivity. Mean activity is shown for each of the 6 finger muscle compartments, averaged across all trials and all subjects. Error bars represent SD. B: when extensor digitorum communis of finger 2 was the intended muscle, activation of EDC2 (blue bars) was observed with coactivity from EDC1 (red bars) that was not significantly different at the 3 target activity levels (10, 20, and 30% MVC). Mean activity is shown for each of the 6 finger muscle compartments, averaged across all trials and all subjects. Error bars represent SD. FDP1 and FDP2, flexor digitorum profundus of fingers 1 and 2; FDS1 and FDS2, flexor digitorum superficialis of fingers 1 and 2.