Myoelectric, Myo-Oxygenation, and Myotonometry Changes during Robot-Assisted Bilateral Arm Exercises with Varying Resistances

Abstract

Robot-assisted bilateral arm training has demonstrated its effectiveness in improving motor function in individuals post-stroke, showing significant enhancements with increased repetitions. However, prolonged training sessions may lead to both mental and muscle fatigue. We conducted two types of robot-assisted bimanual wrist exercises on 16 healthy adults, separated by one week: long-duration, low-resistance workouts and short-duration, high-resistance exercises. Various measures, including surface electromyograms, near-infrared spectroscopy, heart rate, and the Borg Rating of Perceived Exertion scale, were employed to assess fatigue levels and the impacts of exercise intensity. High-resistance exercise resulted in a more pronounced decline in electromyogram median frequency and recruited a greater amount of hemoglobin, indicating increased muscle fatigue and a higher metabolic demand to cope with the intensified workload. Additionally, high-resistance exercise led to increased sympathetic activation and a greater sense of exertion. Conversely, engaging in low-resistance exercises proved beneficial for reducing post-exercise muscle stiffness and enhancing muscle elasticity. Choosing a low-resistance setting for robot-assisted wrist movements offers advantages by alleviating mental and physiological loads. The reduced training intensity can be further optimized by enabling extended exercise periods while maintaining an approximate dosage compared to high-resistance exercises.

Keywords: electromyogram; exercise intensity; heart rate; mental fatigue; muscle fatigue; muscular oxygenation; myotonometry; robot-assisted bilateral arm training.

Figure 1

(a) During the experiment, a subject engaged in repetitive robot-assisted wrist pronation and supination movements using the Bi-Manu-Track, while being concurrently monitored with near-infrared spectroscopy (NIRS) and electromyogram (EMG) measurements on the left and right flexor carpi radialis and extensor carpi radialis. Resting myotonometry was performed on (b) the upper section of the muscle belly of the left extensor carpi radialis and (c) the corresponding area of the left flexor carpi radialis separately.

Figure 2

Tissue oxygen saturation rate (StO₂) and the relative concentrations of oxyhemoglobin (HbO₂) and deoxyhemoglobin (HHb) in the left flexor carpi radialis during a high-resistance bilateral robot-assisted wrist exercise carried out by a participant.

Figure 3

The 8-cycle moving–averaging median frequency (MF) of the electromyogram recorded from the right extensor carpi radialis during low- and high-resistance bilateral robot-assisted wrist exercises performed by a participant were examined. To quantify the decline in MF, the regression slopes within the first 5 and 10 min (S₅ and S₁₀), along with the Fatigue Progress Measure (FPM) were utilized. These metrics provided insights into the change in MF during the exercises.

Figure 4

Borg Rating of Perceived Exertion, mean heart rate, and Fatigue Progress Measure (FPM) for both the right flexor and right extensor muscles, recorded every minute during bilateral robot-assisted wrist exercise. The data are presented as mean ± standard error of mean. A significant trend over the stages is indicated by # (p < 0.05).

Figure 5

Tissue oxygen saturation (StO₂) and the relative concentrations of oxyhemoglobin and deoxyhemoglobin (HbO₂ and HHb) at one-minute intervals during bilateral robot-assisted wrist exercise. The data are presented as mean ± standard error of the mean. A significant trend over the stages is indicated by #.