Toward a wearable monitor of local muscle fatigue during electrical muscle stimulation using tissue Doppler imaging

Abstract

Electrical muscle stimulation (EMS) is widely used in rehabilitation and athletic training to generate involuntary muscle contractions. However, EMS leads to rapid muscle fatigue, limiting the force a muscle can produce during prolonged use. Currently available methods to monitor localized muscle fatigue and recovery are generally not compatible with EMS. The purpose of this study was to examine whether Doppler ultrasound imaging can assess changes in stimulated muscle twitches that are related to muscle fatigue from electrical stimulation. We stimulated five isometric muscle twitches in the medial and lateral gastrocnemius of 13 healthy subjects before and after a fatiguing EMS protocol. Tissue Doppler imaging of the medial gastrocnemius recorded muscle tissue velocities during each twitch. Features of the average muscle tissue velocity waveforms changed immediately after the fatiguing stimulation protocol (peak velocity: -38%, p = .022; time-to-zero velocity: +8%, p = .050). As the fatigued muscle recovered, the features of the average tissue velocity waveforms showed a return towards their baseline values similar to that of the normalized ankle torque. We also found that features of the average tissue velocity waveform could significantly predict the ankle twitch torque for each participant (R2 = 0.255–0.849, p < .001). Our results provide evidence that Doppler ultrasound imaging can detect changes in muscle tissue during isometric muscle twitch that are related to muscle fatigue, fatigue recovery, and the generated joint torque. Tissue Doppler imaging may be a feasible method to monitor localized muscle fatigue during EMS in a wearable device.

Clinical, Safety, and Engineering Perspectives on Wearable Ultrasound Technology: A Review

“…SMG has advantages over sEMG, including spatial selectivity enabled by the depth resolution of US, as well as potentially superior SNR [58]. US can also be used to analyze muscle twitch response during functional electrical stimulation, which is not practical with sEMG [59]. New research has also shown the ability of US to isolate spatially-resolved motor unit action potentials [60].…”

Surface‐Level Muscle Deformation as a Correlate for Joint Torque

“…[32,33,46] Another important consideration for any muscle sensing modality is muscle fatigue, a time-dependent phenomenon that affects both the amplitude and timing of force production, and often complicates the interpretation of measured signals. [47][48][49] The objective of this study was to correlate changes in surfacelevel muscle deformation with changes in volitionally-generated joint torque using a sensor-agnostic, high-resolution, and noncontact approach. A 2D laser profilometer was used to capture a high-resolution line profile of the skin's surface, measuring micron-scale changes in height relative to the laser head by scanning over a fixed distance and providing detailed height data.…”