The Impact of Muscle Lengthening and Electrical Nerve Stimulation on Torque Production

Academic Background

In rehabilitation and training programs, Neuromuscular Electrical Stimulation (NMES) is an effective method for enhancing skeletal muscle function. However, traditional high-intensity NMES, while capable of generating high torque, often comes with significant discomfort. In recent years, Wide-Pulse Low-Intensity NMES has emerged as an alternative, capable of producing high torque at low stimulation intensities without causing discomfort. Nevertheless, how to further optimize the torque output of NMES, especially under different frequencies and muscle lengthening amplitudes, remains a topic worthy of investigation.

This study aims to explore the impact of combining Wide-Pulse NMES with muscle lengthening at different amplitudes on torque production. Specifically, the research team hopes to further optimize torque output by combining NMES with muscle lengthening and to investigate the underlying neural and muscular mechanisms. This research not only contributes to understanding the mechanisms of NMES but may also provide new strategies for rehabilitation and training.

Source of the Paper

This paper was co-authored by Antoine Pineau, Alain Martin, Romuald Lepers, and Maria Papaiordanidou from the Inserm UMR1093-CAPS laboratory at the University of Burgundy, France. The paper was first published on December 6, 2024, in the Journal of Neurophysiology, with the DOI 10.1152/jn.00383.2024.

Research Process

Participants and Experimental Design

The study recruited 15 healthy volunteers (4 females, average age 30.1 years), all of whom were physically active and free from injuries in their right lower limb over the past 3 months. The experiment was divided into two main parts: low-frequency (20 Hz) and high-frequency (100 Hz) NMES stimulation. Under each frequency, three conditions were tested: NMES alone, NMES combined with 10-degree ankle rotation (NMES + Len10), and NMES combined with 20-degree ankle rotation (NMES + Len20).

Experimental Procedure

1. Warm-Up and Maximal Voluntary Contraction (MVC) Testing: Participants first underwent a standardized warm-up, including 8-10 submaximal voluntary contractions of the right plantar flexors. Subsequently, participants performed at least 3 MVC tests to ensure the stability of the results.

2. NMES Stimulation: Wide-pulse NMES was delivered to the posterior tibial nerve at rest, with a pulse width of 1 ms and a stimulation intensity of 5-10% of MVC. The stimulation frequencies were 20 Hz and 100 Hz, with 12 stimulation trains of 15 seconds each under each frequency, with a 2-minute rest interval between trains.

3. Muscle Lengthening: In the NMES + Len10 and NMES + Len20 conditions, the ankle joint was rotated from initial positions of 90 degrees and 100 degrees, respectively, to a reference position of 80 degrees at a speed of 300 degrees/second. The lengthening was initiated 3 seconds after the start of stimulation and maintained for the remaining 12 seconds of stimulation.

4. Data Collection: Surface electrodes were used to record electromyographic (EMG) activity of the right triceps surae muscles (soleus, lateral gastrocnemius, and medial gastrocnemius), and torque data were recorded using an isokinetic dynamometer.

Data Analysis

1. Torque-Time Integral (TTI): The torque-time integral during the 15-second stimulation train under each condition was calculated to evaluate torque output.

2. Sustained EMG Activity: The root mean square (RMS) value of the EMG signal over a 500-ms period after the cessation of stimulation was calculated and compared with the EMG RMS value during MVC to assess sustained neural activity.

3. Extra-Torque Phenomenon: The presence of the extra-torque phenomenon was assessed by comparing the actual TTI with the theoretical TTI (calculated based on the torque value during the first second after stimulation onset).

Key Findings

Low-Frequency NMES (20 Hz)

Under low-frequency NMES, the TTI for NMES + Len10 and NMES + Len20 was significantly higher than that for NMES alone (233.2 ± 101.5 N·s and 229.2 ± 92.1 N·s vs. 187.5 ± 74.5 N·s, respectively). However, there was no significant difference in sustained EMG activity across conditions, suggesting that the increase in torque may be related to muscular mechanisms.

High-Frequency NMES (100 Hz)

Under high-frequency NMES, the TTI for NMES + Len10 was significantly higher than that for NMES alone (226.6 ± 115.3 N·s vs. 173.9 ± 94.9 N·s), but there was no significant difference between NMES + Len20 and NMES alone. Additionally, the sustained EMG activity in the NMES + Len10 condition was significantly higher than in the NMES + Len20 condition, indicating that neural mechanisms played an important role in high-frequency NMES.

Conclusion

This study demonstrates that combining Wide-Pulse Low-Frequency NMES with muscle lengthening can significantly increase torque output, and this increase is likely related to enhanced muscular mechanisms. In contrast, under high-frequency NMES, only a 10-degree muscle lengthening significantly increased torque output, and this increase was accompanied by sustained neural activity, suggesting that neural mechanisms play a key role in high-frequency NMES. However, increasing the lengthening amplitude (20 degrees) did not further enhance torque output, possibly due to presynaptic inhibitory mechanisms.

Research Highlights

1. Torque Enhancement Mechanisms: This study is the first to systematically explore the impact of combining Wide-Pulse NMES with muscle lengthening at different amplitudes on torque production, revealing distinct torque enhancement mechanisms under low- and high-frequency NMES.

2. Distinguishing Neural and Muscular Mechanisms: By analyzing sustained EMG activity, the study successfully distinguished the roles of neural and muscular mechanisms in torque enhancement.

3. Application Value: The findings provide new strategies for rehabilitation and training, particularly in optimizing NMES stimulation parameters and combining muscle lengthening.

Additional Valuable Information

The research team also provided a data availability statement, indicating that source data are available upon reasonable request to the corresponding author. Furthermore, the study was supported by the French Ministry of Higher Education, Research, and Innovation, and the authors declared no conflicts of interest.

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Through this study, we have gained a deeper understanding of the mechanisms of torque enhancement through the combination of NMES and muscle lengthening, while also providing new insights for future rehabilitation and training strategies. This research holds significant theoretical and practical importance in the fields of neurophysiology and exercise science.