Ann Rehabil Med.  2024 Jun;48(3):203-210. 10.5535/arm.230025.

Measurement of Knee Extensor Torque During Repetitive Peripheral Magnetic Stimulation: Comparison of the Forces Induced by Different Stimulators

• Kamiue M
• Tsubahara A
• Ito T
• Koike Y

• KMID: 2557067
• DOI: http://doi.org/10.5535/arm.230025

Abstract

Objective
To investigate the factors that induce strong contractions during repetitive peripheral magnetic stimulation (rPMS) and compare the muscle torque induced by two stimulators (Stim A and Stim B) with different coil properties.
Methods
rPMS was applied to the right vastus lateralis of 30 healthy young adults. Stim A contained a 10.1 cm² rectangular iron core coil, while Stim B contained a 191 cm² round coil. The knee extensor torque (KET) induced by rPMS at 30 Hz was measured isometrically and divided by the maximum voluntary contraction (MVC) to obtain a relative value of MVC (%MVC). KET at 100% intensity of Stim A (A100%, 1.08 T) was compared to those at 100% or 70% intensity of Stim B (B100%, 1.47 T vs. B70%, 1.07 T). Additionally, we conducted a comprehensive literature search for studies that measured the KET during rPMS.
Results
Both the mean values of %MVC using B100% and B70% were significantly greater than that using A100%. Furthermore, the KET induced by Stim B was found to be larger than that described in previous reports, unless booster units were used to directly stimulate the main trunk of the femoral nerve.
Conclusion
Stim B induced a stronger muscle contraction force than Stim A did. This may be because the larger the coil area, the wider the area that can be stimulated. Additionally, a circular coil allows for deeper stimulation.

Keyword

Muscle strength; Strengthening; Contraction force; Magnetic coil; Disuse atrophy

Figure

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  Fig. 1. Magnetic stimulators used for muscle contraction. (A) Air-cooled type stimulator manufactured by IFG Co. Ltd. (Stim A). (B) Oil-cooled type stimulator manufactured by REMED Co. Ltd. (Stim B).

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  Fig. 2. Muscle contraction induced by repetitive peripheral magnetic stimulation and measurement of muscle torque. (A) Contraction induced by the stimulator manufactured by IFG Co. Ltd. (Stim A). (B) Contraction induced by the stimulator manufactured by REMED Co. Ltd. (Stim B).

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  Fig. 3. Knee extensor torque waveform during repetitive peripheral magnetic stimulation. Torque value: mean torque for 1 second from 500 to 1,500 ms before the waveform returned to baseline after contraction.

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  Fig. 4. Comparison of relative value of maximum voluntary contraction (%MVC) during repetitive peripheral magnetic stimulation between each stimulation condition. %MVC was calculated by dividing the knee extensor torque during stimulation by the MVC. A100%, maximum output intensity of the stimulator manufactured by IFG Co. Ltd. (Stim A); B70%, 70% of maximum output intensity of the stimulator manufactured by REMED Co. Ltd. (Stim B); B100%, maximum output intensity of the Stim B. **p<0.01.

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  Fig. 5. Comparison of pain during repetitive peripheral magnetic stimulation in each stimulation condition. A100%, maximum output intensity of the stimulator manufactured by IFG Co. Ltd. (Stim A); B70%, 70% of maximum output intensity of the stimulator manufactured by REMED Co. Ltd. (Stim B); B100%, maximum output intensity of the Stim B; VAS, visual analogue scale. *p<0.05, **p<0.01.

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