Measurement of maximal muscle contraction force induced by high-frequency magnetic stimulation: a preliminary study on the identification of the optimal stimulation site

Abstract

Tsubahara A, Kamiue M, Ito T, Kishimoto T, Kurozumi C. Measurement of maximal muscle contraction force induced by high-frequency magnetic stimulation: a preliminary study on the identification of the optimal stimulation site. Jpn J Compr Rehabil Sci 2021; 12: 27-31.

Purpose: To identify the optimal stimulation site and technique for inducing strong muscle contraction using a high-frequency magnetic stimulator.

Methods: High-frequency magnetic stimulation was administered to the right vastus lateralis (VL) of eight healthy adults at maximal intensity within the range of tolerable pain. The stimulation sites were as follows: section A, the area between the lateral edge of the base of the patella (LEBP) and the distal one-third of the thigh (point D); section B, the area between point D and the proximal one-third of the thigh (point P). Isometric maximal muscle contraction forces induced by magnetic stimulation (Stim-MCF) were compared between the two sections.

Results: The Stim-MCF was significantly higher in section B than in section A. Additionally, the sites susceptible to stimulation were confined to a narrow area near point D in section A and the central part between points D and P in section B. The degree of pain was very low in both sections.

Conclusion: The optimal site for magnetic stimulation of the VL was limited to the central part of the thigh. In addition to the superficial proximal sub-branch, the deep proximal sub-branch and/or deeply clustered motor nerve endings may have been stimulated. Our results suggested that moving the probe was a useful way to identify the site that elicited the strongest muscle contraction force.

Keywords: high-frequency magnetic stimulation; motor points; muscle contraction force; quadriceps femoris; strengthening.

Figure 1.. Places of magnetic stimulation and nerve distributions corresponding to each section.

ASIS: anterior superior iliac spine, LEBP: lateral edge of the base of the patella, point D: a point at the distal one-third length on the straight line connecting the LEBP and the ASIS, point P: a point of the proximal one-third length on the straight line connecting the LEBP and the ASIS, Section A: between the LEBP and point P, Section B: between points D and P, 1) femoral nerve trunk, 2) branch of the rectus femoris (RF), 3) branch of the vastus medialis (VM), 4) branch of the vastus intermedius (VI), 5) branch of the vastus lateralis (VL), 6) superficial proximal sub-branch of the VL, 7) deep proximal sub-branch of the VL, 8) mid-distal sub-branch of the VL, 9) distal sub-branch of the VL.

Figure 2.. Probe of magnetic stimulator and scene of magnetic stimulation.

The size of the prototype probe is 19.5 × 21.5 cm, the surface area of the coil inside the probe is 18.2 cm², and the maximal output intensity is 1.3 Tesla.

Figure 3.. Comparison of maximal muscle contraction force (muscle torque) induced by magnetic stimulation.

The values represent “mean ± standard deviation.”

Figure 4.. Sites susceptible to stimulation in sections A and B.

The vertical axis shows the position on the straight line connecting the lateral edge of the base of the patella (LEBP) and the anterior superior iliac spine (ASIS) as a percentage (0%: LEBP, 100%: ASIS). Point D denotes the location of the distal one-third of the thigh, and point P denotes the location of the proximal one-third of the thigh.