Abstract: BACKGROUND: Exposure to transient low-frequency pulsed magnetic field can induce and activate classical transient receptor potential channel 1, and enhance the maximal voluntary contraction force and strength endurance of local muscles, such as biceps brachii.
OBJECTIVE: To use a specific low-frequency pulsed magnetic field that can activate classical transient receptor potential channel 1 as a means of improving muscle strength and to observe the attenuation process of the maximal voluntary contraction force and strength endurance of human biceps brachii after short-term stimulation.
METHODS: A total of 27 normal adult healthy subjects were selected and randomly divided into training group, exposure group, and training+exposure group. The training+exposure group received resistance training immediately after 10-minute exposure to low-frequency pulsed magnetic field; the exposure group was only exposed to low-frequency pulsed magnetic field for 10 minutes; and the training group only received resistance training. The trial duration was 8 weeks. At 1-12 days of the trial, short-term muscle strength enhancement scheme was performed and post-test muscle strength was measured. The attenuation process of maximum voluntary contraction force and strength endurance was then observed in the following 6 weeks.
RESULTS AND CONCLUSION: (1) The maximum voluntary contraction force value of all subjects changed significantly over time (P < 0.01), with obvious time-dependent interaction effect but no interaction effect between groups, and there was no significant difference between time- and grouping-dependent interaction effects. (2) In the 1st, 2nd, 3rd, and 4th weeks, the maximum voluntary contraction force values for strength attenuation in the exposure group were significantly higher than the initial value. In the 1st and 4th weeks, the maximum voluntary contraction force values for strength attenuation in the training group were significantly higher than the initial value. In the 5th and 6th weeks, the maximum voluntary contraction force values for strength attenuation in the exposure group and training group were significantly lower than the post-test value. In the 1st, 5th, and 6th weeks, the maximum voluntary contraction force values for strength attenuation in the training+exposure group were significantly lower than the post-test value. (3) The correlation analysis of the maximal voluntary contraction force curves in the three groups indicated that the overall changes in the maximal voluntary contraction force in the exposure group were highly positively correlated with the variation trend of maximum voluntary contraction force in the training group and training+exposure group. Their variation trend was highly consistent. Compared with the exposure group, the training group showed stronger positive correlation with the training+exposure group in terms of the maximum voluntary contraction force. (4) The median frequency value of the subjects in each group changed significantly with time (P < 0.01), and there was significant time-dependent interaction effect. Time- and grouping-dependent interactions led to significant changes in the median frequency values (P < 0.01). (5) In the 2nd week, the median frequency values of muscle strength attenuation in the exposure group and training+exposure group was significantly higher than the initial value. However, compared with the post-test value, the median frequency values of muscle strength attenuation showed no significant changes in the exposure group. In the 1st and 4nd weeks, the median frequency values of muscle strength attenuation in the training group were significantly lower than the post-test value. In the 1st and 2th weeks, the median frequency values of muscle strength attenuation in the training+exposure group were significantly lower than the post-test value. (6) The correlation analysis of the median frequency change curves in the three groups indicated that the training+exposure group showed a lower positive correlation with the training group and exposure group. However, compared with the training group, the exposure group had a higher correlation with the training+exposure group. (7) After exposure to the short-term pulsed magnetic field with the intensity of 1.5 mT and the frequency of 3 300 Hz, the maximum voluntary contraction force value of muscle strength attenuates to the initial value within 6 weeks, consistent with the attenuation period of resistance training and attenuation speed after enhancement of muscle strength. During the attenuation process, there is less fluctuation in the two resistance training groups, without fatigue accumulation, in which muscle strength can be better maintained. Improved strength endurance can be maintained for at least 6 weeks. Compared with the resistance training, low-frequency pulsed magnetic field stimulation can avoid fatigue accumulation, and the anti-fatigue ability continues to increase and maintain at a better level. Resistance training combined with pulsed magnetic field has a certain benefit for endurance level. To reduce the fluctuation during the attenuation process is beneficial to the maintenance of the endurance level.

Key words: pulsed magnetic field, classical transient receptor potential channel 1, TRPC1, strength quality, muscle strength attenuation, short-term effect, endurance, biceps brachii, contraction, muscle strength