Abstract: BACKGROUND: Strength quality is an essential element for human physical activity. Short-term low-frequency pulsed electrical magnetic field (PEMF) stimulation can activate classical transient receptor potential channel 1 (TRPC1) and trigger skeletal muscle growth and remodeling in mice, thereby producing a series of physiological support effects on muscle tissue. However, there is no report on whether this mechanism will alter the physiological structure and working ability of human skeletal muscle and act as a new way to improve muscle strength.
OBJECTIVE: To select a specific low-frequency PEMF that can activate TRPC1 as an exogenous stimulus to observe and verify the effect of short-term stimulation on the maximum voluntary contraction force and strength endurance of human biceps brachii.
METHODS: A total of 27 normal adult healthy subjects were selected and randomly divided into exercise group, PEMF group, and exercise+PEMF group, with 9 cases in each group. The training group was subjected to resistance training. The exercise+PEMF group received 10-min low-frequency PEMF stimulation (intensity 1.5 mT, frequency 3300 Hz) immediately after resistance training. The irradiation group only received 10-minute low-frequency PEMF stimulation. Training or irradiation was performed every 48 hours for 9 days. Electromyography information on the maximum voluntary contraction force was collected before and after five training sessions in the exercise group and the exercise+PEMF group, to observe whether the combination of low-frequency PEMF and resistance training will produce a gain effect. In the PEMF group, the maximum voluntary contraction force was tested only at the 1st, 3rd, and 5th sessions to track the changes in muscle strength. The maximum voluntary contraction force, one-repetition maximum force, endurance duration and median frequency were observed in each group.
RESULTS AND CONCLUSION: (1) During the trial, there were significant interaction effects between the changes in maximum voluntary contraction force and time in all the subjects (P < 0.01) as well as in each group as time goes by (P < 0.05). However, there was no interaction effect between groups. (2) There was a significant improvement in the maximum voluntary contraction force, one-repetition maximum force, endurance duration, and median frequency in each group after completion of the trial. These indicators were successively improved by 19%, 23%, 28%, and 18% in the exercise group, 11%, 10%, 53%, and 18% in the exercise+PEMF group, and 28%, 18%, 27%, and 6% in the PEMF group. (3) The median frequency of the exercise+PEMF group was significantly higher than that of the PEMF group (P < 0.05), but there was no significant difference between the exercise+PEMF group and exercise group. (4) Compared with the exercise+PEMF group, the exercise group showed a significant decrease in the root mean square of maximum voluntary contraction after the first two exercise sessions (P < 0.05). (5) All these findings indicate that the short-term PEMF stimulation with intensity of 1.5 mT and frequency of 3 300 Hz can significantly improve the maximum voluntary contraction force and strength endurance of human biceps brachii. That is, low-frequency PEMF can induces TRPC1 to promote the working ability of muscle tissue, which has been effectively verified in human body. Low-frequency PEMF stimulation shares similar effects with conventional resistance training in terms of enhancing the maximum strength. Resistance training combined with PEMF stimulation shows better anti-fatigue ability and stabilizes the increase of muscle strength. This combination can stimulate more motor units of local muscle groups under the same load in the early stage of training and improve the overall training efficiency. In terms of strength endurance improvement, low-frequency PEMF stimulation can prolong isometric muscle contraction time and improve anti-fatigue ability. Compared with simple low-frequency PEMF stimulation, low-frequency PEMF stimulation combined with resistance training is more effective to develop strength endurance.  

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