Healthcare 2023, 11(6), 922; https://doi.org/10.3390/healthcare11060922
Received: 6 February 2023 / Revised: 13 March 2023 / Accepted: 21 March 2023 / Published: 22 March 2023
(This article belongs to the Collection Sport and Exercise Medicine)
Purpose: PEMF stimulation results in a higher O2 muscle supply during exercise through increased O2 release and uptake. Given the importance of oxygen uptake in sport activity, especially in aerobic disciplines such as cycling, we sought to investigate the influence of PEMF on muscle activity when subjects cycled at an intensity between low and severe. Methods: Twenty semi-professional cyclists performed a constant-load exercise with randomized active (ON) or inactive (OFF) PEMF stimulation. Each subject started the recording session with 1 min of cycling without load (warm-up), followed by an instantaneous increase in power, as the individualized workload (constant-load physical effort). PEMF loops were applied on the vastus medialis and biceps femoris of the right leg. We recorded the electromyographic activity from each muscle and measured blood lactate prior the exercise and during the constant-load physical effort. Results: PEMF stimulation caused a significant increase in muscle activity in the warm-up condition when subjects cycled without load (p < 0.001). The blood lactate concentration was higher during PEMF stimulation (p < 0.001), a possible consequence of PEMF’s influence on glycolytic metabolism. Conclusion: PEMF stimulation augmented the activity and the metabolism of muscular fibers during the execution of physical exercise. PEMF stimulation could be used to raise the amplitude of muscular responses to physical activity, especially during low-intensity exercise.
The main result of this study is that PEMF stimulation increases the activity of muscle fibers during warm-up but not during high-intensity constant load.
4.1. Effect of PEMF Stimulation on Warm-Up (Low Intensity)
During warm-up, when athletes cycled at a very light aerobic intensity, PEMF stimulation enhanced the activity of both the vastus medialis and biceps femoris (Figure 3A). One possible explanation for this effect arises from the change in the membrane permeability and Ca2+ channel conduction enhancing the ion flux and cellular concentration [36,37]. The increase in the amplitude of the muscular response was probably caused by the effect of stimulation on type-I and type-II muscular fibers. Likely, PEMF stimulation increased the activity of type-II fibers, normally poorly activated during light physical effort, suggesting a possible application of PEMFs during the preparatory phase before competition, in order to raise the magnitude of muscular response.
During the constant-load phase of effort, PEMF stimulation did not affect the amplitude of muscle activity (RMS). The analysis showed significantly increased activity for the vastus medialis with respect to the biceps femoris (Figure 3B). This result is not surprising given that the effective role of the vastus medialis during cycling is well-known, but the role of the biceps femoris is still under discussion: the magnitude of the biceps femoris is more affected by fatigue, pedaling rate, coordination/activation timing (angle), training status, shoe–pedal interface and body position. The biceps femoris is a bi-articular muscle involved in knee flexion and hip extension. According to Hug and Dorel, the biceps femoris seems to be more important for energy transfer between joints during cycling rather than to supply the main force . One of the largest activities and an earliest activation of biceps femoris seem to be related to increased fatigue in both the vastus lateralis and medialis as a consequence of modified coordination and activation patterns . In the present study protocol, the workload was instantaneous and strongly near to maximal, causing an immediate and large response of the main muscles of cycling, such as the vastus medialis, causing a rapid increment of its muscular activity. Thus, the biceps femoris increased its activity later, upon the arrival of fatigue in the vastus medialis.
PEMF stimulation has an effect on muscle activity during low-intensity exercises but does not seem to affect muscle during heavy-load exercises. Possibly, the higher muscle activation covered the effect of stimulation. This is reasonable because our subjects performed a strenuous exercise that required a very high muscle activity. Our dosage of stimulation may not have been sufficient to increase even over the amplitude of muscle activity during exercise. Despite this, the higher blood lactate concentration recorded on exercise during stimulation indicates an effect of PEMF on muscle activity, especially on the contraction mechanism and glycolytic metabolism of type-II muscular fibers strongly involved during exercise.
4.2. Effect of PEMF Stimulation on Lactate Concentration
The results showed that the PEMF stimulation caused an increase in the blood lactate, suggesting a potential mechanism of microstimulation in enhancing the activity of type-II muscular fibers, typically recruited when the intensity of exercise exceeds the ventilatory threshold. Moreover, lactate production is essential to delay muscle fatigue during heavy physical exercise. According to Robergs et al. , lactate production delayed metabolic acidosis and muscle fatigue, preventing the impairment of exercise performance. Lactate prevents pyruvate accumulation and supplies muscles’ production of NAD+, based on ATP regeneration from glycolysis . The high-intensity exercise used in this study, with high increases in power during physical effort, led to a faster reduction in the intramuscular pH, suggesting that PEMF stimulation promoted type-II fiber metabolism and lactate production to delay metabolic acidosis. These results suggest a possible application of stimulation during exercise to enhance the amplitude of muscular fibers in response to physical activity. The results of the present study clearly show an effect of PEMF stimulation on low exercise intensity and on the amplitude of muscular responses (Figure 3A). The higher magnitude of muscular activity seems to suggest that PEMF stimulation could enhance muscular activation during preparatory activity.
In addition to contraction mechanisms, it is possible to hypothesize that PEMF stimulation affected the energetic system inside the muscular fiber, especially glucose utilization. In rats with streptozotocin-induced diabetic muscle atrophy , chronic PEMF treatment affected metabolic enzymes in the quadriceps, with increased succinate dehydrogenase (SDH) and malate dehydrogenase activity (MDH), thus suggesting an increase in the metabolic capacity of muscle. Further, it has been found that PEMF treatment reduced blood glucose and increased serum insulin levels. In insulinoma cells, exposure to PEMF attenuated insulin secretion, suggesting effects on the calcium channels and ion flux . In the present study, the high values of lactate recorded during PEMF stimulation were probably due to the increased overall activity of type-II fibers and boosting of their glycolytic metabolism.
4.3. Practical Applications
The results of this study show that PEMFs can have an effect on muscular activity, suggesting potential applications in sport disciplines. Based on the present results, PEMF stimulation could be used during light physical effort in order to enhance the amplitude of muscular responses to exercise.
PEMFs might be used at a high intensity of physical effort or during hard work-out sessions in order to boost the glycolytic metabolism of type-II fibers in response to heavy workloads and increase the benefits of an exercise program such as peripheral heart action training .
PEMF stimulation could also be applied during warm-up to raise the amplitude of muscular responses during the preparatory activity of different performances such as jumps, shots or sprints. PEMF stimulation could also be applied during light exercise or low aerobic intensity in order to increase the overall muscular response. Finally, due to the effect of PEMFs on succinate and malate dehydrogenase in rat quadriceps, it could be a possible effect of microstimulation on the aerobic activity over short and long distances.
4.4. Limitation of the Lactate Measure
The main critical issue of this study regards the lactate measurement. In the methodological preparation of this study, we chose to take the sample before the beginning of the exercise session (baseline) and at the third minute of the constant-load exercise. We chose this moment because it represents the common time of the end of VO2 kinetics phase II and the start of the slow component [43,44] in order to obtain a more standardized value compared to that of the end of the exercise, given that the time of exercise differed for each athlete. The results showed a strong PEMF effect on the lactate concentration. The authors are aware that taking more lactate samples during the entire phase of both exercise and recovery, building the entire lactate curve, would better clarify the influence of PEMF on the glycolytic metabolism of type-II muscular fibers during exercise. Future experiments should be aimed to measure lactate every minute during the entire phase of exercise and recovery to better clarify the influence of PEMF on the energetic system. This would allow us to uncover the effects of PEMF stimulation on the glycolytic metabolism of type-II muscular fibers during exercise.
To the best of our knowledge, this is the first study to investigate PEMF stimulation during exercise performed between low and severe intensity. This study shows an influence of PEMF stimulation on muscle activity, as well as on the energetic system during exercise. Despite this, more studies are necessary to confirm the influence of pulsed electromagnetic fields in human subjects during physical activity. We believe that these first observations could open new horizons in the field of sport performance. Further studies are necessary to elucidate the stimulation parameters necessary to elicit the most useful physiological response.
Full article: https://www.mdpi.com/2227-9032/11/6/922