Rate of force development (RFD)
In many sports such as basketball, boxing, weight lifting, etc., numerous technical movements require athletes to complete rapid muscle contraction in a short period of time. Explosive force, determined by strength as well as speed, is the ability of the neuromuscular system to exert maximum muscle strength with maximum acceleration in the shortest time. The rate of force development (RFD) is an important index of explosive force, which is defined as the slope of the force-time curve of muscle under the condition of dynamic and static contraction . It is particularly important to seek effective strategies for athletes to increase their RFD. Athletes with a high RFD have faster muscle contraction speed, and are able to complete motor tasks more quickly, and therefore gain a greater competitive advantage.
At present, there are few studies on the enhancement of RFD through tDCS. We found that the RFD of the subjects’ left leg extension and flexion increased significantly immediately after 2 mA direct current stimulation over bilateral M1 area; moreover, the RFD of left leg extension immediately after real stimulation was significantly larger compared with sham condition. Previous studies have shown that the increase of M1 cortical excitability induced by tDCS can alter the firing frequency of neurons, increase the nerve impulse to muscles, and promote the recruitment of motor units . The nerve impulse is an electrical signal sent from the central nervous system to the muscle, which is conducive to driving the recruitment of motor units and triggering the muscle to generate force . Some studies have pointed out that RFD is closely related to the recruitment of nerve to motor units per unit time, the frequency of nerve impulses, and the type of muscle contraction . Hence, the enhancement of RFD in this study may be relevant to the increased cortical excitability induced by tDCS. The results of the present study are also supported by other published studies. Halo Neuroscience found that 15 min of 2 mA bihemispheric tDCS administered via the Halo Sport device aggrandized vastly the RFD of non-dominant hand in healthy right-handed subjects during an isometric pinch force task . Likewise, Cates et al. found the healthy right-handed subjects receiving 2 mA of anodal tDCS for 15 min exhibited prominently enhancement in peak rate of force development (pRFD) of non-dominant ballistic thumb during and after stimulation compared to the sham condition . These results indicate that tDCS may contribute to the increases in RFD of the non-dominant limb.
In addition to the immediate effect of tDCS, the after-effect of tDCS on cortical excitability was also observed in the present study; namely, the RFD of left leg extension 30 min after tDCS intervention was significantly greater than that of the sham condition. Tanaka et al.  observed that following 10 min of anodal tDCS at 2 mA over the contralateral leg motor cortex, the maximal pinch force of the left leg in healthy adult subjects was transiently enhanced, and the augment effect lasted for 30 min after the end of tDCS sessions. The results of the present study are consistent with these studies, which verify that tDCS intervention has after-effects. However, our study found that the RFD improved 30 min after real stimulation, but not as significantly as in the immediate period after stimulation. A previous study found the after-effects of tDCS increasingly decreased with the prolongation of time after stimulation . The duration of the post-stimulation effects was left to the duration, times as well as intensity of the stimulation . It is reported that the stimulation effect is able to last for 30-60 minutes following a single tDCS session for 10-20 min . Repetitive intervention more than 1 week is capable of having effect for 1-2 weeks , and the after-effects of prolonged tDCS stimulation can even be detected few months later . Regarding current intensities, a recent study reported that the intensity of 2 mA may not be sufficient to affect neuronal circuits . Vöröslkos et al.  proposed by testing transcranial alternating current stimulation (tACS) that because a large part of the current is lost as a result of soft tissues, skin as well as resistance of the skull, at least 4.5 mA would be required to affect the neuronal circuits. Nevertheless, there is a relative paucity of data on the effectiveness and safety of higher current intensity on exercise capacity in healthy populations at present [28, 29], therefore, the practice of applying high-intensity electrical stimulation in an attempt to obtain the gain effects needs to be carefully considered. Furthermore, as a result of the high variability among individuals, the most effective measure may be to apply individualized current intensity to the subjects .
We found that tDCS has the capacity to increase the recruitment of motor units by modulating the excitability of cerebral cortex as well as nerve impulses, thus increasing the rate of force development, and its after-effects can last for 30 minutes. These findings demonstrate that tDCS delivered via the Halo Sport may be a safe and effective method to facilitate explosive strength for healthy populations in daily life or in exercise training.
Maximal voluntary contraction
It is generally believed that motor unit recruitment strategy is crucial in the process of maximum force generation . Previous studies have indicated that motor unit recruitment and synchronicity can be modulated by anodal tDCS . As a consequence, it can be inferred that this neuromodulatory technique may contribute to the improvement of MVC force. We found that the MVC of the left flexor and extensor groups immediately after and 30 min after real stimulation increased significantly compared with before stimulation, furthermore, the MVC of the left leg flexion 30 min after tDCS intervention was prominently greater than the sham condition. The results of the present study are consistent with the results reported in previous studies. For instance, Washabaugh et al. found that 2 mA,12 min of anodal tDCS vastly aggrandized the knee extensor torque and MVC ability in healthy subjects than sham stimulation, and the stimulation effect was also remarkable 25 min after stimulation . A plausible explanation for the improvement in muscular strength is that tDCS-induced changes in corticospinal excitability increase the recruitment of motor units, thus leading to greater muscle strength during contraction .
Interestingly, the pronounced effect of the lower limb muscle strength enhancement after tDCS administration was mainly manifested in the non-dominant leg. Muscle strength of the dominant leg did show an upward trend after stimulation, but with no statistical difference compared to the sham. Asymmetric use of the dominant and non-dominant legs might elicit asymmetry of cortical excitability between the dominant and non-dominant hemisphere, namely, the excitability of the non-dominant motor cortex is lower than that of the dominant motor cortex . Boggio et al.  investigated the effect of anodal tDCS of the dominant and non-dominant M1 on the hand motor function in healthy right-handed subjects. Their results showed that non-dominant hand (left hand) motor function was significantly improved by anodal stimulation (1 mA, 20 min), whereas neither anodal nor sham tDCS gave rise to a prominent change in the dominant hand motor performance. The possible reason for the lack of effects on the dominant hemisphere is that there may be a ceiling effect on the stimulation effect of tDCS on the dominant side. Since the cerebral dominant hemisphere is already optimally activated, an additional increase in excitability by anodal tDCS would not provide further behavioral benefits to these subjects . Nonetheless, in a study by Vargas, 20 adolescent female soccer players underwent five MVC tests of bilateral knee extensors after 2 mA anodal tDCS and found significant improvement in MVC force in the dominant limb . These inconsistent findings suggest that further studies are needed to confirm whether there is a ceiling effect of tDCS on the enhancement of athletic abilities. Although the effects of tDCS on the dominant limb muscle strength are controversial, it is clear from the results of this study that tDCS has the potential to augment the muscle strength of non-dominant limb. In future experimental research and actual sports training, tDCS technology can be employed to increase the non-dominant limb muscle strength of athletes, narrow the gap with the dominant limb, avoid the imbalance phenomenon, and further boost the overall exercise capacity, which would have an exceedingly practical significance for muscle strength training and the prevention of sports injury.
Muscle Activation assessment
The main mechanism of enhancing muscle strength by transcranial direct current stimulation is to regulate the nerve factors related to muscle strength. EMG is capable of reflecting the influence of nerve drive and other factors during muscle contraction. Fast-twitch (type II) muscle fibers are commonly innervated by high-threshold neurons. This muscle fiber types are usually closer to the surface, and their contractile changes are able to be well recorded via sEMG signals . The changes of motor unit recruitment, in a sense, can be traced by sEMG . A previous study has argued that the peak torque and EMG amplitude of biceps brachii in healthy subjects during maximum contraction can be significantly increased after 2 mA, 10 min anodal tDCS over the left M1. The author believes that the muscle activation may be related to the changes of motor unit recruitment strategies . The results of the present study showed that the activation level of left biceps femoris and rectus femoris was significantly higher immediately after and 30 min after tDCS administration than before, indicating that tDCS can elevate the activation levels of non-dominant knee and recruit more motor units, so as to enhance muscle strength. A similar phenomenon was observed in a previous study by Frazer et al. , who found the muscle strength and EMG amplitude of the non-dominant (left) biceps brachii in healthy subjects were increased significantly after discharging 2 mA direct current applied to M1 for 20 min. Nevertheless, no significant change was noted in the activation level of the right flexor and extensor muscles after tDCS treatment, which was consistent with the results of muscle strength in this study. Kan et al. found that 2 mA of anodal tDCS for 10 min did not affect the RMS amplitude of biceps brachii sEMG in elbow endurance test of healthy male subjects. The authors summarize that tDCS intervention would not further heighten muscle function when muscle function has reached the maximum . Considering that the subjects selected in this study were all right leg dominant, the muscle activation level has reached the best state in high-intensity exercise, and in this state, the ability of tDCS to increase the number of motor unit recruitment to improve the muscle activation degree is limited, resulting in no significant improvement in muscle strength performance.
There is a certain correlation between sEMG amplitude and muscle strength; generally speaking, muscle strength increases with increasing sEMG amplitude. Several studies point out, however, that the changes of sEMG amplitude and power spectrum are not only related to muscle strength, but also related to fatigue degree . The time-domain EMG signal tends to increase with the enhancement of muscle strength as well as the generation of fatigue, while the frequency-domain EMG signal increases with the improvement of muscle strength, but decreases with the generation of fatigue . Consequently, mean power frequency (MPF) was employed in an attempt to determine whether the increase of RMS value is caused by the augment of muscle strength. Our results showed that the MPF values of left rectus femoris and biceps femoris sEMG were significantly higher immediately after and 30 min after real stimulation compared to before stimulation,meanwhile, the MPF of left biceps femoris sEMG was increased significantly 30 min after real stimulation compared with the sham condition. Also, the corresponding MVC values immediately after and 30 min after tDCS intervention were increased, so the increase of RMS values could exclude the effects of fatigue factors. The changes of EMG amplitude are related to the number of motor unit recruitment, impulse frequency and synchrony. Therefore, tDCS was likely to promote the recruitment of motor units, increase the level of muscle activation, and then enhance muscle strength in this study.