Introduction

Methods to obtain an immediate and long-term increase in muscle strength and function are goals of rehabilitation practitioners, sports therapists, strength and condition coaches and athletes. Long-term improvement in muscle strength results from ongoing sports-specific strength and conditioning training. There is a growing body of evidence to support the use of acupuncture for rehabilitation of muscle function following stroke and there is strong evidence to support the use of acupuncture to enhance muscle strength in healthy individuals [1]. Acupuncture has been shown to affect both acute and chronic improvement in muscle strength. However, the underlying physiological mechanisms by which acupuncture enhances muscle strength are still unclear and, to our knowledge, have never been fully investigated. The purpose of the current review is to critically appraise randomized control trials, systematic reviews, and meta-analyses to determine the underlying physiological mechanisms in which acupuncture enhances muscle strength in both animals and humans. This review will lay the foundation for clarifying the likely mechanisms of acupuncture in enhancing acute and chronic muscle strength, thus allowing clinicians and researchers involved in sports and/or exercise rehabilitation to justify their selection of acupuncture points. This review is reported according to the SANRA (Scale for the Assessment of Narrative Review Articles) guidelines [2].

Discussion

The neurological effect of acupuncture on muscle function

There are a several mechanisms by which acupuncture has been suggested to influence muscle strength (see Figure 1). One mechanism is through the activation of regions of the brain following the stimulation of specific classical acupuncture points. The main system in the body that is affected by acupuncture is the nervous system. It not only transmits signals along central, spinal, and peripheral nerves but also emits a variety of biochemicals that influence other cells of the body. The nervous system has over 30 peptides involved in transmitting signals and is connected to the hormonal system via the hypothalamic-pituitary axis. Therefore, it can make connections to every cell and system of the body [3]. In 1973 Chiang and colleagues [4] found that the effect of acupuncture is mediated by the nervous system. This can be explained by the fact that the classical acupoints are situated near underlying nerves. Liu, et al. [5] found a high correlation between acupuncture points and muscle motor points. Dung [6] found that there were, in close vicinity to acupuncture points, large peripheral nerves, motor points where the nerve enters the muscle, blood vessels near neuromuscular attachments, cutaneous nerves emerging from the deep fascia, bifurcation points of peripheral nerves, ligaments, tendons, joint capsules, and fascial sheets, which are rich in nerve endings. The electrical conductivity of acupoints has been known for several decades thanks to the work of Nakatani [7] and Dr Robert Becker [8] in the 1950s, and Saita [9] and Tiller [10] in the 1970s. The electrical properties of acupuncture points include increased conductance, reduced impedance and resistance, increased capacitance, and elevated electrical potential compared with adjacent non- acupuncture points [11].

The effect of deqi sensation on the motor cortex

In traditional Chinese medicine (TCM), deqi is one of the most important principles and the key to successful acupuncture treatment since it is associated with clinical efficacy [12-14]. The sensations of deqi are transmitted by nerves local to the needling site to the somatosensory cortex where integration and interpretation occur at the spinal cord, brainstem, thalamic and cortical level [15-18]. Nerves local to the needling site consist of low-threshold nociceptors that are connected to fast-conducting A-delta fibers, which generally supply the skin and respond to strong mechanical stimuli [19]. High-threshold nociceptors that conduct impulses through slower unmyelinated C-fibers carry impulses to deep body tissues such as muscles. The sensation of deqi has been reported to be detected in acupuncture and non-acupuncture points; however, a stronger sensation of deqi is reported at classical acupuncture points in comparison with non-acupuncture points. Kong, et al. [20] found that the sensation of deqi from sham acupuncture was significantly lower than that of Manual Acupuncture (MA) or electroacupuncture (EA). Li, et al. [21] investigated the central mechanism of deqi following acupuncture in the treatment of ischemic stroke and found that, compared with the non-deqi group, the deqi group produced marked activation of the right anterior lobe of the cerebellum and right limbic lobe. These findings are supported by several studies that investigated the efficacy and mechanism of different depths of acupuncture for mild cognitive impairment and found that deep acupuncture is necessary to achieve significant clinical results [22-24]. With regards to the activation of the primary motor cortex, it is suggested that a strong deqi sensation is required. Sun, et al. [25] Wang, et al. [26] and Yang, et al. [27] established that acupuncture at ST-36 acupoint produces a strong deqi sensation that activated different nerve fibers which affect the excitability of the motor cortex and was correlated to Motor-Evoked Potential (MEP). Although there is convincing evidence to support the requirement of a strong deqi sensation at classical acupoints to affect the motor cortex and produced a strong MEP, amplitude studies investigating osteoarthritis have reached conflicting conclusions. Furthermore, Maioli, et al. [28] showed that both classical acupoints and non-acupoints had a similar effect in modulating MEP responses in the motor cortex. Some studies have questioned the need for deep stimulation and deqi in achieving acupuncture analgesia [14,29-33]. Nonetheless, the main findings from the above studies suggest that in order to activate the motor cortex and produce a stronger motor- evoked potential, it would be more beneficial to obtain a strong deqi sensation by applying deep needling (10-20 mm) to classical acupoints.

Local response of acupuncture in the production of acute muscle power

The acute effect of acupuncture in the enhancement of explosive muscle force may be brought about by stimulating local nerves in the agonist muscle to elicit the deqi sensation. Using acupuncture to stimulate local nerves in the agonist muscle is reported to induce a Post-Activation Potentiation (PAP) phenomenon [34-36], which is a method that is used for strength and conditioning of athletes to improve strength and performance. Data collected during this review indicates that the acute effect of acupuncture on explosive muscle force may be more associated with Post-Activation Performance Enhancement (PAPE) rather than PAP.

The most accepted physiological mechanism that plays a key role in the PAP phenomenon is an increase in the phosphorylation of myosin regulatory light chains [37-39]. This makes actin and myosin more sensitive to calcium released from the sarcoplasmic reticulum during subsequent muscle contractions [40-43]. As a result, there is an increase in the cross-bridge cycling between actin and myosin filaments [44], thereby increasing the force of each subsequent twitch contraction. PAP has been shown to have a short window of action (effect decreases drastically to one-half after 28s) following its induction and is only beneficial in the first 8-10 minutes after being activated [45]. Post-Activation Performance Enhancement (PAPE) is an acute increase in explosive neuromuscular force during high-speed dynamic muscle contractions. It comes on 3-10 minutes following its activation [46,47] and has a longer window of action (> 15 minutes) [48-51] than PAP. The physiological mechanism is reported to be through an increase in neural drive and high-threshold motor unit activation and an increase in muscle temperature and muscle water content. The effect of PAP and PAPE is greater in type II fast-twitch fibers [52] than in type I slow-twitch fibers [53] as type II fibers have a lower basal Ca²⁺ sensitivity [54,55]. In 2021 Wang and colleagues [26] undertook two studies, one to investigate the immediate effect of acupuncture on the explosive force of the knee joint and the other to investigate the effect of acupuncture on the timeline of male shoulder joint endurance. Both studies demonstrated that selecting and stimulating acupoints around the shoulder joint (LI-14, SJ-14, SJ-13, LU-1, LU-4, LU-3 and SJ-12) or the knee joint (ST-32, ST-34, ST-36, SP-10 and UB-57) activate the nerves and muscles around these joints. Their results showed a significant increase in the maximum torque, average power, average work, and total work of the shoulder and knee joints. Both studies found that acupuncture improved muscle strength for 23-33 minutes following its application. Like PAPE the timeline to the onset of the effect of acupuncture took approximately 10 minutes to appear [56]. Su, et al. [35] compared the acute effect of acupuncture with sham acupuncture on forearm muscle strength. They found that immediately following 20 minutes of acupuncture to LI-4, LI-10, LI-11, SI-8, SI-10, and SJ-5 there was a significant enhancement in forearm muscle force, which lasted approximately 7-20 minutes until the acupuncture efficacy decreased or disappeared. There was no significant enhancement in forearm muscle force in the sham group. De Souza, et al. [57] evaluated the effect of acupuncture on the upper trapezius muscle. They found that following acupuncture there was a significant increase in the upper trapezius muscle strength at 20 minutes following acupuncture and the effects lasted for 5 minutes. The timeline for acupuncture to be effective and the timeline for the efficacy to decrease or disappear coincide with the timeline of the PAPE studies. Therefore, it may be deduced that acupuncture has a similar effect to PAPE on enhancing explosive muscle strength rather than PAP, which has a shorter timeline.

Effect of acupuncture on the primary somatosensory cortex in the enhancement of muscle strength

The somatosensory cortex lies behind the primary motor cortex in the frontal lobe. It functions to detect sensory stimuli such as touch, pain, pressure, and movements such as joint displacement [58], which arises from muscles, bones, joints, skin, cardiovascular system, internal organs, and fascia. Sensory information from muscle spindles, musculotendinous receptors concerning changes in muscle length are transmitted to the medial nuclei of the thalamus which decode and process the information before passing this on to the somatosensory cortex [59]. Early motor mapping studies in primates [60] and humans [61] have described movements that are brought about by stimulation of the primary somatosensory cortex in addition to the motor cortex. In support of these findings, there is growing evidence from animal studies showing that rich fiber pathways interconnect the primary somatosensory cortex with the primary motor cortex [62-64]. A study using Transcranial Magnetic Stimulation (TMS) found that somatosensory electrical stimulation strongly increased the excitability of the primary motor cortex [65]. Boricha and colleagues [66] undertook a review to investigate the role of the primary somatosensory cortex in motor control, motor learning and functional recovery. Their findings indicated that without correct processing and translation of sensory input, both before and during movement, motor outputs were abnormal and/or inaccurate. Research on humans has suggested that there is an increased peripheral somatosensory inflow that facilitates functional reorganization of the primary motor cortex [67] and that non-invasive stimulation of the somatosensory cortex induces shorter latencies to initiate movements [68]. These findings support a continuous mutual communication between sensory inflow and motor outflow [69,70]. Non-invasive stimulation of the somatosensory cortex can be brought about through tactile-pressure stimulation as it shares some similarities with non-painful electrical stimuli [25] or acupuncture. Insertion of a needle into the skin or muscle will stimulate free nerve endings, encapsulated cutaneous receptors (Merkel, Meissner receptors, Ruffini, and Pacinian corpuscles), sarcous sensory receptors (muscle spindles and tendon organs), and their afferent fibers [71]. Applying manual manipulation or electrical stimulation to the needle will produce a stronger activation of various neural and neuroactive components thereby producing a strong deqi sensation. This will produce an action potential in C-fibers and A-delta fibers located in the skin and muscle which will travel to the central nervous system, where integration and interpretation occur at the thalamus and somatosensory cortex. This can provide feedback that is required for motor control during exercise rehabilitation or in cases of enhancing sports performance. In healthy humans, it has been found that electrical stimulation over the agonist muscle during exercise results in improved training effects compared with either electrical stimulation or exercise only [72]. Wei, et al. [73] used fMRI to examine the activation and connectivity of the brain of 22 right-handed healthy male volunteers under three different conditions: (1) Right ankle dorsiflexion as the control; (2) Ankle dorsiflexion coupled with tactile-pressure stimulation to the agonist (tibialis anterior) muscle; and (3) To a control area proximal to the right medial malleolus away from the agonist (tibialis anterior) muscle. Tactile stimulation was administered along the tibialis anterior muscle by brushing the skin of subjects in a back-and- forth manner with a probe at a frequency of 1 Hz and a shift distance of 3 cm. They found that increased somatosensory inputs during movement activated links between the primary sensory centers (thalamus and cerebellum) and major motor centers.

In clinical applications, sensory inflow through repetitive stimulation of the peripheral nerves combined with voluntary muscle contractions has been found to enhance motor function in the rehabilitation of post-stroke patients [74-78]. This method of rehabilitation has been shown to induce a more significant muscular adaptation than muscle contraction alone [79,80]. In sports, it has been demonstrated that stimulating the muscles or nerves through an intense exercise- based warm-up will produce a voluntary force or power enhancement for a short time as seen in the PAPE phenomenon. Acupuncture stimulation of the peripheral nerves of the agonist (prime mover) has been shown to improve motor performance by enhancing somatosensory input prior to muscle activity. Maioli and colleagues [28] investigated the effects of acupuncture upon upper-limb Motor-Evoked Potentials (MEPs), elicited by transcranial magnetic stimulation of the primary motor cortex in humans. They reported that the simple insertion of the needle is an adequate somatosensory stimulus to induce a significant modulation of MEP amplitude, the sign of which is specific to the investigated muscle and to the point of needle insertion. It has been demonstrated that when applying electroacupuncture or electrical stimulation to induce a motor response, it is vital that parameters creating a motor response that mimics a voluntary muscle contraction are used [81,82]. De Brito, et al. [81] demonstrate that peripheral electroacupuncture or electrical stimulation that is sufficient to induce a motor contraction (30 Hz and 100 Hz) increased corticomotor responsiveness (P = 0.002), whereas stimulation at 10 Hz did not.

Effect of acupuncture on the motor cortex in the enhancement of muscle strength

It has been shown that the effect of acupuncture on muscle function is brought about through enhancing functional connectivity between the bilateral primary motor cortex, the cerebellum, and the primary somatosensory cortex [83-86]. The motor cortex forms part of the cerebral cortex and is made up of the primary motor cortex, which is responsible for initiating motor movement, and the premotor and supplementary cortex, which are involved with aspects of planning, initiating, and selecting the correct movement.

Assessment of the effect of acupuncture on the motor cortex has primarily involved animal studies or post-stroke patient studies (see Table 1). Functional Magnetic Resonance Imaging (fMRI) and transcranial magnetic stimulation (TMS) have been used to assess the effect of acupuncture on brain activity. While fMRI show real-time hemodynamic changes in brain activity during and following acupuncture, TMS measures changes in the excitability and inhibition of the human motor cortex during and following acupuncture. Several studies [87-91] have shown that there is a significantly greater activation change in the motor-related area, improved blood flow to ischemic areas, and enhanced stroke recovery following true acupuncture compared with sham acupuncture. Yang, et al. [27] used TMS to investigate the neuroplasticity changes in the human motor cortex induced by acupuncture. The acupuncture treatment consisted of 10 acupoints (LI-4, LI-10, LI-11, SJ-5, ST-36, SP-6, GB-34, and the last three Baxie points). They found that acupuncture induced a significant modulation of Motor-Evoked Potential (MEP) amplitudes of the motor pathways that depart from the contralateral primary motor cortex. Changes in MEP amplitude also occur in the ipsilateral primary motor cortex following acupuncture. Sun and colleagues [25] used TMS to explore the effect of ST-36 and sham points on motor cortical excitation and inhibition on 20 healthy volunteers. They found that acupuncture at ST-36 significantly increased motor cortical excitation and reduced motor cortical inhibition with no significant difference from baseline in the sham acupuncture group. Furthermore, deqi sensation was correlated to Motor-Evoked Potential (MEP) amplitude. Like Yang, et al. [27] Sun, et al. [25] also found that when acupuncture was applied to the left-sided ST-36 acupoint, the effects of acupuncture influenced the excitability of the ipsilateral and contralateral hemisphere. In 2019 He, et al. [92] undertook a study to determine whether the changes in the cortical excitability before, during and after acupuncture are time dependent. They used acupoints LI-11 and SJ-5 in the left arm. Like earlier studies [25,27], they found that changes in the MEP amplitudes occurred on both the contralateral and ipsilateral sides. The finding from the TMS studies agrees with fMRI studies that show acupuncture exerts an effect on the supplementary motor area [28,93,94].

The Underlying Metabolic Effect of Acupuncture in the Enhancement of Muscle Performance

Effect of acupuncture on creatine, carnitine, and glutathione levels

Strength training forms an integral part of an athlete's preparation for competition, return to sport following injury, or the prevention of injuries [95]. Muscles in athletes who partake in regular intensive exercise programs undergo major adaptive changes, which include increased muscle mass, mitochondrial contents, and respiratory capacity of the muscle fibers. The main metabolic adaptation that occurs within the muscles following intense exercise is a slower utilization of muscle glycogen and blood glucose, a greater reliance on fat oxidation, and less lactate production during exercise of a given intensity [96]. Several studies [97-100] have demonstrated that during intensive exercise, athletes who have received acupuncture are able to work harder and longer at a near maximal intensity and will produce a higher level of lactate. Furthermore, our previous research [101] found strong evidence showing that acupuncture is significantly more effective than no treatment or sham acupuncture in enhancing lactate removal following intense exercise thus demonstrating that following acupuncture, athletes will recover much quicker from symptoms of delayed onset muscle soreness. These findings can be explained by the effect of acupuncture on the release of carnitine, glutathione, and creatine into circulation and mitochondrial cells in the muscles. During energy metabolism, carnitine plays a major role in transporting long-chain fatty acids into mitochondrial cells [102]. Like carnitine, creatine helps to energize cells, particularly muscle cells, and helps to build muscle mass. Both creatine and carnitine will help accelerate recovery, improve athletic performance, and increase energy production. Animal and human studies [130-106] have demonstrated a significant increase in carnitine levels following acupuncture compared with control (see Table 2). In 2017 Lin and colleagues [107] investigated the metabolic basis of meridian specificity using proton nuclear magnetic resonance. They used points on the Stomach meridian (ST-2, ST-21, and ST-36) and the Gall Bladder meridian (GB-14, GB-24 and GB-34) and found that EA stimulation produced a significant change in energy metabolism. Stimulation of points along the Stomach meridian produced a marked metabolic increase in glycolysis, while stimulation of points along the Gall Bladder meridian produced a significant change in glutamine and glutamate. Lin, et al. [107] found that there was an increase in lysine and creatine levels following acupuncture. Lysine and methionine are essential amino acids that allow the body to naturally produce carnitine and creatine [108]. Overall, the findings indicated that EA significantly induced a change in energy metabolism with an enhanced aerobic capacity through altered levels of the Krebs cycle intermediates (glucose, lactate, fumarate, citrate, α- KG, and ATP).

Antioxidant effect of acupuncture

A few studies have demonstrated that high-intensity exercise led to an over production of Reactive Oxygen Species (ROS) in muscle tissue, which in excess lead to a decrease in mitochondria respiratory capacity and sarcoplasmic reticulum integrity [109-111]. Oxidative stress is associated with tissue inflammation, fatigue, muscle injury, impaired recovery after high- intensity exercise, and reduced immune function [112-117], all which may impair athlete performance [112]. Mitochondrial cells are particularly vulnerable to oxidative damage during and following intense exercise. With oxidative stress, mitochondria membranes are seriously impaired, and in general, a reduction of mitochondria biogenesis is reported [118]. Investigation to identify micronutrients and natural compounds that can prevent or attenuate exercise- induced oxidative stress and inflammation [119] continues. There is growing evidence that has demonstrated that acupuncture reduces oxidative stress in different conditions [120-125]. Carnitine and glutathione are antioxidants that have a protective role in reducing the overproduction of free Reactive Oxygen Species (ROS) within mitochondrial cells, which in turn reduce damage to muscle cells during intensive exercise. Carnitine preserves the integrity of the mitochondrial membrane by reducing the accumulation of reactive oxygen species, it reduces the production of lactate [126], and allows the mitochondrial cells to utilize energy more efficiently. Shin [105] and Kim, et al. [106] found that when manual acupuncture and electroacupuncture are applied, carnitine levels significantly increased, compared with a sham acupuncture group. In 2007 and 2011, Toda [103,104] demonstrated the effects of EA and MA at ST-36 and ST-41 on carnitine and glutathione levels in the circulation and muscles. Results showed that carnitine levels in muscle tissue following MA were significantly higher than in the EA and control groups, with no significant difference seen between EA and control group. With regards to glutathione levels in muscle tissue, there was no significant difference between EA and MA groups and both groups were significantly higher than the control group [103]. Ma, et al. [99] used a Nuclear Magnetic Resonance- (NMR-) based metabolomics approach to investigate the antifatigue effects of acupuncture on healthy young male athletes who had done exhaustive physical exercises. It was observed that during strenuous exercise there was an increase in reactive oxygen species, enhanced glutathione synthesis along with the production of 2-hydroxybutyrate. 2-Hydroxybutyric acid, also known as alpha-hydroxybutyrate, is a marker that relates to oxidative stress. 2-hydroxybutyrate catabolizes L-threonine or synthesizes glutathione. Ma, et al. [99] found that after the treatment with acupuncture, the levels of 2-hydroxybutyrate, 3-hydroxyisovalerate, lactate, pyruvate, citrate, dimethylglycine, choline, glycine, hippurate, and hypoxanthine were recovered at an accelerated speed in the acupuncture group in comparison with the control group.

Effect of acupuncture on Ca²⁺(-) ATPase and Ca²⁺ in mitochondrial cells

Normal levels of Ca²⁺ are important for vital processes within cells, for example, contraction of muscles, cell proliferation, gene expression, secretion of hormones and neurotransmitters, exocytosis, and chemotaxis. A significant accumulation of Ca²⁺ in mitochondrial cells can be very toxic leading to mitochondrial destruction, aerobic energy metabolism dysfunction, and inhibition of ATP synthesis [127-131]. In 2005 and 2008 Gao [100,102] and colleagues observed the effects of MA and EA on antioxidant enzyme and Ca²⁺(-) ATPase activities and Ca²⁺ content in mitochondria and the sarcoplasmic reticulum of skeletal muscle cells in rats following acute swimming exercise. They found that the Ca²⁺-ATPase activity value was significantly higher in the EA and MA group compared with the swimming group with the highest value in the MA group. Furthermore, Ca²⁺ was significantly higher in the EA and swimming groups than in the control and MA groups indicating that MA was more effective at reducing the accumulation of Ca²⁺ resulting from intense swimming. These findings can be explained by the fact that they found superoxide dismutase, glutathione, and glutathione peroxidase activity to be significantly (P < 0.05 P < 0.01) higher in the MA group than in the other groups.

The purpose of this review was to determine the underlying physiological mechanisms in which acupuncture enhances muscle strength. The overall finding of this review indicates that acupuncture has an acute effect on enhancing muscle strength similar to the PAPE phenomena. Acupuncture has a chronic effect on improving muscle strength by reducing the build-up of oxidants, enhancing glycolysis within mitochondrial cells, and improving connectivity within the central nervous system. Furthermore, MA is more effective than EA at increasing carnitine, creatine, and glutathione levels, reducing the accumulation of Ca²⁺ and enhancing the respiratory capacity of mitochondrial cells in muscles. ST-36, GB-34, LI-4 and LI-10 (see Figure 2) were found to be the most common points used to enhance the connectivity of the primary somatosensory cortex and the primary motor cortex to induce a strong motor-evoked potential. ST-36 was the primary point used to slow the utilization of glycogen, enhance pyruvate oxidization, and reduce the risk of damage to mitochondrial cells during intense exercise.

Conclusion

Acupuncture enhances muscle strength in several ways, which include improving the efficiency in which mitochondrial cells function, reducing inflammation, and releasing antioxidants thereby reducing the build-up of reactive oxygen species and damage to cells. Furthermore, acupuncture produces a strong motor-evoked potential. Most studies in this review were undertaken on post-stroke patients, therefore further studies with a large sample size are required on healthy athletes to help determine the underlying physiological effect of acupuncture on improving muscle strength in sports.

Limitations

The limitations were that studies that were not in English were not included in this review and the methodological quality of each of the RCTs and systematic reviews was not assessed due to the nature of this narrative review.

Conflict of Interest

The author declares that they have no conflict of interest.

References

1. Bailey SD (2022) Effects of acupuncture on enhancing muscle strength: A systematic review. Int J Sports Exerc Med 8: 221.
2. Baethge C, Goldbeck-Wood S, Mertens S (2019) SANRA – A scale for the quality assessment of narrative review articles. Res Integr Peer Rev 4: 5.
3. Bailey SD (2018) Dry needling and traditional Chinese acupuncture. Manual of acupuncture points, trigger points and auricular points for the management of musculoskeletal and associated disorders. Steve Bailey Acupuncture.
4. Chiang C, Chang C, Chu H, Yang L (1973) Peripheral afferent pathway for acupuncture analgesia. Scientia Sinica.
5. Liu KY, Varela M, Oswald R (1975) The correspondence between some motor points and acupuncture loci. Am J Chin Med 3: 347-358.
6. Dung H (1984) Anatomical features contributing to the formation of acupuncture points. Am J Acupuncture 12: 139-143.
7. Nakatani Y, Yamashita K (1977) Ryodoraku acupuncture Tokyo: Ryodoraku Research Institute 1-12.
8. Becker R, Reichmanns M, Marino A, Spadaro J (1976) Electrophysiological correlates of acupuncture points and meridians. Psychoenergetic Systems 1: 105-112.
9. Saita H (1973) Modern scientific medical acupuncture. Journal of American Orthopaedic Association 72: 685-696.
10. Tiller W (1973) Some energy field observations of man and nature. 71-112.
11. Ahn A, Martinsen O (2007) Electrical characterization of acupuncture points: Technical issues and challenges. J Altern Complement Med 13: 817-824.
12. Xu SB, Huang B, Zhang CY, Du P, Yuan Q, et al. (2013) Effectiveness of strengthened stimulation during acupuncture for the treatment of Bell palsy: A randomised controlled trial. Canadian Medical Association Journal 185: 473-478.
13. Bai L, Zhang M, Chen S, Ai L, Xu M, et al. (2013) Characterizing acupuncture de qi in mild cognitive impairment: Relations with small-world efficiency of functional brain networks. Evid Based Compl Alt 1-8.
14. Witt C, Brinkhaus B, Jena S, Linde K, Streng A, et al. (2005) Acupuncture in patients with osteoarthritis of the knee: A randomised trial. The Lancet 366: 136-143.
15. Liebeskind JC, Paul LA (1977) Psychological and physiological mechanism of pain. Annual Review of Psychology 28: 41-60.
16. Mountcastle VB (1980) Medical physiology.
17. Bishop B (1980) Pain: Its physiology and rationale for management Part 1. neuroanatomical substrate of pain. Phys Ther 60: 13-20.
18. Watson J (1981) Pain mechanisms - A review: 1. characteristics of the peripheral receptors. Australian Journal of Physiotherapy 27: 135-143.
19. Low J, Reed A (1990) Electrotherapy explained: Principles and Practice.
20. Kong J, Fufa DT, Gerber AJ, Rosman IS, Vangel MG, et al. (2005) Psychophysical outcomes from a randomized pilot study of manual, electro, and sham acupuncture treatment on experimentally induced thermal pain. Journal of Pain 6: 55-64.
21. Li MK, Li YJ, Zhangetal GF, Chen JQ, Zang JP, et al. (2015) Acupuncture for ischemic stroke: Cerebellar activation may be a central mechanism following deqi. Neural Regeneration Research 10: 1997-2003.
22. Feng Y, Bai L, Ren Y, Chen S, Wang H, et al. (2012) FMRI connectivity analysis of acupuncture effects on the whole brain network in mild cognitive impairment patients. Magn Reson Imaging 30: 672-682.
23. Chen J, Huang Y, Lai X, Tang C, Yang J, et al. (2013) Acupuncture at Waiguan (TE5) influences activation/deactivation of functional brain areas in ischemic stroke patients and healthy people: A functional MRI study. Neural Regen Res 8: 226-232.
24. Zhang J, Li Z, Li Z, Li J, Hu Q, et al. (2021) Progress of acupuncture therapy in diseases based on magnetic resonance image studies: A Literature Review. Front Hum Neurosci 15: 694919.
25. Sun ZG, Pi YL, Zhang J, Wang M, Zou J, et al. (2019) Effect of acupuncture at ST-36 on motor cortical excitation and inhibition. Brain and Behavior 9: e01370.
26. Wang J, Wang IL, Hu R, Yao S, Su Y, et al. (2021) Immediate effects of acupuncture on explosive force production and stiffness in male knee joint. Int J Environ Res Public Health 18: 9518.
27. Yang Y, Eisner I, Chen S, Wang S, Zhang F, et al. (2017) Neuroplasticity changes on human motor cortex induced by acupuncture therapy: A preliminary study. Neural Plasticity 1-8.
28. Maioli C, Falciati L, Marangon M, Perini S, Losio A (2006) Short- and long-term modulation of upper limb motor-evoked potentials induced by acupuncture. European Journal of Neuroscience 23: 1931-1938.
29. White P, Prescott P, Lewith G (2010) Does needling sensation (de qi) affect treatment outcome in pain? Analysis of data from a larger single-blind, randomized controlled trial. Acupunct Med 28: 120-125.
30. Berman BM, Lao L, Langenberg P, Lee WL, Gilpin AM, et al. (2004) Effectiveness of acupuncture as adjunctive therapy in osteoarthritis of the knee: A randomized, controlled trial. Annals of Internal Medicine 141: 901-910.
31. Scharf HP, Mansmann U, Streitberger K, Witte S, Kramer J, et al. (2006) Acupuncture and knee osteoarthritis: A three-armed randomized trial. Annals of Internal Medicine 145: 12-20.
32. Witt CM, Jena S, Brinkhaus B, Liecker B, Wegscheider K, et al. (2006) Acupuncture in patients with osteoarthritis of the knee or hip: A randomized controlled trial with an additional nonrandomized arm. Arthritis Rheum 54: 3485-3493.
33. Foster NE, Thomas E, Barlas P, Hill JC, Young J, et al. (2007) Acupuncture as an adjunct to exercise-based physiotherapy for osteoarthritis of the knee: Randomized controlled trial. BMJ 335: 436.
34. Wang I-L, Hu R, Chen YM, Chen CH, Wang J, et al. (2021) Effect of acupuncture on timeliness of male shoulder joint endurance. Int J Environ Res Public Health 18: 5638.
35. Su Y, Yao S, Zhou ZJ, Wu C, Wang IL, et al. (2022) Effect of acupuncture on time-dependent of muscle endurance in female elbow joint: A randomized controlled trial. Evid Based Complement Alternat Med 2022: 8052256.
36. Wang I-L, Chen YM, Hu R, Wang J, Li ZB (2020) Effect of acupuncture on muscle endurance in the female shoulder joint: A pilot study. Evidence-Based Complementary and Alternative Medicine 2020: 9786367.
37. Hamada T, Sale DG, MacDougall JD, Tarnopolsky MA (2003) Interaction of fiber type, potentiation and fatigue in human knee extensor muscles. Acta Physiol Scand 178: 165-173.
38. Hodgson M, Docherty D, Robbins D (2005) Post-activation potentiation: Underlying physiology and implications for motor performance. Sports Med 35: 585-595.
39. Sale DG (2002) Post activation potentiation: Role in human performance. Exerc Sport Sci Rev 30: 138-143.
40. Judge LW (2009) The application of post activation potentiation to the track and field thrower. Strength Cond J 31: 34-36.
41. Grange RW, Vandenboom R, Houston ME (1993) Physiological significance of myosin phosphorylation in skeletal muscle. Can J Appl Physiol 18: 229-242.
42. Sweeney HL, Bowman BF, Stull JT (1993) Myosin light chain phosphorylation in vertebrate striated muscle: Regulation and function. Am J Physiol 264: C1085-C1095.
43. Vandenboom R, Grange RW, Houston ME (1995) Myosin phosphorylation enhances rate of force development in fast-twitch skeletal muscle. Am J Physiol 268: 596-603.
44. Lorenz D (2011) Post activation potentiation: An introduction. The International Journal of Sports Physical Therapy 6: 234-240.
45. Vandervoort AA, Quinlan J, McComas AJ (1983) Twitch potentiation after voluntary contraction. Exp Neurol 81: 141-152.
46. Blazevich AJ, Babault N (2019) Post-activation Potentiation Versus Post-Activation Performance Enhancement in Humans: Historical Perspective, Underlying Mechanisms, and Current Issues. Front Physiol 10: 1359.
47. Boullosa D, Beato M, Dello Iacono A, Cuenca-Fernandez F, Doma K, et al. (2020) A new taxonomy for post activation potentiation in sport. Int J Sports Physiol Perform 19: 1197-1200.
48. Bevan HR, Cunningham DJ, Tooley EP, Owen NJ, Cook CJ, et al. (2010) Influence of post activation potentiation on sprinting performance in professional rugby players. J Strength Cond Res 24: 701-705.
49. Wilson JM, Duncan NM, Marin PJ, Brown LE, Loenneke JP, et al. (2013) Meta- analysis of post activation potentiation and power: Effects of conditioning activity, volume, gender, rest periods, and training status. J Strength Cond Res 27: 854-859.
50. Seitz LB, De Villarreal ES, Haff GG (2014) The temporal profile of post activation potentiation is related to strength level. J Strength Cond Res 28: 706-715.
51. Nuzzo JL, Barry BK, Gandevia SC, Taylor JL (2016) Acute strength training increases responses to stimulation of corticospinal axons. Med Sci Sports Exerc 48: 139-150.
52. Moore RL, Stull JT (1984) Myosin light chain phosphorylation in fast and slow skeletal muscles in situ. Am J Phys 247: C462-C471.
53. Fink RH, Stephenson DG, Williams DA (1986) Potassium and ionic strength effects on the isometric force of skinned twitch muscle fibres of the rat and toad. J Physiol 370: 317-337.
54. Gardetto PR, Schluter JM, Fitts RH (1989) Contractile function of single muscle fibers after hindlimb suspension. J Appl Physiol 66: 2739-2749.
55. Metzger JM, Moss RL (1990) Calcium-sensitive cross-bridge transitions in mammalian fast and slow skeletal muscle fibers. Science 247: 1088-1090.
56. Wang IL, Wang J, Chen YM, Hu R, Su Y, et al. (2021) Effects of acupuncture on the timeline of explosive forces generated by the male shoulder joint. Evidence-Based Complementary and Alternative Medicine 2021: 5585605.
57. De Souza LL, De Araujo FLB, de Silva FAM, Mucciaroni TS, de Araujo JE (2016) Unilateral and immediate stimulation of acupuncture points Xiaohai (SI8) and Jianwaishu (SI14) of the small intestine meridian increases electromyographic activity and strength in the ipsilateral and contralateral upper trapezius muscle. Journ Acupunct Meridian Stud 9: 250-256.
58. Tyldesley B, Grieve JI (1989) Muscles, nerves and movement: Kinesiology in daily living. Boston: Blackwell Scientific Publication.
59. Yen CT, Lu PL (2013) Thalamus and pain. Acta Anaesthesiologica Taiwanica 51: 73-80.
60. Welker WI, Benjamin RM, Miles RC, Woolsey CN (1957) Journ Neurophysiol 20.
61. Penfield W, Boldrey E (1937) Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation. Brain 60: 389-443.
62. Veinante P, Deschenes M (2003) Single-cell study of motor cortex projections to the barrel field in rats. J Comp Neurol 464: 98-103.
63. Donoghue JP, Parham C (1983) Afferent connections of the lateral agranular field of the rat motor cortex. J Comp Neurol 217: 390-404.
64. White EL, DeAmicis RA (1977) Afferent and efferent projections of the region in mouse SmL cortex which contains the posteromedial barrel subfield. J Comp Neurol 175: 455-482.
65. Veldman MP, Maffiuletti NA, Hallett M, Zijdewind I, Hortobágyi T (2014) Direct and crossed effects of somatosensory stimulation on neuronal excitability and motor performance in humans. Neurosci Biobehav Rev 47: 22-35.
66. Boricha MR, Brodieb SM, Graya WA, Iontac S, Boyd LA (2015) Understanding the role of the primary somatosensory cortex: Opportunities for rehabilitation. Neuropsychologia 79: 246-255.
67. Hamdy S, Rothwell JC, Aziz Q, Singh KD, Thompson DG (1998) Long-term reorganization of human motor cortex driven by short-term sensory stimulation. Nat Neurosci 1: 64-68.
68. Meehan SK, Dao E, Linsdell MA, Boyd LA (2011) Continuous theta burst stimulation over the contralesional sensory and motor cortex enhances motor learning post-stroke. Neurosci Lett 500: 26-30.
69. Kleinfeld D, Ahissar E, Diamond ME (2006) Active sensation: Insights from the rodent vibrissa sensorimotor system. Curr Opin Neurobiol 16: 435-444.
70. Lee S, Carvell GE, Simons DJ (2008) Motor modulation of afferent somatosensory circuits. Nat Neurosci 11: 1430-1438.
71. Zhou F, Huang D, Xia Y (1972) Neuroanatomical basis of acupuncture points. Acupuncture Therapy for Neurological Diseases 32-80.
72. Willoughby DS, Simpson S (1998) Supplemental EMS and dynamic weight training: effects on knee extensor strength and vertical jump of female college track and field athletes. J Strength Cond Res 12: 131-137.
73. Wei P, Bao R, Lv Z, Jing B (2018) Weak but critical links between primary somatosensory centers and motor cortex during movement. Front Hum Neurosci 12: 1.
74. Celnik P, Hummel F, Harris-Love M, Wolk R, Cohen LG (2007) Somatosensory stimulation enhances the effects of training functional hand tasks in patients with chronic stroke. Arch Phys Med Rehabil 88: 1369-1376.
75. Conforto AB, Ferreiro KN, Tomasi C, dos Santos RL, Moreira VL, et al. (2010) Effects of somatosensory stimulation on motor function after subacute stroke. Neurorehabiln Neural Repair 24: 263-272.
76. Klaiput A, Kitisomprayoonkul W (2009) Increased pinch strength in acute and subacute stroke patients after simultaneous median and ulnar sensory stimulation. Neurorehabilit Neural Repair 23: 351-356.
77. Knutson JS, Harley MY, Hisel TZ, Makowski NS, Fu MJ, et al. (2012) Contralaterally controlled functional electrical stimulation for stroke rehabilitation. Annu Int Conf IEEE Engineering Medical Biology Society 2012: 314-317.
78. Wu CW, Seo HJ, Cohen LG (2006) Influence of electric somatosensory stimulation on paretic-hand function in chronic stroke. Arch Phys Med Rehabil 87: 351-357.
79. Paillard T (2008) Combined application of neuromuscular electrical stimulation and voluntary muscular contractions. Sports Med 38: 161-177.
80. Kattenstroth JC, Kalisch T, Peters S, Tegenthoff M, Dinse HR (2012) Long-term sensory stimulation therapy improves hand function and restores cortical responsiveness in patients with chronic cerebral lesions Three single case studies. Front Hum Neurosci 6: 244.
81. De Brito FX, Luz-Santos C, Camatti JR, Suzarth G, Campos Moraes LM, et al. (2022) Electroacupuncture modulates cortical excitability in a manner dependent on the parameters used. Acupuncture in Medicine 40: 178-185.
82. Chipchase LS, Schabrun SM, Hodges PW (2011) Corticospinal excitability is dependent on the parameters of peripheral electric stimulation: A preliminary study. Arch Phys Med Rehabil 92: 1423-1430.
83. Bai L, Tao Y, Wang D, Wang J, Sun C, et al. (2014) Acupuncture induces time-dependent remodeling brain network on the stable somatosensory first-ever stroke patients: Combining diffusion tensor and functional MR imaging. Evidence-based Complementary and Alternative Medicine 2014: 740480.
84. Han X, Bai L, Sun C, Niu X, Ning Y, et al. (2019) Acupuncture enhances communication between cortices with damaged white matters in post-stroke motor impairment. Evidence-Based Complementary and Alternative Medicine 2019: 4245753.
85. Fu C, Li K, Ning Y, Zang Y, Han X, et al. (2017) Altered effective connectivity of resting state networks by acupuncture stimulation in stroke patients with left hemiplegia: A multivariate granger analysis Medicine 96: e8897.
86. Ning Y, Li K, Fu C, Ren Y, Zang Y, et al. (2017) Enhanced functional connectivity between the bilateral primary motor cortices after acupuncture at Yanglingquan (GB34) in right-hemispheric subcortical stroke patients: A resting-state fMRI study. Frontiers in Human Neuroscience 11: 178.
87. Schaechter JD, Connell BD, Stasonetal WB, Kaptchuk TJ, Krebs DE, et al. (2007) Correlated change in upper limb function and motor cortex activation after verum and sham acupuncture in patients with chronic stroke. The Journal of Alternative and Complementary Medicine 13: 527-532.
88. Shen Y, Li M, Wei R, Lou M (2012) Effect of acupuncture therapy for postponing Wallerian degeneration of cerebral infarction as shown by diffusion tensor imaging. The Journal of Alternative and Complementary Medicine 18: 1154-1160.
89. Huang Y, Xiao H, Chen J, Qu S, Zheng Y, et al. (2011) Needling at the Waiguan (SJ5) in healthy limbs deactivated functional brain areas in ischemic stroke patients a functional magnetic resonance imaging study. Neural Regeneration Research 6: 2829-2833.
90. Chen J, Wang J, Huang Y, Lai X, Tang C, et al. (2014) Modulatory effect of acupuncture at Waiguan (TE5) on the functional connectivity of the central nervous system of patients with ischemic stroke in the left basal ganglia. PLoS One 9: e96777.
91. Qi J, Chen J, Huang Y, Lai X, Tang Y, et al. (2014) Acupuncture at Waiguan (SJ5) and sham points influences activation of functional brain areas of ischemic stroke patients: A functional magnetic resonance imaging study. Neural Regeneration Research 9: 293-300.
92. He XK, Sun QQ, Liu HH, Guo XY, Chen C, et al. (2019) Timing of acupuncture during ltp-like plasticity induced by paired-associative stimulation. Behav Neurol 2019: 9278270.
93. Mazoyer B, Zago L, Mellet E, Bricogne S, Etard O, et al. (2001) Cortical networks for working memory and executive function sustain the conscious resting state in man. Brain Research Bulletin 54: 287-298.
94. Zhang J, Lu C, Wu X, Nie D, Haibo Y (2021) Neuroplasticity of acupuncture for stroke: An evidence-based review of MRI. Neural Plasticity 2021: 2662585.
95. Payton S, Bailey SD (2017) The effect of manual acupuncture and electroacupuncture on lower limb muscle strength. J Acupunct Tuina Sci 15: 47-53.
96. Holloszy JO, Coyle EF (1984) Adaptations of skeletal muscle to endurance exercise and their metabolic consequences. J Appl Physiol Respir Environ Exerc Physiol 56: 831-838.
97. Dhillon S (2008) The acute effect of acupuncture on 20-km cycling performance. Clin J Sport Med 18: 76-80.
98. Ngai SPC, Jones AYM, Hui-Chan CWY (2011) Acu-tens and postexercise expiratory flow volume in healthy subjects. Evid Based Complement Alternat Med 2011: 726510.
99. Ma H, Liu X, Wu Y, Zhang N (2015) The intervention effects of acupuncture on fatigue induced by exhaustive physical exercises: A metabolomics investigation. Evidence-Based Complementary and Alternative Medicine 2015: 508302.
100. Gao M, Yang HY, Le K, Liu TY, Gu XJ (2008) Effects of manual acupuncture and electroacupuncture on Ca²⁺ content and Ca²⁺ -ATPase Activity in sarcoplasmic reticulum of skeletal muscle cells in rats during acute swimming exercise. Zhen Ci Yan Jiu 33: 13-16.
101. Bailey SD, Liddington B (2021) Effect of acupuncture on physiological response to exercise: A systematic review. Int J Sports Exerc Med.
102. Gao M, Yang HY, Liu TY, Kuai L (2005) Effects of manual acupuncture and electroacupuncture on mitochondria of skeletal muscle cells in rats of acute swimming exercise. Zhongguo Zhen Jiu 25: 421-424.
103. Toda S (2011) Investigation of electroacupuncture and manual acupuncture on carnitine and glutathione in muscle. Evid Based Complement Alternat Med 2011: 297130.
104. Toda S (2007) Effects of electroacupuncture on carnitine in serum and brain. World J Acupunct Moxibustion. 17: 22-24.
105. Shin WC (2018) Effects of acupuncture on serum metabolic parameters in premenopausal obese women.
106. Kim KW, Shin WC, Choi MS, Cho JH, Park HJ, et al. (2021) Effects of acupuncture on anthropometric and serum metabolic parameters in premenopausal overweight and obese women: A randomized, patient- and assessor-blind, sham-controlled clinical trial. Acupunct Med 39: 30-40.
107. Lin C, Wei Z, Cheng KK, Xu J, Shen G, et al. (2017) H NMR- based investigation of metabolic response to electro-acupuncture stimulation. Sci Rep 7: 6820.
108. Bremer J (1983) Carnitine metabolism and functions. Physiol Rev 63: 1420-1480.
109. Saxton JM, Donnelly AE, Roper HP (1994) Indices of free-radical-mediated damage following maximum voluntary eccentric and concentric muscular work. Eur J Appl Physiol 68: 189-193.
110. Ji LL (1999) Antioxidant and oxidative stress in exercise. Proc Soc Exp Biol Med 222: 283-292.
111. Guissoni FM, Deminice R, Ovidio PP, Martinez EZ, Mialich MS, et al. (2020) Nutritional profile and oxidative stress in adolescent soccer players. Central European Journal of Sport Sciences and Medicine 32: 51-59.
112. Michailidis Y, Jamurtas AZ, Nikolaidis MG, Fatouros IG, Koutedakis Y, et al. (2007) Sampling time is crucial for measurement of aerobic exercise-induced oxidative stress. Medicine and Science in Sports and Exercise 39: 1107-1113.
113. Nieman DC, Bishop NC (2006) Nutritional strategies to counter stress to the immune system in athletes, with special reference to football. Journal of Sports Sciences 24: 763-772.
114. Shing CM, Peake JM, Ahern SM, Strobel NA, Wilson G, et al. (2007) The effect of consecutive days of exercise on markers of oxidative stress. Applied Physiology, Nutrition, and Metabolism 32: 677-685.
115. Zanella AM, Souza DRS, Godoy MF (2007) Influência do exercício físico no perfil lipídico e estresse oxidativo. Arquivos de Ciência da Saúde 14: 111-116.
116. Hadžović-Džuvo A, Valjevac A, Lepara O, Pjanić S, Hadžimuratović A, et al. (2014) Oxidative stress status in elite athletes engaged in different sport disciplines. Bosnian Journal of Basic Medical Science 14: 56-62.
117. Silva JR, Rebelo A, Marques F, Pereira L, Seabra A, et al. (2014) Biochemical impact of soccer: An analysis of hormonal, muscle damage, and redox markers during the season. Applied Physiology Nutrition and Metabolism 39: 432-438.
118. Peternelj TT, Coombes JS (2011) Antioxidant supplementation during exercise training: Beneficial or detrimental? Sports Med 41: 1043-1069.
119. Simioni C, Zauli G, Martelli AM, Vitale M, Sacchetti G, et al. (2018) Oxidative stress: Role of physical exercise and antioxidant nutraceuticals in adulthood and aging. Oncotarget 9: 17181-17198.
120. Wang T, Liu CZ, Yu JC, Jiang W, Han YX (2009) Acupuncture protected cerebral multi-infarction rats from memory impairment by regulating the expression of apoptosis related genes Bcl-2 and Bax in hippocampus. Physiology and Behavior 96: 155-161.
121. Shi G, Liu C, Li Q, Zhu H, Wang L (2012) Influence of acupuncture on cognitive function and markers of oxidative DNA damage in patients with vascular dementia. Journal of Traditional Chinese Medicine 32: 199-202.
122. Liu C, Yu J, Zhang X, Fu W, Wang T, et al. (2006) Acupuncture prevents cognitive deficits and oxidative stress in cerebral multi-infarction rats. Neuroscience Letters 393: 45-50.
123. Chen Y, Zhou J, Li J, Yang SB, Mo LQ, et al. (2012) Electroacupuncture pretreatment prevents cognitive impairment induced by limb ischemia-reperfusion via inhibition of microglial activation and attenuation of oxidative stress in rats. Brain Res 1432: 36-45.
124. Oh YI, Yang EJ, Choi SM, Kang CW (2012) The effect of electroacupuncture on insulin- like growth factor–I and oxidative stress in an animal model of renal failure-induced hypertension. Kidney Blood Press Res 35: 634-643.
125. Kim ST, Moon W, Chae Y, Kim YJ, Lee H, et al. (2010) The effect of electroacupuncture for 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine-induced proteomic changes in the mouse striatum. J Physiol Sci 60: 27-34.
126. Siliprandi N, Di Lisa F, Menabo R (1990) Clinical use of carnitine Past, present and future. Adv Exp Med Biol 272: 175-181.
127. Brown MR, Sullivan PG, Geddes JW (2006) Synaptic mitochondria are more susceptible to ca2+overload than nonsynaptic mitochondria. J Biol Chem 281: 11658-11668.
128. Visavadiya NP, McEwen ML, Pandya JD, Sullivan PG, Gwag BJ, et al. (2013) Antioxidant properties of neu2000 on mitochondrial free radicals and oxidative damage. Toxicol In Vitro 27: 788-797.
129. Scholpa NE, Schnellmann RG (2017) Mitochondrial-based therapeutics for the treatment of spinal cord injury: Mitochondrial biogenesis as a potential pharmacological target. J Pharmacol Exp Ther 363: 303-313.
130. Lo YL, Cui SL, Fook-Chong S (2005) The effect of acupuncture on motor cortex excitability and plasticity. Neurosci Lett 384: 145-149.
131. Zunhammer M, Eichhammer P, Franz J, Hajak G, Busch V (2012) Effects of acupuncture needle penetration on motor system excitability. Neurophysiologie Clinique 42: 225-230.

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