Energy Practices and Mindfulness Meditation

The article explores the potential impact of insight meditation and mindfulness practices on the body’s energy system. Basic principles of energy psychology, whose efficacy has been corroborated in current research, are surveyed from the viewpoint of their potential to help in dealing with hypersensitivity resulting from mindfulness practices, combined with presenting a set of specific techniques relevant to hindrances in meditation.

Of additional interest in this growing field of research could be the impact of mindfulness-related practices on the body's energy system, in the way this can be understood from the viewpoint of traditional Chinese medicine. Such exploration has the potential of offering substantial help to practitioners who are in one way or another experiencing energy-related effects of their cultivation of mindfulness, often without an adequate framework of understanding or even a basic idea of what is taking place.

Energy in Early Buddhist Soteriology
The English term "energy" is the standard rendering for Pāli viriya and Sanskrit vīrya (with its Chinese and Tibetan counterparts in 精進 and brtson 'grus). Such type of energy plays a central role in early Buddhist soteriology, evident from its repeated occurrence among the standard list of qualities pertinent to awakening (Anālayo 2020(Anālayo /2022. Such "energy" concerns mental endeavor and the making of an effort; it does not refer to the type of energy that is an equivalent to the pneuma of ancient Greek medicine or the concept of qì (氣) in traditional Chinese medicine. A term that comes closer to the ideas expressed by pneuma and qì would be the element "wind" (Pāli vāyo, Sanskrit vāyu, Chinese 風, Tibetan rlung), sometimes alternatively translated as "air." In the early Buddhist analysis of material phenomena, the element wind features alongside the elements of earth, water, and fire. The whole set of four elements represents basic qualities of matter in terms of solidity, cohesion, temperature, and motion (Anālayo 2021c). A description of the compass of the element wind, extant in a Pāli discourse and its Chinese parallel, takes the following form: Up-going winds, down-going winds, winds in the belly, winds in the bowels, winds that course through the limbs, inhalations, and exhalations. (MN 28: uddhaṅgamā vātā,adhogamā vātā,kucchisayā vātā,koṭṭhasayā vātā,aṅgamaṅgānusārino vātā,assāso passāso).
The notion of such winds being found in the human body has in turn considerable relevance to ancient Indian medicine, as reflected in early Buddhist texts, which conceives of certain diseases in terms of "affliction by wind," "wind in the limbs," and "wind in the joints" (Zysk 1991, pp. 92-96).
An awareness of such energetic processes could underlie descriptions of deep concentrative experiences as resulting in a pervasion of the whole body with mental qualities like joy or happiness and the conception of the mind-made body (Anālayo 2021b). The last refers to a feat believed to be possible based on considerable meditative expertise, where the practitioner leaves the human body behind, presumably still seated in meditation, and travels with a body that has been created mentally to places even far away, being able to pass through solid objects if need be. Such a mind-made body is described as similar to the physical body in appearance but differing from it in not being solid.
Nevertheless, the early Buddhist texts do not seem to reflect any attempt at harnessing the type of energy encompassed by the wind element as an active aid in meditation. The possibly closest instance for such a type of practice appears to be the reported undertaking of breath control by the Buddha-to-be during his quest for awakening (Anālayo 2021a), something he then is on record for abandoning as not conducive to progress to liberation, on a par with other ascetic practices he had tried.

Vipassanā Meditation
Proceeding from ancient India to the contemporary setting, the above provides a background for appreciating an energy-related aspect of the widespread interest in insight meditation, usually referred to with the Pāli term vipassanā. The starting point of the diffusion of such insight meditation appears to have been an initiative by the eminent Burmese scholar monk Ledi Sayādaw (1846Sayādaw ( -1923 to teach Abhidharma widely to lay Buddhists (Braun 2013). Two particularly well-known traditions of vipassanā meditation emerging from this starting point are generally referred to by the name of their most famous respective teachers, which are Mahāsi Sayādaw (1904-1982) and S. N. Goenka (1924. Mahāsi Sayādaw (2016, p. 264) presented the primary object of his approach to insight meditation as requiring that one "should focus his or her mind on the abdomen. You will feel it rising and falling … you should concurrently and continuously note the movements of the abdomen as 'rising' and 'falling' from moment to moment," followed by explaining that the "rise and fall of the abdomen is a manifestation of the air element" (p. 265), alternatively also referred to as the wind element.
The explicit reference to the air or wind element can be explored further from the viewpoint of what in traditional Chinese medicine features under the concept of qì (氣). From that perspective, an important energy center, referred to as the dāntián (丹田), is precisely located in the abdomen area. Moreover, according to a basic principle in qìgōng (氣 功) and tàijíquán (太極拳) practices, the mind can influence the flow of such energy (以心行氣). It follows that, if seen from the viewpoint of ancient Chinese medicine, the practice of focusing on the rise and fall of the abdomen can be expected to stimulate energy in the dāntián and hence have the type of repercussions that can be expected from such stimulation. This assumption is borne out by the description of some "odd experiences" given by Mahāsi Sayādaw (2016, pp. 268-269), which indeed appear to reflect the effects of a (to all appearances unintentional) stimulation of energy: You may feel strong pressure, as if an airbag were being inflated inside your chest, a sharp pain, as if being stabbed with a dagger, a stinging pain, as if being pricked with many small needles, or an overall irritation, as if insects were crawling all over your body. You may feel fierce heat, severe itchiness, unbearable aching, extreme cold, or a variety of other unpleasant sensations … If a meditator finds him-or herself swaying unintentionally, he or she should not be afraid of it, nor should he or she encourage it … If the swaying becomes very strong, you can practice while sitting against a wall or other firm support … Follow the same procedure if you experience trembling in the body. Sometimes you may get goose bumps or a chill may run up your back or through your whole body.
The effects described are indeed what one would expect from a stimulation of the qì caused by attention being focused on the dāntián. Yet, given the lack of framework for understanding the energy dimensions of meditation practices in early and Theravāda Buddhism, it is hardly surprising that such effects are simply seen as odd experiences that are best endured with mental balance.
Energy-related bodily experiences resulting from stimulating the flow of qì also appear to result from the approach to vipassanā taught by S. N. Goenka, which involves executing a body scan. The basic instruction given by Goenka (1987Goenka ( /1997, as part of the standard teaching of a 10-day course, is "to fix your attention in a certain area [of the body], and as soon as you feel a sensation, to move ahead." Such a procedure would indeed stimulate the flow of qì in the part of the body that is being attended to. Goenka (1987Goenka ( /1997 further explained that, on doing this repeatedly in the form of a body scan, it can be expected that "gross, solidified, unpleasant sensations begin to dissolve into subtle vibrations. One starts to experience a very pleasant flow of energy throughout the body." Note that here the concept of "energy" comes up explicitly, although this seems to be simply a way of expressing the meditative experience in a way attuned to a contemporary audience, without being further pursued as an actual effect of the meditation practice. With ongoing practice of the body scans, then, eventually "throughout the body there is nothing but subtle vibrations" (p. 38). At this point, any "gross, consolidated sensations have dissolved into subtle vibrations, arising and falling with great rapidity, and the solidity of the mental-physical structure disappears" (p. 59). Here, the results of (quite probably unintentionally) stimulating the energy dimensions of the body are not just odd effects but constitute a central element in the cultivation of insight into impermanence.
In sum, it seems fair to propose that the approaches to vipassanā meditation taught by Mahāsi Sayādaw and S. N. Goenka involve a stimulation of the qì in various ways, although both traditions do not appear to be fully aware of the implication of this. The significance of this proposal goes beyond practitioners of insight meditation, as the body scan is also part of the standard curriculum of mindfulness-based stress reduction (MBSR). According to Dreeben et al. (2013, p. 395), the "body scan, in effect, provides the pedagogical basis of all the practices introduced later within both the MBSR and MBCT programs." Kabat-Zinn (1990 reported that his development of MBSR was influenced by a teacher of vipassanā meditation who, similar to S. N. Goenka, was a student in the tradition of insight meditation taught by the Burmese lay meditation teacher U Ba Khin (1899Khin ( -1971. In other words, a fuller appreciation of the qì dimensions of the body scan holds considerable potential for a better understanding of the impact of the body scan, in the way it has developed from its Buddhist antecedents (Anālayo 2020 ), as well as offering insights into the mechanisms responsible for the efficacy of MBSR and other mindfulness-based programs that incorporate the body scan or a focusing on the abdomen.

Energy Psychology
Traditional Chinese medicine has for a long time been employing acupuncture or acupressure, which rely on the concept of a qì energy system in the body: the meridian system. In this system, meridians are seen as pathways of energy or qì. The meridian system is based on distinguishing 12 primary meridians, considered to be responsible for the circulation of qì in the body, and eight collective vessels, whose function is to connect the primary meridians. The basic approach consists in understanding physical and psychological symptoms as being related to a dysfunction in the energy system. This dysfunction can be treated by different kinds of stimulation: acupuncture, acupressure, stroking the meridian pathway, or mentally concentrating on a specific meridian or meridian point.
The functioning of the energy system could be compared to the vascular (blood) system. The vascular system needs to be in balance to assure the proper functioning of all body parts. When the blood pressure is too high, this can lead to a headache or even a stroke. When the blood pressure is too low, the result can be dizziness, fatigue, and the risk of fainting. The body functions at its best when the vascular system is in balance. In the case of blockage of blood flow, the functionality of the blocked part(s) of the body will be impaired and eventually not function at all and die. The blood flow needs to be balanced and in free circulation so that the whole system can work properly.
Similar to the blood system, when the energy system in the body is impaired and the energy is too high or too low, or even completely blocked, the person will feel out of balance and not be able to function, think, and feel properly. The resultant physical, emotional, and mental symptoms can be addressed by stimulating specific acupuncture points on the impaired meridian (energy pathway) in order to re-enable the flow of energy and help the body to get back into balance. In the case of energy psychology, a treatment by way of stimulation of meridian points does not employ acupuncture but acupressure, massage, or mental stimulation.
Energy psychology is a generic term for the combination of psychology with traditional Chinese medicine. Following applied kinesiology (Goodheart & Schmidt 1964), behavioral kinesiology (Diamond 1979), Thought Field Therapy (Callahan 1994), and Emotional Freedom Techniques (Craig & Fowlie 1995), energy diagnostic and treatment methods have been developed by Gallo (2000Gallo ( , 2002, using the stimulation of acupoints in the treatment of trauma, phobia, OCD (obsessive compulsive disorder), addictive disorders, depression, and other psychological issues.
Energy psychology addresses traumatic experiences, phobia, and other issues by combining a psychological approach with the stimulation of meridian points. The procedure consists in bringing the issue (e.g., a phobia) to mind and then locating the meridian or the cluster of meridians that are linked to the issue. The meridians will then be stimulated either by touching specific acupoints (patients will tap, massage, or simply touch the point on their own bodies) or by concentrating on and thereby mentally stimulating the acupoint.
Identification of the relevant meridian points can take place in two ways: One of these is an individual diagnostic approach that uses kinesiological muscle testing to identify the particular meridians relevant for the specific issue experienced by an individual. An alternative to such a tailormade approach is simply by way of using a selection of meridian points that are known to be linked to certain issues and which are effective for most patients. The treatment consists in letting the patient stimulate the acupoints while staying tuned to the issue until relief of the physical, emotional, and mental aspects is evident. The therapist will then support the patient in consolidating the change through different integration techniques, and the patient becomes able to confront the situation in daily life. Should the treatment not have sufficed, the meridian points can be stimulated again to dissolve any residue.

Research on Energy Psychology
The efficacy of energy psychology has been supported by several studies. These studies focus on acupoint protocols and standardized procedures of energy psychology, as these are more adapted to replication in scientific research than the individual, tailormade approaches. In a RCT (randomized controlled trial) study with war veterans (Church et al. 2013), six sessions of EFT (emotional freedom techniques) produced drops to subclinical levels of PTSD (post-traumatic stress disorder), with the average subject remaining subclinical at 3-and 6-month follow-up. The waiting-list control group showed no symptom reduction. The EFT subjects evidenced significantly reduced psychological distress (p < 0.0012) and PTSD symptom levels (p < 0.0001) after the intervention. Additionally, 90% of the EFT group no longer met criteria for PTSD, compared with 4% in the control group.
After the wait period, the controls received EFT. In a within-subjects longitudinal analysis, 60% no longer met PTSD criteria after three sessions, which increased to 86% after six sessions for the 49 subjects who received EFT. Benefits remained at 86% at 3 months and at 80% at 6 months. Sebastian and Nelms (2017) conducted a meta-analysis of seven randomized controlled trials comparing EFT treatment to usual care or waitlist. The authors found a large treatment effect with a weighted Cohen's d = 2.96 (95% CI: 1.96-3.97, p < 0.001). They concluded that EFT is effective for the treatment of PTSD with a variety of populations.
Based on a survey of more than 100 peer-reviewed outcome studies, Feinstein (2019) concluded that the evidence shows acupoint-based energy psychology protocols to produce beneficial outcomes in the treatment of anxiety, depression, and PTSD. Feinstein (2021) surveyed 245 clinical trials, meta-analyses, systematic evaluations, and theory pieces examining energy psychology protocols. According to his assessment, there is sufficient empirical support for 6 premises about the method's efficacy, speed, durability, and physiologic effects (p. 17): acupoint tapping protocols (a) are effective in treating a range of clinical conditions, (b) are rapid compared to conventional treatments, (c) lead to durable benefits, (d) produce changes in biologic markers that corroborate the subjective assessments of clients, (e) are a critical ingredient for the demonstrated clinical effects and (f) send signals that can increase or decrease arousal in specific areas of the brain.

The Five Hindrances
Given the apparent efficacy of energy psychology, it would seem reasonable to attempt to integrate some of its approaches with contemporary teachings of mindfulness meditation practices. One approach for doing so could be based on a central early Buddhist teaching for successfully developing one's meditation, which concerns five so-called hindrances. The expression "five hindrances" here refers to a particular set of mental conditions identified in early Buddhist thought as particularly detrimental for the functioning of the mind. These are sensual desire, anger or irritation, sloth-and-torpor, restlessness-and-worry, and doubt.
Overcoming of the five hindrances features in early Buddhist soteriology as an indispensable requirement for being able to cultivate mental tranquility as well as liberating insight. According to two Pāli discourses, the Buddha explained the effect of the five hindrances on the mind to a brahmin who was puzzled at the fact that at times he would forget what he had memorized repeatedly and at other times he would remember what he had not repeatedly memorized (SN 46.55 and AN 5.193; see also Anālayo 2013, pp. 189-191). The vivid similes employed in these discourses to illustrate the effect of these five hindrances are also extant in a Sanskrit fragment (Tripāṭhī 1995, p. 127). The brahmin's difficulties in memorization, a key aspect of learning in the ancient Indian setting, suggest that the relevance of the five hindrances extends beyond meditation practice and could be of considerable interest to education in general (Anālayo 2022).
From the viewpoint of energy psychology, since the proposed application is not part of a general therapy but more specifically a technique for meditators, the stimulation of meridian points can be done not only by direct touching but also by mentally focusing on the specific acupoint. The following methods, a demonstration of which can be found in Scott (2022), could be applied to the case of each of the five hindrances: For the case of sensual desire, a procedure also used in addiction issues can be relied on. This involves stimulating the stomach meridian (acupoint stomach 1, which is under the eye on the infraorbital ridge directly below the center of the pupil) to calm craving and the kidney meridian (acupoint kidney 27, which is at the juncture of the first rib, the clavicle, and the sternum under the collarbone) to recenter on health (Fig. 1).
In case of anger and irritation, holding the wrist, and thereby accessing a chord of several meridians, is a commendable option. The small intestine meridian can be stimulated to treat potentially underlying feelings of sadness and despair (side of the wrist) (Fig. 2). To stimulate the pericardium meridian helps to let go of the past, balancing the heart meridian is helpful to rouse forgiveness, and the lung meridian may be needed to reinforce tolerance (all are located on the inner part of the wrist).
If sloth-and-torpor manifest, stimulation of the thymus point, found on the middle of the breast (Tarzan point), can help increase the body's global energy level. This point can either be tapped with the fingertips or the hand or the fist. It can be helpful to use a body posture like Tarzan: upright, knees a bit bend, leaning a little forward, feet rooted on the ground. In addition, the conception vessel can be stimulated by running the hand upwards from the pubic bone (CV 1) or from the navel (CV 8) up to the hollow between the chin and the bottom lip (CV 24) to boost energy flow. This movement of the hand and arm can continue upwards in an open gesture, something used by opera singers to get more energy before starting to sing. The opposite hindrance of restlessness-and-worry calls for a reduction of the energy flow in the body. The natural direction of energy flow in the conception vessel goes upwards, but when this vessel is traced upside down, it will reduce energy. One starts at the hollow under the bottom lip and traces the conception vessel down toward the navel or pubic bone (this can be done by stroking or tracing with the hand palm downwards in front of the body's midline or else by just visualizing the movement).
When experiencing nervousness or stress (high arousal), a helpful technique can be the frontal occipital holding (FOH) to calm the system. Placing the palm of one hand on the forehead, the other hand holds the back of the head (occipital, just between the head and neck). Breathing with closed eyes, one will soon feel the resulting calm. Another technique to calm nervousness is to practice the emotional stress release (ESR): This begins with the tips of the fingers placed on the neurovascular reflex points on the forehead (Fig. 3). These points are located underneath the forehead humps, in the middle of the forehead, between the eyes and the hairline. They can be touched lightly with the tips of two fingers, and one tries to feel the pulsation while being aware of the breath. Often one will at first sense the pulsation on one side, then the other, and the rhythms will differ. With the continuation of calm breathing, however, one can often observe how the two pulsations start to synchronize.
The fifth hindrance of doubt can be faced with several techniques. One of these is called the "elaborated eye roll." It requires stimulating the triple warmer 3 (which is located on the back of the hand between the ring finger and the little finger; see Fig. 4) while moving the eyes vertically from the floor to the ceiling (the head stays straight, only the eyes are Fig. 3 The neurovascular reflex points on the forehead (located underneath the forehead humps, in the middle of the forehead, between the eyes and the hairline) Fig. 4 The triple warmer 3 (located on the back of the hand between the ring finger and the little finger) moving). When the eyes have been completely raised, one takes a deep breath while lowering the eyelids on the eyes that are kept raised. Then one stops tapping at triple warmer 3 and relaxes the eyes while exhaling. The immediate effect will be a sensation of relaxed rootedness.
Another possibility is the "hook up," a cross-over movement which stimulates both hemispheres of the brain, resulting in feeling calm and centered. This requires extending the legs, crossing the left ankle over the right one, and holding the hands in front with arms extended, with the palms facing each other. Next, the right hand is raised up and placed over the left hand, so that the backs of the hands touch. Then the hands are turned outwards so that the thumbs come down and the palms touch, interlocking the fingers. Then the interlocked hands are turned in and up so that the hands rest on the thymus point on the chest (for some it may be more comfortable to do it the other way round: cross the right ankle over the left; raise the left hand over your right, etc. The rest of the procedure is the same for everyone.). Putting the tip of the tongue behind the upper teeth, at the roof of the mouth, at the center of the palate, and closing the mouth (the teeth do not touch), one breathes and stays like this for at least 2 min.

Energetic Protection
Besides providing support for working with the five hindrances, energy work can also be employed in a general way as a form of protection (guided instructions on which can be found in Steffens-Dhaussy 2022a). This can help especially counter the vulnerability practitioners can experience when shifting from intensive meditation, perhaps in a retreat setting, to having to return to the world and deal with its various challenges. One such approach is the "energy protection," and another the "second skin," with a variant in the form of the "invisible suit or cloak," which is a protection technique using visualization combined with acupoint stimulation.
Any of these approaches can help in case of high sensitiveness or when finding it difficult to deal with highly stimulating situations like, for example, a crowded space in an airport. There is a risk of feeling overwhelmed by the sheer amount of visual, auditory, olfactory, and other sensual stimulations. Especially when leaving a protected space like a meditation center, the world outside with its different rhythms and stressful behavior of people (e.g., abrupt movements, pushy behavior, loud/aggressive voices) can be experienced almost like an attack.
The energy protection technique begins by rubbing the hands against each other, shaking them out, and rubbing them again in order to stimulate the energy. Then the hands move along the body on the skin level, starting from the top of the head to the back of the head as if combing one's hair (stimulating the governing vessel and bladder meridian for calmness) and then touching the forehead, hands close the eyelids as they pass over the eyes, and the palms rest a moment on the eyes while the fingertips touch the neurovascular points on the forehead. Next the hands run down over the face towards the shoulders, crossing over each other in the chest area (stimulating the thymus), moving to the lower back and then along the back of the legs, and bending down as needed until reaching the feet. This downward progression of movement stimulates calm and peacefulness (stimulating governing vessel and bladder meridian). The next step is to create an energy protection around the body, slowly straightening the body and standing in a tree-like bodily posture, palms facing upwards, keeping in mind what quality might be needed, such as centeredness/rootedness, like a tree being rooted in the earth, or the quality of patience, or energy in general, etc. The quality can be aroused in the mind and then related to the open palms. The hands are brought up, raised over the head, and then turned so that the palms show downward. The quality/energy is then allowed to flow down from above the head to cover and surround the whole body, either by just holding the hands palm down over the head like taking a shower or by moving the hands palms facing towards the body around the head and the upper torso, like creating a protective shield. This shield can be close to the body or at some distance.
Another protection technique for facing difficult situations uses visualization combined with the stimulation of specific acupoints (guided instructions on which can be found in Steffens-Dhaussy 2022b). The technique starts by imagining oneself drawing an invisible boundary line along one's body. This line can be close to the skin or else at a bit of a distance, in the sense of being drawn around the aura of the body. In a second step, the imaginary boundary line can be thickened, which is almost as if a second skin were to be enveloping the whole body. The function of this "second skin" is indeed to some extent comparable to the natural skin, as its purpose is to protect the inner body, the organs, and their inner functioning, from outward influences. Comparable to the natural skin that does prevent dust, rain, wind, or other outward stimulations from entering the inner part of the body, likewise the visualization of a second skin can be effective in protecting the (mental) inner functions from intrusive outward stimulants that may potentially perturb these inner functions. This involves using the weather metaphor that is also employed in the "mountain meditation" described in Kabat-Zinn (1990: "We are sitting in stillness, just being what we are, just as a mountain 'sits' unmoved by the changing of day into night and the changes of the weather and of the seasons." In the same way, changes of weather will only affect the outside of the mountain (that is, of the meditator) but not be able to penetrate and influence the inside. Several variants of the "second skin" are possible: a meditator could bring to mind a neoprene suit, for example, or a deep-water diving suit with a helmet or an astronaut suit. Such variations would enable choosing the thickness and the texture of the protection.
An alternative to this technique would be visualizing an invisible cloak. This is an approach developed from daily experience; before leaving the house, one will naturally check for the specific weather conditions and choose clothing appropriate for these conditions. When it is cold outside, one will put on a thick jacket before moving into the outer world; when it is warm and windy, the choice could instead be a light but windproof jacket; when it is raining, the jacket should also be waterproof. This everyday habit can be transferred to being in contact with different kind of energies that originate from other people (e.g., a stormy person) or places (e.g., a cold or slightly hostile atmosphere).
It would be possible to create different cloaks for different situations or to imagine a "general" cloak that is adaptable to all kinds of situations. The variants could be, e.g., to recall a traditional shepherd's cloak (heavy linen or wool, earth or forest colors, so large that it even falls over the toes and protects the feet from being stamped on); or a wizard's cloak like in the story of Harry Potter (a very light and soft tissue, that can be thrown over by snapping the fingers); or a simple cloak with a special breast protection, a bit like what knights wore in the Middle Ages. The mental image of any such personal protection gear can then be activated just before going into a situation that could be potentially stressful or disturbing. Like in everyday life, when coming back inside the house (being back in a safe place), it can be taken off again.
The technique of the invisible suit or cloak can be combined with a simple energy technique like the "zip up." The zip up is a movement that stimulates the conception vessel and heightens energy flow. The conception vessel is located between the pubic bone and the lips. The direction of energy flow is upwards, from the lower part of the body towards the lips. Going with the natural energy flow of the conception vessel means moving upwards, like closing a zipper.
This "zip" for heightening energy and protection can be more effective by adding a turning movement (above the lips). This movement can be compared to turning a key in a keyhole. "Zipping up" and "turning the key to keep oneself safe" can be used as an additional mental procedure to improve personal protection. On the energy level, this has the benefit of stimulating the acupoint "governing vessel 24" (in the philtrum, below the nose) and in this way activating the governing vessel as well. As the governing vessel starts at the coccyx and follows the midline over the back and the skull, ending on the philtrum, the lips can be considered the junction between these two central vessels. Stimulating both central vessels (conception and governing vessel) strengthens the energy level and is an effective way to complete one's personal protection.

Declarations
Ethical Approval This article does not contain any studies performed by the authors with human participants or animals.

Conflict of Interest
The authors declare no competing interests.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.