Biophysics For Physical Therapy Students PDF

BIOPHYSICS FOR PHYSICAL THERAPY STUDENTS PROF. DR. IBRAHIM HAGER  Contents Chapter 1: Electric and magnetic fields effect and therapy for muscles, joints and bones Chapter 2: Laser Therapy for Joints and Muscles Chapter 3: Ultrasound Therapy for J...

BIOPHYSICS FOR PHYSICAL THERAPY STUDENTS PROF. DR. IBRAHIM HAGER  Contents Chapter 1: Electric and magnetic fields effect and therapy for muscles, joints and bones Chapter 2: Laser Therapy for Joints and Muscles Chapter 3: Ultrasound Therapy for Joints and Muscles  Chapter 1 Electric and Magnetic Fields Effect and Therapy for Muscles, Joints and Bones Electricity Within The Body  Electricity plays an important role in medicine.  There are two aspects of electricity and magnetism in medicine:  1. Electrical and magnetic effects generated inside the body.  2. Applications of electricity and magnetism to the surface of the body.  The electricity generated inside the body serves for the control and operation of nerves, muscles, and organs.  Electric fields are an integral part of our daily lives, even though we may not always be aware of their presence.  These fields are generated by various sources, such as power lines, electrical appliances, and electronic devices.  One of the most common sources of electric fields in our environment is power lines.  These high voltage lines carry electricity from power plants to our homes and businesses.  As electricity flows through these lines, it creates an electric field around them.  Although the electric fields generated by power lines are relatively weak, they can still have an impact on our bodies. Electric Fields from Electrical Appliances  Electrical appliances, such as refrigerators, televisions, and computers, also generate electric fields.  These fields are produced by the flow of electricity within the appliances and can extend a short distance beyond them.  When we use these appliances, we are exposed to their electric fields However, the strength of these fields is generally much lower than that of power lines. Electric Fields from Electronic Devices  In today's digital age, electronic devices have become an integral part of our lives.  Devices such as smartphones, tablets, and laptops emit electric fields due to the electrical currents flowing through their circuits.  These fields are relatively weak compared to power lines and electrical appliances but can still have an impact on our bodies, especially when we use these devices for extended periods. - - Minimizing Exposure to Electric Fields  While the effects of electric fields on the human body are still being studied, it is advisable to take certain precautions to minimize exposure. Here are some safety guidelines to consider 1. Maintain Distance  When possible, maintain a safe distance from high voltage power lines and electrical substations.  The strength of electric fields decreases rapidly with distance, so keeping a reasonable distance can reduce your exposure. 2. Limit Device Usage  Limit the durtion of your exposure to electronic devices, especially when using them close to your body.  Take breaks and give your body time to rest from the electric fields emitted by these devices 3. Use Shielding  Consider using shielding materials, such as metal screens or special fabrics, to reduce your exposure to electric fields.  These materials can help block or redirect the electric fields away from your body. Effects of Electric Fields on the Human Body Electric Fields and Health  Electric fields can have both positive and negative effects on the human body, depending on various factors such as: 1. field strength 2. duration of exposure 3. human sensitivity Figure An external electric field causes an electrical charging of the body surface in humans and (in the case of an alternating field) very low internal body currents. The effects of the electric field are therefore generally limited to the body surface. Figure An external extremely low frequency magnetic field causes eddy currents in the human body. The field penetrates the body. The simplified figure shows the eddy currents of an alternating magnetic field perpendicular to the body axis. In healthy conditions the effects of induced fields and currents are not perceptible below the limit values of induced fields and currents in our everyday lives. 1. Direct Effects (rare) A) At low levels, electric fields have minimal direct effects on the human body. They do not cause any immediate harm or noticeable physiological changes. B) At extremely high levels, such as those experienced during electrical accidents or lightning strikes, electric fields can cause severe injuries, including burns, tissue damage, and even death. 2. Indirect Effects (more common)  While direct effects are rare, indirect effects of electric fields on the human body are more common.  These effects primarily occur through the interaction of electric fields with other environmental factors or the body's own electrical systems. Effects of Electric Fields on Cell Behavior  Electric fields have been found to affect different cell types involved in wound healing, including: 1. fibroblasts 2. keratinocytes 3. endothelial cells 1. Fibroblasts: play a crucial role in wound healing by :  producing extracellular matrix components  promoting tissue remodeling.  Electrical stimulation can: A) enhance fibroblast migration and proliferation. B) leading to accelerated wound closure. 2. Keratinocytes: are the primary cells responsible for (re-epithelialization): the process of forming a new epithelial layer over the wound  Electric fields can: A) stimulate keratinocyte migration and proliferation B) facilitating the re epithelialization process C) reducing the healing time 3. Endothelial cells are essential for (Angiogenesis): the formation of new blood vessels  Electric fields can: A) promote endothelial cell migration and tube formation B) facilitating the development of a functional vascular network in the wound bed. This enhanced angiogenesis, improves oxygen, and nutrient supply to the healing tissue, further promoting wound healing. - Electric Fields and the Nervous System  The nervous system is one of the most electrically sensitive systems in the human body.  Electric fields can influence the functioning of nerve cells and have various effects on the nervous system. 1) Sensory Perception Electric fields can affect sensory perception, altering the way we perceive touch, taste, and smell. Studies have shown that exposure to certain electric fields can enhance or diminish sensory perception, leading to: A) changes in sensitivity , B) the perception of phantom sensations. Figure Effect of alternating magnetic fields on people: they generate more or less pronounced eddy currents. Figure Magnetic Field around human body. Figure Proposed axial and longitudinal current flow generation within a neuron. Different from Current models which approximated the electric current flow through the neuron as a continual flow of ions through sodium, potassium and leak channels which is constant in time during an action potential, we propose an additional term of applied current to be added to each consecutive axonal segment. This axial current is a product of the continuous ion flow through the cell’s membrane and its transversal components and, with that, aids action potential propagation and attenuation. 2) Nerve Cell Excitation  Electric fields can also influence the excitation of nerve cells.  When exposed to an electric field, nerve cells may become more excitable, leading to increased firing rates and altered neural activity.  This effect has been utilized in medical applications such as deep brain stimulation for the treatment of neurological disorders like Parkinson's disease. Electric Fields and the Cardiovascular System  The cardiovascular system, which includes the heart and blood vessels, relies on electrical signals for proper functioning.  Electric fields can affect the cardiovascular system in various ways. 1) Heart Rhythm  Electric fields can influence the rhythm of the heart,  Exposure to certain electric fields can disrupt the normal electrical signals in the heart, leading to irregular heartbeats or arrhythmias  This effect is particularly significant in individuals with pre-existing heart conditions. 2) Cardiac Health  On the other hand, electric fields can also be used for therapeutic purposes in the cardiovascular system.  Electric stimulation techniques, such as cardiac pacing, can help regulate the heart's electrical activity and restore normal heart rhythm.  These techniques are commonly used in the treatment of cardiac arrhythmias and other heart related conditions. - - Electric Fields and Muscles  Muscles are another vital component of the human body that can be influenced by electric fields.  Electric fields can affect muscle contraction and have implications for muscle health and rehabilitation. 1) Muscle Contraction  Electric fields can stimulate muscle contractions.  This principle is utilized in various therapeutic techniques such as: A) transcutaneous electrical nerve stimulation (TENS) B) neuromuscular electrical stimulation (NMES)  These techniques involve the application of electric currents to specific muscles or nerve pathways to promote muscle activation, improve blood circulation, and aid in muscle recovery. 2) Muscle Rehabilitation  Electric stimulation techniques have proven to be effective in muscle Rehabilitation.  They can help prevent muscle atrophy, improve muscle strength and coordination, and facilitate the recovery process after injuries or surgeries.  These techniques are commonly used in physical therapy settings to enhance muscle function and promote overall rehabilitation. Electrical Stimulation of The Nervous System Figure: Electrodes touch a frog, and the legs twitch into the upward position. - Electric Fields and the Brain  The brain is a complex organ that depends on intricate electrical signaling for its functioning  Electric fields can influence brain activity and have implications for brain function and cognitive enhancement. Brain Stimulation  Electric fields can be used to stimulate specific regions of the brain: A. Transcranial electrical stimulation (TES) techniques B. transcranial direct current stimulation (TDCS) C. transcranial magnetic stimulation (TMS)  involve the application of electric currents or magnetic fields to the scalp to modulate brain activity  These techniques have shown promise in various applications, including the treatment of depression, pain management, and cognitive enhancement. - - - - Electric Signals in Neurons  Neurons communicate with each other through electrical signals called action potentials.  These action potentials are generated by the movement of ions across the neuron's cell membrane.  At rest, the inside of the neuron is negatively charged compared to the outside.  This difference in charge is maintained by the selective permeability of the cell membrane and the action of ion channels.  When a neuron receives a signal from another neuron, the permeability of its cell membrane changes, allowing positive ions, such as sodium (Na) and potassium (K) to flow in and out of the cell.  This movement of ions creates a temporary reversal of the charge across the cell membrane, known as depolarization.  If the depolarization reaches a certain threshold, an action potential is generated.  Once an action potential is initiated, it travels along the axon of the neuron, propagating the electrical signal.  At the end of the axon, the electrical signal is converted into a chemical signal, allowing the neuron to communicate with the next neuron or target cell through synapses. - - - - - - - Potential Effects of Electric Fields  However, some studies have suggested that exposure to high levels of electric fields may lead to certain health concerns include: 1. Skin Sensations  Some individuals may experience tingling or prickling sensations on their skin when exposed to strong electric fields.  This phenomenon, known as electro sensitivity is not well understood and is still a topic of debate among scientists. 2. Thermal Effects  Electric fields can generate heat when they interact with the body's tissues.  Prolonged exposure to high levels of electric fields may lead to localized heating, which can potentially cause burns or tissue damage.  However, it is important to note that the electric fields encountered in everyday life are typically too weak to cause significant thermal effects. - - Interference with Medical Devices  Strong electric fields can interfere with the proper functioning of certain medical devices, such as pacemakers and implantable cardioverter defibrillators (ICDS).  These devices rely on electrical signals to regulate the heart's rhythm (Pulses) and external electric fields can disrupt their operation  It is crucial for individuals with such medical devices to be cautious and avoid close proximity to strong electric fields. Electric Stimulation for Medical Purposes  Electric stimulation, also known as electrotherapy, is a medical technique that utilizes electric fields to stimulate various parts of the human body for therapeutic purposes  This non invasive procedure has been used for decades to treat a wide range of medical conditions and promote healing.  By applying controlled electrical currents to specific areas of the body, electric stimulation can have profound effects on the nervous system, muscles, and other bodily functions. How Electrical Stimulation Works  Electric stimulation works by delivering low level electrical currents to targeted areas of the body.  These currents are typically generated by a device called an electrical stimulator, which is designed to produce specific waveforms and frequencies.  The electrical stimulator is connected to electrodes, which are placed on the skin over the area to be treated.  When the electrical currents are applied, they interact with the body's own electrical signals, influencing the behavior of cells, tissues, and organs.  The electrical currents can modulate nerve activity, promote muscle contractions, increase blood flow, and stimulate the release of natural pain relieving materials. - - Applications of Electric Stimulation  Electric stimulation has a wide range of applications in the field of medicine.  It is commonly used in physical therapy, rehabilitation, and pain management.  Some of the specific medical purposes for which electric stimulation is utilized include: 1.Pain treatment  Electric stimulation can be an effective method for managing pain, particularly chronic pain Conditions.  By stimulating the nerves and altering pain signals, electric stimulation can: - provide relief from various types of pain, including musculoskeletal pain, neuropathic pain, - and postoperative pain It can also help reduce the need for pain medications, which can have unwanted side effects 2.Muscle Rehabilitation  Electric stimulation is frequently used in muscle rehabilitation programs to help restore muscle strength and function.  By stimulating the muscles, electric currents can promote muscle contractions and prevent muscle atrophy.  This is particularly beneficial for individuals who have experienced muscle weakness or loss of function due to injury, surgery, or neurological conditions. 3.Wound Healing  Electric stimulation has been shown to enhanace the healing process of wounds.  By increasing blood flow and promoting the production of growth factors, electric currents can accelerate tissue repair and regeneration  This can be especially beneficial for chronic wounds, such as diabetic ulcers, that are difficult to heal using conventional treatments. 4.Nerve Stimulation  Electric stimulation can be used to stimulate nerves and improve their function.  It is often employed in the treatment of conditions such as peripheral neuropathy, where the nerves in the extremities are damaged or dysfunctional.  By stimulating the affected nerves, electric currents can help alleviate symptoms such as numbness, tingling, and pain. Basic principle of electricity and electrical stimulation current  Figure 01 illustrate an electrical current, showing that hypothetically, the electrons move superficially in the skeletal muscles, in a linear fashion, from one pole to the other, that is, from one electrode to the other, according to polarity.  We can observe that the skeletal muscle contraction produced by the current is not maximal. In fact, an electrical current capable of producing a maximum muscle contraction would certainly be harmful, or with a high probability of intercurrences such as an electrical burn.  Furthermore, the feeling of an electric shock would certainly be unbearable for the patient. Figure 01: Schematic representation of the effects of electrical currents on skeletal muscle. The illustration shows the electric current acting more superficially on the musculature, causing a submaximal muscle contraction.  Figure 02 illustrate an application of Pulsed electromagnetic field, that potentially evolves the whole muscle, being able to recruit the muscle and to promote maximal muscle contraction.  Besides, PEMF does not produce the sensation of electric shock, being significantly more bearable for the patient, especially if we consider elderly individuals. Figure 02: Schematic representation of the effects of PEMF on skeletal muscle contraction. The illustration shows the PEMF acting deeply on the muscle, due to its 3D effect, causing a supramaximal muscle contraction.  The pulsed electromagnetic field (PEMF) uses alternating magnetic fields, based on the law of electromagnetic induction, promotes electrical currents that depolarize the neuromuscular tissue may resulting in supramaximal contractions.  Motor neurons are activated due to their large diameter and therefore less resistance compared to other types of neurons. Since nociceptors are not activated, the application of magnetic stimulation is not painful.  This is an important point since the classic discomfort of therapeutic electrical currents does not exist with PEMF.  The new PEMF equipment normally comprises a circular coil located in the applicator, which is placed over the treatment area.  Starting a treatment, an alternating electric current run into the circular coil and the alternations in the electric current induce rapidly changing magnetic waves which propagate into the underlying tissue, inducing consequently a secondary electric current that will depolarize the muscle-innervating motor neurons and induce muscle contractions.  Under normal conditions, the greatest amount of tension that could be developed and performed physiologically is called maximal voluntary contraction (MVC).  It usually only lasts for a fraction of a second.  PEMF possesses the ability to generate sustained supramaximal contractions for several seconds, which significantly increases stress/workload, if muscle adaptation takes place.  In supramaximal contraction, proteins are degraded, but amino acids are reused in the synthesis process at the expense of intense energy expenditure.  However, the evolution of research has demonstrated that besides the muscle stimulations, PEMF are able to interact with different structures of the locomotor system, including tendons, cartilage and bones. - - - Pulsed Electromagnetic Field (PEMF) and Tendon Diseases  Tendinopathies are part of the so-called group of the most common musculoskeletal diseases in modern society.  There are several causes, whether due to daily activities, work-related or even repetitive movements or overcharge in sports.  Chronic pain in the tendons is relatively common, especially considering the Achilles, patellar and elbow tendons.  Tendinopathies are changes in the health of the tendon, which are generally frequent and difficult to treat, disabling professional and recreational athletes as well as ordinary people in their workplaces.  The high prevalence, along with the fact that they often become chronic, make these diseases a major socio- economic problem where medical interventions and therapies for rehabilitation are limited. - - -  The PEMF technology has attracted increasing interest since consistent evidence of its therapeutic properties has been demonstrated to treat musculoskeletal conditions.  Concerning on tendon disorders, some scientific studies have investigated the efficacy of PEMF in tendon healing.  In vivo studies showed that PEMFs was able to improve tendon healing through a reduction of inflammation, improvement of mechanical properties and an induction of faster collagen alignment.  Taken together, these results suggest a reparative role PEMFs in tendinopathies.  Rosso et al also demonstrated with in vitro studies on human tendon cells an increased cell proliferation after PEMF stimulation.  The effects of electromagnetic fields were analyzed in human tendon stem cells isolated from patients undergoing surgeries and the treatment presented positive effects on stem cell marker expression, as treated cells maintained a higher expression of these markers during culturing.  In their study, the authors hypothesized that PEMF application after rotator cuff detachment and repair could produce beneficial effects on biomechanical properties, tissue morphology and bone density.  PEMF was able to enhance early postoperative tendon-to-bone healing Pulsed Electromagnetic Field (PEMF) and Joint Diseases  Osteoarthritis (OA) is highly prevalent in elderly population.  According to the scientific literature, the majority of OA patients do not receive appropriate management therapies.  However, moderate evidence for electrophysical agents, such as photobiomodulation has been demonstrated.  Pulsed Electromagnetic Field therapy has also been suggested as an alternative treatment for OA.  According to Shupak et al, PEMF promotes joint benefits based on basic principles of physics: Wolff’s law, (Walff's law describes the nature of bone remodeling regarding stresses), the piezoelectric properties of collagens, and the concept of streaming potentials.  Although the effects of PEMF have previously been reported to increase morphogens and promote osteogenesis, the real therapeutic effects of PEMF on osteoarthritis still on debate. Pulsed Electromagnetic Field (PEMF) and Cartilage  PEMF has been studied for at least 20 years for Muscle-skeletal and joint disorders.  In 2005, it was found that PEMF has been positive effects in patients younger than 65 years old, there were significant and beneficial effect of treatment related to stiffness.  Fini et al, studied the pulsed electromagnetic field stimulation on knee cartilage, of some animals.  PEMF stimulation significantly changed the progression of OA lesions in all examined knee areas, even in the presence of severe OA lesions.  PEMF does not lead to a higher percentage of patients who resume sports or to earlier resumption of sports after arthroscopic debridement and microfracture of talar OCDs.  Furthermore, no differences were found in bone repair between groups.  In 2013, it has been found that with PEMF treatment increased mineralization of ADSC (adipose‐derived stem cells) and enhanced chondrogenic differentiation of ADSCs cultured in a chondrogenic microenvironment.  PEMF enhanced both osteogenesis and chondrogenesis under the same conditions.  Also, PEMF was effective to reduce pain in knee OA patients and also improved pain threshold and physical functioning.  Besides, twenty-six per cent of patients in the PEMF group stopped taking NSAIDs or any other analgesic drug, and no adverse events were detected.  Recently Parate et al demonstrated that PEMF stimulation was able to modulate paracrine function of mesenchymal stem cells (MSCs) for the enhancement and re-establishment of cartilage regeneration in states of cellular stress. PEMF and Muscle Hypertrophy  PEMF was capable of inducing hypertrophic muscle alterations.  The authors report an increase in muscle mass density of 20.56%.  An increase in muscle fiber density (hyperplasia) of 8.0% was observed.  Mean individual muscle fiber size increased by 12.15%  The authors suggest that PEMF can be used for non-invasive induction of muscle growth.  The classic hypothesis is that of depolarization of peripheral motor neurons, with release of acetylcholine in the myoneural plate.  However, there are evidence that PEMF is able to stimulate at least two molecular mechanisms related to the expression of Voltage-dependent Calcium Channels and increase of intracellular calcium concentrations in bone tissue.  However, the effect observed in bone cells may be repeating itself in muscle tissue, which would explain the greater effectiveness of PEMF in relation to electrical currents.  This is because, the effect of muscle contraction, in fact, can be triggered by a direct change in voltage of the membrane of the skeletal muscle cell, leading to the opening of voltage-gated calcium channels and consequent muscle contraction, however without depending on neuronal depolarization, that is, a direct effect on the muscle. - PEMF and postpartum abdominal diastasis  During pregnancy, the abdominal muscles are stretched and separated extensively to accommodate the growing fetus.  However, the abdominal muscles often do not fully return to their original position and may remain separated after delivery.  Results by using PEMF protocol which obtained from magnetic resonance imaging showed an average fat reduction of 17% at one month and 20% after 3 months.  The authors also reported a mean increase in muscle thickness of 20.5% after 1 month  The patients' weight did not change significantly.  That study did not have a control group, but the initial results suggest a beneficial effect on fat reduction and muscle strengthening. Stimulating Protein Synthesis in Natural Systems  The body building may work via the electrical currents that flow through tissues when nerve and muscle membranes carry electrical impulses.  When action potentials go along the membrane, the currents pass around the muscle nuclei, the elements of the cell that contain the DNA, and the frequency of the repeated currents may signal the DNA that it should make certain kinds of protein.  There is an actual shift in the proteins, so that the changes in function reflect changes in composition of the muscle.  In control experiments, eliminating the nerves totally and just stimulating with electrodes, it is possible to show exactly the same effect. Fast stimulation will get a fast-type muscle protein and slow stimulation will get a slow type.  The electrical stimulation can change the composition of muscle.  We also have some preliminary data on muscle which suggest that magnetic stimulation causes changes in protein similar to the changes found in other cells that we have studied.  Stimulation with magnetic fields (0.3 mT "mli Tesla") changes the proteins  It appears possible to have the muscle make proteins that it was not originally intended to make.  Such stimulation would undoubtedly have been important in overcoming the disuse atrophy (muscle tissue loss) that normally occurs in immobilized limbs. - Changes in the Na, K-ATPase  The last subject I would like to talk about is the Na, K-ATPase, the “ion pump” enzyme.  The membrane Na, K-ATPase is activated by the binding of Na and K ions on opposite faces of the enzyme, and ion pumping occurs when ATP is split.  Enzyme function is also affected by very low levels of magnetic and electric fields.  The effects of EM fields on these processes suggest a way of transducing a signal across the membrane, since activation of the enzyme on the outer surface causes the enzyme to pump ions across the membrane at a different rate and influences the contents and function of the cell.  Since the enzyme is relatively simple, it has been possible to obtain certain insights to the mechanism by which external fields affect molecular function. The results indicate the following: (1) The effect of alternating current (a.c.> depends upon the basal level of enzyme activity. (Basal enzyme activity is related to conductivity within the molecule that coordinates ATP splitting with changes at the ion-binding sites on the two faces of the enzyme.) Under optimal conditions a.c. decreases the activity; when basal activity is lowered by any means (e.g. inhibitors), a.c. increases the activity. - (2) Both inhibition and stimulation by a.c. can be explained by a field-dependent increased binding of activating cations, because an increase in binding causes opposite effects in different ranges of activity. At optimal activity, increased ion concentration lowers activity. Below optimal activity, increased ion concentration increases enzyme activity. When the activity of the enzyme varies with the Na/K ratio, the effect of a.c. correlates directly with the basal enzyme activity. (3) Inhibition and stimulation have similar frequency dependences, with maximal effects at about 100 Hz. The frequency dependence is similar to the one calculated for ion concentration changes at membrane surfaces due to a.c., and varies with ionic properties at the binding sites (e.g. mobility, binding rate). (4) The effect of a.c. on Na,K-ATPase activity differs when imposed with electrodes or induced from EM fields (with a stray magnetic field present). The threshold for inhibition by induced a.c. is about an order of magnitude higher than for currents imposed through electrodes. This suggests that the electric and magnetic fields have opposing effects on the process. - -  The normal operation of the enzyme involves coordinated charge movements and these processes as well as the interaction between them are affected by EM fields.  Ionic currents in the aqueous phases due to the field change the ion binding (e.g. activation) at the enzyme surfaces and modulate the basal level of enzyme activity.  Charge movements within the enzyme due to ATP splitting are probably related to electron transport processes in mitochondria and may be influenced by the magnetic fields.  These charge movements probably have the same effect as gating charges in electrically excitable channels and can change the ion binding as well as open and/or close channels.  A coordinated sequence of these processes leads to active ion transport by the Na, H-ATPase. Effect of magnetism on the nervous system  The central and peripheral nervous systems both play a crucial role in regulation of the human body and neuromuscular control.  In addition, the brain is thought to have a significant effect in controlling or governing the body’s ability to perform at its peak exercise performance.  There have been a few studies that have shown that magnetic fields may have an effect on both the peripheral and central nervous systems. Peripheral nervous system  It has long been known that the nervous tissue responds to a magnetic field and this phenomenon has been used as an alternative to electrical nerve stimulation as it is painless.  Ye et al. conducted a study to investigate the effects of a static magnetic field on the neurons that mediate tail-flip escape behaviour in crayfish.  A permanent magnet (474G-4345G) was placed under the isolated nerve cord for a variable time period (20 sec to 3-h).  An intracellular electrode was implanted on the axon of the lateral giant neuron of the crayfish to record the evoked action potential and excitatory postsynaptic potential.  The exposure to the magnetic field increased the amplitude of the action potential and the magnitude of the excitatory postsynaptic potential.  These responses were dependent upon both the intensity and duration of the magnetic field exposure.  In a study of 10 healthy human volunteers, the nerve conduction velocity and excitability index (measured as the ratio of the amplitude of the sub-maximally evoked compound muscle action potential during or after magnetic exposure to that before exposure) was measured when the nerve was exposed to a static magnetic field of 1T for 15 sec.  In this study, there was no significant change in the nerve conduction velocity over the nerve segment exposed to the magnetic field.  However, the excitability was significantly increased during the magnetic exposure.  This effect was observed as early as 5sec after exposure and had disappeared by 3min after exposure.  It thus appears that in both humans and animals the excitability and sensitivity of peripheral nerves are increased when exposed to a magnetic field.  This may lead to enhanced muscle contraction and sensory feedback which could in turn enhance athletic performance. Figure-Electromagnetic induction can be used to control neural activity. (a) Schematic of the electromagnetic induction in the context of TMS. A butterfly coil is held over the head of a human and a pulsed current is applied, resulting in a rapidly increasing magnetic field that induces a current in the brain (from Wagner et al. 2007). (b) Examples of TMS coils, single and butterfly (from magstim.com). (c) Electromagnetic induction could be used to stimulate deep brain structures via implanted millimeter-scale solenoids (from Bonmassar et al. 2012). (d) Implanted devices may be powered using electromagnetic induction. This device may be implanted into an animal and rectifies the induced voltage from an externally applied alternating magnetic field into a DC current that can stimulate neural activity (from Freeman et al. 2017). Central nervous system  Magnetic fields have been shown to have an effect on both neurotransmitters and hormone levels in the central nervous system.  Weak magnetic fields up to 5470G have been shown to inhibit opioid peptide-mediated analgesia in the land snail Cepaea Nemoralis.  This response was non-linear, in that it was not directly proportional to either the amplitude or frequency of the applied magnetic field, suggesting that the possible mechanism was not that of an induced current phenomenon, but possibly due to a direct magnetic field detection mechanism, related to the resonance of the magnetic field.  In humans, pre-frontal transcutaneous magnetic stimulation has been shown to significantly increase serum thyroid-stimulating hormone levels.  This could increase the metabolic rate and have a positive effect on athletic performance.  In summary,  it appears that magnetic fields may alter neurotransmitter and hormone levels, in addition to increasing the excitability of the peripheral nerves.  The increased excitability may, in turn, result in improved or increased neuromuscular activity, in addition to the direct effects of the magnetic field on the musculoskeletal system, thus potentially affecting athletic performance Musculoskeletal system  The musculoskeletal system comprises the bone, connective tissue and skeletal muscle.  The effects of magnetic fields on these structures will be discussed in this section.  As there has been substantial research on the effects of magnetic fields on fracture healing, this will be discussed more fully in the section on medical conditions that are known to respond to a magnetic field. Muscle  There have been very few studies on the effects of magnetic fields on muscle tissue.  In one study, the effects of magnetic fields on the noradrenergic system in rat skeletal tissue was examined.  In this study, rats exposed to a 128 mT magnetic field for 1-h/day for 5 consecutive days had a significant increase in norepinephrine content in the gastrocnemius muscle (+25%, p