ArticleLiterature Review

Biomagnetism: A new tool in sport and exercise science

Taylor & Francis
Journal of Sports Sciences
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Abstract

Biomagnetometry is a non-invasive technique for detecting magnetic fields associated with the electrophysiology and magnetic susceptibility of body tissues. The aims of this paper are to provide a review of the discipline of biomagnetism and its measurement using a superconducting quantum interference device (SQUID), and to discuss the potential utility of this technique in sport and exercise science. A tutorial section is presented to provide an introduction to the theory and practical application of SQUID biomagnetometry. A review of the biomagnetism literature demonstrates the variety of previous biomagnetic investigations and suggests several potential applications of biomagnetometry in sport and exercise science. A discussion of these includes an assessment of the advantages of measuring biomagnetic fields as opposed to electrical potentials, with particular reference to the improved precision and accuracy of physiological source modelling using biomagnetic data. There is evidence to suggest that SQUID biomagnetometry would provide useful (and perhaps unique) information on functional, anatomical and physiological assessments in sport and exercise science. Further investigations of biomagnetometry in this discipline should focus on three main areas: cardiology, encephalography and neurology, and body composition assessment.

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... They carried out the first recordings of magnetic fields generated by a single skeletal muscle fiber and provided details concerning some cellular properties such as membrane capacitance and intracellular conductivity, which are notoriously helpful in basic physiology. Moreover, some authors also support the idea that measuring magnetic fields from this tissue can provide additional details concerning muscle gradation force mechanisms (Lewis, 2003), which is of utmost importance in clinical and sports applications. Thus, why so few studies? ...
... Even though there is clear evidence of the potential use of the MMG in skeletal muscle contraction analysis, there seems to also be methodological and cost constraints that still restrict a larger use and corroborate the lack of studies. For instance, considering that the earth's magnetic fields can reach values of the order of 10 −6 T and the typical magnetic urban noise can be on the order of 10 −7 T/(Hz) 1/2 , collecting the MMG signal with wound toroidal pickup coils may lead to the need of performing experiments in shielded rooms to avoid this larger background field (Lewis, 2003). Albeit Nantel and Pengelly (1990); Nantel and Pengelly (1991) reported the development of a wound toroidal pickup coil constituted by additional elements (ex: thin copperberyllium foil) and a method of digital filtering, both to improve the MMG signal noise ratio, no progress seems to be performed. ...
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It is well known that whenever a motor unit is recruited, electrical, and mechanical events are generated in skeletal muscle fibers. The first concerns to the action potentials propagating along the sarcolemma while the second is represented by dimensional changes that are interpreted as mechanical twitches. Both allow the regulation of muscle force production. Electrical and mechanical events can be indirectly measured by means of different transducers that can provide details concerning the underlying mechanisms of muscle contraction elicited under different conditions. Therefore, surface or indwelling electrodes, laser distance sensors, microphones, and accelerometers are examples of transducers commonly used by those who are interested in interpreting how muscles control joints and regulate force production, among other properties. Thus, electromyography (EMG) and mechano or vibro or acceleromyography (usually named MMG) are examples of well-established methods that are widely used today in basic, sports and clinical studies. However, another physical quantity, magnetic fields associated with the flux of ions across the active cells membranes, has been well reported in organs such as brain (Hari and Salmelin, 1997, 2012; Nevalainen et al., 2014) and heart (Geselowitz, 1979; Fenici et al., 2005; Leithauser et al., 2011) although much less frequently for the skeletal muscles. Interestingly, though, few previous authors have pointed out that the recording of magnetic fields of this tissue may help to improve our knowledge in respect of its physiology under normal and pathological conditions. As an example of these pioneers, we may mention the study conducted by Van Egeraat et al. (1990). They carried out the first recordings of magnetic fields generated by a single skeletal muscle fiber and provided details concerning some cellular properties such as membrane capacitance and intracellular conductivity, which are notoriously helpful in basic physiology. Moreover, some authors also support the idea that measuring magnetic fields from this tissue can provide additional details concerning muscle gradation force mechanisms (Lewis, 2003), which is of utmost importance in clinical and sports applications. Thus, why so few studies? What are the constraints, disadvantages and advantages of measuring magnetic fields from skeletal muscles? May it provide valuable and feasible data for a better understanding of skeletal muscle physiology?
... Presumably the most serious drawback, which can explain this dearth of data, is that the chances of obtaining meaningful, "clean" data during exercise of any intensity, but particularly during exercise of high intensity, are slim because of the rather extreme susceptibility of this method to movement and muscular artifacts (Petruzzello, Ekkekakis, & Hall, 2006;Thompson, Steffert, Ros, Leach, & Gruzelier, 2008). The high susceptibility to similar types of artifact and the high cost have also limited the use of magnetoencephalography (MEG) for investigating brain activity during acute exercise (Lewis, 2003). ...
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A general expression is developed for the quasi-static magnetic field outside an inhomogeneous nonmagnetic volume conductor containing internal electromotive forces. Multipole expansions for both the electric and magnetic fields are derived. It is shown that the external magnetic field vanishes under conditions of axial symmetry. The magnetic field for a dipole current source in a sphere is derived, and the effect of an eccentric spherical inhomogeneity is analyzed. Finally the magnetic dipole moment is calculated for a current dipole in a conducting prolate spheroid.
Article
Weak alternating magnetic fields outside the human scalp, produced by alpha-rhythm currents, are demonstrated. Subject ard magnetic detector were housed in a multilayer magnetically shielded chamber. Background magnetic noise was reduced by signal-averaging. The fields near the scalp are about 1 x 10-9 gauss (peak to peak). A course distribution shows left-right symmetry for the particular averaging technique used here.
Article
Nearly all metal dusts contain iron, which is one of the most common constituents in steel. Iron has one common ferrimagnetic oxide, magnetite. Although it is considered nontoxic, magnetite can be used as a tracer for more active and pathogenic constituents of inhaled dusts. After the chest area has been magnetized, magnetite produces a weak remanent magnetic field around the body. The field is measured and the amount of dust in the lungs is calculated on the assumption that the composition of lung dust is the same as that of airborne dust. This method has been applied to shipyard and stainless steel welders, to iron and steel workers, to stainless grinders, and to foundry workers. A clear magnetic signal was found for each subject. The maximum amount of lung dust found thus far was 10 g in the lungs of a stainless steel electrode welder. Referents showed signals of 0.5 mg magnetite or less.
Article
The direct-current magnetocardiogram, which shows the T-Q (baseline) shift, is used to clarify the cause of S-T depression induced by stress testing in the human heart. Measurements are made of the amount of baseline shift associated with the S-T depression. Results are presented of a well-documented patient, with typical coronary artery disease, undergoing a two-step exercise test. Before exercise, there was no S-T or baseline shift. During exercise, the S-T segment became depressed and the baseline segment was simultaneously elevated, at about 70% of the S-T amplitude. After termination of exercise, the baseline elevation disappeared somewhat more rapidly than the S-T depression. These results were consistent in repeated tests of this patient. Because the baseline shift is a reflection of an injury current, these results confirm the belief that exercise-induced S-T depression is mostly due to an injury current which is interrupted during the S-T interval. The baseline shift seen here is the first non-invasive measurement of an injury current in the human heart, and its presence and time-course generally agree with measurements in the animal heart. This work also confirms that the direct-current magnetocardiogram, although not practical for clinical purposes, is useful as a research tool.
Article
Magnetic susceptibility plethysmography (MSPG) is a non-invasive technique for detecting changes in the total cardiac volume. The MSPG signal is observed with a sensitive magnetometer when an external magnetic field is applied to the thorax. The signal is due to the differences in magnetic susceptibilities of intracardiac blood, heart muscle, and surrounding tissues. In this work the spatial distribution of the MSPG signal across the chest of normal subjects is presented. The ventricular ejection of blood was clearly exhibited in signals measured above the heart. Relatively large MSPG signals were also observed on the upper and lower thorax during the ventricular ejection, related to the blood flow in the aortic arch and in the abdominal aorta, respectively tively. The temporal behavior of the MSPG is compared with the results obtained from existing clinical methods. The origin of the MSPG signal is analyzed theoretically. When a fixed cardiac magnetic dipole model was used, overall change in cardiac volume as a function of time could be evaluated with reasonable accuracy when the proximity effect of the torso was taken into account.
Article
The theoretical basis for the relationship between the electric and magnetic fields of the heart is examined in terms of impressed currents, Jj, from a biologically active region imbedded in an Ohmic conductor. For quasistatic sources in a uniform conductor, it is shown that the problem of measuring the electrocardiogram or magnetocardiogram is an inverse problem rendered non-unique by the presence of so-called silent sources. An important class of sources, toroidal Jj, are silent electrically and not silent magnetically and these sources result in inherent differences between the information content of the electric and magnetic measurement techniques. A hypothetical example of cardiac activation departing from the conventional uniform double-layer model is presented to indicate that electrically silent sources cannot be ruled out a priori without careful magnetic measurements.
Article
Biomagnetism is essentially the study of the weak magnetic fields generated by biological organisms, in particular the human body. The human thorax is composed of a variety of tissues and organs of slightly different magnetic susceptibility. In an applied magnetic field (of the order of milliTeslas) these small differences in susceptibility lead to measurable field variations (of the order of nanoTeslas) at the body surface which may be of diagnostic value. Physiological processes such as cardiac activity, cardiac output, blood flow and respiratory related lung volume changes also contribute to the observed signal. In this study susceptibility 'maps' were obtained by measuring the magnetic field at several hundred points over the thorax. Results indicate that magnetic susceptibility mapping produces low-resolution images of internal body structures from which is should be possible to detect pathologies that cause alterations in tissue susceptibility.
Article
Superconducting Quantum Interference Devices (SQUIDs) can be used to detect neuromagnetic fields evoked in the peripheral and central nervous system. Up to now, such measurements had to be based on SQUIDs with a low critical temperature (Tc) requiring liquid helium cooling. Recent improvements in high-Tc SQUID technology relying on liquid nitrogen cooling led to a significant reduction in the system's noise level. Hare, first high-Tc recordings of weak neuromagnetic fields are demonstrated. In particular, along the entire somatosensory afferent pathway including peripheral nerves, brachial plexus and primary somatosensory neocortex evoked neuromagnetic activities were detected using conventional recording parameters for bandwidth and number of averages. This opens up a wide perspective for cost-effective high-Tc magnetometry in clinical neuroscience.
Article
Acute lesions of polarized membranes lead to slowly decaying ('near-DC') injury currents driven by the transmembrane resting potential gradient. Here we report the first recordings of injury-related near-DC magnetic fields from human nerve and muscle specimens in vitro using Superconducting Quantum Interference Devices (SQUIDs) operated in a conventional magnetically shielded room in a clinical environment. The specimen position was modulated sinusoidally beneath the sensor array by a non-magnetically fabricated scissors lift to improve the signal-to-noise ratio for near-DC fields. Depending on the specimen geometry the field patterns showed dipolar or quadrupolar aspects. The slow decay of human nerve and muscle injury currents was monitored for several hours from a distance of a few centimeters. Thus DC-magnetometry provides a sensitivity which might allow the remote detection of injury currents also in vivo.
Article
To develop new non-invasive ways of analyzing human skeletal muscle function, biomagnetic measurement was applied to the vastus lateralis and vastus medialis of 3 healthy adult males using a 64 channel superconducting quantum interference device system. Discharges from single motor units were detected by simultaneously recorded surface electromyography. Magnetic signals were averaged 64-158 times at zero-crossing timing in the surface electromyographic signal. Six motor units detected in the 3 subjects produced large magnetic fields with peak-to-peak amplitudes of 1-2 pT. Magnetomyographic isofield maps showed current sources arising from motor endplate regions and propagating in directions opposite to fiber ends. The absolute intensity of current moments in muscle fibers within motor units was estimated based on dipole fitting. The estimated moment was 23.9-114.3 nAm for repolarization dipole. Dividing these moment values by the typical dipole moment of 0.286 nAm in a single muscle fiber, the number of muscle fibers in motor unit was estimated to be 84-400. Magnetic recording may provide a new non-invasive way of analyzing and diagnosing human muscle function.
Article
Because optimal management of iron chelation therapy in patients with sickle cell disease and transfusional iron overload requires accurate determination of the magnitude of iron excess, a variety of techniques for evaluating iron overload are under development, including measurement of serum ferritin iron levels, x-ray fluorescence of iron, magnetic resonance imaging, computed tomography, and measurement of magnetic susceptibility. The most promising methods for noninvasive assessment of body iron stores in patients with sickle cell anemia and transfusional iron overload are based on measurement of hepatic magnetic susceptibility, either using superconducting quantum interference device (SQUID) susceptometry or, potentially, magnetic resonance susceptometry.
Article
We have developed an impedance magnetocardiogram (IMCG) system to detect the change of magnetic field corresponding to changes in blood volume in the heart. A low magnetic field from the electrical activity of the human heart--the so-called magnetocardiogram (MCG)--can be simultaneously detected by using this system. Because the mechanical and electrical functions in the heart can be monitored by non-invasive and non-contact measurements, it is easy to observe the cardiovascular functions from an accurate sensor position. This system uses a technique to demodulate induced current in a subject. A flux-locked circuit of a superconducting quantum interference device has a wide frequency range (above 1 MHz) because a constant current (40 kHz) is fed through the subject. It is shown for the first time that the system could measure IMCG signals at the same time as MCG signals.
Article
Magnetocardiography (MCG) is a novel, non-contact mapping technique to record cardiac magnetic field. We evaluated MCG criteria for myocardial ischemia in stress testing. Multichannel MCG over frontal chest was performed in 44 patients with coronary artery disease (CAD) and 26 healthy controls during supine bicycle exercise test. Of the 44 patients 16 had anterior, 15 posterior, and 13 inferior ischemia documented by coronary angiography and exercise thallium scintigraphy. ST amplitude, ST slope, T-wave amplitude, and ST-T integral were measured. The optimal sites for detecting the ischemia-induced changes on MCG were sought. The orientation of the magnetic field was also determined. The optimal sites for the decrease of ST slope, ST amplitude, T-wave amplitude, and ST-T integral were over the abdomen. The reciprocal increase of these parameters was found over the left parasternal area. The optimal sites were approximately the same for all patient groups. In single-vessel disease patients without previous myocardial infarction (MI), ST slope increase and ST elevation performed the best (area under the receiver operating characteristic curve 92% and 90%, respectively). In post-MI patients with triple-vessel disease the decrease of T-wave amplitude and ST slope performed the best (area under curve 91%, for both). The magnetic field orientation at ST segment performed equally well as the other ST parameters. In stepwise logistic regression analysis, by use of the presence of CAD as the dependent parameter, ST slope increase and ST peak gradient orientation entered the model. Various ST segment and T-wave parameters detect ischemia in MCG. ST amplitude performs especially well in non-MI patients with less severe CAD. In advanced CAD late development of T-wave amplitude might be more sensitive to ischemia than ST amplitude.
Article
The magnetocardiogram results from the detection of magnetic fields generated outside the body by electric current sources in the heart. Let the source current dipole moment per unit volume be Ji, the conductivity g , the electric potential V , and the electric field intensity E . Then it may be shown that the magnetic field H is given by either H = (1/4pi) int J^{i} times nabla(1/r) dv + sum_{i} int (g' - g")(E times dS_{i}/r) , or H = (1/4pi) int J^{i} times nabla(1/r) dv + sum_{i} int (g' - g")Vnabla(1/r) times dS_{i} , where the surface integral is over all surfaces S i separating regions of different conductivity, i.e., g ' and g ", and r is the distance from the point of measurement to the element of volume or surface. The magnetic dipole moment m is given by m = frac{1}{2} int r_{1} times J^{i}dv - frac{1}{2} sum_{i} int (g'-g")Vr_{1} times dS_{i} , where r 1 is a radius vector from an arbitrary origin.
  • Ahlfors S.P.
Biomagnetism: an interdisciplinary approach
  • T Katila
  • P J Karp