Laboratory Investigation of Specific and Placebo Effects of a Magnetic Bracelet on a Short Bout of Aerobic Exercise.

Szabo, Attila ; Szemerszky, Renata ; Gresits, Ivan 等

A placebo is an inert substance or intervention containing no active ingredients that could be accounted for an observed effect. The latter is due to the person's expectation that there is a causal link between the placebo and a desired outcome (Benedetti & Amanzio, 2013). According to the definition of Clark, Hopkins, Hawley and Burke (2000), the placebo effect is a favorable outcome triggered by an expectation. This expectation activates neuropsychobiological mechanisms in the brain that mediate the manifestation of an expected result (Benedetti & Amanzio, 2013; Benedetti, Mayberg, Wager, Stohler, & Zubieta, 2005).

The mind is selective--due to learning and experience--in that non-specific perceptual features (e.g. color) of the placebos affect the attitudes and expectations linked to their efficacy (Szabo, Berdi, Koteles, & Bardos, 2013). Placebos are used in medicine and pharmacology to study the so-called non-specific drug effects that are independent of the pharmacologically active ingredients of the medication. While placebo effects receive little attention beyond medicine, these effects were also noted in sports and exercise, alcohol drinking, caffeine use, etc. (e.g. Beedie, Coleman, & Foad, 2007; Beedie, Stuart, Damian, & Foad, 2006; Domotor, Szemerszky, & Koteles, 2014; Mendelson, McGuire, & Mello, 1984).

In sports, placebos appear to enhance athletic performance. A meta-analysis of 14 studies, revealed a homogeneous variance weighted mean effect size of .31 (Berdi, Koteles, Szabo, & Bardos, 2011). In line with Szabo (2013), a more recent meta-analysis, based on nine studies that investigated the psychological effects of training, concluded that about half of the reported psychological benefits are due to the placebo effect (Lindheimer, O'Connor, & Dishman, 2015). These effects may be psychological aids in sports and exercise. A recent survey shows that 53% of the athletes are ready to accept an unknown, but legitimate product from their coaches, and 67% of them would not mind a placebo-linked deception if that was effective in their performance (Berdi, Koteles, Hevesi, Bardos, & Szabo, 2015). Moreover, most (90%) of the elite sports coaches are aware of the placebo effect and at least 44% of them may use it regularly (Szabo & Muller, 2016).

In controlled exercise settings the placebo response was rarely studied. Wright et al. (2009) found that runners' performance increased by 6.5%, and that slower runners showed a stronger placebo effect after ingesting purported nutritional ergogenic aids. In another work, the placebo effect of caffeine on resistance exercise to failure was studied with 12 men (Duncan, Lyons, & Hankey, 2009). Performance was better when participants expected that they have ingested caffeine. Another study of 12 men, drinking either plain water (control), or a labeled performance enhancer drink (placebo), or fatigue inducing (nocebo) drink, showed a modest placebo effect in peak minute power incremental arm crank exercise (Bottoms, Buscombe, & Nicholettos, 2014).

There is a general assumption about the beneficial effects of the commercially available magnetic bracelets. Magnetic bracelet is a device used in a static magnetic field (SMF) therapy, which is in general applied via a permanent magnet attached to the skin. The results from basic as well as clinical research indicate certain biological effects of SMF therapy, that is, specific and non-specific effects may be also reasonable in case of magnetic bracelets. The biological effects of SMF application may occur within seconds through the modification of transmembrane ionic flux (Rosen, 2003). The most commonly measured physiological outcomes of SMF therapy studies were improvement in muscle strength and physical function (Chaloupka, Kang, & Mastrangelo, 2002; Schall, Ishee, & Titlow, 2003; Tis, Trinkaus II, Higbie, Johnson. & McCarty, 2000), muscle soreness post exercise (Borsa & Liggett, 1998; Mikesky & Hayden, 2005; Reeser et al., 2005), blood flow (Barker & Cain, 1985; Martel, Andrews, & Roseboom, 2002; Mayrovitz & Groseclose, 2005; Mayrovitz, Groseclose, Markov, & Pilla, 2001), and heart rate and blood pressure (Hinman, 2002). Changes of these physiological variables can affect physical performance; however, the results are contradictory. According to Colbert et al. (2009), most of the studies lack a sufficiently detailed description of SMF dosage and treatment parameters to characterize the SMF dose delivered to the target tissue, therefore the conclusions drawn from the reported results might be misleading.

Research has also shown that osteoarthritis pain has decreased when the sufferers wore a magnetic bracelet (Harlow et al., 2004), but the authors warned that it is unclear whether their findings are due to specific or non-specific (i.e., placebo) effects. Indeed, their results were challenged later by opposite findings (Richmond et al., 2009). In sports, the effect of a magnetic bracelet on balance was studied, but the results were negative (Moss Jr, 2013). There is an assumption among athletes and some scientific evidence indicates that SMF therapy may enhance blood flow (Barker & Cain, 1985; Martel et al., 2002; Mayrovitz & Groseclose, 2005; Mayrovitz et al., 2001). Therefore, magnetic bracelets might be able to support the aerobic sport performance. However, such an effect was not investigated to date.

Since there is a shortage in well controlled experimental studies in this area, it was suggested that researchers investigating the placebo effects in sports and exercise use a no-intervention control condition alongside placebo and experimental conditions (Beedie, 2007). This way, so-called non-specific (placebo, i.e. the difference between the placebo and the no-intervention condition) and specific (e.g., pharmacological, difference between the intervention and the placebo intervention condition) effects can be tested separately. In the current work we conducted a double blind placebo controlled laboratory experiment with a no-intervention condition to investigate the specific- and the placebo effects of a magnetic bracelet on a short-duration aerobic exercise performance. We expected that mass information about such products, and the associated expectations about their ergogenic effects, could shape people's motivation or perception of effort in exercise performance, which may result in a placebo effect. Moreover, a specific effect on aerobic performance was also considered plausible.

Methods

Participants

Following ethical approval from Eotvos Lorand university's Research Ethics Board, we have recruited research participants through internal advertisements. A total of 97 young athletes (regular participants in organized sports) showed interest in participating. They all signed an informed consent form. After baseline measurements, they were assigned to one of three groups by lot: a) magnetic bracelet group (n = 33), b) placebo bracelet group (n = 32), and c) a no-bracelet control group (n = 32). Their mean age was 22.02 (SD = 1.82) years and 49 of them were men while 48 were women. Their mean BMI was 22.34 (SD = 2.73), and mean VO, max (as assessed by a graded exercise test on cycle ergometer which was connected to an ergospirometer device (Fitmate PRO, Cosmed Sri, Rome, Italy)) was 41.05 (SD= 10.65) ml/kg/min.

Materials

The magnetic bracelet. We used a commercially available bracelet (Trion:Z Active; http://www.trionz.com/). It contained two opposite polarity magnets with a maximal surface magnetic flux density of+19.67 mT and -20.75 mi, respectively. The spatial gradient between the two magnets was 10.0 mT/mm. The mapping of the magnetic field was performed by a 3D step motor robot system (3-Axis Positioning Table, Arrick Robotics, Tyler, TX, USA), and Lakeshore Model 420 Gaussmeter (LakeShore Cryonics, Westerville, OH, USA) with transverse Hall-probe and a non-metallic positioning system. The sensitivity of the probe is 1 micro Tesla to 3 Tesla. Figure 1 shows the mappings with scanning resolution of 2 mm (x, y, z axis) above the inner surface of the bracelet containing the magnets over 30x20 mm scanning area. The bracelet was worn on the dominant arm and the maximal distance of the magnets from the surface of the skin was 4 mm. The placebo bracelet was prepared by replacing the magnets with two pieces of lead of identical weight.

The aerobic exercise test. A modified version of the Astrand Cycle submaximal exercise test (Beam & Adams, 2011) was designed to rely predominantly upon the aerobic system to supply the majority of the ATP to the muscles. Tests were performed on a cycle ergometer (Daum Ergo Bike Premium 8i bicycle ergometer, manufactured by Daum Electronic GmbH, Furth, Germany). The goal of the test was to pedal at maximal RPM at body weight X 1.65W power during the 6 minutes of exercise. A 2-minutes warm-up before the test and a 2-minute cool down after the test was performed at a power setting of 20 Watts. Heart rate was registered using a Firstbeat Team system (heart rate monitoring belt and Sports software, Firstbeat Technologies Ltd., Jyvaskyla, Finland). Physical performance was recorded by "ergowin premium pro" software (Daum Electronic GmbH, Furth, Germany).

Questions. Three questions were asked. One was adopted to assess one's expectation regarding the efficacy of the magnetic bracelet in enhancing performance on a 5-point Likert-type scale (1: marked decrease in performance, 2: mild decrease, 3: no change, 4: mild improvement, 5: marked improvement). Another question was used to assess perceived change in performance following the second exercise on a similar 5-point Likert-type scale. A third 5-point Likert-type scale was used to gauge perceived fatigue after exercise (both at baseline and post-intervention).

Procedure

BMI and V02 max measurements took place the week before the experiment. On the day of the experiment, participants completed the baseline cycle-exercise test and answered the question concerning perceived fatigue. After that they were randomly assigned to one of the three groups (bracelet, placebo, control). Participants in the magnetic bracelet and placebo bracelet groups read a description on the beneficial effects of the magnetic bracelet on sports performance. Both groups received identical written information based on the manufacturer's description that is available from the authors upon request. Participants in the control group received no additional information. They simply fulfilled the required control measure for potential practice effects that may occur in repeated measures interventions. Subsequently, participants in the magnetic and placebo groups were asked to rate their expectations for the second exercise. After a 40-minute rest period, the participants exercised with magnetic- or placebo bracelet or without a bracelet, repeating the earlier exercise protocol. One of the investigators was present and supervised the intervention. Distance, average and maximum speed were read by this experimenter from the software connected to the ergometer. After exercise, participants indicated the extent to which they expected that their performance has changed, as well as their perceived fatigue.

Results

Groups were homogeneous with respect to gender ([chi square](2) = 563,p = .755), age (Kruskal-Wallis [chi square](2) =1.521, p = .468), BMI (one-way ANOVA, F(2) = 0.499p =.609), and V[O.sub.2] max (one-way ANOVA, F(2) = .891 p = 414).

Data were analyzed with a mixed model multivariate repeated measures analysis of variance (group (3) x time (2) x dependent measures (6): distance, average speed, maximal speed, average heart rate, maximal heart rate, and perceived fatigue). This test yielded a statistically significant multivariate main effect for time (Wilks' Lambda = .666, F(6,80) = 6.69,p < .001, partial [[eta].sup.2]p = .334), but no group by time interaction. Further examination of the multivariate results with univariate tests has revealed that the differences between baseline and post-intervention exercise in all six dependent measures were statistically significant p [less than or equal to] .001, showing consistently a greater value during the second assessment (see Table 1).

An independent samples t-test showed that there were no statistically significant differences between the magnetic and the placebo group with regard to the expectation regarding their second exercise performance (bracelet group: 3.40 [+ or -] 0.497; placebo group: 3.23 [+ or -] 0.496; t(67) = 1.378. p > .05). We also tested the difference from the neutral midpoint value of the Likert-type scale (3, reflecting "no change"), to determine whether overall expectations were in the positive or negative direction, and whether they differed statistically significantly from the "no-change" score. This test was performed with a one-sample t-test that yielded a statistically significant positive expectation effect (3.32 [+ or -] 0.50, t(68) = 5.30, p < .001, effect size (d) = .64). Nevertheless, nearly two-thirds (65%) of the participants did not expect that wearing the magnetic (real magnet: 19 out of 32 and placebo: 22 out of 31) bracelet would change their performance. Finally, we tested the effects of the magnetic bracelet in the sub-group who expected that their performance could be improved by the wearing of the bracelet (positive expectation group, n = 31), but the results were again negative.

Perceived change in performance was tested using a one-way ANOVA for the three groups. According to the results (F(2,95) = 0.061, p > .05), no differences, among the three groups, were found.

Discussion

Similar to the results of a recent study with hologram wristbands (Brazier, Sinclair, & Bottoms, 2014), the main results of this laboratory investigation show that an externally applied magnetic bracelet does not influence short duration aerobic exercise performance in a specific or non specific (placebo) way. Concerning the specific effects, a simple explanation is that the examined magnetic bracelet is weak or useless for enhancing one's aerobic exercise performance. This explanation is plausible, because the magnet was at a relatively great distance from the target tissues, which works hard during exercise and needs enhanced blood supply (i.e., muscles in the legs, arms, and trunk). However, a limitation of the adopted aerobic exercise might be the short duration of the test (6 min). It is conceivable that the emergence of physiological effects of SMF takes more time, and/or that the possible physiological effects (e.g., enhanced blood flow at the level of the entire organism) at the adopted SMF strength does not take place, regardless of exercise duration. As for the non-specific (placebo) effects, one explanation is that the duration of exercise in the current laboratory study might have been too short (6-min). It is conjectured that a placebo effect is more likely to surface in longer duration exercise performances (Wright et al., 2009). However, Brazier et al. (2014) even with a complex set of physical tasks were unable to show any beneficial effects of the hologram wristband on performance. Another possible explanation is that participation in a non-competitive laboratory exercise may not be sufficiently challenging to the participants to generate a placebo effect. Finally, a third explanation is that the ergogenic aid used in this study was accompanied by insufficient expectations to trigger a placebo effect. Indeed, two thirds of the participating athletes did not expect that their performance would be altered by the magnetic bracelet. These findings could be related to the perceived efficacy of the placebo agents, which is an explanation that agrees with the already reported differences in expectations concerning the physical characteristics and the target response to placebos (Szabo et al., 2013).

In the recent past, a "technical" revolution occurred in sports, which has changed the athletes' view on the training aids. According to media information, top performance is paired with the adoption of sophisticated aids (e.g., functional food and drinks, magnetic bracelets, kinesiotapes) and monitoring devices of physical performance and physiological reactions (Koteles, Domotor, Berkes, & Szemerszky, 2015). In placebo-controlled research, these aids and devices usually are not more efficient than their respective placebos, and their effects are not supported by empirical work (Broatch, Petersen, & Bishop, 2014; Chang et al., 2013; Heneghan et al., 2012; Sawkins, Refshauge, Kilbreath, & Raymond, 2007; Vercelli, Ferriero, Bravini, & Sartorio, 2013). However, it is known from empirical research that the placebo effects can enhance sports performance (Berdi et al., 2011). Therefore, these aids and devices could mobilize one's mental resources which could translate into better sport or exercise performance. However, there should be a strong expectation--or even convincement --to observe a placebo effect.

Conclusions

A double blind, placebo controlled, laboratory study has shown that the wearing of a magnetic bracelet during a short-term aerobic exercise did not affect either specific / actual, or non-specific / placebo exercise performance. However, a better performance over time, that was unrelated to either intervention, has emerged in the inquiry, possibly due to experience and/or self-challenges (i.e. "I can do better next time" motivation of the subjects) or to Hawthorne effects (McCarney et al., 2007) common in this research design. Most (2/3) of the participants did not expect that their performance would be altered by the magnetic bracelet. However, even those who expected that the wearing of the magnetic bracelet could improve their exercise performance did not exhibit improved performance as a result of wearing the bracelet. It is very much likely that low-challenge (low-subjective-importance) artificial research conditions do not give rise to placebo effects, which may influence one's exercise performance, or--in accord with past research with holographic bracelets--the bracelet used in the current study could have been perceived as too "weak" to yield a detectable placebo effect.

Attila Szabo, Renata Szemerszky, Zsuzsanna Domotor

ELTE Eotvos Lorand University, Budapest, Hungary

Ivan Gresits

National Public Health Institute, Department of Non-Ionizing Radiation, Budapest, Hungary

and

Ferenc Koteles

ELTE Eotvos Lorand University, Budapest, Hungary

Address correspondence to: Attila Szabo, Ph.D., Professor, Institute of Health Promotion and Sport Sciences, Faculty of Education and Psychology, ELTE Eotvos Lorand University. 1117 Budapest, Bogdanfy u. 10, Hungary. E-mail: [email protected]

Author Note

This research was supported by the New National Excellence Program of the Ministry of Human Capacities (for Zs. Domotor), the Janos Bolyai Research Scholarship of the Hungarian Academy of Sciences (for R. Szemerszky), and the Hungarian National Scientific Research Fund (OTKA K 109549).

References

Barker, A. T., & Cain, M. W. (1985). The claimed vasodilatory effect of a commercial permanent magnet foil: results of a double-blind trial. Clinical Physics and Physiological Measurement: An Official Journal of the Hospital Physicists 'Association, Deutsche Gesellschaft Fur Medizinische Physik and the European Federation of Organisations for Medical Physics, 6(3), 261-263.

Beam, W. C., & Adams, G. M. (2011). Exercise Physiology Laboratory Manual (6th ed.). New York: McGraw-Hill.

Beedie, J. C. (2007). Placebo effects in competitive sport: Qualitative data. Journal of Sports Science and Medicine, 6, 21-28.

Beedie, J. C., Coleman, D. A., & Foad, A. J. (2007). Positive and Negative Placebo Effects Resulting From the Deceptive Administration of an Ergogenic Aid. International Journal of Sport Nutrition & Exercise Metabolism, 77(3), 259-270.

Beedie, J. C., Stuart, M. E., Damian, A. C., & Foad, J. A. (2006). Placebo Effects of Caffeine on Cycling Performance. Medicine and Science in Sports and Exercise, 38(12) 2159-2164.

Benedetti, F., & Amanzio, M. (2013). Mechanisms of the placebo response. Pulmonary Pharmacology & Therapeutics, 26(5), 520-523. http://doi.org/10.1016/]. pupt.2013.01.006

Benedetti, F., Mayberg, H. S., Wager, T. D., Stohler, C. S., & Zubieta, J. K. (2005). Neurobiological Mechanisms of the Placebo Effect. The Journal of Neuroscience 25(45) 10390-10402.

Berdi, M., Koteles, F., Hevesi, K., Bardos, G., & Szabo, A. (2015). Elite athletes' attitudes towards the use of placebo-induced performance enhancement in sports. European Journal of Sport Science, 15(4), 315-321. http://doi.org/10.1080/17461391.2014.955126

Berdi, M., Koteles, F., Szabo, A., & Bardos, G. (2011). Placebo Effects in Sport and Exercise: A Meta-Analysis. European Journal of Mental Health, 6(2), 196-212. http://doi.org/10.5708/EJMH.6.2011.2.5

Borsa, P. A., & Liggett, C. L. (1998). Flexible Magnets Are Not Effective in Decreasing Pain Perception and Recovery Time After Muscle Microinjury. Journal of Athletic Training, 33(2), 150-155.

Bottoms, L., Buscombe, R.. & Nicholettos, A. (2014). The placebo and nocebo effects on peak minute power during incremental arm crank ergometry. European Journal of Sport Science, 14(A), 362-367. http://doi.org/10.1080/17461391.2013.822564

Brazier. J., Sinclair, J., & Bottoms, L. (2014). The effects of hologram wristbands and placebo on athletic performance. Kineziologija, 46(1), 109-116.

Broatch, J. R., Petersen, A., & Bishop, D. J. (2014). Postexercise Cold-Water Immersion Benefits Are Not Greater than the Placebo Effect. Medicine and Science in Sports and Exercise, http://d0i.org/10.1249/MSS.0000000000000348

Chaloupka, E. C.. Kang, J., & Mastrangelo, M. A. (2002). The effect of flexible magnets on hand muscle strength: a randomized, double-blind study. Journal of Strength and Conditioning Research /National Strength & Conditioning Association, 16(1), 33-37.

Chang, H.-Y., Cheng, S.-C., Lin, C.-C., Chou, K.-Y., Gan, S.-M., & Wang, C.-H. (2013). The Effectiveness of Kinesio Taping for Athletes with Medial Elbow Epicondylar Tendinopathy. International Journal of Sports Medicine 34,1003-1006. http://doi.org/10.1055/S-0033-1333747

Clark, V. R., Hopkins, W. G., Hawley, J. A., & Burke, L. M. (2000). Placebo effect of carbohydrate feedings during a 40-km cycling time trial. Medicine and Science in Sports and Exercise, 32, 1642-1647.

Colbert, A. P., Wahbeh. H., Harling, N., Connelly, E., Schiffke, H. C., Forsten, C., ... King, V. (2009). Static Magnetic Field Therapy: A Critical Review of Treatment Parameters. Evidence-Based Complementary and Alternative Medicine, 6(2), 133-139. http://doi.org/10.1093/ecam/neml31

Domotor, Z., Szemerszky, R., & Koteles, F. (2014). Subjective and objective effects of coffee consumption--caffeine or expectations? Acta Physiologica Hungarica, 101(3), 1-9. http://doi.org/10.1556/APhysiol.101.2014.012

Duncan, M. J., Lyons, M., & Hankey, J. (2009). Placebo effects of caffeine on short-term resistance exercise to failure. International Journal of Sports Physiology and Performance, 4(2), 244--253.

Harlow. T., Greaves, C., White, A.. Brown, L., Hart, A., & Ernst, E. (2004). Randomised controlled trial of magnetic bracelets for relieving pain in osteoarthritis of the hip and knee. BMJ (Clinical Research Ed.), 329(7480), 1450-1454. http://doi.org/10.1136/bmj.329.7480.1450

Heneghan, C., Howiek, J., O'Neill, B., Gill, P. J., Lasserson, D. S., Cohen, D., ... Thompson, M. (2012). The evidence underpinning sports performance products: a systematic assessment. British Medical Journal Open, 2(4). http://doi.org/10.1136/bmjopen-2012-001702

Hinman, M. R. (2002). Comparative effect of positive and negative static magnetic fields on heart rate and blood pressure in healthy adults. Clinical Rehabilitation, 16(6), 669-674.

Koteles, F., Domotor, Z., Berkes, T., & Szemerszky, R. (2015). Polar Ownlndex is not a reliable indicator of aerobic training status. Acta Phvsiologica Hungarica, 102(A), 419-427. http://doi.Org/10.1556/036.102.2015.4.9

Lindheimer, J. B., O'Connor, P. J., & Dishman, R. K. (2015). Quantifying the placebo effect in psychological outcomes of exercise training: a meta-analysis of randomized trials. Sports Medicine (Auckland, N.Z.), 45(5), 693-711. http://doi.org/10.1007/s40279-0150303-1

Martel, G. F., Andrews, S. C., & Roseboom, C. G. (2002). Comparison of static and placebo magnets on resting forearm blood flow in young, healthy men. The Journal of Orthopaedic and Sports Physical Therapy, 52(10), 518-524. http://doi.org/10.2519/jospt.2002.32.10.518

Mayrovitz, H. N., & Groseclose, E. E. (2005). Effects of a static magnetic field of either polarity on skin microcirculation. Microvascular Research, 69(1-2), 24-27. http://doi.org/10.1016/j.mvr.2004.11.002

Mayrovitz, H. N., Groseclose, E. E., Markov, M., & Pilla, A. A. (2001). Effects of permanent magnets on resting skin blood perfusion in healthy persons assessed by laser Doppler flowmetry and imaging. Bioelectromagnetics, 22(1), 494-502.

McCarney, R., Warner, J., Iliffe, S., Van Haselen, R., Griffin, M., & Fisher, P. (2007). The Hawthorne Effect: a randomised, controlled trial. BMC Medical Research Methodology, 7(1), 30-35. doi: 10.1186/1471-2288-7-30

Mendelson, J. H., McGuire, M., & Mello, N. K. (1984). A new device for administering placebo alcohol. Alcohol (Fayetteville, N.Y.), 1(5), 417-419.

Mikesky, A. E., & Hayden, M. W. (2005). Effect of static magnetic therapy on recovery from delayed onset muscle soreness. Physical Therapy in Sport, 6(4), 188-194. http://doi.org/10.1016/j.ptsp.2005.08.004

Moss Jr, R. (2013). The effect of Magnetic and Hologram bracelets on balance (Masters Thesis). California University of Pennsylvania, Pennsylvania. Retrieved from http://libweb.calu.edu/thesis/Moss_cup_6020M_10095.pdf

Reeser, J. C., Smith, D. T., Fischer, V., Berg, R., Liu, K., Untiedt, C., & Kubista, M. (2005). Static magnetic fields neither prevent nor diminish symptoms and signs of delayed onset muscle soreness. Archives of Physical Medicine and Rehabilitation, 86(3), 565-570. http://doi.org/10.1016/j.apmr.2004.04.025

Richmond, S. J., Brown, S. R., Campion, P. D., Porter. A. J. L., Moffett, J. A. K., Jackson, D. A., ... Taylor, A. J. (2009). Therapeutic effects of magnetic and copper bracelets in osteoarthritis: a randomised placebo-controlled crossover trial. Complementary> Therapies in Medicine, 77(5-6), 249-256. http://doi.org/10.1016/j.ctim.2009.07.002

Rosen, A. D. (2003). Mechanism of action of moderate-intensity static magnetic fields on biological systems. Cell Biochemistiy and Biophysics, 39(2), 163-173. doi: 10.1385/CBB:39:2:163

Sawkins. K., Refshauge, K., Kilbreath, S., & Raymond, J. (2007). The placebo effect of ankle taping in ankle instability. Medicine and Science in Sports and Exercise, 39(5), 781-787. http://doi.org/10.1249/MSS.0b013e3180337371

Schall, D. M., Ishee, J. H., & Titlow, L. W. (2003). Effect of magnetic therapy on selected physical performances. Journal of Strength and Conditioning Research / National Strength & Conditioning Association, 77(2), 299-302.

Szabo, A. (2013). Acute psychological benefits of exercise: Reconsideration of the placebo effect. Journal of Mental Health, 22(5), 449-455. http://doi.org/10.3109/09638237.2012.734657

Szabo, A., Berdi, M.. Koteles, F., & Bardos, G. (2013). Perceptual characteristics of nutritional supplements determine the expected effectiveness in boosting strength, endurance, and concentration performances. International Journal of Sport Nutrition and Exercise Metabolism, 23(6), 624-628.

Szabo, A., & Muller, A. (2016). Coaches' attitudes towards placebo interventions in sport. European Journal of Sport Science, 76(3), 293-300. http://doi.org/10.1080/17461391.2015.1019572

Tis, L. L., Trinkaus II, M. D., Higbie, E. J., Johnson, B. F., & McCarty, F. A. (2000). Effects of magnets on concentric and eccentric isokinetic force production of the quadriceps group. Isokinetics and Exercise Science, 5(4), 217-221.

Vercelli, S., Ferriero, G., Bravini, E., & Sartorio, F. (2013). How much is Kinesio taping a psychological crutch? Manual Therapy, 18(3), ell. http://doi.org/10.1016/j.math.2012.10.008

Wright, G., Porcari, J. P., Foster, C. C., Felker, H., Koshololek, A., Otto, J., ... Udermann, B. E. (2009). Placebo effects on exercise performance. Gundersen Lutheran Medical Journal, 6(1), 3-7.

Caption: Figure 1. Magnetic flux density (absolute values and the x, y and z components) in mT close to the surface of bracelet containing the magnets (approximately at 1 cm from the magnets). Table 1 Means and standard deviations (in brackets) of six dependent measures during two assessments. The p values and the effect sizes (partial ETA squared ([[eta].sup.2])) betM'een the two conditions are shown in the last two columns. Dependent measure Baseline Intervention value value Distance (km) 3.73 (0.56) 3.81 (0.57) Average Speed (km/h) 37.39 (5.63) 38.12 (5.69) Maximal Speed (km/h) 43.71 (7.44) 45.54 (7.62) Average heart rate (bpm) 150.32 (15.09) 153.80 (15.27) Maximal heart rate (bpm) 170.22 (15.19) 172.15 (15.55) Dependent measure p, partial [[eta].sup.2] Distance (km) =.007, .074 Average Speed (km/h) =.007, .073 Maximal Speed (km/h) <001, .127 Average heart rate (bpm) =.002, .096 Maximal heart rate (bpm) =.047, .040