Becero et al. BMC Veterinary Research (2020) 16:10 https://doi.org/10.1186/s12917-020-2233-x RESEARCH ARTICLE Open Access Capacitive resistive electric transfer modifies gait pattern in horses exercised on a treadmill Mireya Becero1, Aritz Saitua1,2, David Argüelles2, Antonia Lucía Sánchez de Medina3,4, Cristina Castejón-Riber1,5, Cristina Riber1,3 and Ana Muñoz1,3* Abstract Background: Capacitive resistive electric transfer (CRET), a radiofrequency at 448 kHz, resulted in increased superficial and deep temperature and hemoglobin saturation, faster elimination of metabolic and inflammatory products and enhanced sport performance in humans. This research aims to investigate whether the application of CRET affects the locomotor pattern in horses and to assess whether an accumulative effect appears when two CRET sessions are applied two consecutive days. Methods: Nine horses were subjected to two CRET sessions applied in both right and left sides of neck, shoulder, back and croup. The horses were exercised on a treadmill, at walk and at trot, before CRET application and at 2, 6 and 12 h after. A second CRET session was applied next day, and the animals were evaluated again at the same times (i.e. at 26, 30 and 36 h after the first session). Between 5 and 7 days later, the same horses were subjected to a sham procedure and they were evaluated in the same times as in the CRET experiment. During treadmill exercise, locomotor parameters were measured with a triaxial accelerometer fixed in the pectoral region and in the sacrum midline. Results: The sham procedure did not affect any of the accelerometric variables studied. CRET applications resulted in greater total powers, which resulted in absolute increased dorsoventral, mediolateral and longitudinal powers. However, a reduction in dorsoventral power expressed as a percentage of total power was found. Stride regularity increased. The greater total power resulted in longer stride length and because the velocity was kept fixed on the treadmill, stride frequency decreased. An accumulative effect of CRET application was only found in stride length and frequency. Conclusions: It appears that CRET is a useful technique to enhance power and to elongate the stride at defined walk and trot velocities. The effect of these changes on performance should be studied for horses competing in different sport disciplines. Keywords: Accelerometry. Capacitive resistive electric transfer. Exercise. Horse. Locomotion. Performance * Correspondence: pv1mujua@uco.es 1 Equine Sport Medicine Center CEMEDE, School of Veterinary Medicine, University of Córdoba, Campus Universitario de Rabanales, 14004 Cordoba, España 3 Department of Animal Medicine and Surgery, School of Veterinary Medicine, University of Córdoba, Campus Universitario de Rabanales, 14004 Cordoba, España Full list of author information is available at the end of the article © The Author(s). 2020 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Becero et al. BMC Veterinary Research (2020) 16:10 Page 2 of 12 Introduction previous studies in humans, the main objective of the current Gait profile is a key to performance in the equine athlete. study was to investigate whether the application of CRET Locomotor characteristics associated with performance might affect the accelerometric parameters measured at walk have been measured by accelerometry in horses compet- and at trot in sound horses exercised on a treadmill. A sec- ing in technical disciplines, such as dressage and show- ond objective was to evaluate whether an accumulative effect jumping, but also in competitions highly dependent on appears when two CRET sessions were applied on two con- running economy, as galloping and trotting racing and en- secutive days. The proposed hypotheses were: 1) that the ap- durance events [1–3]. Accelerometric analysis is a kinetic plication of CRET in horses would change total power (TP) method which measures instantaneous changes of vel- and would also lead to a redistribution of TP in the three ocity, which appears as a consequence of applying a force body axes (dorsoventral, longitudinal and mediolateral); 2) on a solid. The use of the accelerometers allows the calcu- probably because of these changes, stride spatiotemporal var- lation of the acceleration on the surface on which the de- iables would also be modified and 3) an accumulative effect vice is fixed. This acceleration vector is proportional to would appear when the sessions of CRET are applied on two the force applied to the body in which the accelerometer consecutive days. has been fixed. Consequently, its measurements provide a convenient and practical, easy and low-cost way to study Material and methods the kinetics of a body in motion [4]. Horses Good galloping racehorses exhibit greater propulsive A total of 9 healthy adult horses, with a mean age of power (PP) and lower dorsoventral displacement (DVD) 11.2 ± 3.4 years (range 8–15), 2 Angloarabians and 7 Span- of the center of gravity in order to reduce the energy ish horses, of both sexes (4 mares and 5 geldings), with a cost of locomotion [2, 4]. In addition, they are character- mean body weight of 430 ± 89 kg (range 390–495 kg) and ized by an optimal stride length (SL), because maximum with different level fitness, were enrolled in the study. The gallop velocity (V) is primarily determined by SL. Fur- two Anglo-Arabian horses were in active training and ther, the most successful racing horses are those that are regularly participated in endurance competitions of 120 able to increase their stride frequency (SF) to the highest km with excellent results. Of the 7 Spanish horses, 6 were values at the end of the race, after achieving the longest used for Exercise Physiology and Equine Sports Medicine SL [2]. High PP activity and large DVD values are desir- classes at the Equine Sports Medicine Center of the Veter- able gait characteristics in dressage horses, and positive inary School. They were exercised between 3 and 5 days significant correlations with better scores in performance per week. Exercise consisted in 1 h of walking in a walker tests have been described [3]. Fatigued endurance horses and exercises on a treadmill (10 min walk, 10–15 min trot, commonly experienced a reduction in V and SL [5]. As 3–5 min canter and 1–2 min gallop). The other Spanish a consequence of these relationships, different strategies horse competed in prix St George level dressage. have been implemented in the horse in order to improve The horses were subjected to clinical and lameness these accelerometric characteristics. exams and complete hematological and biochemical tests In human beings, physical therapies based on electro- were carried out, including fibrinogen determination. magnetic stimulation, specifically, a system of capacitive Lameness exam was performed by two experienced resistive electric transfer (CRET) has been demonstrated equine clinicians (D.A. and C.R.) and consisted in evalu- to affect spatiotemporal biomechanical parameters after ation in motion (straight line, circling, inclines, hand and exercise [6]. CRET delivers radiofrequency energy that soft surfaces…), hoof testers and joint flexion tests. Diag- passes between active and inactive electrodes, resulting nostic nerve and joint blocks were not considered neces- in increased deep (muscle) temperature and blood flow sary because only horses with a score of 0 over 5, and flexibility and improved hemoglobin saturation [7, according to the American Association of Equine Practi- 8]. Duñabeitia et al. [6] considered that these physio- tioners AAEP were selected for the research. Further, only logical changes would promote a more efficient running horses without laboratory alterations were included. pattern, as shown by the lower SF and longer SL reached The horses were adapted to treadmill exercise, because in their study after application of a CRET session in rec- treadmill exercise was included in their training program reational runners. These authors speculated that the ob- during at least the last 9 months before starting the re- served improvements in biomechanics would likely lead search. They were kept in medium-size paddocks and to improvements in performance during competitions in during the 2 days before and the 2 days after the study, these runners, even though this affirmation was not the horses were not trained. proved in their research. To the authors’ best knowledge, the effects of this type of Device for capacitive resistive electric transfer CRET radiofrequency on the locomotor pattern of the horse have Indiba® Animal Health Vet905 (Indiba® S.A., Barcelona, not been evaluated yet. Based on the results obtained in Spain) was used for CRET application. This device Becero et al. BMC Veterinary Research (2020) 16:10 Page 3 of 12 operates at a frequency of 448 kHz. A rigid circular me- portable weather station (RTGR328N, Oregon Scientific tallic electrode was used as the active electrode and a TM wireless®). large flexible rectangular metallic plate (size: 200 × 260 mm) was used as the inactive electrode. The device de- Accelerometric data acquisition livers radiofrequency energy in two modes at the active Accelerometric data were obtained with a portable gait electrode: capacitive (CAP) and resistive (RES). The CAP analyzer (Equimetrix, Centaure-Metrix®), consisting of electrode has a polyamide coating that acts a dielectric three orthogonal accelerometers that measure acceler- medium, insulating its metallic body from the skin sur- ation along the three body axes, a data logger and a sci- face, generating superficial heat, near the skin. The RES entific software program (Equimetrix-Centaure 3D®) for electrode is uncoated and therefore, radiofrequency en- processing acceleration signals. The recorder collected ergy travels directly through the body into the inactive data continuously at a sampling rate of 100 Hz. The electrode, generating heat in the deeper parts of the accelerometer was placed in the caudal part of the ster- body. The inactive electrode was placed on the sternum num, between the right and left pectoralis ascendens during CRET applications. muscles at the level of the girth, as previously recom- mended [1–4]. In this location (PECT), the device is near the body center of gravity, with a good stability CRET sessions against the body of the horse and provides more infor- A manufactured supplied conductive gel was applied as a mation about the acceleration parameters in the fore- coupling medium between the electrode and the skin sur- limb. Also, the accelerometric device was attached to the face. Each session had a total duration of approximately 50 skin over the midline of the sacrum region (SML), using min. The protocol of the session combined the use of both an adhesive tape in order to better analyze changes in types of electrodes, CAP and RES following the manufac- the hindlimbs. The same researcher (M. B.) positioned turer’s indications. CRET was applied in both the right and the transducer in all occasions. More information about left side of the animals, starting in one of the sides randomly. this device has been previously reported [1–4]. Bilateral epaxial musculature of the neck (M. trapezius, M. rhomboideus, M. splenius, M. serratus ventralisthoracics Accelerometric parameters and M. brachiocephalicus), shoulder (M. supraspinatus, M. Three groups of accelerometric parameters were studied. infraspinatus, M. triceps brachii), back (M. longissimus dorsi) Stride energetic parameters include dorsoventral power and croup (M. gluteus superficialis, M. gluteus medius, M. DVP (W/kg), propulsion power PP (W/kg) and medio- biceps femoris, M. semitendinous, M. semimembranous) lateral power MLP (W/kg). DVP is the integral of the were treated in a craniocaudal direction. Each session con- power spectrum obtained by Fast Fourier Transform- sisted of 2 min of CAP therapy and 4 min of RES therapy. ation (FFT) from the dorsoventral acceleration signal Potency was fixed at 40% for CAP therapy and at 30% for and measures limb suspension and loading activity. PP RES therapy. Due to the greater development of the muscu- measures the craniocaudal or longitudinal activity and it lature of the croup, the application time of the RES therapy is the integral of the power spectrum obtained by FFT was 5 min. from the longitudinal acceleration signal. PP measures the amount of acceleration and deceleration along the Treadmill exercise, TE longitudinal axis. MLP is the side-to-side activity, calcu- Horses were exercised on a high-speed treadmill (Mus- lated as the integral of the power spectrum obtained by tang 2000, Kagra®), at a constant V for each horse. Each FFT from the lateral acceleration signal. MLP therefore, horse was subjected to the V used habitually for training measures the amount of acceleration and deceleration (walk: 1.5 ± 0.1 m/s; trot: 3.4 ± 0.2 m/s). Exercises were along the lateral axis. The sum of the three powers made with the treadmill without inclination, for at least (DVP, PP and MLP) reflects body kinetics [10] and it 5 min each gait, in order to ensure that the horses had represents the total power TP (W/kg). DVP, PP and time to relax and to adopt a regular locomotor pattern, MLP were also expressed as percentages of TP in order as recommended [9]. The accelerometric parameters to assess whether a redistribution of TP appears. Add- were measured between the 5th and the 7th min of exer- itionally, dorsoventral displacement (DVD) of the center cise at each gait. of gravity was also calculated. The TE were performed in the following times: base- Stride coordination parameters included regularity, line (BL), before application of CRET; at 2, 6 and 12 h REG (dimensionless) and symmetry SYM (dimension- after the first session of CRET; and at 2, 6, and 12 h after less). REG measures the acceleration pattern similarity the second session of CRET (i.e., 26, 30 and 36 h after of successive strides in a period of time and SYM mea- the first session). In the same times, data of ambient sures the similarities between left and right acceleration temperature and relative humidity were obtained with a patterns. Stability (STAB) of the gait was calculated as Becero et al. BMC Veterinary Research (2020) 16:10 Page 4 of 12 the sum of SYM and REG and it was used as an indica- were carried out in two acclimatized rooms (exam and tor of gait quality [11]. treadmill rooms), significant differences in the environ- Stride spatiotemporal parameters included V (m/s), SF (cy- mental conditions when comparing the times of the cles/s or Hz) and SL (m). V was fixed in the treadmill and SF accelerometric recordings and the days of the study were and SL values were calculated by the accelerometer. not found. Mean ambient temperature of 25.3 °C (range Each accelerometric parameter was calculated in each time 22.1 and 29.4 °C) and relative humidity of 45% (range 32 of recording (7 times), for the two gaits (walk and trot), and and 51%) were recorded. with the accelerometer device in the two positions, PECT Significant differences were not found between the dif- and SML. In each of these recordings, 6 measurements were ferent times during the sham experiment, for any of the calculated for each horse and the data are presented as the accelerometric parameters. means of these 6 measurements. Changes in energetic parameters Sham procedure Total power increased from BL at walk and at trot, with the Between 5 and 7 days after the CRET experiment, the same accelerometer located in both anatomical regions (Fig. 1). At 9 horses were subjected to a sham procedure, consisting in walk, the highest TP values were observed at 36 h (acceler- two sessions with the same CRET device without operating, ometer in PECT position) and at 26 h (accelerometer in on the same anatomical areas and during the same period of SML position). At trot, the highest values were found at 2 h time (50 min). Accelerometric recordings were carried out at (PECT) and at 12 and 36 h (SML), as shown in Fig. 1. the same times as the CRET group (BL, at 2, 6, 12, 26, 30 Figure 2 presents data of MLP and PP. In most of the and 36 h after the first session). recording times, MLP and PP values increased signifi- cantly from the BL values. An accumulative effect ap- Statistics peared to happen in MLP and PP at walk, with the Analysis of data was made with a commercial statistic accelerometer in both anatomical positions. Such accu- software (Statistica v. 12.0 for windows). A Shapiro-Wilk mulative effect did not appear to happen in MLP at trot test was used to assess for normality. As W value was with the accelerometer in PECT position neither in PP lower than 0.05, the normality hypothesis was rejected, at trot, with the accelerometer in SML (Fig. 2). and non-parametric methods were used. A Friedman Dorsoventral power increased at walk, reaching the test was carried out in order to evaluate significant dif- maximum values at 12 h with the accelerometer in ferences between times of accelerometric recording in PECT position and at 2 h and 26 h with the accelerom- both experiments (sham and CRET). When significant eter in SML (Fig. 3). Similarly, there was an increase in differences were detected, a post-hoc test was performed DVD at walk, at 36 h with the accelerometer in PECT (Wilcoxon rank test). The comparisons between sham and from 2 to 36 h with the accelerometer in SML. On and CRET experiments in each of the times were made the contrary, there was a decrease in DVP, together a with a Mann-Whitney test. non-significant trend (p < 0.067) towards a reduction in The differences in power percentages in the three cor- DVD at trot with the accelerometer in PECT (Fig. 3). poral axes were evaluated with the Q of Cochran. Corre- Percentages of the distribution of TP in the three body lations between the different variables at the walk and at axes (PML, PP and DVP) for the accelerometer fixed in the trot were calculated with a Spearman Rank R Correl- PECT and SML are presented for the walk and trot in ation, considering the data obtained at all the measure- Tables 1 and 2 respectively. At walk, DVP% decreased at ments’ times. 26 h with the accelerometer in PECT and at 36 h with All these tests were performed separately for the two the accelerometer in SML. MLP% decreased at 6 h but gaits and for the two locations of the accelerometric de- increased at 36 h with the accelerometer in PECT, vice (PECT and SML). Comparisons between gaits and whereas PP% increased at 26 h with the accelerometer in locations of the accelerometric device were out of the PECT and at 2, 26 and 36 h with the accelerometer in scope of the research and were not carried out. Signifi- SML (Table 1). At trot, DVP% decreased at 2, 30 and 36 cant differences in ambient temperatures and relative h with the accelerometer in PECT. MLP% increased at 2 humidity were evaluated with a repeated measures h with the accelerometer in PECT and PP% increased at ANOVA. Values of P < 0.05 were considered significant. 36 h with the accelerometer in PECT but decreased at 36 h with the accelerometer in SML (Table 2). Results None of the horses showed any negative side effects dur- Changes in coordination parameters ing the study. On the contrary, a relaxion state was Coordination parameters are presented in Tables 3 and 4 for noted during the application of CRET, with head lower- walk and trot respectively. REG at walk increased from BL at ing, fall of the lower lip… Because all the procedures 26, 30 and 36 h with the accelerometer in PECT and at 30 Becero et al. BMC Veterinary Research (2020) 16:10 Page 5 of 12 Fig. 1 Mean values and standard deviations of total power at walk (a) and trot (b), with the accelerometer in the sternum, PECT (●) and in sacrum midline, SML (▲), at defined times after the first session of capacitive resistive electric transfer session compared to a sham procedure (Values with different letters are significantly different from each other between times of accelerometric recordings) P < 0.05. (lowercase letters indicate significant differences with the accelerometer in the sternum; capital letters indicate significant differences with the accelerometer in the sacrum midline) Becero et al. BMC Veterinary Research (2020) 16:10 Page 6 of 12 Fig. 2 Mean values and standard deviations of mediolateral power at walk (a) and trot (b) and propulsion power at walk (c) and trot (d) with the accelerometer in the sternum, PECT (●) and in sacrum midline, SML (▲), at defined times after the first session of capacitive resistive electric transfer session compared to a sham procedure (Values with different letters are significantly different from each other between times of accelerometric recordings) P < 0.05. (lowercase letters indicate significant differences with the accelerometer in the sternum; capital letters indicate significant differences with the accelerometer in the sacrum midline) and 36 h with the accelerometer in SML. The only significant Correlations change in this parameter at trot was observed at 30 h with Correlations between the different parameters are pre- the accelerometer in PECT, showing values higher than BL. sented in Table 5. SYM showed a reduction from BL at walk at 6, 12 and 36 h with the accelerometer at SML and at 6 and 26 h at trot with Discussion the accelerometer in PECT. Significant changes in STA were The current research assessed the effect of CRET, a RF not found (Tables 3 and 4). at 448 kHz, on the locomotor pattern of horse evaluated on a treadmill. We confirmed two of the three proposed hypotheses in our study. The first hypothesis was that a Changes in stride spatiotemporal parameters session of CRET would modify TP and would redistrib- Changes in SF and SL are shown in Fig. 4. Significant ute this TP in the three body axes. We found that the decreases in SF and increases in SL were found for the application of CRET resulted in an increased TP and two gaits and with the accelerometer in PECT and SML. consequently, the absolute values of the powers in the SF and SL decreased and increased respectively during three body axes rose, except for DVP% at trot with the the second day (times 26, 30 and 36 h) compared with accelerometer in PECT. Furthermore, when these values the first day (2, 6 and 12 h). The exception of these re- were examined as percentages of TP, a reduction in sults was the SL at trot with the accelerometer at SML, DVP%, together with an increase in PP% and in MLP%, that did not experience any significant change (Fig. 4). were detected. Our second hypothesis was that, changes Becero et al. BMC Veterinary Research (2020) 16:10 Page 7 of 12 Fig. 3 Mean values and standard deviations of dorsoventral power at walk (a) and trot (b) and dorsoventral displacement at walk (c) and trot (d) with the accelerometer in the sternum, PECT (●) and in sacrum midline, SML (▲), at defined times after the first session of capacitive resistive electric transfer session compared to a sham procedure (Values with different letters are significantly different from each other between times of accelerometric recordings) P < 0.05. (lowercase letters indicate significant differences with the accelerometer in the sternum; capital letters indicate significant differences with the accelerometer in the sacrum midline) in TP would affect the stride spatiotemporal variables. In friction forces of charged molecules, such as proteins and this way, we found that CRET led to an increase in SL ions. This increase in temperature has been described at a together with a reduction in SF. The third hypothesis, superficial level (10–15 mm, which is believed to reflect however, could not completely been confirmed. We hy- the state of the subcutaneous adipose tissue and fascia), pothesized that an accumulative effect of the CRET but also at a deeper level (20–30 mm, which is supposed would be observed when two sessions of CRET were ap- to reflect the state of the muscle layer) [7, 8, 13]. The mild plied on two consecutive days. Although an accumula- elevation of the temperature achieved in these studies was tive effect was found in some of the studied variables enough to accelerate metabolic activity and to improve (SF, SL), such effect was not clear in others (TP, DVP, local circulation. It is plausible to assume that these same MLP, PP). effects would have occurred in the horses in the present investigation, leading to an improvement in muscle func- Changes in energetic parameters tion and hence the significant increase in TP. However, Previous studies performed in human beings have shown the above mentioned authors conducted sessions of 15 that CRET exerts diverse thermophysiological responses min of duration (5 min capacitive; 10 min resistive), which have been considered the basis of its therapeutic ef- whereas in our study a shorter time of application was fects [12, 13]. The main underlying mechanism is consid- used (2 min capacitive; 4–5 min resistive), even CRET was ered to be the rise in temperature associated with applied in greater anatomical areas in the horses, and electromagnetic energy, which triggers oscillation and therefore, affecting more muscles. Becero et al. BMC Veterinary Research (2020) 16:10 Page 8 of 12 Table 1 Mean and standard deviation of the percentages of dorsoventral (DVP), propulsion (PP) and mediolateral (MLP) powers at walk, with the accelerometer in the pectoral region and in the sacrum midline at defined times after a sham procedure and after the first session of capacitive resistive electric transfer session (Values with different letters are significantly different from each other between times of accelerometric recordings) P < 0.05 Time DVP% MLP% PP% DVP% %MLP PP% Accelerometer located in the pectoral region (PECT) SHAM procedure After CRET application BL 24.82 ± 7.10 a 41.69 ± 4.91 a 33.49 ± 5.25 a 23.43 ± 6.12 a 41.61 ± 4.91 a 34.96 ± 5.40 a 2h 23.50 ± 6.17 a 41.75 ± 5.01 a 34.75 ± 5.49 a 22.85 ± 7.28 a 40.01 ± 4.85 a 37.14 ± 8.96 a a a a a b 6h 24.03 ± 6.77 41.78 ± 4.43 34.19 ± 6.39 24.57 ± 5.69 37.65 ± 4.51 37.78 ± 7.33 a 12 h 23.62 ± 6.53 a 42.39 ± 4.83 a 33.99 ± 5.50 a 22.34 ± 6.12 a 42.93 ± 4.58 a 34.73 ± 6.07 a a a a b a 26 h 23.78 ± 8.52 41.62 ± 4.99 34.60 ± 6.39 20.98 ± 6.22 38.45 ± 4.46 40.57 ± 6.64 b 30 h 25.72 ± 7.36 a 42.82 ± 4.72 a 31.46 ± 8.58 a 22.85 ± 6.11 a 39.48 ± 6.02 a 37.67 ± 5.27 a a a a a b 36 h 24.54 ± 6.56 43.09 ± 5.54 32.37 ± 5.95 21.05 ± 7.41 43.37 ± 4.25 35.48 ± 3.98 a Accelerometer located in the sacrum midline (SML) SHAM procedure After CRET application a a a BL 36.31 ± 7.12 31.06 ± 9.59 32.63 ± 8.17 35.80 ± 7.96 a 31.44 ± 9.68 a 32.76 ± 8.09 a a a a a a 2h 35.99 ± 7.13 31.27 ± 9.63 32.74 ± 8.29 32.62 ± 3.19 29.87 ± 6.15 37.51 ± 6.22 b 6h 36.01 ± 8.13 a 31.44 ± 9.65 a 32.55 ± 8.02 a 33.21 ± 4.49 a 30.11 ± 6.92 a 36.68 ± 6.14 a a a a a a 12 h 35.89 ± 8.12 31.48 ± 9.68 32.63 ± 8.08 33.02 ± 3.96 31.55 ± 4.03 35.43 ± 6.53 a 26 h 35.92 ± 7.91 a 31.40 ± 9.82 a 32.68 ± 8.13 a 30.92 ± 6.32 b 30.89 ± 6.93 a 38.19 ± 8.96 b a a a a a 30 h 35.50 ± 7.06 31.06 ± 10.3 33.44 ± 9.30 34.23 ± 4.30 29.32 ± 6.17 36.45 ± 4.52 a 36 h 35.22 ± 7.12 a 31.45 ± 10.2 a 33.33 ± 8.29 a 30.85 ± 5.05 b 31.72 ± 6.21 a 37.43 ± 5.58 b Table 2 Mean and standard deviation of the percentages of dorsoventral (DVP), propulsion (PP) and mediolateral (MLP) powers at trot, with the accelerometer in the pectoral region and in the sacrum midline at defined times after a sham procedure and after the first session of capacitive resistive electric transfer session (Values with different letters are significantly different from each other between times of accelerometric recordings) P < 0.05 Time DVP% MLP% PP% DVP% MLP% PP% Accelerometer located in the pectoral region (PECT) SHAM procedure After CRET application a a a BL 47.80 ± 10.7 26.50 ± 8.87 25.70 ± 6.38 48.44 ± 9.04 a 25.51 ± 5.23 a 26.05 ± 4.130 a a a a b b 2h 48.47 ± 9.16 26.25 ± 8.70 25.28 ± 5.40 37.74 ± 14.70 32.14 ± 4.35 30.12 ± 12.85 a 6h 48.76 ± 9.50 a 26.32 ± 8.89 a 24.94 ± 5.47 a 40.64 ± 15.85 a 28.11 ± 6.70 a 31.25 ± 11.11 a a a a a a 12 h 49.00 ± 9.21 26.28 ± 8.80 24.72 ± 5.20 38.41 ± 13.74 28.11 ± 6.70 33.48 ± 18.06 a 26 h 49.30 ± 8.18 a 26.28 ± 8.78 a 25.42 ± 5.14 a 40.40 ± 17.90 a 26.90 ± 7.54 a 32.70 ± 12.28 a a a a b a 30 h 48.85 ± 8.64 26.38 ± 8.89 24.77 ± 5.37 34.67 ± 12.88 30.72 ± 4.81 34.61 ± 11.70 a 36 h 48.82 ± 8.71 a 26.36 ± 8.90 a 24.82 ± 5.24 a 32.19 ± 12.88 b 29.61 ± 5.52 a 38.20 ± 13.08 b Accelerometer located in the sacrum midline (SML) SHAM procedure After CRET application BL 59.85 ± 7.09 a 20.93 ± 5.71 a 19.22 ± 5.39 a 59.83 ± 7.17 a 20.65 ± 5.85 a 19.52 ± 5.44 a 2h 59.99 ± 7.13 a 22.19 ± 11.8 a 17.82 ± 5.56 a 62.45 ± 7.29 a 18.24 ± 7.07 a 19.31 ± 3.63 a a a a a a 6h 59.65 ± 7.14 20.94 ± 5.62 19.41 ± 5.44 61.20 ± 6.06 19.04 ± 4.57 19.76 ± 4.81 a 12 h 60.37 ± 7.18 a 21.47 ± 9.08 a 18.16 ± 5.44 a 59.10 ± 6.61 a 20.96 ± 7.15 a 19.94 ± 6.51 a a a a a a 26 h 60.15 ± 7.05 21.93 ± 10.4 17.92 ± 5.40 62.27 ± 7.40 21.13 ± 8.66 16.60 ± 4.62 a 30 h 60.21 ± 7.11 a 21.79 ± 11.9 a 18.00 ± 5.45 a 60.78 ± 3.98 a 19.56 ± 5.54 a 19.86 ± 4.72 a a a a a a 36 h 60.64 ± 4.92 20.28 ± 5.41 19.08 ± 4.71 58.96 ± 5.70 23.83 ± 7.29 17.21 ± 6.36 b Becero et al. BMC Veterinary Research (2020) 16:10 Page 9 of 12 Table 3 Mean and standard deviation of the stride regularity (REG), symmetry (SYM) and stability (STA) at walk, with the accelerometer in the pectoral region and in the sacrum midline at defined times after a sham procedure and after the first session of capacitive resistive electric transfer session (Values with different letters are significantly different from each other between times of accelerometric recordings) P < 0.05 Time REG SYM STA REG SYM STA Accelerometer in the pectoral region (PECT) SHAM procedure After CRET application BL 244.3 ± 51.37 a 170.1 ± 57.21 a 414.4 ± 73.97 a 244.2 ± 51.03 a 176.3 ± 54.29 a 420.8 ± 89.96 a 2h 252.1 ± 54.22 a 169.6 ± 47.21 a 421.7 ± 81.03 a 257.8 ± 63.33 a 176.4 ± 52.49 a 434.2 ± 97.85 a a a a a a 6h 246.4 ± 48.88 175.9 ± 57.91 422.3 ± 77.16 246.6 ± 59.25 187.9 ± 57.49 434.5 ± 82.50 a 12 h 249.3 ± 53.56 a 170.0 ± 48.88 a 419.3 ± 74.86 a 256.0 ± 63.74 a 183.8 ± 62.56 a 439.7 ± 93.91 a a a a b a 26 h 253.1 ± 54.42 176.4 ± 55.44 419.3 ± 74.86 264.3 ± 55.54 177.1 ± 64.03 441.4 ± 91.78 a 30 h 243.9 ± 54.79 a 176.6 ± 51.45 a 429.5 ± 71.56 a 265.8 ± 53.75 b 168.7 ± 45.32 a 434.4 ± 79.42 a a a a b a 36 h 245.4 ± 50.45 173.3 ± 54.23 418.7 ± 77.51 262.1 ± 58.48 186.6 ± 49.70 448.7 ± 94.73 a Accelerometer in the sacrum midline (SML) SHAM procedure After CRET application a a a BL 348.8 ± 66.08 236.4 ± 45.97 585.2 ± 75.05 305.4 ± 51.76 a 243.9 ± 59.63 a 555.1 ± 99.99 a a a a a a 2h 350.6 ± 60.82 217.6 ± 46.63 568.2 ± 81.12 337.0 ± 58.75 225.4 ± 53.47 562.4 ± 81.47 a 6h 343.7 ± 55.30 a 217.4 ± 42.92 a 561.1 ± 73.47 a 331.1 ± 66.72 a 202.3 ± 41.74 b 533.4 ± 72.15 a a a a a b 12 h 340.6 ± 60.10 232.1 ± 49.36 572.7 ± 76.82 337.0 ± 51.18 195.5 ± 42.32 532.4 ± 62.83 a 26 h 343.9 ± 61.55 a 223.3 ± 41.90 a 567.2 ± 82.80 a 331.1 ± 75.43 a 218.8 ± 50.82 a 549.8 ± 93.12 a a a a b a 30 h 342.5 ± 58.73 206.3 ± 44.94 548.8 ± 73.34 345.7 ± 52.16 223.1 ± 47.69 568.8 ± 82.19 a 36 h 348.4 ± 54.77 a 220.4 ± 50.36 a 568.8 ± 80.17 a 356.1 ± 40.80 b 192.1 ± 41.52 b 548.2 ± 51.42 a Table 4 Mean and standard deviation of the stride regularity (REG), symmetry (SYM) and stability (STA) at trot, with the accelerometer in the pectoral region and in the sacrum midline at defined times after a sham procedure and after the first session of capacitive resistive electric transfer session (Values with different letters are significantly different from each other between times of accelerometric recordings) P < 0.05 Time REG SYM STA REG SYM STA Accelerometer in the pectoral region (PECT) SHAM procedure After CRET application a a a BL 374.2 ± 50.68 240.6 ± 54.07 614.8 ± 80.87 377.4 ± 59.31 a 252.2 ± 48.15 a 620.6 ± 89.98 a a a a a a 2h 374.5 ± 45.52 243.7 ± 56.20 618.2 ± 62.25 369.5 ± 50.75 252.4 ± 61.14 621.9 ± 74.39 a 6h 376.3 ± 41.18 a 253.7 ± 46.41 a 630.0 ± 80.97 a 375.6 ± 38.66 a 227.5 ± 30.76 b 603.1 ± 53.45 a a a a a a 12 h 378.5 ± 42.04 242.8 ± 50.25 621.2 ± 71.72 381.2 ± 46.76 240.9 ± 50.02 622.0 ± 73.09 a 26 h 373.6 ± 40.42 a 243.8 ± 50.65 a 617.3 ± 56.30 a 369.6 ± 51.13 a 229.4 ± 52.99 b 598.9 ± 83.47 a a a a a a 30 h 376.1 ± 41.78 249.5 ± 42.82 625.6 ± 56.60 368.0 ± 54.11 239.8 ± 63.50 607.8 ± 99.57 a 36 h 371.1 ± 43.33 a 253.7 ± 55.77 a 624.8 ± 68.10 a 370.8 ± 43.57 a 239.1 ± 48.74 a 609.9 ± 71.01 a Accelerometer in the sacrum midline (SML) SHAM procedure After CRET application BL 366.9 ± 42.80 a 208.2 ± 50.97 a 575.1 ± 67.65 a 337.7 ± 55.80 a 236.1 ± 61.27 a 573.8 ± 88.38 a 2h 360.7 ± 41.54 a 214.6 ± 51.10 a 575.3 ± 61.50 a 365.7 ± 59.27 a 208.8 ± 38.46 a 574.5 ± 80.22 a a a a a a 6h 363.8 ± 38.81 214.9 ± 48.18 578.7 ± 61.12 349.5 ± 48.49 211.0 ± 42.31 560.5 ± 62.54 a 12 h 360.1 ± 39.26 a 218.6 ± 43.07 a 578.7 ± 59.58 a 371.6 ± 36.63 a 208.1 ± 54.77 a 579.8 ± 71.38 a a a a a a 26 h 363.4 ± 38.54 227.9 ± 51.39 591.3 ± 62.60 342.6 ± 59.44 215.1 ± 73.22 557.6 ± 82.15 a 30 h 368.4 ± 33.36 a 227.1 ± 42.40 a 595.5 ± 64.26 a 375.4 ± 42.57 b 224.1 ± 44.46 a 599.5 ± 62.52 a a a a a a 36 h 363.2 ± 32.24 225.3 ± 50.64 588.5 ± 61.46 368.8 ± 32.78 213.9 ± 41.41 582.7 ± 54.46 a Becero et al. BMC Veterinary Research (2020) 16:10 Page 10 of 12 Fig. 4 Mean values and standard deviations of stride frequency at walk (a) and trot (b) and stride length at walk (c) and trot (d) with the accelerometer in the sternum, PECT (●) and in sacrum midline, SML (▲), at defined times after the first session of capacitive resistive electric transfer session compared to a sham procedure (Values with different letters are significantly different from each other between times of accelerometric recordings) P < 0.05. (lowercase letters indicate significant differences with the accelerometer in the sternum; capital letters indicate significant differences with the accelerometer in the sacrum midline) The beneficial effects of a slight elevation in muscle temperature before exercise on muscle functionality dur- ing exercise have been reported in human athletes [14, Table 5 Spearman rank order correlations between energetic 15], being one of the reasons for warming and stretching stride parameters (total power and power distribution in the before exercise. Unfortunately, in the current research three body axis) and, coordination parameters and stride we have documented the direct consequence of the ap- spatiotemporal parameters in 9 horses after two sessions of plication of CRET technique on muscle power (in- capacitive resistive electric transfer applied on two consecutive creased TP) and on locomotor pattern, but reliable days. a Correlation coefficient significant at P < 0.05. (TP: total hypotheses about its mechanisms of action cannot be power; DVP: dorsoventral power; PP: propulsion power; MLP: provided. mediolateral power; DDV: dorsoventral power) The increase in TP caused by CRET led to an increase TP DVP PP MLP DDV in power in the three body axes in the horses. Despite Symmetry 0.296 0.315 0.216 0.730 0.284 this affirmation, different findings were observed when Regularity 0.541 0.592 0.422 0.430 0.537 evaluating the changes in the percentage of the three Stability 0.340 0.505 0.315 0.590 0.433 powers. CRET resulted in a deviation of TP towards the Stride length 0.887 a 0.707 0.917 a 0.690 0.491 longitudinal and mediolateral axes at the expense of the dorsoventral axis. In our opinion, these findings might Stride frequency 0.825 a 0.735 0.765 0.732 0.136 be partially due to the fact that the horses were exercised Becero et al. BMC Veterinary Research (2020) 16:10 Page 11 of 12 on a treadmill. In an early study, Buchner et al. [9] de- [18, 19]. In our study, the changes in the locomotor pat- scribed that the vertical displacement of the stride was tern were maintained for at least 12 h and in some cases smaller in horses exercised on a treadmill compared to (SL and SF), they even changed more intensely after the asphalt. More recently, we have reported that DVD and second session of CRET. DVP were lower in horses exercised on a treadmill com- A limitation of our study was that we were unable to pared to a soft surface (training arena) and on a hard measure skin and muscle temperature in response to CRET, surface (asphalt) [16]. The horses of the current report that would have provided relevant information regarding the presented a reduction in DVP%, but DDV increased, al- mechanism of action of this technique. Superficial beit only at the walk. The combined consideration of temperature can be measured with infrared thermography, these results seems to reflect that the decrease in DVP% but we could not measure it because the skin was moistened, was due to a redistribution of the TP rather than to a and a conductive gel was applied between the electrode and real reduction of DVP. An investigation of the CRET ef- the skin surface. Muscle temperature measurements would fects on these accelerometric parameters evaluated on a have required implantable thermistor probes inserted into training surface will be concluded soon and might pro- the muscles. However, this technique was not used because vide an explanation on whether this power redistribution electromagnetic field could have directly heated the measure- was due to the effect of CRET or on the contrary, it was ment device, leading to erroneous temperature recordings. a consequence of the locomotor pattern on the In addition, some of the horses included in the research treadmill. belonged to owners that would not have approved the use of invasive methods. Furthermore, the invasive procedure Changes in coordination parameters would trigger an inflammatory response which potentially REG is an accelerometric specific variable referring to might affect the local physiological activity. stride to stride variability, SYM refers to the similarity We chose to carry out the study on a treadmill be- between left and the right phases of the stride and STA cause of the higher standardization of the exercise in is described as an indicator of gait quality [4]. The in- terms of surface and speed compared to a track. How- crease in REG observed mainly at walk was compatible ever, it is well established that equine locomotion signifi- with a more homogenous locomotor pattern. In fact, cantly differs between treadmill and track [9, 20]. lameness is characterized by a reduction in REG [17]. Possibly treadmill exercise affected DVP, PP and DVD, The reason for the reduction experimented by SYM is and in addition, the animals could not freely choose the unknown. velocity during exercise. It is interesting to speculate that, whether the animals had freely chosen the exercise Changes in stride spatiotemporal parameters velocity, the locomotor changes associated with CRET One of the most consistent response to CRET in the could have been even more intense. This aspect deserves current research was the increase in SL and conse- future research. quently, the reduction in SF, as the treadmill velocity remained invariable. These results agreed with those re- Conclusions ported by Duñabeitia et al. [6], who described an in- Our study demonstrated that application of CRET on crease in step length and a reduction in step frequency the horse significantly modifies the accelerometric pat- in recreational runners after CRET. These changes were tern of the horse evaluated on a treadmill, leading to in- attributed to the thermophysiological responses caused creased muscle power, longer SL and lower SF. Further, by CRET. In our study, and according to the correlation the observed modifications (increases of TP, DVP, PP, analysis, the main determinants of the longer SL were MLP, DVD, SL), according to previous studies per- the increases in TP and particularly in PP. formed in different equine athletes, such as racing thor- In addition, a summing effect appeared to happen in the oughbreds [2], jumpers [1] and dressage [3], appear to second session of CRET in these parameters, since SL was relate to better sport performances. However, the evalu- longer on the second day (26, 30 and 36 h after the first ation of the effects of these locomotor changes on per- session) compared to the first day (2, 6 and 12 h). The formance was beyond the scope of this article, but they duration of the increased temperature and other physio- should be studied in a next future for horses competing logical effects caused by CRET have not been defined in different sport disciplines. exactly, and skin, superficial and deep temperature, blood Abbreviations flow and hemoglobin saturation, have been measured up BL: Baseline; CAP: Capacitive; CRET: Capacitive resistive electric transfer; to only to 30 min post-application [7, 8, 13]. Moreover, DVD: Dorsoventral displacement; FFT: Fast Fourier transformation; several in vitro studies have shown that CRET application MLP: Mediolateral power; PECT: Pectoral location; PP: Propulsive power; REG: Stride regularity; RES: Resistive; SF: Stride frequency; SL: Stride length; also exerts non-thermal effects, consisting of a modulation SML: Midline of the sacrum region; STAB: Stride stability; SYM: Stride of cellular activity and changes in membrane transport symmetry; TE: Treadmill exercise; TP: Total power; V: Velocity Becero et al. BMC Veterinary Research (2020) 16:10 Page 12 of 12 Acknowledgements haemoglobin saturation and tissue temperature. Int J Hyperth. 2017;33(6): The authors would like to express their gratitude to Indiba® S.A. (Barcelona, 696–702. Spain) for providing their device for the study. We are also in debt with the 8. Yokota Y, Tashiro Y, Suzuki Y, Tasaka S, Matsushita T, Matsubara K, Kawagoe owners of the horses that kindly accepted the inclusion of the animals in M, Sonoda T, Nakayama Y, Hasegawa S, Aoyama T. Effect of capacitive and this research. resistive electric transfer on tissue temperature, muscle flexibility, and blood circulation. J Novel Physiother. 2017;7(1):1000325. Authors’ contributions 9. Buchner HH, Savelberg HH, Schamhardt HC, Merkens HW, Barneveld A. AM and DA conceived and designed the study. MB, AS and ALSM performed Kinematics of treadmill versus overground locomotion in horses. Vet Q. the experiments with the horses. CCR performed data treatment and 1994;16(S2):87–90. statistics. AM and CR wrote the paper. All authors were involved in critical 10. Paquet JM, Auvinet B, Chaleil D, Barrey E. Analyse des troubles de la marche reading and approval of the submitted version. par une méthode accélérométrique dans la maladie de Parkinson. Rev Neurol. 2003;159:786–9. 11. López-Sanromán FJ, Gómez-Cisneros D, Varela del Arco M, Santiago Funding Llorente I, Santos González M. The use of low doses of acepromazine as an There is not funding for this research. aid for lameness diagnosis in horses: an accelerometric evaluation. Vet Comp Orthop Traumatol. 2015;28(5):312–7. Availability of data and materials 12. Kumaran B, Watson T. Skin thermophysiological effects of 448 kHz Data and materials are available from the corresponding author upon capacitive resistive monopolar radiofrequency in healthy adults: a reasonable request. randomised crossover study and comparison with pulsed shortwave therapy. Electromagn Biol Med. 2015;37(1):1–12. Ethics approval 13. Kumaran B, Watson T. Thermal build-up, decay and retention responses to The study was carried out in accordance with the rules of the Bioethics and local therapeutic application of 448 kHz capacitive resistive monopolar Biosafety Committee for animal experimentation at the University of radiofrequency: a prospective randomised crossover study in healthy adults. Córdoba. The research was approved by the Bioethics Committee of the Int J Hyperth. 2018;31(8):883–95. Veterinary Teaching Hospital of the University of Córdoba. Written consent 14. Stewart IB, Sleivert GG. The effect of warm-up intensity on range of motion was obtained from the owners of the horses included in the study. and anaerobic performance. J Orthop Sports Phys Ther. 1998;27(2):154–61. Consent to participate from human subjects: not applicable. 15. Mohr M, Krustrup P, Nybo L, Nielsen JJ, Bangsbo J. Muscle temperature and sprint performance during soccer matches- beneficial effect of re-warm-up Consent for publication at half-time. Scan J Med Sci Sports. 2004;14(3):156–62. Not applicable. 16. Herrera R, Saitua A, Becero M, Castejón-Riber C, Muñoz A. Stride length and dorsoventral displacement in horses exercised on different surfaces, land treadmill and water treadmill. In: Annual Meeting of the Spanish Association Competing interests of Equine Veterinarians. Alicante, Spain; 2018. The authors declare that they have no competing interests. 17. Barrey E, Desbrosse F. Lameness detection using an accelerometric device. Pferdeheilkunde. 1996;12(4):617–22. Author details 1 18. Hernández-Bule ML, Paíno CL, Trillo MÁ, Úbeda A. Electric stimulation at 448 Equine Sport Medicine Center CEMEDE, School of Veterinary Medicine, kHz promotes proliferation of human mesenchymal stem cells. Cell Physiol University of Córdoba, Campus Universitario de Rabanales, 14004 Cordoba, Biochem. 2014;34(5):1741–55. España. 2Veterinary Teaching Hospital, School of Veterinary Medicine, 19. Hernández-Bule ML, Trillo MA, Martínez-García MA, Abilahoud C, Úbeda A. Autonomous University of Barcelona, Bellaterra, 08193 Barcelona, Spain. 3 Chondrogenic differentiation of adipose-derived stem cells by Department of Animal Medicine and Surgery, School of Veterinary Medicine, radiofrequency electric stimulation. J Stem Cell Res Ther. 2017;7(12):1000407. University of Córdoba, Campus Universitario de Rabanales, 14004 Cordoba, 20. Barrey E, Galloux P, Valette JP, Auvinet B, Wolter R. Stride characteristics of España. 4Veterinary Teaching Hospital, School of Veterinary Medicine, overground versus treadmill locomotion in the saddle horse. Acta Anat. University of Córdoba, Campus Universitario de Rabanales, 14004 Cordoba, 1993;146:90–4. España. 5Department of Corporal Expression, University of Córdoba, 4004 Córdoba, Spain. 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