Training & Testing 1161

Oxygen Uptake, Muscle Activity and Ground Reaction Force during Water Aerobic Exercises

Affiliations

Key words ▶ aquatic exercise ● ▶ underwater EMG ● ▶ impact forces ●

C. L. Alberton1, S. S. Pinto1, E. L. Cadore2, M. P. Tartaruga3, A. C. Kanitz2, A. H. Antunes2, P. Finatto2, L. F. M. Kruel2 1

School of Physical Education, Federal University of Pelotas, Pelotas, Brazil School of Physical Education, Federal University of Rio Grande do Sul, Porto Alegre, Brazil 3 School of Physical Education, Midwest State University of Parana, Guarapuava, Brazil 2

Abstract

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This study aimed to compare the oxygen uptake (VO2), the muscle activity of lower limbs, and the vertical ground reaction force (V-GRF) of women performing water aerobic exercises at different intensities. 12 young women performed the experimental protocol, which consisted of 3 water exercises (stationary running [SR], frontal kick [FK] and cross country skiing [CCS]) at 3 intensities (first and second ventilatory thresholds and maximum effort). A two-way repeated measures ANOVA was used. Regarding VO2, different responses between intensities (p < 0.001) were found, and values between exercises were similar. For electromyographic activity (EMG),

Introduction

▼ accepted after revision October 21, 2013 Bibliography DOI http://dx.doi.org/ 10.1055/s-0034-1383597 Published online: August 21, 2014 Int J Sports Med 2014; 35: 1161–1169 © Georg Thieme Verlag KG Stuttgart · New York ISSN 0172-4622 Correspondence Prof. Cristine Lima Alberton School of Physical Education Federal University of Pelotas Rua Luís de Camões 625, Pelotas Brazil 96055630 Tel.: + 55/53/32732 752 Fax: + 55/53/32733 851 [email protected]

The use of the principles of hydrostatics and hydrodynamics creates challenges that promote health through water exercise while placing low overload on the lower limb joints. Based on the meta-analysis by Batterham et al. [11], aquatic exercise, like land-based exercise, appears to be effective in promoting health, while having characteristics that allow people to perform exercises that they would be unable to perform on dry land. Thus, such programs are widely indicated for people who need to perform exercises with lower impact, such as people with osteoarticular diseases or who are overweight or pregnant [6, 27, 42]. Neuromuscular, kinetic and cardiorespiratory responses of water-based exercises are important components of aquatic rehabilitation and water aerobics. These parameters have received considerable interest in recent studies [2, 9, 31]. Electromyographic activity (EMG) of the muscles of the trunk and upper and lower limbs have been assessed in aquatic exercises such as water walking [9, 31, 33], deep-water running [28, 32],

differences between intensities for all muscles (p < 0.001) were found. Greater EMG signals were observed in the FK compared to SR for rectus femoris, semitendinosus, vastus lateralis and biceps femoris muscles (p < 0.05). Regarding V-GRF, there was an increase in the V-GRF at greater intensities compared to the first ventilatory threshold (p = 0.001). In addition, lower values were found during CCS compared to the SR and FK exercises (p < 0.001). Thus, greater cardiorespiratory and neuromuscular responses were observed with increasing intensity. Exercises such as CCS could be used to attenuate the V-GRF; if the purpose is to reduce the muscular activity of lower limbs at a specific intensity, SR could be recommended.

water-resistance exercises [15, 17, 29, 38] and water aerobic exercises [2, 37]. These studies have compared different environments (i. e., water and dry land) and intensities, by using auto-selected or fixed velocities of motion during the exercises. As mentioned, only 2 studies on EMG analysis in water aerobics were found in the literature. The study by Alberton et al. [2] analysed only one exercise (i. e., the stationary running), and their purpose was to compare the performance of this exercise between aquatic and dry land environments at different preselected submaximal cadences and maximum effort. The study by Pinto et al. [37] also analysed only one water-based exercise, specifically stationary running combined with elbow flexion/ extension, and their purpose was to compare the use of different aquatic devices at different preselected submaximal cadences and the maximum effort. Analysis of vertical ground reaction forces (V-GRF) plays an important role in aquatic exercises. The aquatic environment elicits a reduced underwater weight [4, 20] and impact in the lower limbs during walking [9, 10, 20, 33–35, 41],

Alberton CL et al. Oxygen Uptake, Muscle Activity … Int J Sports Med 2014; 35: 1161–1169

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Authors

1162 Training & Testing Methods

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Subjects 12 physically active and healthy women volunteered to take part in the present study (age: 23.8 ± 2.2 years; height: 162.1 ± 5.6 cm; body mass: 58.3 ± 5.5 kg; fat mass: 27.1 ± 3.4 %; underwater weight reduction: 68.6 ± 4.0 %). The subjects participated in water aerobics programs for at least 3 months and were required to sign an informed consent form. The study was conducted according to the ethical standards of the International Journal of Sports Medicine described by Harriss and Atkinson [19] and was approved by the Local Research Ethics Committee.

Experimental procedure An initial session was held to collect participants’ physical characteristics. Body mass and height measurements were obtained using an analogue medical scale and a stadiometer (FILIZOLA; Sao Paulo, Brazil). Skin folds were measured using a plicometer (LANGE; Cambridge, United Kingdom) to estimate the body density according to the protocol proposed by Jackson et al. [26]. Body fat was subsequently calculated using the Siri equation [43]. Participants completed a training session to familiarise themselves with the water aerobic exercises, in which the details regarding range of movement and other information pertaining to the exercises were explained. 3 typical water aerobic exercises were used in this study: stationary running (SR), frontal kick (FK) and cross country skiing (CCS). These exercises, commonly used during aquatic programs, were previously described in detail [4]. Each exercise was divided into 2 phases, and each segmental action (hip flexion or extension) was performed in one beat. An elastic band was fixed to lateral supports to control the range of motion, limiting it within the adequate amplitude for hip flexion during each exercise (SR = 90 °; FK = 45 °; CCS = 60 °) ▶ Fig. 1. These angles were determined individuas shown on ● ally on dry land with a goniometer (CARCI; Sao Paulo, Brazil). Additionally, all of these exercises involve displacement of the lower limbs in the sagittal plane through flexion and extension of the hips and knees and dorsi- and planter flexion of the ankles. To determine the cadences corresponding to VT1 and VT2 for each exercise, 3 sessions were performed randomly, each with one maximal test and an interval of 48 h between sessions. Each aquatic maximal test was conducted at an initial cadence of 80 b. min − 1 for 2 min with progressive increases in cadence of 10 b. min − 1 every minute until maximum effort was obtained. Tests were stopped when participants indicated their exhaustion using a hand signal or if they were unable to maintain the stage’s cadence. This protocol was adapted from Alberton et al. [3]. The cadences were set by a digital metronome (MA-30, KORG; Tokyo, Japan). The assessment was considered valid when some of the following criteria were met at the end of the test [24]: plateau in VO2 despite an increase in exercise intensity, respiratory exchange ratio greater than 1.15 and maximal respiratory rate of at least 35 breaths per minute. VT1 and VT2 were determined using the first and second break points in the ventilation-byintensity graph and confirmed using the ventilatory equivalents slopes for oxygen (VE/VO2) and carbon dioxide (VE/VCO2), respectively [45]. 3 experienced, independent physiologists determined the corresponding points and cadences by visual inspection in a blind procedure. The individual cadences that corresponded to VT1 and VT2 in each exercise were used in the

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jumping [16, 44] and water aerobic exercises [4, 12] when compared to dry land. In contrast to the dry land environment, some studies developed in the aquatic environment have shown similar V-GRF values between different self-selected velocities during walking in shallow water [33, 41]. On the other hand, recent studies observed that the V-GRF was significantly higher as the cadence or speed was increased during water aerobic exercises [4, 12] or shallow-water walking [22]. Although recent experiments have simultaneously investigated the EMG and V-GRF responses during shallow-water walking [9, 10, 33], comparisons between different water aerobic exercises performed at different intensities have been scarcely investigated. Regarding the oxygen uptake (VO2) in water aerobic exercises, several studies over the last 2 decades have investigated its responses at submaximal cadences [1, 2, 7, 8, 14, 39]. Significant differences have been found in the VO2 when comparing different types of water-based exercises, and increases in these responses were observed as the cadence increased. Among these studies, the work of Alberton et al. [2] and Pinto et al. [37] verified the EMG signal of lower limbs’ muscles simultaneously to the cardiorespiratory measurements during the stationary running exercise performed in the aquatic environment at cadences of 60, 80 and 100 b.min − 1. These pre-selected cadences yielded intensities ranging from 24–42 % of VO2max and 43–46 % of VO2max, respectively, values considerably low for the water aerobics prescription. Consequently, when comparing the intensities, no significant difference was found in the EMG signal between the pre-selected submaximal cadences in either study. Recently, another study by Alberton et al. [3] investigated the cardiorespiratory parameters during maximum progressive tests performed in 3 water aerobic exercises (stationary running, frontal kick and jumping jacks). The authors showed that the first ventilatory threshold (VT1) corresponded to approximately 50 % of VO2max and second ventilatory threshold (VT2) to 70 % of VO2max. Therefore, determining the cadences corresponding to these ventilatory thresholds is fundamental for measuring the muscle activity and ground reaction forces at intensities that demarcate the predominantly aerobic training zone and that can be used to prescribe the aquatic fitness training sessions with this purpose. However, to the best of authors’ knowledge, no study has investigated and compared neuromuscular and kinetic responses between different water aerobic exercises performed at the VT1 and VT2 intensities. Because VO2, muscle activity and ground reaction force during water aerobic exercises are crucial factors for a better aquatic training prescription, a study analysing the pattern of these variables simultaneously is extremely important for understanding their global performance from a physiological and mechanical point of view. In addition, the people who seek this kind of modality are predominantly women, and most studies in the literature investigated the aforementioned parameters during water aerobics in this gender [1–4, 8, 37, 39]. Therefore, the purpose of this study was to compare VO2, muscle activity of the lower limbs and peak V-GRF (V-GRFpeak) of women performing water aerobic exercises at intensities corresponding to VT1, VT2 and maximal effort (MAX). It was hypothesised that muscle activity of the lower limbs and V-GRFpeak values would be different between exercises based on their characteristics and that VO2 responses would be similar. Moreover, it was hypothesised that significant differences would be found between intensities for all variables based on the cadences used to represent these different intensities.

Fig. 1 Range of motion control during water aerobic exercises (i. e., frontal kick).

experimental protocol. The MAX intensity was performed without control of cadence, and the subjects were asked to perform each exercise as fast as they could, without the help of the metronome. Tests were performed while the subjects were immersed in water to the level of the xiphoid process at a constant temperature of 32 °C. Several restrictions were imposed on the volunteers: 1) no food was allowed to be consumed 3–4 h before the experimental protocol, and 2) neither the use of stimulants nor intense physical activity was allowed 12 h before the experimental protocol. Sessions were performed between the 8th and 20th day after the start of the last menstrual period to control hormonal levels. Each session was also performed at the same time of day to avoid variations related to circadian rhythms. One session corresponding to the experimental protocol was performed to collect the variables. Initially, subjects remained at rest on the plate to measure their apparent weight in the aquatic environment. Water aerobic exercises (SR, FK and CCS) were then performed at the 3 intensities (VT1, VT2 and MAX). Exercises were performed for 4 min at submaximal intensities, while VO2, the EMG signal and V-GRFpeak were recorded from the 3rd to 4th minute. At MAX intensity, the exercises were performed for 15 s, and only the neuromuscular and kinetic data were measured during this time. The intensities were performed over a 5-min interval, and the exercises were performed over a 15-min interval in a random order. To evaluate the ventilatory data collected during the maximal tests and experimental protocol, a gas analyser (VO2000, MedGraphics; Ann Arbor, USA) was used, which was calibrated beforehand according to manufacturer’s specifications. The sampling rate used to collect values was 10 s (Aerograph software).

The activity of the following 6 muscles on the body’s right side were obtained via surface EMG: rectus femoris (RF), semitendinosus (ST), vastus lateralis (VL), short head of biceps femoris (BF), tibialis anterior (TA) and gastrocnemius lateralis (GL). Hair was shaved off the electrode placement sites, and the skin in these areas was abraded and cleaned with alcohol to keep the interelectrode resistance low ( < 3 kΩ). Surface monopolar electrodes with 15 mm radii (model Mini Medi-Trace 100, Kendall Ag/AgCl; Tyco, USA) were placed in bipolar configuration over the belly muscle parallel to the orientation of the muscle fibres, according to the SENIAM project [23]. The inter-electrode distance was maintained at 30 mm. The reference electrode was positioned on the clavicle. An insulation procedure was performed in order to avoid interference by artefacts due to the contact of electrodes with water [40], according to the method performed in previous studies [2, 36, 37]. The insulation was done with waterproof transparent adhesive tape (model Tegaderm, 3M; St. Paul, MN, USA). Silicone glue was placed at the exit point of the cables (dried for approximately 1.5 h) to prevent the entry of water. The cables and preamplifiers were fixed with adhesive tape. EMG signals were collected using two 14-bit electromyographs (model Miotool400, MIOTEC Biomedical Equipment; Porto Alegre, Brazil) with a common-mode rejection ratio of 110 dB and 2 000 Hz per channel, each having a 4-channel system. Data were transferred to an A/D converter before being uploaded onto a personal computer (Miograph software). In addition, reflective markers were placed on the greater trochanter, lateral femoral epicondyle and lateral malleolus so that performances could be videotaped to determine the position of the thigh and leg segments and align them to the EMG signal. A waterproof video camera (VPC-WH1, SANYO, Osaka, Japan) set to a sampling rate of 60 Hz was used for filming. The camera was positioned in water and fixed by an external device on the sagittal plane of the subjects’ right side at a distance of 5 m. Maximal voluntary contraction (MVC) was used to normalise the magnitude of the EMG signal. EMG signals of each muscle were recorded on dry land during MVC while simultaneously measuring maximal force before implementing the experimental protocol. The subjects performed 3 sets of MVC isometric contraction for each of the tested muscles. The duration of the test was set to 5 s for each muscle [18]. Contraction angles were measured with a goniometer (CARCI; Sao Paulo, Brazil), and the corporal segments were maintained while force production was measured through a load cell. For the hip flexor and extensor and ankle dorsi and planter flexor muscle groups, subjects remained seated with the hip and knee flexed at 90º and fixed by Velcro strips at the hip and trunk. During MVC of the hip flexor and extensor, the right thigh segment was allowed to rest partially on a chair, with the distal portion (popliteal fossa) strapped with a non-elastic belt to the inferior and superior part of a steel device. During MVC of the ankle dorsi and planter flexor, the right foot segment was maintained at neutral position with the distal portion (5th metatarsus) strapped to the inferior and superior part of a steel device with a non-elastic belt. To measure the knee flexor and extensor muscle groups, subjects remained standing with the anterior trunk supported. During MVC of the knee flexor and extensor, right knee flexion was maintained 90 ° with the distal portion of shank segment (10 cm above the heel) strapped with a non-elastic belt to the inferior and superior parts of a steel device. To evaluate the maximal force during MVC, a load cell was used (MIOTEC, Porto Alegre, Brazil). The cell has a nominal capacity of

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Training & Testing 1163

1164 Training & Testing

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Fig. 2 Analysis of oxygen uptake (VO2) a and percentage of the maximal VO2 ( %VO2max) b between different water aerobic exercises (Exe) (SR: stationary running; FK: frontal kick; CCS: cross country skiing) and intensities (Int) (VT1: first ventilatory threshold; VT2: second ventilatory threshold). Different letters indicate significant differences between intensities: b > a.

200 kg, a sensitivity of 2 ± 10 %, a combined error of less than 0.03 % and a useful working temperature ranging between − 5 to + 60 °C. The load cell was calibrated prior to data collection according to the manufacturer’s instructions. Data were acquired using Miograph software. To evaluate V-GRFpeak corresponding to the right lower limb in each situation, a waterproof force plate (OR6-WP, AMTI, Watertown, USA) was used that was calibrated beforehand according to the manufacturer’s specifications. Its maximum measurement capacity for V-GRF values is 8900N, the sensitivity is 0.08 μV/[V.N] and the useful working temperature ranges from − 17 to + 52 °C. The sampling rate was set to 2 000 Hz (AMTIForce software). To align the systems, a light signal was triggered and videotaped at the onset of simultaneous collection of EMG and force plate data.

Data analysis Submaximal VO2 was calculated based on the mean values of VO2 collected from 3rd to 4th minute. These values were also expressed as percentage of the maximal VO2 ( %VO2max) of the maximal VO2 obtained for each corresponding exercise. The force production values and EMG signals were analysed using SAD32 software. The force signal (Newton) during MVC for each muscle group was filtered using a third-order Butterworth low-pass filter with a cutoff frequency of 8 Hz. This measure was used to select the period of 1 s corresponding to the steady force production signal during the central 2–4 s of MVC in order to align and determine the EMG signal slices. The EMG digital signal (μV) was filtered using a third-order Butterworth band-pass filter with cutoff frequencies ranging between 20 and 500 Hz. The EMG signal in the MVC was sliced (1 s) according to the force production signal, and the root mean square (RMS) value was calculated for each muscle. The greatest values among 3 trials for each muscle were used to normalise the EMG data collected during the experimental protocol [18]. Analysis of the EMG signals recorded during the experimental protocol was based on videotaped performances. The reflective markers corresponding to the first 10 repetitions of each situation were manually digitalized (Dvideow software, Laboratory of Biomechanics & Institute of Computing, UNICAMP; Campinas, Brazil). The frames corresponding to the individual starting and

finishing points (successive right heel contact) of each repetition were considered to the alignment of the EMG signal. Based on this analysis, slices in the EMG signal were made to limit each one of the 5 central repetitions (among the 10 repetitions analysed), and the corresponding RMS value was calculated. The average RMS value for each muscle was then calculated from these 5 selected repetitions. The average value for each muscle group was normalised and expressed as a percentage of the respective MVC ( %MVC) for each participant in each situation. The V-GRF signal was analysed using SAD32 software. The digital signal was filtered using a third-order low-pass Butterworth filter with a cutoff frequency of 10 Hz. Underwater weight reduction percentage was calculated using actual body weight and apparent body weight in water. Slices in the V-GRF signal corresponding to the first 10 total repetitions collected for each exercise were made to determine the corresponding V-GRFpeak. The V-GRFpeak is defined as the maximal value obtained from the V-GRF signal, occurring at any time during one entire cycle. These data were normalised using body weight measurements that were taken outside the water. Next, the 5 central valid repetitions were averaged to obtain the mean value for each participant in each situation.

Statistical analysis Results are reported as the mean ± SD. Normality of the distribution of data was assessed with the Shapiro-Wilk test. The VO2, %VO2max and V-GRFpeak presented normal distribution (p > 0.05). Because EMG signal of all muscles failed to fit normality, a logarithmic transformation was made to make the distribution normal. Statistical comparisons were made using repeated measures two-way ANOVA (factors: intensity and exercise) with the Bonferroni post hoc test. The significance level was defined as α = 0.05. SPSS statistical software package (version 19.0) was used to analyse all data.

Results

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The main effect of intensity was significant for VO2 (p < 0.001) as well as for %VO2max (p < 0.001), whereas the main effect of exercise was not significant. Furthermore, there was no significant

Alberton CL et al. Oxygen Uptake, Muscle Activity … Int J Sports Med 2014; 35: 1161–1169

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Fig. 3 Analysis of electromyographic activity of rectus femoris (RF) a, semitendinosus (ST) b, vastus lateralis (VL) c, short head of biceps femoris (BF) d, tibialis anterior (TA) e and gastrocnemius lateralis (GL) f between different water aerobic exercises (Exe) (SR: stationary running; FK: frontal kick; CCS: cross country skiing) and intensities (Int) (VT1: first ventilatory threshold; VT2: second ventilatory threshold; MAX: maximum effort). Different letters indicate significant differences between intensities: c > b > a. * indicates significant difference between FK and SR. † indicates significant differences between SR and other exercises. ‡ indicates significant differences between SR and CCS. § indicates significant differences between CCS and other exercises.

exercise * intensity interaction for any cardiorespiratory variable, which indicates that the cardiorespiratory response patterns between exercises is independent of the intensity of the exercise ▶ Fig. 2). (● Analysis of the EMG signal from all muscles yielded a significant main effect of intensity (p < 0.001) as well as exercise (RF, VL and BF: p < 0.01; ST, TA and GL: p < 0.05). Furthermore, there was no significant exercise * intensity interaction for the EMG signal of ▶ Fig. 3). Thus, significant increases in the any muscle group (● EMG signal occurred in all muscles analysed as the intensity of the exercise increased, except for the GL muscle. No difference was observed in this muscle when exercises were performed between the VT1 and VT2 intensities. Analysis of the EMG signal yielded a significant difference between SR and FK exercises for

the RF and ST muscles. Analysis of data collected during the SR exercise yielded lower EMG values in the VL and BF muscles when compared with the others exercises. Regarding the TA muscle, a significant difference was observed between the CCS and SR exercises. Greater EMG values were observed for CCS in particular. Moreover, for GL muscle, significant differences were found between CCS and the other exercises and lower activity observed in this muscle during CCS. For the V-GRFpeak, there was a significant main effect of intensity (p = 0.001) and exercise (p < 0.001). There was no significant ▶ Fig. 4). These findings indicate exercise * intensity interaction (● that there was a significant increase in the V-GRFpeak from VT1 to the greater intensities for the exercises analysed and that there was no difference between VT2 and MAX intensities. In addition,

Alberton CL et al. Oxygen Uptake, Muscle Activity … Int J Sports Med 2014; 35: 1161–1169

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Fig. 4 Analysis of peak vertical ground reaction force (V-GRFpeak) between different water aerobic exercises (Exe) (SR: stationary running; FK: frontal kick; CCS: cross country skiing) and intensities (Int) (VT1: first ventilatory threshold; VT2: second ventilatory threshold; MAX: maximum effort). Different letters indicate significant differences between intensities: b > a. § indicates significant differences between CCS and other exercises.

significantly lower values of V-GRFpeak were found during CCS when compared to the SR and FK exercises.

Discussion

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In the present study, each of the analysed muscle groups with exception of the GL muscle presented increases in the EMG signal as the intensity of the exercise increased. In this muscle, the only difference observed occurred between the MAX and the 2 submaximal intensities (VT1 and VT2). Our results corroborate with previous studies in which an increase in the EMG signal from the muscles of the lower and upper limbs was observed when submaximal and maximal intensities were compared during water aerobic (i. e., stationary running) and water-resistance (i. e., shoulder abduction from 0 to 90 °) exercises [2, 29, 37]. In contrast to previous studies that demonstrated no increase in the EMG signal of the RF, VL, ST and BF muscles between different submaximal fixed cadences [2, 37], the present study found differences between the submaximal intensities for these muscles. The distinction between the present results and those from the literature may be attributed to the difference in the intensities chosen for analysis, i. e., the cadences corresponding to VT1 and VT2 in the present study and the pre-selected fixed cadences used in the aforementioned studies. In the study by Alberton et al. [2], the SR exercise was performed at submaximal cadences of 60, 80 and 100 b.min − 1, corresponding to approximately 24, 32 and 42 % of VO2max, respectively. Similarly, Pinto et al. [37] analysed SR combined with elbow flexion/extension exercises at submaximal cadences of 80 and 100 b.min − 1. During the situation without equipment, these cadences corresponded to 43 and 46 % of VO2max, respectively. Our current findings showed that the VT1 intensity cadences corresponded to 102.5 b.min − 1 for SR, 97.5 b.min − 1 for FK and 97.5 b.min − 1 for CCS, corresponding to intensities around 52–56 % of VO2max, while the VT2 intensity cadences corresponded to 135.0 b.min − 1 for SR, 123.3 b.min − 1 for FK and 127.5 b.min − 1 for CCS, intensity around 73–79 % of VO2max. One previous study analysed only the cardiorespiratory responses during the 3 water aerobics investigated in the present

study at pre-selected cadences ranging from 110 to 140 b.min − 1 [39] and found important results: their data showed that the CCS elicited intensities between 20–39 % of VO2max at cadences 110– 120 and 120–130 b.min − 1 and between 40–59 % of VO2max at cadences 130–140 b.min − 1 and the FK exercise elicited intensities between 40–59 % of VO2max at cadences 110–120 and 120– 130 b.min − 1 and between 60–84 % of VO2max at cadences 130–140 b.min − 1. However, it is important to highlight that the maximal test in the present study was performed in each specific exercise, and in the other studies in the literature the % of VO2max was based on maximum tests performed on dry land. Thus, is possible that the cadences used in the aforementioned studies were similar to or lower than those that defined the VT1 in the present study. It is therefore possible that neuromuscular activity at intensities greater than VT1 and similar to VT2 have not been investigated. The literature reports that there is a relationship between the ventilatory and neuromuscular thresholds obtained from the EMG signals recorded from the lower limbs during cycle ergometer exercise performed on dry land [25, 30]. Hence, starting from VT1 and especially after VT2, a non-linear increase in the amplitude of EMG signals occurs as the intensity increases [25, 30]. This relationship is similar to that observed in the ventilation slope. Although this analysis has not been performed during water aerobic exercises, the association found in the aforementioned studies supports our results because differences were observed in the EMG signal among the 3 intensities evaluated. On the other hand, the GL muscle showed a different pattern compared to the others muscles, because there was no EMG increase from the VT1 to VT2 intensity in this muscle. This may probably be explained by this muscle being as important during the VT1 as during VT2 intensity for producing vertical propulsion, especially for SR and FK exercises. A similar pattern can be observed in the study by Masumoto et al. [32], who investigated the EMG signal of the GL muscle during the deep-water running at increasing intensities based on the rate of perceived exertion (11, 13 and 15 according to the 6-20 RPE Borg’s Scale). While no statistical comparison was performed among the intensities, the EMG values found ranged from 6.1 ± 4.4 to 8.2 ± 7.2 % of MVC, demonstrating similar values in the different submaximal speeds of motion. A different pattern was found for the shallow water walking in the study by Miyoshi et al. [33], with an increase being observed in the EMG signal of the gastrocnemius medialis when comparing comfortable, slower and faster auto-selected intensities. Notwithstanding, in this modality of exercise, the gastrocnemius is the main muscle responsible for overcoming the drag forces and displacing the body in the pool, where a low increase in speed elicits a great horizontal propulsive force because the drag forces acting against the movement depend on the velocity and the projected area, represented by all body immersed. Analysis of the V-GRFpeak showed that exercises performed at VT1 yielded significantly lower values than those obtained at greater intensities, although there was no difference between VT2 and MAX. These results agree with those found in a recent study [4] which also analysed the V-GRFpeak during water aerobic exercises at the cadences corresponding to VT1, VT2 and MAX intensities. In addition, these findings corroborate with the study by Brito-Fontana et al. [12], which analysed the V-GRFpeak during the SR in water at fixed cadences (90, 110 and 130 b. min − 1), which were similar to the submaximal cadences obtained for this exercise in the present study (i. e., VT1: 102.5 b. min − 1; VT2: 135.0 b.min − 1). These authors also showed signifi-

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1166 Training & Testing

cant increase in the V-GRFpeak from the cadence corresponding to 90 b.min − 1 compared to the higher intensities, with no differences between 110 and 130 b.min − 1. These patterns can be explained by hydrodynamic properties. As the intensity increases from VT1 to VT2, angular velocity is enhanced when the same exercise is performed within a controlled range of motion. Consequently, an increase in acceleration during the contact of the foot with the plate follows, resulting in a greater V-GRF. In addition, with the increased intensity, an increase in the V-GRF is observed due to the requirement of a greater propulsive to overcome the drag force [22]. However, in contrast to the shallow-water walking, for this kind of exercise there is an absence of horizontal propulsion, with vertical propulsion being necessary for these movements, especially for the SR and FK exercises. Nevertheless, when using maximal velocities for this type of analysis, an advantage is provided by buoyancy and the drag force generated by the leg contralateral to the support leg. This increase in the pace of execution reduces the time corresponding to the support phase [34]. The buoyancy and drag forces evoke an alteration in the foot support phase pattern, which corresponds to the transition from a total support in the lower intensities to a forefoot support in the greater intensities [21]. This pattern facilitates performance in the MAX intensity and influences the acceleration provided to the body when engaging to touch the plate. Regarding the comparison between exercises, the FK and CCS exercises present common hip flexion and extension movements performed with the knee extended during one complete cycle. This pattern is opposite to the SR, which is performed with hip motion associated with knee flexion and extension. The projected area against the water flow is thereby greater for FK and CCS, and, consequently, the drag force is greater in these exercises [1, 5]. To overcome drag introduced by water resistance, the RF (hip flexor and knee extensor) and ST (hip extensor and knee flexor) muscles were elicited at greater magnitude, because the exercises should be performed at the same physiological intensity. In addition, although these exercises are performed with restricted motion of the knee joint, the RF, ST, VL and BF muscles were activated to maintain knee isometry, because the thigh and leg segments move against water resistance. Similarly, these exercises also generated greater EMG signals in the VL and BF muscles. In contrast, SR is assisted by buoyancy and water turbulence present during both phases, because knee flexion occurs during the ascendant movement (hip flexion) and knee extension occurs during the descendent movement (hip extension). Thus, even performing the dynamic knee flexion and extension at greater amplitude, the SR exercise resulted in lower VL and BF activity than FK and CCS exercises. Based on these results, it can be concluded that SR presents a neuromuscular economy in the hip and knee flexors and extensors when compared to the FK and CCS exercises. Neuromuscular economy can be defined as decreased muscle activation, represented by the EMG signal amplitude, necessary for performing the same absolute load [13]. In the present study, the load can be controlled by the intensity (VT1, VT2 or MAX). Thus, SR can be used in aquatic programs at the same cardiorespiratory intensity as the other exercises. However, it most likely evokes less peripheral fatigue. When analysing the TA and GL muscles, a distinct pattern of activation was observed between exercises. CCS should be performed with ankle dorsi flexion throughout the cycle due to the absence of the flight phase and the use of sliding to transfer support from one lower limb to the other. Thus, this exercise was

probably performed keeping the TA shortened and the GL lengthened throughout the cycle. As a consequence, there was a greater EMG signal in the TA muscle while less activity in the GL muscle was observed. In contrast to CCS, the SR and FK exercises are characterised as having distinct support and flight phases. Therefore, these exercises require GL muscle activation to assist in vertical propulsion during the support phase and possibly to absorb impact during the touch of the foot on the plate, resulting in a greater EMG signal being elicited from this muscle. During the flight phase of SR, the foot is maintained with the ankle in a neutral position. However, it is extended in the FK exercise in order to increase the projected area of the leg, which adds another factor explaining the observed increase in GL activation and decrease in TA activation. With respect to V-GRFpeak, independent of intensity, CCS elicited lower responses compared to the SR and FK exercises consistent with the findings by Alberton et al. [4]. This pattern was caused by different characteristics necessary to perform these exercises. Because CCS is performed with bipedal support, the weight is divided between the 2 lower limbs. In addition, this exercise does not have a flight phase because the change of foot support phase is performed by sliding. Thus, there is no significant vertical oscillation of the centre of mass, and, consequently, the acceleration is attenuated when the foot touches the plate [4]. On the other hand, both the SR and FK exercises are characterised as single support, meaning that the whole body weight is carried by leg support. Furthermore, they include a flight phase that induces an appreciable vertical displacement of the centre of mass, which increases acceleration generated as the body contacts the touch plate in addition to the impact absorption. Thus, no differences were found in the V-GRFpeak between them, corroborating with previous studies by Alberton et al. [4], who also investigated exercises with these characteristics. The study by Triplett et al. [44] analysed the V-GRFpeak during single-leg jump in young female handball players performed with the maximum effort. They found values of 557.7 N and 829.1 N during its performance at xiphoid process depth with and without aquatic devices. In addition, the study by Brito-Fontana et al. [12] analysed the SR in immersion and observed values of V-GRFpeak corresponding to 0.98–1.12 BW at chest depth during the submaximal cadences of 90, 110 and 130 b.min − 1. In the present study, the SR and FK exercises presented similar characteristics to those in single-leg jump analysed by Triplett et al. [44], and one of the present exercises was also analysed by BritoFontana et al. [12]. Our results show similar values ranging from 0.96–1.25 BW, i. e., 549–714 N (based on the BW = 571.92 N) at intensities from VT1 to maximum effort.

Conclusion

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Greater cardiorespiratory and neuromuscular responses were observed as the intensity of the exercises increased. The VT1 intensity elicited lower cardiorespiratory, neuromuscular and kinetic responses for all water aerobic exercises. The MAX intensity evoked an increase in the EMG activity for all muscles analysed, without a rise in the V-GRFpeak, when compared to VT2. The CCS exercise yielded lower V-GRFpeak values with an increase in neuromuscular activity in almost all of the muscles evaluated. On the other hand, even though SR yielded V-GRFpeak values that were greater than those evoked by CCS, EMG activity during SR was lower than the other exercises performed at the same effort

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intensity. Based on our findings, we suggest that water aerobic exercises may be recommended as a component of aquatic rehabilitation and training protocols for optimizing the recruitment of the flexors and extensors muscles of hip and knee and dorsi and planter flexor muscles of ankle, with reduced V-GRF action on the lower limbs. Exercises with similar characteristics to CCS (displacement through sliding) could be used to attenuate the V-GRFpeak. If the purpose is to reduce muscular activity of lower limbs at a specific intensity, SR should be recommended over the FK and CCS exercises. Therefore, the choice of exercise and intensity during aquatic programs is fundamental for adequate practice.

Acknowledgements

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This study was supported by CAPES and CNPq, Brazil. The authors wish to thank MIOTEC and INBRAMED companies for their invaluable contribution to this study.

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