Effects of Aquatic Exercise Training on Physical Performance and Mood States in Male Collegiate Soccer Players : Translational Journal of the American College of Sports Medicine

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Controlled Trial

Effects of Aquatic Exercise Training on Physical Performance and Mood States in Male Collegiate Soccer Players

Michishita, Ryoma1,2; Hide, Taijiro3; Kawakami, Shotaro1,2; Kanegawa, Yuta1; Takayanagi, Kohei1; Inui, Masahiro1; Uehara, Yoshinari1,2; Higaki, Yasuki1,2

Author Information
Translational Journal of the ACSM 8(1):e000218, Winter 2023. | DOI: 10.1249/TJX.0000000000000218
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Abstract

INTRODUCTION

Aquatic exercise training involves underwater whole-body exercises performed using the upper and lower limbs. Unlike on land, the body is affected by the physical characteristics of water, such as buoyancy, resistance, pressure, water temperature, and thermoregulation. The high-density environment underwater exerts a greater resistance than that exerted by the environment on land. It is believed that during aquatic exercises, an individual’s muscle strength can improve if the resistance applied to the body is controlled using various tools to change the magnitude and speed of upper and lower limb movements and using the resistance of the water (1–4). Reportedly, long-term aquatic exercise training increases physical fitness, improves musculoskeletal disorders, reduces stress, and helps prevent falls in middle-aged and older people (5–10). Thus, aquatic exercise training is recommended as safe and effective for improving muscle strength and health promotion in middle-aged and older people (1), but not much has been reported in other populations.

Many of the studies on aquatic exercise training for athletes have been carried out as rehabilitation tools for the purpose of early return to sports activities, such as recovery from physical performance and muscle-strength-involving sports injuries (11,12). Soccer is a sport that requires physical performance, such as change of direction, dexterity, muscle strength, and endurance. The effects of aquatic exercise training to improve physical performance, muscle strength, and mood states in soccer players remain unelucidated, but as mentioned above, long-term aquatic exercise training may be effective for physical fitness, muscle strength, and mood states in nonathletic populations (1,5–9). The present study was designed to examine the effects of aquatic exercise training on the physical performance and mood states of male collegiate soccer players. We propose that if an appropriate load can be applied in aquatic exercise training, that may demonstrate its applicability for improving the physical performance and mood states in these soccer players who require physical performance, such as change of direction, dexterity, muscle strength, and endurance.

METHODS

Participants and Study Design

Twenty Japanese national-level male collegiate soccer players (age = 19.9 ± 0.9 yr, range = 19 to 21 yr, body mass index (BMI) = 21.9 ± 1.6 kg·m−2) voluntarily participated in the program. Participants with serious injuries or diseases who could not play sports were excluded from the study. No experience of aquatic exercise training was required, and all participants were inexperienced in water exercise. The participants were randomly allocated to the training or control groups, which included 10 subjects each (age = 19.9 ± 1.0 yr, BMI = 21.8 ± 1.2 kg·m−2, and age = 19.8 ± 0.9 yr, BMI = 22.0 ± 1.9 kg·m−2, respectively). Table 1 shows the baseline characteristics of the participants in both groups. All the participants completed the trial. Anthropometric measurements, range of motion (ROM), physical performance (muscle strength, change of direction, and endurance), and mood states were examined at baseline and after the 11-wk intervention.

TABLE 1 - Participants’ Baseline Characteristics in the Training and Control Groups.
Training Group (n = 10) Control Group (n = 10) P
Age (yr) 19.9 ± 1.0 19.8 ± 0.9 0.82
Height (cm) 172.5 ± 3.5 171.7 ± 7.4 0.76
Body weight (kg) 65.0 ± 4.8 64.9 ± 6.2 0.94
BMI (kg·m−2) 21.8 ± 1.2 22.0 ± 1.9 0.82
Body fat mass (kg) 6.9 ± 1.4 8.0 ± 2.9 0.76
Lean body mass (kg) 58.1 ± 4.0 56.9 ± 4.5 0.91
Number of exercise participation (times) 20.0 ± 1.6
Data are expressed as the mean ± SD.

After the baseline assessments, the participants in the training group commenced the 11-wk aquatic exercise training program in addition to their daily sports activities (soccer training). Athletes in both the training and the control groups belonged to the same team and did the same daily soccer training, excluding aquatic exercise training. All participants were instructed not to engage in any specific physical activity except daily soccer training and aquatic exercise training. The aquatic exercise training program was introduced during the athletic season from July to October 2021.

All participants provided informed consent for participation after agreeing to its purpose, methods, and significance. This study conformed to the Declaration of Helsinki and was approved by the ethics committee of Fukuoka University (No. 20-07-01).

Anthropometry and ROM Measurements

Anthropometric measurements were carried out after at least 2 h of fasting. Participants’ heights and body weights were measured, and their BMI values were calculated as the ratio of body weight (kg) to height squared (m2). Chest circumference was measured in the standing position at the level of the nipple using a tape measure. The upper arm and thigh circumferences were measured at the maximum level for each upper arm (maximal bump parts of biceps belly) and thigh (half way between the superior border of the patella and anterior superior iliac spine) in the nonflexed position. Body fat mass and lean body mass were measured using the bioelectrical impedance method (MC-780A-N; TANITA Inc., Tokyo, Japan). It has been reported that the algorithm of this analyzer has a high correlation with lean body mass measured by the dual-energy x-ray absorptiometry method and is widely used in Japan (13).

ROM was measured by a skilled athletic trainer using a plastic goniometer as the maximum tolerable range of active movement. The ROM of the shoulder joint on the dominant arm side (flexion, extension, abduction, internal rotation, and external rotation), hip joint on the dominant foot side (flexion, extension, abduction, internal rotation, and external rotation), and knee joint (flexion) were measured in the supine position according to the methods recommended by the Japanese Orthopedic Association and the Japanese Association of Rehabilitation Medicine (14).

Aquatic Exercise Training Program

The participants underwent aquatic exercise training two times per week for 11 wk, for a total of 22 sessions. This training method has been described previously (2–4,14). Our aquatic exercise training program consisted of warm-up exercises (jogging and moderate-intensity water aerobics; 5 min), high-intensity interval training (aqua run; composite exercise consist of forward and backward run and swim) and high-intensity circuit training (composite exercise incorporating boxing and speed skating movements; 10 min), resistance training using tools (aqua kickboard, aqua fitness tube, and aqua noodles; 10 min), and cooldown exercises (stretching and relaxation; 5 min) (Fig. 1). A rest period of about 30 s to 1 min was provided between each exercise. High-intensity interval training consisted of four types of 30-s high-intensity exercises followed by a 30-s rest, for a total of 2–3 sets. Aqua run consisted of 3–4 rounds of 25 m jogging. High-intensity circuit training consisted of 20 s of high-intensity exercise followed by 10 s of rest, for a total of 8 sets. Participants used aqua shoes and aqua mitts underwater. This aquatic exercise training program was invented by MIZUNO Inc. (MIZUNO AQUA; MIZUNO Inc., Osaka, Japan) for health promotion in middle-aged and older populations and to increase athletes’ physical performance. The program was designed to be completed within 30 min. The safety of the training program, which involves moderate- to high-intensity exercise, has been confirmed in previous studies (2–4,15). The training was performed under the supervision of a fitness instructor at the university’s indoor pool (water temperature = 28.0°C–29.0°C, depth = 1.2–1.5 m). This training was conducted on the same day as the participants’ soccer training. In this study, a heart rate (HR) monitor (Polar Verity Sense; Polar Electro GmbH, Büttelborn, Germany) was used to periodically confirm the participants’ HR during aquatic exercise. Percent HR reserve was calculated using the following formula (16): percent HR reserve (%) = (HR during exercise – resting HR)/([220 – age] – resting HR) × 100.

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Figure 1:
Aquatic exercise training program. All participants consented to have their images used in this paper.

Muscle Strength Measurements

The muscle strength of each participant was evaluated based on the isokinetic muscle strength of knee flexion and extension and back muscle strength. The isokinetic muscle strength of knee flexion and extension was measured using a Biodex System 3 Pro isokinetic dynamometer (Biodex Medical System Inc., Shirley, NY) (17). Seated in a chair at 90°, each participant’s lower limbs were weighed before the test, after which the results were adjusted in accordance with the weight of the lower limbs to exclude the effect of gravity during limb movement. The total ROM was 100°, set from 100° to 0° of leg flexion. After adequate warm-up, a participant’s muscle strength was measured twice at high-speed concentric velocities (180°·s−1), and after an interval of 30 s, the muscle strength was measured twice at low-speed concentric velocities (60°·s−1). Similarly, the muscle strength of the opposite leg was measured, and the peak torque (N·m) was recorded. Isokinetic muscle strength of knee flexion and extension was calculated as peak torque divided by body weight (N·m/weight) because leg muscle strength is influenced by body mass.

The back muscle strength of each participant was assessed by measuring the maximal isometric strength of the trunk muscles, with the participants in a standing position with 30° of lumbar flexion, using a digital back muscle strength meter (T.K.K.5402; Takei Scientific Instrument Inc., Niigata, Japan). The maximum value of the two trials was recorded. In this study, we used the value of back muscle strength divided by body weight (kg/weight).

Assessment of Change of Direction and Endurance

In this study, the t-test was performed to determine change of direction ability, and the Yo-Yo Intermittent Recovery Test was performed to determine endurance ability. The t-test is designed to assess athletic performance in multiple movements and is used to assess the agility of sports athletes (18). Four markers were prepared, and participants performed the following exercises: (a) forward sprinting from marker 1 to marker 2, (b) lateral shuffling to the left from marker 2 to marker 3, (c) lateral shuffling to the right from marker 3 to marker 4, (d) lateral shuffling to the left from marker 4 to marker 2, and (e) backpedaling from marker 2 to marker 1. The exercises were evaluated based on the time required to complete the task (a–e). The test was carried out twice, and the mean value of the two trials was recorded (s).

The Yo-Yo Intermittent Recovery Test is used to assess the endurance of sports athletes (19). This test was performed indoors in a gymnasium. Participants performed the Yo-Yo Intermittent Recovery Test after at least 2 h of fasting. This test (level 1) consisted of 40-m running stages (20-m outward and 20-m back) starting at 10 km·h−1 with a gradual increase. The speed increase was marked by audio beeps, which were used by the athletes to determine the required running speed. There was an active recovery time of 10 s for each stage, during which the individuals were required to walk around a cone located 5 m behind the start line. The test was terminated if a participant was unable to reach the line in time for two consecutive stages. Endurance performance was evaluated based on the maximal oxygen uptake (V˙O2max) calculated using the following formula (18): V˙O2max (mL·kg−1⋅min−1) = total distance (m) × 0.0084 + 36.4.

Assessment of Mood States

The mood states of the participants were evaluated using the Japanese translation of the Profile of Mood States, Second Edition (POMS 2; KANEKOSHOBO Inc., Tokyo, Japan) (20–22). POMS 2 is a mood inventory that contains 65 adjectives that describe seven moods, as follows: anger–hostility, confusion–bewilderment, depression–dejection, fatigue–inertia, tension–anxiety, vigor–activity, and friendliness. Participants were asked to indicate their mood states on a five-point scale ranging from “not at all” to “extremely” during the previous 1-wk period. The scores were calculated for each subscale. The total mood disturbance (TMD) score was calculated as follows: (anger–hostility + confusion–bewilderment + depression–dejection + fatigue–inertia + tension–anxiety) − vigor–activity. Many studies have reported that POMS 2 allows for the detailed assessment of mood changes, and the Japanese version has been reported as adequately reliable and valid based on a previous study which indicated a Cronbach’s alpha coefficient ranging between 0.779 and 0.926 for the mood subscales (20–22).

Statistical Analyses

Data are expressed as mean and standard deviation. All statistical analyses were performed using StatView J-5.0 software (SAS Institute, Cary, NC). Intergroup comparisons were performed using the Mann–Whitney U-test for continuous variables. The data at baseline and after the 11-wk intervention were compared using the Wilcoxon signed-rank test for continuous variables. Differences in the changes in anthropometric and ROM values, muscle strength, change of direction, endurance performance, and mood states of the two groups were included in a two-way repeated-measures analysis of variance and Scheffé’s method as a post hoc test for the intervention and the group–time interaction. The effect size was calculated using Cohen’s d value with the G*Power 3 free software package (23). In this study, the d value was defined as follows: 0.20 (small effect), 0.50 (medium effect), and 0.80 (large effect) (24). A probability value of <0.05 indicated statistical significance.

RESULTS

There were no significant differences in baseline characteristics between the two groups. The mean number of trainings performed by the participants in the training group was 20.0 ± 1.6 (participation rate = 90.9%, range = 18–22 sessions). In the present study, no adverse events were observed during the intervention period (Table 1).

Figure 2 shows the HR and percent HR reserve response during aquatic exercise training (the change in HR of the subjects in the last training). The mean HR and the percent HR reserve for 30 min of aquatic exercise training were 119.7 ± 10.2 bpm and 33.0% ± 9.3%, respectively. The mean maximum HR and the percent HR reserve during 30 min of aquatic exercise training were 157.4 ± 18.3 bpm and 64.2% ± 14.3%, respectively. The mean HR and the percent HR reserve during the warm-up period, high-intensity interval training, resistance training using tools, and cooldown period were 103.1 ± 9.6 bpm and 19.8% ± 7.6%, 124.7 ± 13.1 bpm and 34.2% ± 11.4%, 125.1 ± 11.2 bpm and 36.9% ± 10.4%, and 116.0 ± 8.8 bpm and 33.3% ± 7.7%, respectively.

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Figure 2:
HR and percentage of HR reserve (%HRR) response during aquatic exercise training. This figure shows the change in HR and percent HR reserve of the 10 subjects during the last training. Data are expressed as the mean values. The solid line indicates HR, and a dotted line indicates %HRR. %HRR was calculated using the following formula: %HRR = (HR during exercise − resting HR)/([220 − age] − resting HR) × 100. All participants consented to have their images used in this paper.

Table 2 shows anthropometric and ROM values, muscle strength, agility, and endurance performance at baseline and after the 11-wk intervention in both groups. After the intervention, the maximal thigh circumference; ROM of shoulder joint extension and hip joint extension, abduction, and external rotation; and knee joint flexion improved in the training group (P < 0.05 each). A significant group–time interaction effect was seen between the two groups in the measured values for ROM of shoulder joint extension and hip joint extension, abduction, and external rotation (P < 0.05 each). In terms of isokinetic muscle strength of high-speed concentric velocities (180°·s−1), the right knee flexion and extension, left knee extension, and back muscle strength increased in the training group (P < 0.05 each), whereas a significant group–time interaction effect was seen between the two groups in the measured values for right knee flexion and extension, left knee extension, and back muscle strength (P < 0.05 each). However, there were no significant differences in the measurements obtained at low-speed concentric velocities (60°·s−1). In terms of agility and endurance performance, the results of the t-test improved in both groups (P < 0.05 each). However, there was no significant interaction effect for group–time in the t-test between the two groups. No significant between-group differences were observed in the V˙O2max.

TABLE 2 - Differences in the Anthropometric and ROM Values, Muscle Strength, Agility, and Endurance Performance before and after 11-wk Intervention in Training and Control Groups.
Training Group (n = 10) Control Group (n = 10) Group–Time Interaction (P)
Baseline 11 wk P Effect Size Baseline 11 wk P Effect Size
Anthropometry measurements
 BMI (kg·m−2) 21.8 ± 1.2 22.0 ± 1.2 0.26 0.17 22.0 ± 1.9 22.4 ± 1.7 0.17 0.22 0.43
 Body fat mass (kg) 6.9 ± 1.4 7.0 ± 1.3 0.84 0.07 8.0 ± 2.9 8.9 ± 2.5 0.08 0.33 0.09
 Lean body mass (kg) 58.1 ± 4.0 58.6 ± 3.9 0.11 0.13 56.9 ± 4.5 57.0 ± 4.7 0.58 0.02 0.26
 Chest circumference (cm) 88.1 ± 4.8 88.3 ± 4.0 0.72 0.04 87.4 ± 4.9 86.7 ± 3.7 0.26 0.16 0.29
 Upper arm circumference (cm) 27.5 ± 1.7 27.8 ± 1.4 0.37 0.19 27.1 ± 2.5 26.6 ± 1.8 0.21 0.22 0.17
 Thigh circumference (cm) 54.3 ± 1.0 55.6 ± 1.7 0.01 0.88 54.5 ± 2.5 54.7 ± 2.7 0.39 0.08 0.15
ROM
 Shoulder joint flexion (°) 145.1 ± 10.9 148.4 ± 10.5 0.51 0.31 141.7 ± 11.0 136.9 ± 15.3 0.14 0.35 0.11
 Shoulder joint extension (°) 85.3 ± 6.1 90.5 ± 4.6 0.03 0.95 86.2 ± 9.2 82.8 ± 6.1 0.26 0.42 0.02
 Shoulder joint abduction (°) 178.5 ± 2.3 179.6 ± 2.5 0.35 0.46 175.4 ± 12.9 172.4 ± 10.7 0.19 0.25 0.14
 Shoulder joint internal rotation (°) 44.0 ± 6.5 44.9 ± 11.1 0.92 0.09 34.7 ± 10.0 38.7 ± 8.2 0.11 0.43 0.57
 Shoulder joint external rotation (°) 88.6 ± 12.7 94.4 ± 13.1 0.06 0.45 87.8 ± 16.9 92.3 ± 12.7 0.10 0.29 0.85
 Hip joint flexion (°) 77.7 ± 8.9 78.1 ± 9.7 0.91 0.04 78.2 ± 11.8 76.2 ± 11.1 0.07 0.17 0.47
 Hip joint extension (°) 16.8 ± 2.1 21.1 ± 3.1 0.02 1.57 18.7 ± 2.8 18.6 ± 3.8 0.72 0.03 0.04
 Hip joint abduction (°) 36.5 ± 5.9 42.4 ± 7.6 0.04 0.85 39.5 ± 6.7 29.1 ± 6.9 0.007 1.53 0.003
 Hip joint internal rotation (°) 28.0 ± 3.9 34.2 ± 8.1 0.06 0.88 27.5 ± 9.2 30.2 ± 7.8 0.33 0.31 0.41
 Hip joint external rotation (°) 30.8 ± 6.2 37.1 ± 5.8 0.01 1.05 28.5 ± 6.9 29.1 ± 7.6 0.50 0.08 0.02
 Knee joint flexion (°) 134.5 ± 8.3 133.6 ± 5.1 0.81 0.12 134.1 ± 8.9 126.8 ± 6.3 0.08 0.92 0.08
Isokinetic muscle strength
 Right knee flexion (180°·s−1) (N·m/weight) 1.09 ± 0.16 1.30 ± 0.21 0.005 1.11 1.26 ± 0.19 1.24 ± 0.18 0.84 0.11 0.004
 Right knee extension (180°·s−1) (N·m/weight) 1.99 ± 0.24 2.11 ± 0.24 0.04 0.50 1.95 ± 0.29 1.91 ± 0.27 0.88 0.14 0.001
 Left knee flexion (180°·s−1) (N·m/weight) 1.06 ± 0.19 1.22 ± 0.23 0.07 0.75 1.22 ± 0.20 1.15 ± 0.22 0.45 0.33 0.08
 Left knee extension (180°·s−1) (N·m/weight) 1.95 ± 0.22 2.07 ± 0.15 0.02 0.62 1.90 ± 0.23 1.85 ± 0.25 0.11 0.21 0.03
 Right knee flexion (60°·s−1) (N·m/weight) 1.82 ± 0.31 1.86 ± 0.29 0.65 0.13 1.77 ± 0.25 1.74 ± 0.25 0.42 0.12 0.60
 Right knee extension (60°·s−1) (N·m/weight) 3.30 ± 0.25 3.41 ± 0.29 0.11 0.40 3.10 ± 0.31 2.97 ± 0.35 0.09 0.39 0.09
 Left knee flexion (60°·s−1) (N·m/weight) 1.64 ± 0.32 1.73 ± 0.29 0.39 0.29 1.72 ± 0.26 1.61 ± 0.21 0.09 0.46 0.13
 Left knee extension (60°·s−1) (N·m/weight) 3.05 ± 0.32 3.16 ± 0.34 0.09 0.33 3.02 ± 0.39 2.88 ± 0.36 0.14 0.37 0.08
Back muscle strength (kg/weight) 2.29 ± 0.52 2.43 ± 0.37 0.04 0.30 1.94 ± 0.28 1.80 ± 0.22 0.04 0.55 0.004
Change of direction and endurance performance
t-test (s) 10.9 ± 0.7 10.4 ± 0.5 0.01 0.80 10.9 ± 0.7 10.3 ± 0.3 0.009 0.99 0.72
 V˙O2max (mL·kg−1⋅min−1) 55.9 ± 2.7 56.6 ± 3.2 0.05 0.24 56.1 ± 2.7 56.5 ± 2.5 0.09 0.15 0.62
Data are expressed as the mean ± SD.
Effect size is Cohen’s d value. The thresholds for small, medium, and large effects were defined as d values below 0.20, 0.50, and 0.80, respectively.
V̇O2max, maximal oxygen uptake.

Table 3 shows the mood states at baseline and after the 11-wk intervention in both groups. The depression–dejection and TMD scores decreased whereas the vigor–activity scores increased in the training group (P < 0.05 each). A significant interaction effect for group–time was observed between the POMS 2 vigor–activity scores in the two groups (P < 0.05).

TABLE 3 - Differences in Mood States Before and After 11-wk Intervention in the Training and Control Groups.
Training Group (n = 10) Control Group (n = 10) Group–Time Interaction (P)
Baseline 11 wk P Effect Size Baseline 11 wk P Effect Size
Anger–hostility (points) 5.4 ± 4.5 4.2 ± 3.6 0.11 0.29 3.5 ± 3.1 2.6 ± 2.3 0.16 0.32 0.88
Confusion–bewilderment (points) 11.1 ± 4.4 9.8 ± 3.7 0.52 0.32 11.8 ± 5.5 10.6 ± 4.6 0.12 0.24 0.63
Depression–dejection (points) 5.2 ± 6.1 2.4 ± 3.0 0.03 0.53 5.8 ± 5.3 5.3 ± 4.3 0.45 0.10 0.09
Fatigue–inertia (points) 6.1 ± 2.6 5.1 ± 3.5 0.40 0.32 5.2 ± 3.4 3.7 ± 2.3 0.09 0.50 0.85
Tension–anxiety (points) 12.4 ± 5.2 9.1 ± 5.9 0.02 0.59 11.0 ± 5.6 10.4 ± 5.9 0.59 0.10 0.27
Vigor–activity (points) 19.3 ± 6.4 24.5 ± 5.6 0.007 0.86 17.4 ± 8.4 17.2 ± 7.1 0.58 0.03 0.005
Friendliness (points) 12.6 ± 2.9 15.0 ± 3.5 0.11 0.74 14.4 ± 4.4 14.5 ± 3.8 0.94 0.03 0.08
TMD score (points) 20.9 ± 20.1 6.1 ± 17.7 0.007 0.78 19.9 ± 22.4 15.4 ± 16.9 0.17 0.22 0.05
Data are expressed as the mean ± SD.
Effect size means Cohen’s d value. The thresholds for small, medium, and large effects were defined as d values below 0.20, 0.50, and 0.80, respectively.

DISCUSSION

The major findings of present study were that the ROM of the shoulder and hip joints increased in the training group, and a significant group–time interaction effect was observed between the two groups in the ROM of the shoulder and hip joints. Moreover, isokinetic and back muscle strength increased in the training group, and a significant interaction effect for group–time was observed between the two groups in isokinetic and back muscle strength.

Reportedly, long-term aquatic exercise training is effective for physical fitness (5,6), muscle strength (7,25), and physical functions, including balance ability and aerobic performance (10,26). Most of the reports on the effects of aquatic exercise training have focused on middle-aged and older adults, and the effect of aquatic exercise training on the ROM and muscle strength in male collegiate soccer players has been unelucidated. Pöyhönen et al. (25) examined the effects of an aquatic resistance training program on neuromuscular performance and muscle mass of knee extensors and flexors in healthy women. The authors demonstrated that aquatic resistance training resulted in significant improvement in the muscle torque of the knee extensors and flexors, accompanied by a proportional improvement in neural activation and a significant increase in the lean muscle mass of the trained muscles (25). The results of our study demonstrate that although there were no significant improvements in the isokinetic muscle strength during low-speed concentric velocities (60°·s−1) in both groups, the isokinetic muscle strength during high-speed concentric velocities (180°·s−1) and back muscle strength increased in the training group. Our aquatic exercise training program consisted of complex movements, including warm-up exercises, high-intensity interval training, resistance training using tools, and cooldown activities. These types of complex movements require trunk muscle strength to support the trunk in unstable water and high-speed muscle contraction with a higher intensity than that at low-speed muscle contraction. Therefore, we believe that the isokinetic muscle strength during high-speed concentric velocities (180°·s−1) and the back muscle strength in the intervention group may have increased in relation to underwater movements during our aquatic exercise training.

Additionally, a previous study investigated the effects of a high-intensity aquatic interval training program on physical performance and showed that the program elicited a stimulus of sufficient strength to improve flexibility and anaerobic power (27). In the present study, the ROM of shoulder joint extension and hip joint extension, abduction, and external rotation increased in the training group. Extension of the shoulder joint is required when drawing water with the arm, whereas extension and abduction of the hip joint are required when kicking water with the leg or opening and closing the leg. Thus, improvements in the shoulder joint extension and hip joint extension and abduction movements may have occurred in the training group because the aquatic exercise training included complex underwater upper and lower limb movements. Our results confirmed what has been reported in previous studies and guidelines. Therefore, although the effects of aquatic exercise training have only been confirmed in middle-aged and older participants, the present findings suggest that aquatic exercise training may help improve ROM and muscle strength in male collegiate soccer players.

Previous studies have demonstrated that water-based exercise elicits significant improvements in cardiorespiratory fitness, muscle strength, body fat, and blood lipid profiles in older women (5,6,28). We hypothesized that underwater exercise training would increase change of direction and aerobic capacity in our population. However, although t-test and V˙O2max increased in both groups, there was no significant group–time interaction effect in the t-test and V˙O2max between the two groups. The participants of this study were all male collegiate soccer players, whose competitions require high change of direction ability to dodge opponents and aerobic ability to continuously chase the ball. Because both the intervention and the control groups were instructed to continue their daily sports activities, the t-test and V˙O2max may have improved in both groups due to the influence of those activities.

Aquatic exercise also has relaxation effects in middle-aged and older participants (9,29). Katsura et al. (9) investigated the efficacy of aquatic exercise training using water resistance equipment in older adults and found that it may improve balance, walking ability, and mood states (tension and anxiety). In the current study, the POMS 2 vigor–activity scores increased in the training group, whereas a significant interaction effect for group–time was observed for this item in the two groups. The effects of aquatic exercise training on positive mood have not been clarified. Polman et al. (30) investigated the influence of exercise training on the moods of pregnant women participating in an aqua- or studio-based exercise classes and found that exercise enhanced the participants’ positive moods. In the current study, all participants in the training group received aquatic exercise training together. In a previous study, we demonstrated that the implementation of short-term group exercise training may help improve the POMS 2 vigor–activity scores (31). Thus, the present findings suggest that performing aquatic exercise training together enhances participants’ positive moods, including those related to vigor–activity.

This study has several limitations. First, the study was limited to a small number of male collegiate soccer players. Thus, there was potential selection bias, and it is unclear whether our results are applicable to female athletes and other competitive sports players. Second, although the control group in this study was chosen for comparison with the training group, direct comparisons could not be drawn among our aquatic exercise training and other underwater and land exercise. Therefore, it was not possible to clarify whether the improvements observed in this study were caused by the effects of our aquatic exercise or those of other exercise. Moreover, athletes in both the training and the control groups belonged to the same team and practiced the same daily soccer training (excluding aquatic exercise training). This extra training was not matched in the control group with exercise outside of the pool similar to the aquatic training. Thus, improvements in physical performance may have been caused by an increased volume load in the training group and not by the effects of aquatic training in itself. Finally, despite our requests for adherence to the training, members of the training group did not attend every session (20.0 ± 1.6 times). In addition, the intervention period in our study was only 11 wk, which is insufficient to investigate the longer-term effects of aquatic exercise training on physical performance and mood states.

Despite these study limitations, the current study is unique in that it demonstrates the effects of aquatic exercise training on physical performance and mood states in male collegiate soccer players. This aquatic exercise training program was developed with the aim of health promotion in middle-aged and older populations and increasing physical performance in athletes (2–4,15). Many previous studies have demonstrated that long-term aquatic exercise training is effective in increasing physical fitness and improving muscle strength and physical function in middle-aged and older participants (5–7,10,25,26). The results of our study demonstrate the effects of aquatic exercise training and support the hypothesis that the inclusion of aquatic exercise training in daily sports activities helps improve the physical performance and mood states of male collegiate soccer players. Therefore, our findings suggest that aquatic exercise training may be proposed as a new training modality aimed at increasing physical performance, sports levels, and mood states in male collegiate soccer players. Given our results, we believe that it is necessary to promote aquatic exercise training to improve competition performance and mood states in male collegiate soccer players. Further research in a larger number of participants, including other competitive athletes, is necessary to clarify the mechanisms underlying the association of aquatic exercise training with physical performance and mood states, along with its implications for sports science.

The authors thank Hiroyuki Tanaka, Miho Miyashige, Miyuki Kanaya, Yukiko Ishida, and all the staff members of MIZUNO Inc. for their assistance in promoting this study. They thank all the members of the Laboratory of Exercise Physiology, Fukuoka University, for their help with data analysis. They are grateful to the participants of this study. The results of the study are presented clearly, honestly, without fabrication, falsification, or inappropriate data manipulation and do not constitute endorsement by the American College of Sport Medicine.

The authors declare no conflicts of interest in association with this study. This work was supported by a consignment study from MIZUNO, Inc. (no. 200185GJ).

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