The effects of dumbbell versus TRX suspension training on shoulder strength,vertical jump,and spike speed in volleyball players

Abstract

BACKGROUND:

Limited research exists on how various resistance training methods, such as TRX and dumbbells, impact sport-specific technical skills and muscle performance in young athletes.

OBJECTIVE:

This study aimed to compare the effects of 8-week Total-Body Resistance Exercise (TRX) suspension training and Traditional Dumbbell Training (TDT) on shoulder muscle strength, leg power, and spike speed in young male volleyball players.

METHODS:

Twenty-five male volleyball players were randomly assigned to either the TRX suspension group ( $n=$ 13) or the TDT group ( $n=$ 12). Anthropometric measures, countermovement jump (CMJ) height, spike speed, and shoulder joint isokinetic strength tests were conducted pre- and post-intervention. Both groups continued their volleyball training (5 days/week) and added TRX or TDT sessions for 2 months (60–90 minutes/day, 3 times/week), including 8 multi-joint exercises (1–3 sets/12 reps, 60 min rest). Statistical analysis involved Mann-Whitney and Wilcoxon tests, with effect size calculations.

RESULTS:

Compared to the TDT group, the TRX group showed significant improvements in CMJ height ( $+$ 12.17% vs. $+$ 0.83%; $p=$ 0.001), CMJ power ( $+$ 9.55% vs. $-$ 0.24 %; $p=$ 0.01), and spike speed ( $+$ 11.49% vs. $+$ 6.82%; $p=$ 0.03) with a small to moderate effect size.

CONCLUSIONS:

In young male volleyball players, TRX suspension training may be more effective than dumbbell training in enhancing jump performance and spike speed.

Keywords

Countermovement jump spike velocity strength team sports total body resistance exercise

1. Introduction

Volleyball is a sport characterized by dynamic, explosive, interval-based multidirectional movements that require the display of technical skills such as various defensive (block) and offensive (attack, spike, pass, and serve) maneuvers [1]. The most effective offensive action closely related to the outcome of the match is to hit the ball with the spiking technique [9] at the highest possible speed after the vertical high jump [20]. Therefore, to improve performance, volleyball players need to jump higher and spike balls with more power and speed than in other sports [32]. Developing specific technical skills and muscle strength of this nature is important for young players to achieve success [10]. To spike successfully, various factors, including arm swing, jump height, and range of motion, are core aspects [26]. During spiking, strength is transferred from the body through the shoulder, elbow, and wrist to the end of the upper limb. Such techniques require dynamic stabilization of the shoulder region [55]. When stabilization is limited, the possibility of injury to the glenohumeral joint [55], which includes the external and internal rotator (ER and IR) muscles of the shoulder, may increase. A well-designed strength and conditioning program is important to contribute to injury reduction [25]. Therefore, volleyball players need to engage in extensive resistance training (RT) to improve their force-velocity characteristics of performance [14]. To date, there is a lack of studies investigating the effect of a resistance training intervention on tissue structural, morphological, or compositional parameters in the specific population of highly trained adolescent athletes.

One of the commonly used training modalities is traditional resistance exercises. This training modality involves free-weight exercises, mostly bilateral equipment, such as dumbbells, barbells, and kettlebells [39]. Dumbbell equipment allows for performing both single-arm and alternating-arm movements similar to those used in volleyball, such as serving with one hand. It is emphasized that dumbbell training is important for volleyball players as it improves balance and body control, and these characteristics can be transferred very effectively to the demands of competition (e.g., spiking, blocking, etc.) [25].

Dumbbell training, a common method for improving shoulder strength, has been linked to enhanced rotator cuff strength [23] and sports performance, like tennis spike speed [57]. Treiber et al. [57] observed that resistance training with therabands and light dumbbells improved shoulder rotation moment and functional performance in tennis players. No studies have validated the impact of dumbbell training on the strength and technical skills, such as jumping and spiking speed, in young volleyball players, despite systematic reviews [48] demonstrating the effectiveness of plyometric jump training for volleyball-specific actions like spike jumps and countermovement jumps with arm swing.

Another effective method for improving muscle strength is suspension exercise systems or functional exercise bands, known as whole-body resistance training (TRX) [3, 24, 28, 42, 45, 47]. TRX training is known to enhance strength, balance, flexibility, core stability, endurance, coordination, and explosive power within a single session [35]. It has benefited athletes in golf, handball, soccer, and softball [53]. Marta et al. [36] found that an 8-week regimen of suspension and plyometric training improved upper and lower-body strength and power in untrained children, with suspension training particularly effective for lower body power. Another study showed that 5 weeks of TRX training improved energetic capacities and certain jumping parameters in young athletes [56]. While TRX training is well-studied for stability, core strength, and vertical jumping [35, 36, 56], its effects on shoulder muscle strength and spiking technique, crucial in volleyball, remain understudied [9, 20, 52].

Table 1
Participants’ characteristics (mean $\pm$ SD) by groups^§

	TRX group ( $n=$ 13)			Dumbbell group ( $n=$ 12)
	Min.	Max.	Mean	Min.	Max.	Mean	^*p
Age (y)	14.08	16.11	15.21 $\pm$ 0.67	15.00	16.11	15.47 $\pm$ 0.51	0.496
Body height (cm)	169.4	190.5	180.45 $\pm$ 6.21	175.5	193.4	182.77 $\pm$ 6.13	0.415
Body mass (kg)	51.60	91.40	69.16 $\pm$ 11.39	62.00	89.00	70.91 $\pm$ 8.32	0.744
BMI (kg/m²)	17.98	28.85	21.17 $\pm$ 2.99	17.38	24.45	21.23 $\pm$ 2.27	0.703
Sport age (y)	2	5	3.46 $\pm$ 0.88	2	4	3.00 $\pm$ 0.85	0.205

^§Data are presented as mean $\pm$ SD for all variables. TRX $=$ Total-Body Resistance Exercise; Age $=$ chronological age; BMI $=$ Body mass index. ^*There was no significant difference in physical measurements and training history among the groups.

While strength and power are vital attributes in volleyball [43], the relationship between jump height and ball velocity [5] remains debated. Forthomme et al. [20] found a moderate link between players’ jumping ability and spike speed across male volleyball players at different competitive levels, potentially explaining lower spike speeds among lower-level players. Conversely, no correlation was observed between vertical jump and spiking velocity in NCAA Division I female volleyball players [18]. Moreover, prior studies have shown a positive connection between isokinetic force in internal shoulder rotators and spike speed among elite male volleyball players [20]. These findings underscore the need for further research to understand the impact of jump height [57] and strength-related factors on spiking velocity. Yet, the applicability of strength gains from general resistance training to sport-specific performance, especially in crucial skills like spike speed [1], remains uncertain, particularly among young athletes.

To the best of our knowledge, there are currently no studies on the benefits or effectiveness of TRX training compared to traditional resistance training with dumb-bells in young volleyball players. Based on the above-mentioned research gaps, the aim of this study was to compare the effects of an 8-week TRX suspension and traditional dumbbell training (TDT) on CMJ jump height and power, shoulder internal–external isokinetic strength, and jump spike speed performance in young male volleyball players.

2. Methods

2.1 Participants

Twenty-five young participants aged 14 to 16, who were competing in the pre-junior volleyball league and were members of the same regional teams, voluntarily took part in the study (Table 1). Sample size calculation was performed using the G* Power 3.1.9.2 software [16] and defined as 1- $\beta$ ( $\beta=$ type II error probability). The impact size (d) was found as 1.3, and the calculation was performed to obtain 80% power at $\alpha=$ 0.05. Accordingly, evidence indicated that each group should have at least 11 participants.

This study involved an 8-week resistance training programme whereby participants were randomly divided into two homogeneous groups – TRX ( $n=$ 13) and TDT ( $n=$ 12) according the results of an isokinetic pretest.

The main inclusion criteria were as follows: participants’ age range (14 to 16 years old), volleyball training frequency (5 days per week for 2 hours each day), training experience (a minimum of 2 years in both volleyball and TRX training and at least 3 years of regular strength and conditioning training), strength-power training sessions per week (2–3 days per week), and no injuries sustained in the six months prior to the study.

The exclusion criteria for participating in the study were as follows: (a) a history of injury or musculoskeletal disorders in the past six months that could prohibit full participation in training; (b) participants who had experienced glenohumeral joint pain that limited their participation in volleyball play or practice for three consecutive days in the year before testing; (c) injuries during training resulting in absence from training; and (d) not meeting the training experience characteristics as outlined in the participation criteria.

Adherence to the training interventions was assessed through participants’ workout logs, which were reviewed weekly by the study staff. Compliance was determined based on the number of participants who completed both pre- and post-intervention assessments.

All participants and their parents signed an informed consent document and the study was approved by the Research Ethics Committee of the School of Medicine, Marmara University (Protocol number: 09.2017.654). At the enrolment visit, baseline demographic data were recorded. All participants were right-handed and practiced the spike by the right hand.

2.2 Study design and procedure

In this study, a pre-and post-intervention experimental design was used. Performance variables were the dependent variables (vertical jump height and power, shoulder internal–external rotators isokinetic muscle strength, jump spike speed), and the types of exercises (traditional dumb-bell training and TRX suspension training) were the independent variables.

The study was conducted during the first pre-competition period of the participants’ annual training programme and within a macro-cycle during the month. All participants were assessed before and after an 8-week training programme in the same condition on a closed volleyball court.

Table 2
Study design and trainings procedure

Study design and procedure-Pre Tests-1 week (1^st week)
Days	Hours	Tests		Test procedure
DAY 1^*	9:00–10:00	Completed a questionnaire & Anthropometric measurements		Demographic information, medical conditions, and training experiences Height, body weight, body mass index
	10:00–12:00 17:00–19:00	Tradional dumbbell training 10 RM measurement		$-$ 10 min Standart warm-up- ^¶Morning exercises: chest press, lunge, crunch, internal–external rotation $-$ 10 min Standart warm-up- ^¶ Afternoon exercises: rowing, shoulder front raises, hip trust, external-internal rotation
DAY 2^*	9:00–10:00	Vertical jump test		$-$ 10 min Standart warm-up- Countermovement jump test
	15:00–16:00	Jump spike speed performance test		$-$ 10 min Standart warm-up- Spiking speed performance test
DAY 3^*	10:00-12:00	Isokinetic muscle strength test		$-$ 10 min Standart warm-up- Shoulder internal and external rotator muscle groups strenght at 60 ${{}^{\circ}}$ /sec and 180 ${{}^{\circ}}$ /sec
After the 3rd day, participants were divided into two groups according to the results of isokinetic and 10 RM dumbbell strength tests. Only participants in The TRX group were included in the Trx Suspention Training Intensity Test
DAY 4^**	9:00–11:00 16:00–18:00	TRX suspention training intensity test		$-$ 10 min Standart warm-up- ^¶Morning exercises: chest press, lunge, jack-knife pike for TRX, internal rotation $-$ 10 min Standart warm-up- ^¶Afternoon exercises: rowing, TRX front deltoid, hip trust, external-internal rotation
^All participants; ^Only TRX Suspention Training participants. ^¶Morning Exercises: (4 exercises: Chest Press –pectorals and triceps; Lunge– gluteals and quadriceps; Crunch for Dumb-bell –rectus abdominus/Jack-knife Pike for TRX –abdominals, iliopsoas, and deltoids; Internal–External Rotation –* teres major, pectoralis major, subscapularis, latissimus dorsi, and anterior deltoid). ^¶Afternoon Exercises: (4 exercises: Rowing – latissimus dorsi and biceps; Dumb-bell Shoulder Front Raises – anterior deltoid/TRX Front Deltoid – deltoids; Hip Thrust– hamstrings, gluteals, and erector spinae; External-Internal Rotation –mid-trapezius, rhomboids, and deltoid).
Training procedures $-$ 8 weeks (2^nd – 9^th weeks)
Hours	Monday	Tuesday	Wednesday	Thursday	Friday	Saturday	Sunday
17.00–18.00	Technique & Tactic		Technique & Tactic		Technique & Tactic	Technique &	Technique &
18:00–19.00/19.30	TRX or TDT		TRX or TDT		TRX or DT	Tactic	Tactic
Loading intensity for Tradional Dumbbell Training (TDT) and TRX Suspention Training was determined every two weeks.
Study design and procedure – Post Tests-1 week (10 th week)
Days	Hours	Tests		Test procedure
DAY 1^*	9:00–10:00	Completed a questionnaire & Anthropometric measurements		Demographic information, medical conditions, and training experiences Height, body weight, body mass index
DAY 2^*	9:00–10:00	Vertical jump test		$-$ 10 min Standart warm-up- Countermovement jump test
	15:00–16:00	Jump spike speed Performance test		$-$ 10 min Standart warm-up- Jump spike speed performance test
DAY 3^*	10:00–12:00	Isokinetic muscle strength test		$-$ 10 min Standart warm-up- Shoulder internal and external rotator muscle groups strenght at 60 ${{}^{\circ}}$ /sec and 180 ${{}^{\circ}}$ /sec

The pretest (4 days) and post-test (3 days) sessions occurred on separate days, with a minimum of a 48-hour and maximum 7-day gap in-between. A familiarization session was conducted one week before baseline measurements. The study design is outlined in Table 2. On all three test days (Day 1, Day 2, Day 4), participants completed a 10-minute standard warm-up, including light cardiovascular exercise (5 minutes of jogging and 5 minutes of dynamic upper- and lower-body warm-up exercises). This warm-up preceded vertical jump, jump spike speed tests (Day 2), and dumbbell (Day 1) and TRX suspension (Day 4) tests. A different warm-up procedure was used for isokinetic testing, described in Section 2.7.

2.3 Anthropometric measurements

The participants’ body heights and body mass were measured while wearing shorts and a T-shirt with no footwear on Day 1. Using an X-scan body composition analyzer (Jawon Medical, Korea], height was measured to the nearest 0.1 cm, and body mass was determined to the nearest 0.05 kg.

2.4 Muscular strength testing

General guidelines and proper techniques [8, 22, 24, 25, 38, 54] were employed throughout the strength testing and training sessions, conducted by the same trained researchers. Baseline general strength and external loading intensity were assessed using 10-repetition maximum (10-RM) strength testswith dumbbells or plates [38, 54] on Day 1. All participants exhibited similar 10-RM strength levels during the pretest. After Day 3, participants were grouped based on isokinetic and 10-RM dumbbell strength test results. On Day 4, the TRX group underwent an intensity test for TRX suspension training.

Baseline evaluations were conducted every two weeks to monitor changes in exercise loading intensity for both training groups. To prevent fatigue, participants performed half of the exercises in the morning and the remaining half in the afternoon. The exercise order and targeted muscle groups are detailed in Table 2.

2.4.1 Ten repetition maximum test for dumb-bell exercises

Dynamic muscle strength performance (kg) of the upper and lower body was assessed using the 10-RM test, a validated and reliable method [51]. The 10-RM represents the maximum load at which the subject can perform no more than 10 consecutive maximal repetitions with either a plate or dumbbell.

Participants began with a standardized warm-up, consisting of 15 repetitions using a self-selected load, roughly equivalent to 50% of their normal training load. Subsequently, they underwent 10RM testing with 10-minute rest intervals between exercises. Repetitions followed a metronome-based tempo of 4 seconds per repetition (2 seconds for the concentric phase and 2 seconds for the eccentric phase) [54].

To determine the 10-RM, participants initially performed lifts in the range of 14–16 repetitions while maintaining proper form. Once this range was achieved, the load was increased by 10%, and the repetition range was adjusted accordingly until reaching their ’repetition maximum.’ A 5-minute recovery period followed each 10-RM assessment [27]. This process entailed 3–4 trials per lift for each participant.

2.4.2 The intensity of the load of TRX exercises

The level of intensity in TRX suspension training was adjusted based on three primary principles: a) stability; b) vector resistance; and c) pendulum [8, 22, 24, 45]. Exercise intensity and the overall workload were determined by the angle between the floor and the TRX straps, as well as the difficulty level (level 1–30 ${{}^{\circ}}$ angle, level 2–45 ${{}^{\circ}}$ angle, level 3–60 ${{}^{\circ}}$ or 75 ${{}^{\circ}}$ angle).

Angular measurements for each movement were taken using tapes placed on the ground (Fitness Anywhere LLC, San Francisco, CA) [45]. The maximum number of repetitions performed with correct technique at level 1 was evaluated for each exercise. Similar to TDT, there was a 10-minute rest interval between TRX exercises [54], and participants adhered to a metronome-driven pace: 4 seconds for the concentric phase and 2 seconds for the eccentric phase [38].

In line with previous research [47], we also assessed exercise intensity based on the movement’s difficulty level and monitored it using the rate of perceived exertion (RPE) scale (6 to 20). We selected moderate exercise intensity for the entire session, with an RPE score of 12, based on the values determined at the end of the tests.

2.5 Jump performance

Vertical jump height and explosive power of the lower extremity extensor muscles were assessed using the reliable countermovement jump (CMJ) test with arm swing [1, 34]. The Swift Performance Speed Mat (Swift Performance, Brisbane, Australia) measured participants’ vertical jump height, recording data using the Swift Performance SpeedLight iPad app, version-493 (Swift Performance, Brisbane, Australia), based on flight time calculations. A one-minute recovery followed each trial, and participants completed three jumping trials (knee flexion up to approximately 90^∘). The analysis focused on the highest jump height (in centimeters) and peak power (in watts) from these trials [34].

2.6 The jump spike speed performance

The purpose of this test is to assess ball speeds during the spike [18, 43]. This protocol, combined with the monitoring of jump ability, will allow coaches to measure the ability to use strength in the technical actions of spike [43]. Spiking performance was evaluated in a gym on a standard court with a net set at the regulation height of 2.43 meters. The volleyball used in the testing adhered to the standards set by the International Volleyball Federation (Mikasa MVA200), and its pressure was verified before each testing session, maintaining a range of 0.30 to 0.325 kg/cm2 [1].

Each player participated in the jump spike speed test after executing the spike technique three times with at least 95% of their maximum perceived effort. Subsequently, they completed a five-minute specific warm-up. Following the warm-up, each player performed three spike attempts, with one-minute rest intervals between each attempt. These tests took place on an indoor court and were overseen by the same trained coach who served as the ball tosser.

The tester was positioned on a referee stand, located 3 meters from the net and 1 meter outside the sideline, with a radar gun placed at an approximate height of 3 meters [18]. The ball was tossed 3–4 meters upwards from 0.5 meters behind the net [43]. Participants were instructed to swiftly spike the ball into a designated target area (4.5 $\times$ 4.5 meters in the 5th zone; 1 $\times$ 1 meter in the middle of the marked area) by taking a step from 5 meters behind the net and transitioning from the 4th position to the 5th position as rapidly as possible. The highest ball speed achieved, measured in kilometers per hour, was recorded using a professionally calibrated radar gun device (Sports Radar Gun SR3500; Sports Electronics, Inc., Homosassa, FL, USA) and used for subsequent statistical analyses [18].

2.7 Isokinetic tests

Maximum isokinetic concentric external rotation strength of the shoulder muscles (infraspinatus, teres minor, and posterior deltoid) and internal rotation strength (subscapularis, pectoralis major, teres major, and anterior deltoid) were measured for the dominant arm using a Biodex System 4 Pro Multijoint System isokinetic dynamometer (Biodex Medical System, Shirley, NY, USA) in the research laboratory. The measurements were conducted by the same experienced examiner.

Shoulder isokinetic strength was assessed at two different angular velocities (60 ${{}^{\circ}}$ /s and 180 ${{}^{\circ}}$ /s), as commonly employed in the literature [15, 32]. The selected isokinetic speeds involved concentric contractions, and the order in which subjects initiated the test was randomized. Participants began with a supervised 5-minute warm-up on a cycle-style arm pedal machine (MagneTrainer, 3D Innovations, Greeley, CO) at a moderate pace (50–100 W). This was followed by a 5-minute warm-up involving low-intensity dynamic range of motion exercises for the upper limbs, including shoulder flexion, extension, abduction, adduction, internal rotation, and external rotation.

During testing, participants were seated in an isokinetic testing chair, with a restraining strap securing their trunk to minimize unnecessary body movements. The back angle of the dynamometer chair was set at 70 ${{}^{\circ}}$ for stabilization. The machine was calibrated according to the manufacturer’s instructions. Participants maintained a right angle (90 ${{}^{\circ}}$ ) abduction of the humerus from the trunk, with the elbow flexed at 90 ${{}^{\circ}}$ .

Participants completed 2–3 submaximal warm-up trials before each test, followed by a 60-s rest. The test included 5 repetitions at 60 ${{}^{\circ}}$ /s and 10 repetitions at 180 ${{}^{\circ}}$ /s for internal and external rotations [13, 15], with a 3-minute rest. The software selected the best three contractions in each test [15].

The isokinetic measures used for statistical analysis included peak moment (PM in Newton-metres, Nm), PM normalized by body weight (PM/BW in Nm/kg), total work (TW in joules), and the internal rotation/external rotation ratio (ER/IR in %). Ratios for IR:ER were calculated for each athlete from the relative strength values and reported as a single number.

2.8 Training procedures

All participants attended daily technical-tactical training sessions lasting either one hour (including a 15-minute warm-up, 40 minutes of technical-tactical training, and a 5-minute cool-down) or two hours (including a 20-minute warm-up, 90 minutes of technical-tactical training, and a 10-minute cool-down), five days a week. Both groups completed a total of 40 training sessions during the research period.

In addition to these sessions, participants attended either TRX suspension or TDT resistance training, totaling 24 sessions, over an 8-week period (3 days a week, lasting 60–90 minutes each day). These resistance training sessions took place after the technical-tactical training sessions. Further details of the training procedures can be found in Table 2.

The TRX suspension and dumbbell exercise protocol was developed by the researcher, who possesses extensive coaching experience in this domain. The goal was to enhance strength in various areas of the upper and lower body, as well as the hip and trunk muscles. Exercise volume and intensity were adjusted following the FITT (frequency, intensity, time spent, and type of exercise) principles [7, 42]. The typical resistance training program was structured based on recommendations from meta-analyses, which suggest 2–3 sets of 8–15 repetitions with loads ranging from 60% to 80% of the 1RM across 6–8 exercises for youth strength development [7]. Both training programs adhered to the suggestion of training at least 2–3 times a week [42] and using resistances between 70% and 100% of the 1RM to improve power [25].

The loading value for each exercise in both the TDT and TRX suspension training programs was determined during the pretest, with 70% of the values used for exercise intensity (details provided in each exercise section). Within these characteristics, the intensity and volume of exercises in both TDT and TRX workouts have been maintained, as much as possible, at an equivalent level, and the exercise routines in both workouts have been designed to result in an equal amount of work being performed.

Each session included a warm-up (10–15 minutes), the primary exercise phase (40–60 minutes), and a cooldown (5–15 minutes). The duration of the primary session increased every two weeks, gradually extending the total training time from 60 to 90 minutes. No injuries were reported during the 8-week training sessions, and both groups refrained from engaging in other physical activities during the study. Both groups were supervised by the primary investigator, and they demonstrated compliance by attending all training sessions regularly.

Table 3
Eight weeks exercise protocol of TRX suspension training program

TRX training program exercises protocol
Exercises	Weeks	Set	Rep	Stability	Vector resistance	Pendulum	Resttime (s)	Total time (min)
1TRX chest press	1–2 3–4–5 6–7–8	1 2 3	12 12 12	Low Low Low	75 ${{}^{\circ}}$ 60 ${{}^{\circ}}$ 45 ${{}^{\circ}}$		60 60 60	12 24 36
2TRX rowing	1–2 3–4–5 6–7–8	1 2 3	12 12 12	Low Low Low	75 ${{}^{\circ}}$ 60 ${{}^{\circ}}$ 45 ${{}^{\circ}}$		60 60 60	12 24 36
3TRX lunge	1–2 3–4–5 6–7–8	1 2 3	12 12 12	Low Low Low	0 ${{}^{\circ}}$ 0 ${{}^{\circ}}$ 0 ${{}^{\circ}}$	Low Middle High	60 60 60	12 24 36
4-TRX front deltoid	1–2 3–4–5 6–7–8	1 2 3	12 12 12	Low Low Middle	75 ${{}^{\circ}}$ 60 ${{}^{\circ}}$ 60 ${{}^{\circ}}$		60 60 60	12 24 36
5-TRX pike	1–2 3–4–5 6–7–8	1 2 3	12 12 12	Low Low Low	0 ${{}^{\circ}}$ 0 ${{}^{\circ}}$ 0 ${{}^{\circ}}$	Low Middle High	60 60 60	12 24 36
6-TRX internal external rotation	1–2 3–4–5 6–7–8	1 2 3	12 12 12	Low Low Middle	75 ${{}^{\circ}}$ 60 ${{}^{\circ}}$ 60 ${{}^{\circ}}$		60 60 60	12 24 36
7-TRX hip trust	1–2 3–4–5 6–7–8	1 2 3	12 12 12	Low Low Low	0 ${{}^{\circ}}$ 0 ${{}^{\circ}}$ 0 ${{}^{\circ}}$	Low Middle High	60 60 60	12 24 36
8-TRX external internal rotation	1–2 3–4–5 6–7–8	1 2 3	12 12 12	Low Low Low	75 ${{}^{\circ}}$ 60 ${{}^{\circ}}$ 45 ${{}^{\circ}}$		60 60 60	12 24 36

^§Note: TRX $=$ Total-Body Resistance Exercise; Rep $=$ Repetition.

2.8.1 TRX training programme exercises

The TRX training program utilized a TRX suspension training system (Fitness Anywhere LLC, San Francisco, CA), anchored 2.44 cm above the ground using iron handles fixed to the gym walls. The TRX suspension training program (as detailed in Table 3) consisted of eight multi-joint exercises, varying between one to three sets based on the training week. Participants performed 12 repetitions per exercise with a one-minute passive rest between exercises and sets. The TRX training protocol was adapted from previous studies [8, 17, 22, 30, 35, 36, 45, 47].

The intensity of TRX suspension training is based on three principles: stability, vector resistance, and pendulum [8, 22, 24, 45]. Exercise intensity and workload depend on the body or attached body part angle relative to the ground. Progression involves increasing exercise difficulty, categorized as 1 (easy), 2 (moderate), and 3 (difficult) based on perceived exertion (RPE) [8, 30, 45]. This is done by altering body stance, transitioning from bipedal to unipedal, and adjusting angles. Resistance is controlled with angles of 30 ${{}^{\circ}}$ (level-1), 45 ${{}^{\circ}}$ (level-2), 60 ${{}^{\circ}}$ (level-3), and 75 ${{}^{\circ}}$ (level-3) for each subject, measured using floor markings. Initially, suspension point distance ranged from 40–50 cm for level-1 exercises (weeks 1–2) to 95–100 cm for level-3 exercises (weeks 3–4) over four weeks. The RPE scale (6–20) was used to monitor exercise intensity, with an RPE of 12 chosen as moderate exercise intensity for all sessions, consistent with previous studies [47].

Table 4
Eight weeks exercise protocol of TRX suspension training program

Dumbbell training program exercises protocol
Exercises	Weeks	Set	Rep	Intensity %	Dumbbell /plate weight	Rest time (s)	Total time (min)
1-Dumbbell chest press	1–2 3–4–5 6–7–8	1 2 3	12 12 12	70% 70% 70%	(10/12,5/ 15/17,5 kg.) $\times$ 2 dumbbell	60 60 60	12 24 36
2-Dumbbell rowing	1–2 3–4–5 6–7–8	1 2 3	12 12 12	70% 70% 70%	(10/12,5/ 15/20 kg.) single	60 60 60	12 24 36
3-Dumbbell lunge	1–2 3–4–5 6–7–8	1 2 3	12 12 12	70% 70% 70%	(5 /7,5/10 kg.) $\times$ 2 plate	60 60 60	12 24 36
4-Dumbbell shoulder front raises	1–2 3–4–5 6–7–8	1 2 3	12 12 12	70% 70% 70%	(3/5/7,5 kg.) single plate	60 60 60	12 24 36
5- Crunch	1–2 3–4–5 6–7–8	1 2 3	12 12 12	70% 70% 70%	(5/7,5/10 kg.)single plate	60 60 60	12 24 36
6-Dumbbell internal –external rotation	1–2 3–4–5 6–7–8	1 2 3	12 12 12	70% 70% 70%	(3/5/7,5 kg.) $\times$ 2 dumbbell	60 60 60	12 24 36
7-Dumbbell hip trust	1–2 3–4–5 6–7–8	1 2 3	12 12 12	70% 70% 70% ${{}^{\circ}}$	(5/7,5/10 kg.) single plate	60 60 60	12 24 36
8-Dumbbell external –internal rotation	1–2 3–4–5 6–7–8	1 2 3	12 12 12	70% 70% 70%	(3/5/7,5 kg.) $\times$ 2 dumbbell	60 60 60	12 24 36

Rep $=$ Repetition.

2.8.2 Traditional dumb-bell training programme exercises

The intensity of traditional dumbbell training (TDT) was determined based on the 10-repetition maximum (10 RM) at 70% in the pretest, following references [19, 51, 54]. The loading intensity for TDT was progressively increased every two weeks. Participants performed a total of eight multi-joint exercises, with repetitions ranging from one to three sets of 12. A one-minute passive rest was allowed between each exercise and set. Dumbbells (with an average weight of 3–20 kg each) and plates (with an average weight of 5–10 kg each) were used for training, as detailed in Table 4.

2.9 Statistical analyses

Descriptive statistical methods utilized mean values, standard deviations (SDs), and both minimum and maximum scores. To assess data normality, a Shapiro–Wilk test was applied. The homogeneity of variances was confirmed using the Levene test. For intra-group pre- and post-test evaluations, the Wilcoxon Signed Ranks test was employed, while inter-group pre- and post-test comparisons were conducted using the Mann–Whitney U test.

Additionally, effect sizes (ES) were computed to gauge the intervention effects, employing the standardized mean difference formula (ES $=$ [Mean Post – Mean Pre]/SD of the pre-value)], as recommended by Rhea [49] for recreationally trained subjects. Effect sizes were categorized as trivial, small, moderate, or large, with values falling into the ranges of $<$ 0.35, 0.35–0.80, 0.80–1.50, and $>$ 1.50, respectively. All analyses were carried out using SPSS version 19.0 (SPSS, Inc., Chicago, IL, USA) and significance was set at p-value $\leqslant$ 0.05.

Table 5
Statistical analysis of CMJ and spike speed measurements

Motor skills measure-ments	TRX Group ( $n=$ 13)		Pre-/post-intervention differences within the TRX group		Dumbbell Group ( $n=$ 12)		Pre-/post-intervention differences within the dumbbell group		Baseline differences between groups	Pre-/post-intervention differences between groups
	Pre	Post	^bp	^¶Effect size	Pre	Post	^bp	^¶Effect size	^ap	^ap
CMJ height (cm)	47.48 $\pm$ 6.14	53.11 $\pm$ 5.70		0.92^#	45.58 $\pm$ 7.49	45.79 $\pm$ 6.70		0.03^£	0.369	0.001^∗∗
Differences $\Delta$ (%)	12.17 $\pm$ 3.97		0.001^∗∗		0.83 $\pm$ 6.69		0.078
CMJ power (watt)	1206.37 $\pm$ 219.77	1309.12 $\pm$ 193.39		0.47^□	1208.41 $\pm$ 234.46	1197.38 $\pm$ 199.40		$-$ 0.05^£	0.62	0.01^∗
Differences $\Delta$ (%)	9.55 $\pm$ 8.13		0.001^∗∗		$-$ 0.24 $\pm$ 7.26		0.17
Spike speed (km/h)	67.87 $\pm$ 8.61	75.23 $\pm$ 6.20		0.85^#	66.30 $\pm$ 8.79	70.44 $\pm$ 7.10		0.47^□	0.765	0.032^†
Differences $\Delta$ (%)	11.49 $\pm$ 6.41		0.001^∗∗		6.82 $\pm$ 6.88		0.003^∗

^§Note: SD, Standard Deviation; TRX $=$ Total-Body Resistance Exercise; CMJ; Countermovement Jump; ^aMann Whitney U Test; ^bWilcoxon Signed Ranks Test; Indicates statistical significance ${}^{{\dagger}}p\leqslant$ 0.05; ^∗p0.01; ${}^{\ast\ast}p\leqslant$ 0.001; ^¶Effect size, ^‡Large,^#Moderate, ^□Small, ^£Trivial.

3. Results

3.1 Physical characteristics, CMJ, and spike speed measurements

Baseline physical characteristics and training levels for each group are presented in Table 1. At baseline, there were no significant differences between the groups in terms of physical characteristics (height, weight, body mass index [BMI]), years of volleyball training (Table 1), or motoric skills measurements (Table 5).

When comparing the results within each group, a statistically significant ( $p\leqslant$ 0.05) increase in spike speed from the pre- to the post-innervation was observed in both the TRX ( $+$ 11.49%) suspension and TDT ( $+$ 6.82%) groups (Table 5). Furthermore, CMJ height ( $+$ 12.17%) and CMJ peak power ( $+$ 9.55%) also exhibited significant increases over the 8-week period but were observed only in the TRX suspension training group, as indicated in Table 5.

The results of the Mann–Whitney U-test indicated significant differences between the groups in spike speed, CMJ height, and peak power when comparing the pre- and post-intervention ( $p<$ 0.001), with the TRX group showing superior performance (Table 5). Effect size calculations revealed a moderate effect size for improvements in jump speed and CMJ height in the TRX group, while the effect size for CMJ peak power was small (Table 5).

Table 6
Statistical analysis of external and internal rotators peak moment (PM) values in 60 ${{}^{\circ}}$ /s and 180 ${{}^{\circ}}$ /s angular velocities

Isokinetic strength variables	TRX Group ( $n=$ 13)		Pre-/post-intervention differences within the TRX group		Dumbbell Group ( $n=$ 12)		Pre-/post-intervention differences within the dumbbell group		Baseline differences between groups	Pre-/post-intervention differences between groups
	Pre	Post	^bp	^¶Effect size	Pre	Post	^bp	^¶Effect size	^ap	^ap
pm external rotators – (60 ${{}^{\circ}}$ /s) (Nm)	28.48 $\pm$ 4.59	33.09 $\pm$ 4.30		1.00^#	27.44 $\pm$ 5.33	33.16 $\pm$ 4.65		1.07^#	0.514	0.242
Differences $\Delta$ (%)	14.01 $\pm$ 7.67		0.002^∗		17.46 $\pm$ 7.65		0.002^∗
PM internal rotators – (60 ${{}^{\circ}}$ /s) (Nm)	42.42 $\pm$ 7.72	46.16 $\pm$ 8.59		0.48^□	40.42 $\pm$ 6.97	44.97 $\pm$ 7.65		0.65^□	0.480	0.744
Differences $\Delta$ (%)	7.77 $\pm$ 8.24		0.013^∗		9.46 $\pm$ 11.46		0.019^∗
PM external rotators – (180 ${{}^{\circ}}$ /s) (Nm)	28.48 $\pm$ 4.59	33.09 $\pm$ 4.30		1.00^#	27.44 $\pm$ 5.33	33.16 $\pm$ 4.65		1.07^#	0.514	0.242
Differences $\Delta$ (%)	14.01 $\pm$ 7.67		0.002^∗		6.82 $\pm$ 6.88		0.002^∗
PM internal rotators – (180 ${{}^{\circ}}$ /s) (Nm)	41.24 $\pm$ 6.61	46.93 $\pm$ 7.33		0.86^#	39.57 $\pm$ 5.57	46.55 $\pm$ 5.06		1.25^#	0.568	0.478
Differences $\Delta$ (%)	12.08 $\pm$ 4.80		0.001^∗∗		14.87 $\pm$ 8.49		0.002^∗

^§Note: SD, Standard Deviation; PM $=$ Peak Moment; TRX $=$ Total-Body Resistance Exercise; CMJ; Countermovement Jump; ^aMann Whitney U Test; ^bWilcoxon Signed Ranks Test; Indicates statistical significance ${}^{{\dagger}}p\leqslant$ 0.05; ${}^{\ast}p\leqslant$ 0.01; ${}^{\ast\ast}p\leqslant$ 0.001; ^¶Effect size, ^‡Large,^#Moderate, ^□Small, ^£Trivial.

Table 7

Statistical analysis of external and internal rotators peak moment body weight (PM/BW) and ER/IR ratio values in 60 ${{}^{\circ}}$ /s and 180 ${{}^{\circ}}$ /s angular velocities

Isokinetic strength variables	TRX Group ( $n=$ 13)		Pre-/post-intervention differences within the TRX group		Dumbbell Group ( $n=$ 12)		Pre-/post-intervention differences within the dumbbell group		Baseline differences between groups	Pre-/post-intervention differences between groups
	Pre	Post	^bp	^¶Effect size	Pre	Post	^bp	^¶Effect size	^ap	^ap
PM/BW External Rotators – (60 ${{}^{\circ}}$ /s) – (Nm/kg, in %)	46.77 $\pm$ 7.28	50.41 $\pm$ 5.64		0.50^#	44.92 $\pm$ 6.35	50.57 $\pm$ 5.59		0.89^#	0.550	0.221
Differences $\Delta$ (%)	7.48 $\pm$ 7.18		0.009^∗		11.26 $\pm$ 6.77		0.003^∗
PM/BW Internal Rotators– ( 60 ${{}^{\circ}}$ /s) – (Nm/kg, in %)	62.14 $\pm$ 7.23	67.60 $\pm$ 7.78		0.76^#	57.41 $\pm$ 6.64	63.90 $\pm$ 7.53		0.98^#	0.077	0.724
Differences $\Delta$ (%)	7.80 $\pm$ 8.19		0.23^†		9.47 $\pm$ 11.40		0.23^†
PM/BW External Rotators – (180 ${{}^{\circ}}$ /s) – (Nm/kg, in %)	42.00 $\pm$ 5.95	48.85 $\pm$ 5.56		1.15^#	38.95 $\pm$ 5.14	47.32 $\pm$ 5.84		1.63^‡	0.211	0.314
Differences $\Delta$ (%)	13.98 $\pm$ 7.61		0.002^∗		17.40 $\pm$ 7.63		0.002^∗
PM/BW Internal Rotators – (180 ${{}^{\circ}}$ /s)– (Nm/kg, in %)	60.83 $\pm$ 8.50	69.17 $\pm$ 8.68		0.98^#	56.30 $\pm$ 4.56	66.46 $\pm$ 6.10		2.23^‡	0.201	0.415
Differences $\Delta$ (%)	12.08 $\pm$ 4.80		0.001^∗∗		14.84 $\pm$ 8.51		0.002^∗
ER/IR Ratio – (60 ${{}^{\circ}}$ /s) – (%)	0.75 $\pm$ 0.07	0.75 $\pm$ 0.06		0.00^£	0.79 $\pm$ 0.09	0.80 $\pm$ 0.08		0.11^£	0.398	0.586
Differences $\Delta$ (%)	$-$ 1.10 $\pm$ 11.37		0.752		0.93 $\pm$ 11.68		0.894
ER/IR Ratio – (180 ${{}^{\circ}}$ /s) – (%)	0.69 $\pm$ 0.06	0.71 $\pm$ 0.06		0.33^£	0.69 $\pm$ 0.07	0.71 $\pm$ 0.08		0.29^£	0.978	0.785
Differences $\Delta$ (%)	2.31 $\pm$ 4.96		0.157		2.62 $\pm$ 8.48		0.238

^§Note: SD, Standard Deviation; TRX $=$ Total-Body Resistance Exercise; CMJ; Countermovement Jump; PM/BW $=$ Peak Moment Body Weight; ER $=$ External Rotation; IR $=$ Internal Rotation; ^aMann Whitney U Test; ^bWilcoxon Signed Ranks Test; Indicates statistical significance ${}^{{\dagger}}p\leqslant$ 0.05; ${}^{\ast}p\leqslant$ 0.01; ${}^{\ast\ast}p\leqslant$ 0.001; ^¶Effect size, ^‡Large, ^#Moderate, ^□Small, ^£Trivial.

3.2 Isokinetic measurements

The statistical analyses of intra- and inter-group differences in the pre- and post-intervention mean values of the components of the internal and external isokinetic shoulder rotator muscles for both TRX and TDT are outlined in Table 6 (PM) and Table 7 (PM/BW). Initially, there were no statistically significant differences between the groups concerning the PM (Table 6) and PM/BW (Table 7).

In both groups, after 8 weeks, significant increases were observed between the pre- and post-tests for both rotations strength components (PM, PM/BW) at both angular velocities (60 and 180 ${{}^{\circ}}$ /s), except for the ER/IR ratio values. The effect sizes for the within-group change differences were determined to be small for PM internal rotators at 60 ${{}^{\circ}}$ /s and medium for all other components (Table 6). For all angular velocities, it was found that the within-group effect sizes for changes in the pre- and post-test results of PM/BW variables were moderate in the TRX group for all variables and moderate (PM/BW external and internal rotators at 60 ${{}^{\circ}}$ /s) and high (PM/BW external and internal rotators at 180 ${{}^{\circ}}$ /s) in the TDT group (Table 7).

4. Discussion

The primary finding of this study indicates that an 8-week TRX training program (24 sessions; 3 days/week; 60–90 min/day in total) had a noticeable impact on enhancing spike speed (moderate effect), vertical jump height (moderate effect), and jump power (small effect) in young male volleyball players compared to TDT programs. Both resistance training methods contributed to the improvement of isokinetic PM and PM/BW strength values for internal and external shoulder rotational muscles, with varying effect sizes. However, neither training approach demonstrated superiority over the other. Neither of the training methods significantly affected the ER/IR ratio values at either velocity. While the exercise routines in both training programs were designed with apparently equal amounts of work, it was concluded that the TRX training model could enhance lower extremity jump height and power in young male volleyball players, as well as increase spiking speed, which is an important technical skill [10] in volleyball.

Prior studies [4, 11, 56] have shown that various training methods, including weight training, plyometrics, and heavy resistance strength training, can enhance vertical jump performance by 5–24%. Current results support these findings, demonstrating the effectiveness of an 8-week TRX program in significantly improving vertical jump height (12.2%) and power (9.6%) in young male volleyball players. However, an 8-week TDT program alongside volleyball training did not impact jump performance noticeably. While more research is needed, these initial findings suggest the potential benefits of TRX training for improving jumps.

The current study’s results align with Nalbant and Kınık’s [40] research, which incorporated a 6-week (2 days/week) TRX program alongside basketball training for young basketball players, resulting in a significant improvement in jumping ability. However, present findings contrast with Janot et al.’s [28] study, where a 7-week (3 days/week) resistance training program (26.5%) demonstrated a higher percentage increase in lower-leg strength in young women compared to TRX training (13.1%). Nevertheless, it is essential to exercise caution when directly comparing Janot et al.’s [28] study to the current research due to the gender differences in the studied populations. Differences in jump height and power adaptations between TRX and TDT in this study may result from various factors. Although both groups practiced different resistance strength training methods, their technical and tactical training was uniform. The exclusive improvement in jump height and power in the TRX group likely reflects the effectiveness of TRX training, independent of shared technical and tactical training.

Maté-Muñoz et al. [37] stressed the importance of explosive resistance exercises for enhancing both strength development and speed of movement, regardless of the load. In current study, although there were no direct jumping exercises, both programs were designed to activate upper and lower extremity muscle groups through multi-joint movements. Exercises like lunges, pikes, and hip thrusts effectively targeted lower extremity muscle group strength. The precise mechanisms responsible for these observed changes are not straightforward and likely involve multiple factors.

The improvements in muscle strength and vertical jump performance in the TRX suspension training group can be attributed to physiological and musculoskeletal adaptations, including enhanced mechanoreceptor stimulation and neural adjustments unique to this exercise form. TRX, with its closed-kinetic chain exercises, promotes axial joint load, joint motion perception, and stabilizing muscle contractions [41]. It efficiently activates neuromuscular function, surpassing traditional weight training [33], contributing to increased muscle activation and strength [29]. Central muscles, crucial for stabilizing the spine, abdomen, waist, and pelvis, optimize the distribution of force by engaging internal trunk muscles [2]. Strengthening these central muscles through TRX enhances athletic performance more effectively than traditional resistance training [2]. This study supports the idea [56] that TRX training can enhance energy capacity in trained muscles, leading to overall strength improvement.

In this study, the TRX group significantly improved their pretest scores for vertical jump height (from 47.48 cm to 53.11 cm) and leg muscle explosive power (from 1206.37 watts to 1309.12 watts) during the post-test, while the TDT group showed no enhancements in vertical jump performance. Generating maximum muscle power is crucial for sports performance, encompassing various defensive and offensive maneuvers in volleyball [44] training and games [59]. Notably, both groups had baseline vertical jump heights exceeding reference values [52]. Remarkably, following the intervention, TRX suspension players’ vertical jump height values surpassed the average reported values (40.3–49.7 cm) for volleyball players in the same or older age groups across different countries [52].

This research highlights that the TRX training model (11.5%, moderate effect size: $d=$ 0.85) produced a significantly greater spike speed increase compared to the traditional TDT training model (6.82%, small effect size: $d=$ 0.47). These results demonstrate that resistance training with an unstable system can substantially enhance maximum throwing velocity, consistent with studies in various sports emphasizing targeted shoulder girdle training [46, 50]. Current findings align with Saeterbakken et al.’s [50] research, where a 6-week sling suspension training program on unstable surfaces and closed kinetic chain movements led to a 4.9% increase in throwing velocity. Similarly, Prokopy et al. [46] reported a 3.4% increase in ball throwing velocity in NCAA Division I female softball players after a 12-week closed-kinetic chain resistance training program (3 days/week).

The volleyball spike is a complex skill requiring various elements of movement, technical expertise, and muscular attributes [20]. Therefore, the improved throwing velocity in the TRX group can be attributed to several factors, including engagement of proximal segments, force generation, proximal-to-distal force transfer, segmental deceleration capacity, segmental function, and postural stability [50]. Exercises that enhance core strength and stability can enable athletes to activate their muscles more cohesively and generate greater force [50]. TRX exercises enhance coordination between trunk and shoulder joint stabilization, leading to increased muscle activation and force transmission [58], thus contributing to spike speed improvement.

Volleyball requires higher jumps and more powerful spikes compared to other sports [32]. This study emphasizes the importance of TRX exercises in effectively improving jump height, power, and spike speed in volleyball players. The force generated in proximal segments, transferred between upper and lower limbs [6], offers opportunities for performance enhancement. As a result, the increased jump height and explosive power may have contributed to maximum ball speed [5], potentially influencing overall hitting ability, including both upper and lower-body strength.

An evaluation assessed the impact of TRX and TDT exercises on shoulder isokinetic muscle strength, including internal and external aspects. Both methods increased isokinetic shoulder muscle strength in young volleyball players, as measured by PM and PM/BW. While a direct comparison between TRX and TDT effects was not possible, these findings align with other sports research. Prokopy et al. [46] reported that a 12-week TRX program improved shoulder strength and power in female softball players more than traditional training. Dannelly et al. [12] found significant muscle strength gains with TRX exercises in female students, similar to conventional training. The current study showed increased isokinetic strength in upper extremity IR and ER for both groups, but disparities from previous research [12, 46] may be due to gender, age, methodological, or sport-specific differences.

This study aligns with prior researchs [21, 31], confirming that volleyball players often exhibit higher shoulder IR values than ER values, particularly at angular velocities of 60 ${{}^{\circ}}$ /s and 180 ${{}^{\circ}}$ /s. This discrepancy likely arises from the dominant arm’s increased internal rotation muscle strength, developed through repetitive acceleration movements in volleyball [21]. Notably, technical actions like spiking and serving in volleyball tend to activate internal rotation muscles more than external rotation muscles [21]. Therefore, this study emphasizes the need for customized training programs focusing on enhancing volleyball players’ ER muscles. Volleyball-specific movements can create an imbalance between shoulder ER and IR muscles, increasing the risk of injury by destabilizing the joint [21].

In the present study, no significant changes in shoulder ER/IR isokinetic muscle strength development were found at two angular velocities (60 and 180 ${{}^{\circ}}$ /s) during the 8-week resistance training period. The recommended optimal ratio between IR and ER muscles for dominant volleyball shoulders is 3:2 [31], and on average, the current study’s results align with these recommendations. The ER to IR muscle strength ratios for participants in present study (60 ${{}^{\circ}}$ /s 0.75–0.80; 180 ${{}^{\circ}}$ /s 0.69–0.71) exceeded ratios suggested for the general population (60 ${{}^{\circ}}$ /s 0.57) [31]. Notably, mean ER/IR values for the dominant shoulder in both TRX and TDT groups’ athletes fell within the conventional maximum range (66–75%), considered ideal for injury prevention [13].

The limitations of the current study include its exclusive focus on the team members of a single club without assigning a true control group from other teams, resembling a case study. Another limitation is that only young male volleyball players were included, and the sample size was limited. Additionally, the study did not quantify the competition load of the players. Furthermore, the work expenditure was made as similar as possible but a more accurate method is critically needed for rendering this variable strictly comparable.

5. Conclusion

This study, conducted over 8 weeks (3 days/week, 60–90 min/day), found that TRX training led to greater improvements in lower-extremity power and spike speed compared to TDT. Shoulder muscle strength gains were similar in both groups. Therefore, for young male volleyball players, we recommend incorporating 8-week (2 days/week) TRX suspension training alongside technical-tactical training to enhance spike speed, shoulder strength, and lower extremity power. This information is valuable for strength and conditioning coaches guiding their players toward improved performance.

Author contributions

CONCEPMION: Ani Agopyan and Soner Ozdamar.

PERFORMANCE OF WORK: Soner Ozdamar, Ani Agopyan, and Selda Uzun.

INTERPRETATION OR ANALYSIS OF DATA: Soner Ozdamar and Ani Agopyan.

PREPARATION OF THE MANUSCRIPM: Ani Agopyan and Soner Ozdamar.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Selda Uzun.

SUPERVISION: Ani Agopyan.

Ethical considerations

This study was approved (Protocol number: 09.2017. 654) by the Research Ethics Committee of the School of Medicine, Marmara University, Istanbul, Turkey. All participants and their parents provided written informed consent for participation in this study before engaging in the trials. The study protocol was conformed to the recommendations of the Declaration of Helsinki involving Human Subjects.

Funding

The authors reported there is no funding associated with the work featured in this article. The present study was conducted as a Master Theses.

Footnotes

Acknowledgments

We appreciate all the athletes for their participation.

Conflict of interest

No potential conflict of interest was reported by the authors.

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