1. Introduction
Balance performance refers to the human ability to maintain the body’s centre of gravity over the base of support. Balance depends on rapid and continuous feedback from the somatosensory, visual, and vestibular systems to produce smooth and coordinated neuromuscular actions (Hrysomallis, 2011; Shumway & Woollacott, 2007). Due to the wide range of unpredictable actions in the football match, balance plays a crucial role for fundamental motor skills, such as jumping, shooting, accelerating, decelerating, sprinting, and changing directions at various angles (Evangelos et al., 2012). Moreover, the high number of dynamic unilateral technical skills in which players used their dominant (D) leg for shooting, passing, juggling, dribbling, and receiving the ball, but a non-dominant (ND) leg simultaneously provides stabilization of body position. Barone et al. (2011) and Chew-Bullock et al. (2012) suggested that players should improve their balance proficiency. The capacity for balance performance is significantly different between professional and youth players (Pau et al., 2015), and in different levels of competition (Butler et al., 2012; Paillard et al., 2006) and depends on playing positions (Jadczak et al., 2019).
Balance capacity is provided by the central processing of the proprioceptive senses coming from the ankle proprioceptors to enhance sport performance and to reduce the number of falls and injuries (Han et al., 2015). Studies performed by Kim & Kim (2018) and Trajkovic (2021) confirmed that ankle strength and range of motion (ROM) were related to balance control ability. Regarding football performance, precise ankle rotation was considered to be linked to balance proficiency (stability of the supporting limb) and accuracy of ankle movements in shooting or passing (Cameron & Adams, 2003; Cè et al., 2018). Moreover, balance supported by ankle mobility and explosive strength of the lower extremity most contributes to various movements and different situations that are necessary within a game, particularly change of direction (Guzman et al., 2022). Ankle dorsiflexion plays a role in absorbing and transferring force, while balance ability may enable faster COD performance at various angles from 45 to 180 (Rouissi et al., 2018). However, deficit ankle dorsiflexion ROM was associated with an increased number of landing errors during football-specific tasks (Akbari et al., 2023).
Based on the observations of Surakhamhaeng et al. (2020), balance training alone led solely to an enhancement in balance in the Y-dynamic balance test for the lower body. The plyometric exercises additionally improve not only jumping performance but also result in enhancement of balance characteristics: reduction of body centre of pressure movement amplitude and speed in anteroposterior and mediolateral directions measured using a 3D motion analysis system. Therefore, the combination of balance and plyometric exercises should be more effective in improving balance characteristics than solely balance training.
Pediatric fitness programs should include various neuromuscular exercises to enhance motor skills and fitness, which are critical for football practice (Hammami et al., 2023). Recent studies have focused on developing novel neuromuscular training for sprint, agility, jump characteristics, change of direction, balance control, and technical skill in young football players (Chaouachi et al., 2017; Hammami et al., 2016, 2023; Muehlbauer et al., 2019). For instance, implementation of eight weeks of neuromuscular training (e.g. balance, plyometric, strength, change of direction training) (Hammami et al., 2023) significantly improved dynamic balance (p < 0.001, d = 1.90), 1RM back squat (p = 0.001, d = 1.70), jump performance (p < 0.001–0.02, d = 0.80–2.40), linear sprint (p < 0.01,d = 1.6) and change of direction (p < 0.05, d = 0.70) in youth male football players. Additionally, various training strategies to incorporate balance and plyometric exercises have been shown to enhance overall physical fitness among young football players (Chaouachi et al., 2017; Hammami et al., 2016, 2023; Muehlbauer et al., 2019).
Plyometric training effectively enhances the knee extensor muscle peak isometric strength and power (vertical jump height) (Váczi et al., 2013), which were related to peak force production and acceleration in dynamic squats and jumps (Ramirez-Campillo et al., 2013). Davies et al. (2015) in their review article highly recommended inclusion of the plyometric exercises in late stage of rehabilitation of athletes. Balance training refers to the neurophysiological adaptation of the central nervous system to improve the spinal and supraspinal structures’ functions in providing motor control, which was concluded in the review article (Taube et al., 2008). Hrysomallis (2011) mentioned in his review that balance training was beneficial for motor performance, rehabilitation, and injury prevention. While the previously mentioned studies have demonstrated the effectiveness of plyometric and balance training on overall physical fitness in football players, we focused on investigating how this training modality enhances balance performance and lower leg characteristics, including factors such as ankle flexibility, peak isometric strength of shin muscles, and explosive strength of the lower leg which is not investigated in wide scale. Evidence has revealed that injuries in male youth football players occur mainly in the lower extremities (71–80%) (Read et al., 2016), with a 20% proportion of ankle instability due to impaired neuromuscular control during dynamic movements (Francia et al., 2022; Park et al., 2024). Therefore, our study aimed to investigate the effects of integrative balance and plyometric training on static and dynamic balance performance, active ankle range of movements, shin muscle strength, and jumping performance in young footballers. We hypothesized that providing balance and plyometric training (including progression of difficulty, variation of resistive load movements, and duration) would lead to significant improvement in balance performance, ankle mobility, and explosive lower extremity strength.
2. Methods
2.1. Participants
The sample size was estimated by G*Power 1.9.4 analysis. Based on an effect size (f) value of >0.80 derived from the dynamic balance results from a previous study (Hammami et al., 2023) which was with 80% statistical power (β = 0.8) and α = 0.05, the analysis indicated a minimum requirement of 12 participants per group. To account for a potential dropout rate of 20%, 30 well-trained adolescent male players from a football club in Riga, Latvia, were recruited for this study (Figure 1).
Figure 1
CONSORT diagram of participants, recruitment, allocation, follow-up, and analysis.
Eligible participants met the following criteria: aged 14–16 years and classified at pubertal stages 3 and 4 according to the Tanner scale (Beunen et al., 1992). Players had regular training sessions four times per week (7.5 ± 2.1 hours per week). They have at least five years of training experience in football (8.1 ± 2.6 years) and participate in competitions on weekends. Exclusion criteria: pain in the lower limbs during study, ankle sprain within the previous six months, surgery within the last 12 months, visual and vestibular disorders, ongoing treatment for inner ear or sinus infection, or a concussion within three months before the study.
The cohort was stratified into a randomized control study with an intervention (IG) and a control (CG) group. The IG (N: 15, age: 15.8 ± 1.7 years, body mass of 67.8 ± 11.9 kg, height of 177.2 ± 5.9 cm, body mass index BMI of 21.9 ± 2.8 kg/m2), and CG (N: 15, age: 15.7 ± 0.6 years, body mass of 69.6 ± 4.8 kg, height of 180.3 ± 5.6 cm, BMI of 21.8 ± 1.4 kg/m2). No differences in anthropometric characteristics were observed at baseline between groups (p > 0.05). Before the study commencement, subjects and their parents provided informed consent. This study was conducted following the requirements of the Helsinki Declaration, and the protocol was approved by the Ethics Committee of the Latvian Academy of Sports Education (Meeting protocol No. 6, decision no. 1/51813, February 24, 2023).
2.2. Training program
The training intervention was conducted twice weekly on alternate days during eight weeks (April–May 2024) at the beginning of the competitive season. Each session lasted 30–45 minutes and began with a standardized 15-minute warm-up, which included general and local exercises, as well as active stretching for the legs and trunk.
The balance training consisted of four types of exercises: 1) semi-squat position on one or both legs while holding a medicine ball, 2) squat while throwing the medicine ball vertically upwards, then stand up and catch the ball, 3) standing and keeping a balance position on the balance wobble board with a resistance band positioned around the thighs above the knee, 4) standing and keeping a balance position on the roller balance board with a resistance band positioned around the shins below the knee. The dose-response for balance training followed the protocol outlined by Gebel, Prieske et al. (2020). Each exercise lasted 20–40 seconds, and 3 sets were performed. A 30-second break between the sets, and the rest periods between different exercises were 120 seconds.
The plyometric training consisted of four types of exercises: 1) countermovement jump on both legs over the hurdle, 2) lateral hurdle hop, 3) single leg jumps forward and backward, side to side in front of cone, 4) Skipping rope with single and both legs. Plyometric exercises were performed with 2–4 sets of 6–15 repetitions per set, as it was recommended for youth football players (Bedoya et al., 2015). Rest intervals between sets of different exercises were established at 60–120 seconds. The initial number of ground contacts per session was set at 50–60, with progression leading to a maximum of 80–120 contacts per session.
During the intervention, balance training was performed before plyometric exercises to achieve optimal functional movement and muscular fitness outcomes (Behm et al., 2008; Granacher & Behm, 2023). Training intensity increased biweekly by adjusting the number of repetitions, duration, and difficulty of exercises (Table 1 and Figure 2). Throughout the training session, coaches provided instructions on proper technique for balance and plyometric exercises. The coach verbally encouraged the players to exert maximal efforts during the training. Regarding the control group, they participated in their regular training sessions (e.g., 30–40 minutes of physical training and 60–90 minutes of technique and game play), which consisted of game-based training without any specific intervention.
Table 1
Description an eight week of integrative neuromuscular training program.
| MATERIALS FOR BALANCE TRAINING | SET × REPETITION | TRAINING DESCRIPTIONS | |||
|---|---|---|---|---|---|
| Week 1, 2 | Week 3, 4 | Week 5,6 | Week 7, 8 | ||
| Static and dynamic balance | |||||
| Medicine ball 1.5 Kg + hoops (static balance performance, core stabilization + leg muscle strength endurance) | 2 × 40s | 3 × 40s | 2 × 30s | 3 × 30s | Semi-squat position on one or both legs while holding a medicine ball. Place both legs together or stand on a single leg. Stand in a semi-squat position (knee flexion angle to 45°). The head, neck, and back should be on a line. The head should be straight, and the gaze should be forward. Hold a medicine ball with both hands in front of the body for some time, the eyes should be open. |
| Medicine ball 1.5 Kg (dynamic balance, core stabilization) | 2 × 40s | 3 × 40s | 2 × 30s | 3 × 30s | Squat while throwing the medicine ball vertically upwards, then stand up and catch the ball. Stand with feet positioned at shoulder-width to produce maximum power. The position of the head, neck, and back should be in one line. Bend hip and knee joints to 90° (squat position) and return to a stabilized position while throwing the medicine ball vertically. Use the core, shoulders, and arms to throw the ball, engaging your core muscles. |
| Balance wobble board + resistance band (dynamic balance) | 2 × 30s | 3 × 30s | 2 × 40s | 3 × 40s | Standing and keeping a balanced position on the balance wobble board with a resistance band positioned around the thighs above the knee. Put on a resistance band around the thighs above the knees, and stand on both feet on the balance wobble board. Bend the hip and knee joints to 40°. Keep the back straight. Distribute the weight on the body center of pressure. |
| Balance board + roller+ resistance band (dynamic balance) | 2 × 30s | 3 × 30s | 2 × 40s | 3 × 40s | Standing and keeping a balanced position on the roller balance board with a resistance band positioned around the shins below the knee. Put on a resistance band around the shins below the knees, and stand on both feet on the balance wobble board. Bend hip and knee joints to 45° (squat position). Keep the back straight. Sway your body sideways for some time. Shift your weight from side to side to move the board in those directions. |
| Plyometric training | |||||
| Hurdle jump++ resistance band. (leg muscle explosive power) | 2 × 8 | 2 × 10 | 3 × 8 | 3 × 10 | Counter-movement jump on both legs over the hurdle. Start with feet hip-width apart and knees slightly bent. Jump forward and backward over the hurdles. In the landing phase, bend ankles, knees, and hips to absorb impact energy. |
| Hurdle (leg muscle explosive power) | 2 × 8 | 2 × 10 | 3 × 8 | 3 × 10 | Lateral hurdle hop. Perform the hop by pushing off the right foot to land on the left foot over the hurdle, and vice versa. In the landing phase, bend ankles, knees, and hips to absorb the impact energy. |
| Cone triangle hop (Displace triangle cones with a distance of 1 m) (leg muscle explosive power) | 2 × 6/leg | 2 × 8/leg | 3 × 4/leg | 3 × 5/leg | Single leg jumps forward and backward, and side to side in front of the cone. Jump forward by taking off on one leg and land on the opposite leg in front of the cone. Bend your knee to 900 to touch the cone with the knee of the landed leg while the opposite leg hangs. Jump backward to the starting point. Second step: hop to the right from one leg to the other, then hop to the left. Repeat the hops starting with the opposite leg. |
| Skipping Rope (leg muscle explosive power) | Na | Na | 3 × 30s | 2 × 40s | Skipping rope with single and both legs. Stand with both legs in the middle of the rope. Feet are shoulder-width apart. Jump with a consistent bound and steady rhythm. Use wrists to swing the rope over the head. Align the upper body, hips, knees, and feet. Keep the pelvis horizontal. Continue skipping rope on the right and left leg, respectively. |
Figure 2
Illustration of balance and plyometric training.
2.3. Study measurement
2.3.1. Procedure for data collection
The measurements were conducted in the afternoon (from 4 to 6 p.m.) at the Health Care Research Center of Riga Stradiņš University, Latvian Academy of Sports Education. The ambient temperature was maintained between 20 and 22°C, with relative air humidity levels of 35–45%. Players were instructed to wear indoor sportswear during testing. A familiarization session was conducted for all testing protocols to prevent potential learning effects two weeks before the baseline assessments. The pre-test was administered before the beginning of the training program. Post-test measurement was conducted in the following week after finishing the training intervention. For objectivity, the assessment was performed by two blinded investigators who were unaware of which participant was from the intervention or control group. Leg dominance was determined by instructing players to kick the ball with maximal force (Baldon et al., 2012; Kollock et al., 2015). A 10–15 minute standardized warm-up exercise protocol was performed before testing. One day before testing, players were advised to rest and avoid training or match play.
2.3.2. Anthropometric data collection
Height was measured using the SECA 2020 telescopic stadiometer (SECA, Hamburg, Germany), with an accuracy of ±1 mm. Body weight was assessed using a SECA 874 digital scale (SECA, Hamburg, Germany), with a precision of measurement of ±0.05 kg. Body mass index (BMI) was calculated with the standard formula: BMI = mass (kg)/height (m2) (Gallagher et al., 2000).
2.3.3. Static Balance Test
The static and dynamic balance performance was assessed using the ProKin 252 stabilometric platform (TecnoBody, Dalmine, Italy). The platform features a 55 cm diameter surface, a 20 Hz sampling rate, and a sensitivity of 0.1°. A trunk sensor was positioned on the chest at the midpoint of the sternum. Balance was measured within 30 seconds of one-legged stance, barefoot with eyes open. The foot’s longitudinal axis was detected as the tangent line between the midpoint of the middle of the toe and the central point of the heel. Subjects were instructed to place their hands on their hips and focus their sight on a crosshair target (at a distance of 1 m at eye-level height). The supporting leg was maintained at a knee flexion angle of 30°, while the opposite leg was flexed at an angle of 45° (Mauch & Hakin, 2011). It was not allowed to touch the supporting leg with the opposite leg. After completing the 30s stance on each leg, the device recorded the following static balance parameters: ellipse area (EA) covered by the centre of pressure (CoP) movements, representing at least 90–95% of the sway lines. A smaller EA indicates better balance performance (Asseman et al., 2004). Perimeter length (P) is the total length of the circumference of the CoP sway lines. Average anterior-posterior speed (AAPS) and medial-lateral speed (AMLS) of CoP movements were determined in mm/s.
2.3.4. Dynamic Balance test
During the dynamic balance test, the movable balance platform of the system operates with air piston servo motors and can perform measurements in all directions within an operating angle of 15°. Total stability index (TSI) and total trunk deviation angle (TTD) were recorded as dynamic balance parameters. TSI refers to the distance between the body pressure centre and the centre of the movable platform. If the participant’s centre of pressure (CoP) is closer to the centre of the movable platform, it indicates better dynamic balance performance. Similar to TSI, TTD refers to the sways of the trunk (anterior-posterior and medial-lateral) during a 30-second single-leg stance dynamic balance test. TSI values were classified: for athletes, TSI must be 0–0.83; for untrained participants, the norm was 0.84–2.32, and poor balance performance was detected if the TSI value exceeded 2.33. Higher scores of TSI and TTD indicated weaker dynamic balance performance. Following previous protocols (Meiners & Loudon, 2020; Sinulingga et al., 2024), participants were allowed one additional trial if they failed to complete the 30-second test without errors.
2.3.5. Ankle joint range of movement test
Active ankle range of movement (ROM) for plantar flexion (PF) and dorsiflexion (DF) was measured using a Jamar Plus digital goniometer (Performance Health, United Kingdom), following protocols outlined by Norkin and White (2016). Participants were seated with their knees flexed at a 90° angle. Two points were marked, 2 cm distal and 10 cm proximal to the lateral malleolus. The goniometer’s pivot point was aligned with the lateral malleolus, while the fixed arm was positioned parallel to the lateral midline of the fibula. The movable arms were aligned with the lateral midline of the fifth metatarsal bone. ROM (o) was measured across three trials per participant, with mean values used for analysis (Maior et al., 2020).
2.3.6. Ankle muscle strength test
The peak isometric forces of the plantar flexor (PF) and dorsiflexor (DF) muscles were measured by a handheld dynamometer (MicroFET2 wireless; Hoggan Health Industries, West Jordan, USA). The reliability of the hand-held dynamometry procedure was excellent for both intrarater (ICC3,1 = 0.78–0.94) and interrater (ICC3,1 = 0.77–0.88) (Spink et al., 2010). PF muscle force was measured with the dynamometer contacting the plantar surface of the foot (proximal to the first metatarsal bone head). DF muscle force measurements were taken from the dorsal surface of the foot (proximal to the metatarsal bones’ heads) (Spink et al., 2010). Subjects were seated on a standard table, their hips flexed at the right angle (90°), and their knees extended. The investigator applied counterforce against the ankle DF and PF participants’ muscle force using his dominant hand. Verbal encouragement was given during each contraction. The duration of each trial was 3 s, with a 1-minute rest interval between the trials. The maximum force (N) was selected for statistical analysis. Absolute peak torque (N∙m) was calculated by multiplying the peak force by the length of lever arms (m) [the distance from the medial malleolus to the distal head of the first metatarsal bone]. Relative peak torque (Nm/kg) was calculated by dividing the absolute peak torque by the body mass of the participant: peak torque (PT)/body mass (BM).
2.3.7. Countermovement Jump test
Before starting the countermovement jump test (CMJ), participants were instructed to place their hands fixed on their hips (Aragon-Vargas, 2000). Participants had to remain upright before jumping, starting with a counter-movement until the hips and knees were bent to the right angle of 90° (Petrigna et al., 2019). Players started a counter-movement jump maximally in one motion immediately after a verbal command given by the investigator. Jump height and flight time were measured using an Optojump (Microgate, Srl., Bolzano, Italy). Maximum force production in the jump test was recorded using a force plate BTSP-6000 (BTS Bioengineering, Garbagnate Milanese, Italy).
2.4. Statistical analysis
All data were calculated using SPSS version 27 (IBM, Armonk, NY, USA). Shapiro-Wilk and Levene’s tests were used to verify the data distribution normality and homogeneity. Results were presented as mean ±SD (see Table 2). Within-group changes (pre- (t0) vs. post- (t1) the intervention were assessed using the paired t-test, with effect size calculated by Cohen’s d (trivial: <0.2; small: <0.5; moderate: <0.8; large: >0.8 (Cohen, 2013). Percentage changes were calculated as: [(pre–post)/pre] × 100. Between-groups differences were analyzed using ANCOVA (2 × 2), with effect sizes reported as partial eta-squared (ηp2) (small: 0.01; medium: 0.06; large: 0.15).
Table 2
Statistical results of between-group and within-group differences by presenting mean absolute and relative percentage value for pre- and post- test.
| VARIABLE | BALANCE AND PLYOMETRICS TRAINING GROUP | CONTROL GROUP | ANCOVA | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| x̄1 | x̄2 | Δ% | p | d | x̄1 | x̄2 | Δ% | p | d | F | p | ηp2 | |
| Static Balance | |||||||||||||
| Ellipse area [mm²] D | 741.94 ± 171.49 | 645.42 ± 166.95 | –13.01 | 0.033* | 0.57 | 746.95 ± 172.73 | 799.56 ± 179.27 | 7.04 | 0.331 | 0.29 | 6.620 | 0.017§ | 0.224 |
| ND | 699.28 ± 145.3 | 598.70 ± 117.76 | –14.38 | 0.045* | 0.76 | 684.39 ± 156.76 | 723.42 ± 172.75 | 5.70 | 0.549 | 0.23 | 4.622 | 0.042§ | 0.167 |
| Perimeter [mm] D | 1259.77 ± 169.33 | 1081.75 ± 138.9 | –14.13 | 0.001* | 1.14 | 1271.87 ± 162.87 | 1295.56 ± 155.08 | 1.86 | 0.641 | 0.14 | 18.303 | 0.001§ | 0.443 |
| ND | 1213.68 ± 187.1 | 1037.44 ± 166.85 | –14.52 | 0.001* | 0.99 | 1282.21 ± 201.04 | 1288.13 ± 181.33 | 0.46 | 0.908 | 0.03 | 14.780 | 0.001§ | 0.391 |
| Anterior-Posterior Speed [mm/s] D | 24.70 ± 4.14 | 21.65 ± 3.7 | –12.37 | 0.015* | 0.77 | 25.44 ± 4.8 | 28.14 ± 6 | 10.59 | 0.172 | 0.49 | 10.816 | 0.003§ | 0.320 |
| ND | 23.99 ± 3.62 | 20.19 ± 3.32 | –15.84 | 0.001* | 1.09 | 25.37 ± 6.14 | 27.24 ± 5.13 | 7.38 | 0.312 | 0.33 | 17.794 | 0.001§ | 0.436 |
| Medial-Lateral Speed [mm/s] D | 28.85 ± 3.27 | 25.88 ± 4.23 | –10.29 | 0.023* | 0.78 | 30.24 ± 5.69 | 33.15 ± 6.55 | 9.62 | 0.286 | 0.47 | 10.599 | 0.003§ | 0.315 |
| ND | 27.56 ± 6.57 | 23.65 ± 4.17 | –14.20 | 0.038* | 0.71 | 27.82 ± 8.55 | 31.23 ± 6.05 | 12.23 | 0.258 | 0.45 | 13.521 | 0.001§ | 0.370 |
| Dynamic Balance | |||||||||||||
| Total stability index [°] D | 2.29 ± 0.63 | 1.59 ± 0.74 | –30.29 | 0.002* | 1.01 | 2.33 ± 0.9 | 2.34 ± 0.87 | 0.43 | 0.994 | 0.01 | 5.793 | 0.025§ | 0.201 |
| ND | 2.15 ± 0.79 | 1.50 ± 0.44 | –30.41 | 0.013* | 1.02 | 2.17 ± 0.82 | 2.27 ± 0.78 | 4.89 | 0.771 | 0.12 | 7.675 | 0.011§ | 0.250 |
| Trunk Total Deviation [°] D | 2.56 ± 0.67 | 1.91 ± 0.59 | –25.59 | 0.013* | 1.03 | 2.60 ± 0.99 | 2.70 ± 1.49 | 4.00 | 0.721 | 0.08 | 4.506 | 0.045§ | 0.164 |
| ND | 2.36 ± 0.67 | 1.74 ± 0.72 | –26.14 | 0.007* | 0.89 | 2.37 ± 0.97 | 2.50 ± 1.38 | 5.35 | 0.685 | 0.13 | 4.444 | 0.046§ | 0.162 |
| Talocrural Ankle ROM | |||||||||||||
| Plantarflexion [°] D | 42.51 ± 5.09 | 47.71 ± 4.76 | 12.22 | 0.002* | 1.05 | 42.49 ± 5.7 | 43.90 ± 3.72 | 3.32 | 0.474 | 0.29 | 5.432 | 0.029§ | 0.191 |
| ND | 43.11 ± 6.61 | 48.85 ± 3.81 | 13.31 | 0.027* | 1.06 | 42.47 ± 5.38 | 44.53 ± 5.85 | 4.83 | 0.322 | 0.36 | 4.751 | 0.040§ | 0.171 |
| Dorsiflexion [°] D | 11.07 ± 2.51 | 14.32 ± 2.57 | 29.37 | 0.001* | 1.27 | 10.99 ± 2.75 | 12.09 ± 2.25 | 9.99 | 0.134 | 0.43 | 7.645 | 0.011§ | 0.249 |
| ND | 11.39 ± 3.55 | 15.03 ± 2.41 | 32.03 | 0.001* | 1.19 | 10.28 ± 3.29 | 12.05 ± 2.57 | 17.29 | 0.067 | 0.59 | 5.284 | 0.031§ | 0.187 |
| Shin muscle peak Isometric | |||||||||||||
| Plantarflexor Peak Force [N] D | 136.32 ± 33.79 | 150.52 ± 29.04 | 10.42 | 0.006* | 0.45 | 136.71 ± 22.25 | 142.30 ± 23.35 | 4.09 | 0.253 | 0.24 | 5.246 | 0.031§ | 0.186 |
| ND | 133.50 ± 35.04 | 147.81 ± 22.35 | 10.72 | 0.038* | 0.48 | 133.70 ± 23 | 138.34 ± 14.66 | 3.47 | 0.433 | 0.24 | 3.096 | 0.092 | 0.119 |
| Plantarflexor Peak Torque [N.m] D | 27.93 ± 6.3 | 32.23 ± 6.02 | 15.38 | 0.001* | 0.69 | 28.94 ± 5.83 | 31.00 ± 6.04 | 7.11 | 0.101 | 0.34 | 2.156 | 0.156 | 0.086 |
| ND | 27.37 ± 6.77 | 31.66 ± 4.56 | 15.68 | 0.005* | 0.74 | 28.27 ± 5.74 | 30.05 ± 3.88 | 6.31 | 0.232 | 0.36 | 2.347 | 0.139 | 0.093 |
| Plantarflexor Relatif Peak Torque[N.m/kg]D | 0.46 ± 0.11 | 0.52 ± 0.10 | 14.18 | 0.001* | 0.62 | 0.46 ± 0.10 | 0.51 ± 0.13 | 11.74 | 0.164 | 0.46 | 0.160 | 0.693 | 0.007 |
| ND | 0.45 ± 0.11 | 0.51 ± 0.09 | 14.36 | 0.013* | 0.56 | 0.45 ± 0.1 | 0.49 ± 0.11 | 10.56 | 0.139 | 0.45 | 0.268 | 0.610 | 0.012 |
| Dorsiflexor Peak Force [N] D | 135.23 ± 27.71 | 160.66 ± 27.83 | 18.80 | 0.001* | 0.91 | 136.19 ± 15.99 | 151.65 ± 26.29 | 11.35 | 0.049* | 0.71 | 1.434 | 0.243 | 0.059 |
| ND | 131.32 ± 28.34 | 157.53 ± 32.41 | 19.96 | 0.001* | 0.86 | 133.38 ± 21.06 | 142.70 ± 22.35 | 6.99 | 0.119 | 0.42 | 5.144 | 0.033§ | 0.183 |
| Dorsiflexor Peak Torque [N.m] D | 27.87 ± 5.84 | 34.43 ± 5.70 | 23.51 | 0.001* | 1.13 | 28.72 ± 3.80 | 32.95 ± 6.14 | 14.74 | 0.014* | 0.82 | 1.435 | 0.243 | 0.059 |
| ND | 27.01 ± 5.78 | 33.72 ± 6.52 | 24.84 | 0.001* | 1.08 | 28.14 ± 4.81 | 31.09 ± 5.77 | 10.47 | 0.025* | 0.55 | 5.075 | 0.034§ | 0.181 |
| Dorsiflexor Realatif Torque [N.m/kg] D | 0.45 ± 0.08 | 0.55 ± 0.09 | 22.81 | 0.001* | 1.21 | 0.45 ± 0.07 | 0.54 ± 0.12 | 19.19 | 0.012* | 0.91 | 0.191 | 0.666 | 0.008 |
| ND | 0.43 ± 0.07 | 0.54 ± 0.09 | 24.22 | 0.001* | 1.32 | 0.44 ± 0.07 | 0.51 ± 0.13 | 15.66 | 0.029* | 0.67 | 1.028 | 0.321 | 0.043 |
| Countermovement Jump | |||||||||||||
| Height [cm] | 39.15 ± 1.75 | 43.19 ± 4.12 | 10.34 | 0.000* | 1.27 | 39.35 ± 2.59 | 41.56 ± 2.77 | 5.63 | 0.008* | 0.82 | 2.389 | 0.136 | 0.094 |
| Time Flights [s] | 0.55 ± 0.04 | 0.59 ± 0.05 | 6.74 | 0.017* | 0.82 | 0.57 ± 0.02 | 0.58 ± 0.03 | 2.23 | 0.324 | 0.48 | 0.543 | 0.468 | 0.023 |
| Maximum force [N] | 1643.62 ± 189.60 | 1876.79 ± 242.60 | 14.19 | 0.004* | 1.07 | 1717.62 ± 154.61 | 1816.89 ± 185.11 | 5.78 | 0.195 | 0.580 | 0.774 | 0.388 | 0.033 |
[i] Abbreviation: x̄1 = mean pre-test, x̄2 = mean post-test, Δ % = percentage change from pre to post test, D = Dominant, ND = Non-dominant, mm² = Millimeter square, mm = Millimeter, mm/s = millimeter per second, ° = degree, s = Second. N = Newton, cm = Centimeter, m = meter, kg = kilograms, ηp2 = Partial eta square. * Significant difference: p < 0.05 between pre- and post-test within groups; § significant difference: p < 0.05 between intervention and control groups.
3. Results
All participants followed the assigned intervention conditions as allocated. The final sample size for analysis was reduced to 26 players: a) two players missed the post-intervention test; b) two participants did not fully complete this study due to injury Figure 1. Obtaining an adherence rate of 86.67%, the data were eligible for further analysis.
3.1. Static and Dynamic Balance
IG demonstrated a significant improvement in static balance performance (10–15%, p = 0.001–0.04, d = 0.57–1.14). Post-test values revealed a significant improvement in static balance parameters: ellipse area (EA) decreased by 96–101 mm², perimeter length (P) reduced by 176–178 mm, and average anterior-posterior/medial-lateral speed (AAPS/AMLS) declined by up to 3 mm/s compared to baseline. Conversely, CG exhibited no significant changes in their training routine, p = 0.17–0.90. Between-groups comparison revealed that IG surpassed CG in all variables of static balance with F = 4.62–18.30, p = 0.001–0.04, ηp2 = 0.14–0.44 (Table 2, Figure 3A and B). IG significantly improved in dynamic balance, with a 25–30% reduction in total stability index (TSI) and trunk total deviation (TTD) decrease of 0.62°–0.70°, p = 0.002–0.01, d = 0.89–1.03. In contrast, CG exhibited no significant changes in both parameters, p = 0.68–0.99. Significant between-groups differences were shown in TSI and TTD with F = 4.44–7.67, p = 0.01–0.04, ηp2 = 0.16–0.25 (Table 2, Figure 3C and D).
Figure 3
Absolute (A, C, E, F, H, J, L) and relative percentage (B, D, G, I, K, M) value change (pre-to-post) for all variables test.
Abbreviation: ellipse area [EA], perimeter length [P], average anterior-posterior speed [AAPS], average medial-lateral speed [AMLS], total stability index [TSI], total trunk deviation angle [TTD], Plantar flexion [PF], dorsiflexion [DF], peak force (PR), peak torque (PT), relative peak torque(RPT), height jump [HJ], flight time [FT], maximum force [MF]). IG.D intervention group dominant leg, CG.D control group dominant leg, IG.ND intervention group non-dominant leg, CG.ND control group non-dominant leg.
3.2. Ankle Mobility (Range of motion and strength)
IG demonstrated significant change in active ankle range of motion (ROM), with an increase of 12–32% in plantarflexion (PF) and dorsiflexion (DF) (p = 0.001–0.02, d = 1.05–1.27). Post-intervention results showed a significant improvement in active ROM for IG, with an increase of 5°–7° in PF and of 3°–4° in DF. However, CG exhibited no significant changes from pre- to post-test = 0.06–0.47. IG showed significantly greater than CG in PF (F = 4.751–7.675, p = 0.029–0.040, ηp2 = 0.171–0.191) and DF (F = 5.284–7.645, p = 0.011–0.031, ηp2 = 0.187–0.249) (Table 2, Figure 3E–G).
Ankle plantar flexion peak force, absolute and relative torque significantly improved in IG (10–19%, p = 0.001–0.03, d = 0.45–0.91), while dorsiflexion (DF) increased in both groups (10–24%, p = 0.001–0.04, d = 0.55–1.32). Between-group analysis revealed significant differences between IG and CG in peak force of PF for D leg (F = 5.246; p = 0.031; ηp2 = 0.186), peak force of DF for ND leg (F = 5.144; p = 0.033; ηp2 = 0.183), and peak torque of DF for ND leg (F = 5.075; p = 0.034; ηp2 = 0.181 (Table 2, Figure 3H–K).
3.3. Explosive strength of lower limbs
Jump height significantly increased in both groups (5–10%, p = 0.000–0.008, d = 0.82–1.27). Furthermore, IG demonstrated significant improvements in time flight (6.74%; p = 0.017;d = 0.82) and maximum force production (14.19%; p = 0.004;d = 1.07), while CG showed no significant changes p = 0.19–0.32. ANCOVA revealed no significant group × time interaction for any countermovement jump characteristics, p = 0.13–0.46 (Table 2, Figures 3L and M).
4. Discussion
This study aimed to evaluate the effect of integrative balance and plyometric training on balance, ankle mobility, and explosive strength in young football players. After eight weeks of balance and plyometric intervention, IG showed improvement in their static and dynamic balance, ankle active range of motion (ROM), shin muscle strength, and vertical jump performance. The control groups (CG) who performed only the training routine improved in shin muscle peak isometric DF force for the non-dominant leg, absolute and relative peak torque for both legs, and height jump performance. Within-group analysis revealed that the IG achieved significant enhancements across all tests, surpassing the performance of the CG counterparts.
4.1. Static and Dynamic Balance
Combination neuromuscular training (e.g., balance, plyometric, resistance exercise) is more effective than stand-alone to enhance balance and muscular fitness in youth athletes (Granacher & Behm, 2023). We conducted an eight-week integrative balance and plyometric training program, which led to improvements in static and dynamic balance (p = 0.001–0.045, d = 0.5–1.14) in young players. IG showed significant improvement in static and dynamic balance compared to CG, with a large effect size. Static balance demonstrated reduced centre of pressure (CoP) displacement (EA) (±14%), perimeter length (P) (±14%), and decreased sway speed in anterior-posterior (AP) and medial-lateral (ML) directions (±15%). In the previous study Dafkou et al. (2021) reported a 33% reduction in CoP sway after eight weeks of eccentric, balance, and core exercises implementation in football training routines. In line with this, improvements in AP and ML CoP velocity (eyes open/closed) following proprioceptive training in taekwondo poomsae athletes (Yoo et al., 2018), and among individuals with functional ankle instability was observed (Surakhamhaeng et al., 2020).
The study conducted by Cè et al. (2018) determined an enhancement in total stability index (TSI) on bipedal (ES = 2.37) and unipedal tests (ES = 1.95) after 12 weeks of balance training (three times per week, 20 minutes per session) in 11-year-old male football players. These findings support our results, where the improvement in dynamic balance characteristics: total stability index (TSI) and sway trunk deviation (TTD), p = 0.002–0.013 (0.62o –0.70o lower than baseline). The current study provides materials (e.g., wobble board, roller balance board, medicine ball, resistance band), surface (e.g., stable/unstable), patterns of intervention (e.g., static and dynamic), dose, and response (Gebel, Prieske, et al., 2020; Schedler et al., 2020) that improve sensory components for center of pressure displacement: proprioceptive (e.g. somatosensory), visual, and vestibular sense acuity. Progression of difficulty levels during balance training (Gebel, Lehmann, et al., 2020; Schedler et al., 2020), as implemented in our intervention (e.g. bipedal/unipedal stance, unstable/stable platform, and sway stability on roller board, with/without arm support), enhance neuromuscular activation required for postural stability. Conversely, plyometric may enhance balance performance, similar to the role of balance training (Ramachandran et al., 2021), but with different training characteristics. Plyometric training resulted in improved motor control and reduced rate of force development, which applied to the reduction of the CoP perturbation during football-specific actions.
4.2. Ankle Mobility (Range of motion) and Strength
Ankle mobility represents a key determinant of balance improvement, as ankle flexibility and increased strength help enhance balance capacity (Kim & Kim, 2018; Trajković et al., 2021). In the present study, following an eight-week balance and plyometric training, the range of motion for plantar flexion increased by 5° (13%), and dorsiflexion by 4° (32%) in the IG (p = 0.001–0.02). In contrast, there was no significant change in the CG (p = 0.06–0.47). Furthermore, there is a significant difference in plantar and dorsiflexion between the groups in both the dominant (p = 0.01–0.002, ηp2 = 0.19–0.24) and non-dominant leg (p = 0.03–0.04, ηp2 = 017–018). Similar to a study conducted by (Alahmari et al., 2021) a 6-week program of strength and balance exercises improves talocrural joint mobility by 3.7° (44.3%) in PF and 3.5° (45.2%) in DF among individuals with functional ankle instability (FAI). Current studies indicate that balance and plyometric training affect ankle proprioception, thereby enhancing sensory feedback related to muscle length changes, joint position sense, and movement speed (Alahmari et al., 2021). This training regimen also induces muscle relaxation to promote increased stretch magnitudes during balance intervention, resulting in larger dorsiflexion ROM, as well as improved functional movement and stability (Lazarou et al., 2018). Kim & Kim (2018) observed that the ROM of ankle dorsiflexion negatively correlated with CoP sway distance in the static balance test, standing on both feet, improving the balance; controversially, the ankle plantar flexion ROM positively correlated with CoP sway distance, worsening the static balance performance.
Maximal voluntary isometric strength of the shin muscle increased in plantar flexor +14 N (10%), absolute torque +5 N/kg (15%), and relative torque +0.6 N.m/kg (14%), with moderate effect size (p = 0.001–0.038, d = 0.45–074). Moreover, IG also improved in dorsiflexion strength of +25 N (19%), absolute torque +7 N/kg (24%), and relative torque + 0.10 N.m/kg (22%), with a large effect size. In contrast to CG, only their dominant leg showed an increase in peak force and torque p = 0.01–0.04, while relative torques increased in both legs p = 0.001–0.02. Significant differences between groups were observed in the plantar flexor of the dominant leg (p = 0.031, ηp2 = 0.18), and in dorsiflexor force and torque of the non-dominant leg (p = 0.034, ηp2 = 0.18). One of the most highlighted improvements in shin muscle isometric strength may be explained by combining stable and unstable platforms, enhanced muscle strength and coordination, particularly for joint stability (Anderson & Behm, 2005). Balance training on stable and unstable surfaces, particularly with the addition of band resistance, leads to improved ankle strength and a reduction in valgus deformity of the ankles (Kim et al., 2017). In line with this, a 4-week balance training consisting of dynamic unstable surface exercises (e.g. mini squat on BOSU ball) improves ankle force production in PF (ES = 0.80 [0.25, 1.34]) and DF (ES = 0.75 [0.21, 1.29] in healthy young adults (Cuǧ et al., 2016).
Plyometric exercise with various jump conditions (CMJ, hop, leap, skip) also contributes to improving force production and muscular coordination, which affect ankle joint position sense (Huang et al., 2021). The current study agrees that combining plyometric and balance exercises is more effective in mitigating ankle joint position sense errors and enhancing neuromuscular control, compared to plyometric training alone (Huang et al., 2021), even for athletes with functional ankle instability (Park et al., 2024). Improvement in ankle mobility directly contributes to somatosensory regulation during upright posture and balance control. This occurs through its influence on muscle spindles, Golgi tendon organs, and plantar cutaneous receptors, thereby enhancing balance control (Alahmari et al., 2021; Viseux et al., 2019).
4.3. Countermovement Vertical Jump
The improvement in jump performance in our study might be attributed to plyometric training, which enhances motor recruitment and stretch-shortening cycle (SSC) (Davies et al., 2015; Sammoud et al., 2024). Incorporating balance and plyometric training over six weeks improves jump height in youth football players (p = 0.002, d = 2.21) (Muehlbauer et al., 2019). As mentioned in the previous investigations, plyometric exercise variation (e.g., CMJ, hurdle jump, ankle jump, lateral hops) and balance (e.g., standing or squatting on single/bilateral on Swiss ball) lead to improvements in jump performance in young footballers within an eight-week intervention (Chaouachi et al., 2017; Hammami et al., 2016; Makhlouf et al., 2018). Additionally, variables such as flight time and peak force production also improved in our study. Plyometric training on a stable surface improves neuromuscular adaptation (Granacher & Behm, 2023), including enhanced motor unit recruitment, optimized synergistic muscle activation, and reduced antagonistic muscle activity (Markovic & Mikulic, 2010). Additionally, musculotendinous tissue adaptations (increased tendons’ stiffness that is connected to muscle capacity for rapid force production) may further contribute to enhancing jump characteristics in young football players (Arntz et al., 2022; Moran et al., 2023).
4.4. Strengths, Limitations, and Future Study Directions
The present study demonstrated that integrative balance and plyometric training can significantly improve static and dynamic balance, ankle active range of motion, peak isometric strength of the shin muscle, and jump performance in young players. This study has some limitations: 1) small number participant, 2) only specific group of participants (14–16 years old male football payers), meaning the results cannot be generalized to other age and sex groups, 3) mesocycles intervention (8 weeks intervention), where long-term effects of the balance and plyometric training was not evaluated, and 4) lack of methodological approaches that should be included in future studies: (1) monitoring cortical activity via electroencephalography (EEG), (2) analyzing electromyography (EMG) patterns of muscle activation, and (3) employing isokinetic dynamometry. Future studies should consider 1) a larger sample size and various populations, 2) a longer period of intervention, 4) methodological improvements using more precise measurement methods (isokinetic dynamometry, electromyography, etc.), and 4) reformulation of training modalities (dose and response) under different sample conditions.
5. Conclusion
Eight weeks of integrative balance and plyometric training resulted in significantly improved static and dynamic balance, active ankle range of motion, and enhanced peak isometric strength of the plantar and dorsal flexor muscles in young football players, as well as improved vertical jump characteristics. To ensure the effectiveness of the training regimen, the progression of balance and plyometric exercises (e.g., volume, duration, and difficulty levels) must increase every two weeks of the intervention period. This training program may be considered for inclusion in training routines as a fundamental to improve balance performance, ankle proprioception, flexibility, and strength. This finding would contribute to improving player performance and overall quality of play in football matches.
Acknowledgements
The authors are grateful to players, parents, and coaches. Thanks to Anna Zuša and Miks Laksbergs for helping with data collection.
Competing Interests
The authors have no competing interests to declare.
Author Contributions
Conceptualization, methodology, writing-original draft, review and editing ARS and IP, Data collection and curation, ARS, AL, AS; Investigation and formal analysis: ARS and KS. All authors read and agreed to the published version of the manuscript.