Evaluation of the Effect of Intensive Resistance Exercises on Anaerobic and Balance Performance in Young Football Players. Doctoral Thesis

Abstract

Football is a dynamic team sport that requires intermittent effort levels from low to high intensities, indicated by a substantial volume of motor actions and unpredictable movement patterns. In football, physical performance is reflected in the quality of play and skill, while functional movement influences technique and prevents sports injuries. The purpose of this work was to evaluate the effects of an eight-week combined RE program integrating self-loading, balance, and plyometric on anaerobic and balance performance in well-trained young male football players aged 14-16 years competing at regional and national levels. The study was conducted in the Riga region, Latvia, from 2022 to 2024. In the first and second studies, we conducted a pilot to analyse 1) anaerobic and balance performance based on playing position, and 2) detection of asymmetry between the dominant (D) and non-dominant (ND) leg. 43 players (age = 15.81±1.33 years, height (H) = 179.50±6.14 cm, and body mass (BM) = 69.73±9.35 kg) participated. They had more than five years of football experience, 7-8 hours of regular training per week, and competed regularly. The players were classified into 4 groups: Goalkeeper (GK) (N: 5, H: 185.42±2.20 cm, BM: 76.20±9.85 kg), Defenders (DEF) (N: 12, H: 182.48±2.20 cm, BM: 73.70±4.73 kg), Midfielders (MD) (N: 20, H: 174.89±5.03 cm, BM: 63.77±6.92 kg), and Forwards (F) (N: 6, H: 184.0±3.69 cm, BM: 75.28±12.25 kg). 1) A significant difference in peak power output (PPO) was observed across playing positions (p = 0.018, η² = 0.226), where F significantly greater in power output compared to MD (p = 0.03). Vertical jump characteristics, including jump height, flight time, and peak force, showed no significant differences across playing positions (p = 0.11–0.31). Average medial-lateral speed (AMLS) showed significant differences between ND (p = 0.025, η² = 0.211) and D leg (p = 0.033, η² = 0.198) based on the players' positions. AMLS showed statistically significant better results (slower movement speed) in GK compared to F in their ND (p = 0.022) and D leg (p = 0.040). However, other parameters of static and dynamic balance showed no statistical significance in the D/ND leg for 30s one-leg stance across groups. 2) No significant differences were found between the D and ND leg in all parameters of static balance, p = 0.33–0.72. Similarly, dynamic balance results revealed no significant differences between the D and ND legs for the parameters: total stability index (TSI) (p = 0.736) and total trunk deviation (TTD) (p = 0.789). In addition, no correlation was found between static and dynamic balance tests, although minor links existed between TTD and EA (r = 0.332; p = 0.03), TTD and MLD (r = 0.335; p = 0.02), and TTD and TD (r = 0.423; p = 0.01). In the third and fourth studies, we evaluated the effect of various RE on 3) anaerobic and 4) balance performance, ankle mobility, and explosive strength. 30 young players were randomly allocated to an intervention (IG) and a control (CG) group. The IG (N: 15, age: 15.8±1.7 years, BM: 67.8±11.9 kg, H: 177.2±5.9 cm), and CG (N: 15, age: 15.7±0.6 years, BM: 69.6±4.8 kg, H: 180.3±5.6 cm). 3) Both groups demonstrated significant improvements in PPO during the Wingate Anaerobic test (WAnT) and six-second peak power output test (6 PPO), with percentage change ranging from 12–31 % (p = 0.000–0.02; d = 0.97–1.5). Absolute mean power (MP) significantly increased IG (21.8 %, p = 0.001) and CG (11.8 %, p = 0.01). However, no statistically significant improvement was observed in the fatigue index (FI) of participants from both groups. 4) IG significantly improved static and dynamic balance by −12 to −30 % (p = 0.001–0.04, d = 0.57–1.14), active ankle ROM in PF and DF by 12–32 % (p = 0.001–0.02, d = 1.05–1.19), isometric shin muscle force, torque and relative in PF and DF by 10–24 % (p = 0.001–0.03, d = 0.48–1.31), and jump performance by 6–14 % (p = 0.004–0.01, d = 0.82–1.27). No significant changes were observed in the CG, except for peak and relative torque DF by 10–19 % (p = 0.01–0.02, d = 0.55–0.91), peak force DF in the dominant leg by 11 % (p = 0.04, d = 0.71) and jump height by 5 % (p = 0.008, d = 0.82). The statistically significant interactions between groups in all balance variables, ankle ROM, and ankle force of PF in the dominant leg, peak force and torque of DF in the dominant leg were calculated using ANCOVA analysis. Based on research findings, variation in peak power output was revealed among player positions but not for jump and balance performance. Regularly monitoring physical fitness and functional movement is essential to ensure young players reach their peak performance and prevent injuries. The implementation of resistance exercises with progression, regularity, overload, and supervision is appropriate for young players to enhance their anaerobic and balance performance. RE should be included in regular training to improve physical fitness and functional mobility in young footballers. The Doctoral Thesis was developed at Rīga Stradiņš University Latvian Academy of Sport Education (RSU LASE), Latvia. Defence of the Doctoral Thesis will take place at the public session of the Promotion Council of Health and Sports Sciences on 24 August 2026 at 15.00 via online platform Zoom. The research is carried out within the framework of the European Union Recovery and Resilience Mechanism Plan and the state budget funded project “RSU internal and RSU with LASE external consolidation” No. 5.2.1.1.i.0/2/24/I/CFLA/005, implemented under Doctoral Grant. 3.3. Health and Sports Sciences; Sub-Sector – Sports Pedagogy. Keywords: resistance exercise; plyometric; self-loading training; balance training; peak power output; static and dynamic balance; ankle range of movement; shin muscle peak isometric force and torque; explosive strength.