Preventing Non-Contact Hamstring Injuries in Elite Footballers: Eccentric vs Concentric Training

Background and Rationale Football is associated with one of the highest injury rates among competitive sports, with hamstring muscle injuries representing a major cause of time loss, reduced performance, and increased medical burden. Epidemiological evidence indicates that approximately 80% of hamstring injuries in football occur without direct contact, most commonly during high-speed running, sprinting, acceleration, deceleration, and sudden changes of direction. These injuries are associated with high recurrence rates and may significantly impact player availability and long-term athletic performance. Hamstring muscle function plays a critical role in knee and hip stability, particularly during the terminal swing phase of sprinting when the hamstrings are subjected to high eccentric loads. Muscle weakness, fatigue, reduced flexibility, and imbalances between concentric and eccentric strength have been identified as key intrinsic risk factors for hamstring strain injuries in footballers. Eccentric hamstring strengthening exercises-such as the Nordic hamstring exercise-are widely advocated for injury prevention due to their ability to increase muscle strength, fascicle length, and neuromuscular control. However, emerging evidence suggests that concentric strengthening and combined eccentric-concentric programs may also contribute to improved muscle performance and fatigue resistance. Despite extensive research on eccentric training, direct comparisons between eccentric, concentric, and combined hamstring strengthening programs in elite male football players remain limited, particularly with long-term follow-up across an entire competitive season. Therefore, this randomized controlled trial aims to systematically evaluate and compare the long-term effectiveness of eccentric, concentric, and combined hamstring strengthening programs in preventing non-contact hamstring injuries, improving muscle performance and flexibility, reducing injury recurrence, and enhancing psychological satisfaction in elite male footballers. Aim of the Study To compare the long-term effects of eccentric, concentric, and combined hamstring strengthening programs-each conducted over a three-month intervention period-on non-contact hamstring injury prevention, muscle performance, flexibility, injury recurrence, and psychological satisfaction in elite male football players over a 12-month follow-up period. Specific Objectives 1. To evaluate changes in the incidence of non-contact hamstring injuries following eccentric, concentric, and combined hamstring strengthening programs. 2. To assess improvements in hamstring muscle performance and flexibility in response to different strengthening protocols. 3. To determine the recurrence rate of hamstring injuries during the 12-month follow-up period. 4. To assess psychological satisfaction and perceived sports performance among players undergoing each intervention. 5. To identify the most effective hamstring strengthening approach for injury prevention and performance enhancement in elite male footballers. Hypotheses 1. A three-month eccentric hamstring strengthening program will produce significant long-term improvements in injury prevention, muscle performance, flexibility, reduced recurrence rates, and psychological satisfaction over a 12-month period. 2. A three-month concentric hamstring strengthening program will result in significant long-term improvements in the same outcome measures. 3. A combined eccentric and concentric hamstring strengthening program will yield significant long-term benefits across all outcome measures. 4. Significant differences will exist among eccentric, concentric, combined, and control groups with respect to injury incidence, muscle performance, flexibility, recurrence rates, and psychological satisfaction over the 12-month follow-up period. Study Design This study is designed as a single-blind randomized controlled trial conducted over a 12-month period. Eligible participants will be randomly allocated into four intervention groups. Study Setting The study will be conducted in collaboration with selected elite football clubs across the United Arab Emirates. All exercise interventions and injury management will be supervised by qualified physiotherapists at the Physiotherapy and Sports Medicine Center, Ras Al Khaimah. Randomization and Blinding Randomization will be performed using an automated system by an independent individual not involved in the study. Participants will be allocated sequentially into one of four groups to ensure balanced distribution. Due to the nature of the interventions, the study will follow an open-label design; however, outcome assessors will remain blinded to group allocation to reduce assessment bias. Intervention Groups Group A: Eccentric Training + Regular Football Training Participants will perform: * Nordic hamstring curls * Dumbbell stiff-leg deadlifts * Barbell Romanian deadlifts Group B: Concentric Training + Regular Football Training Participants will perform: * Seated leg curls * Lying leg curls * Prone hamstring curls with resistance bands Group C: Combined Eccentric and Concentric Training + Regular Football Training Participants will perform all exercises listed in Groups A and B. Group D: Control Group Participants will continue with regular football training only, without additional hamstring-specific exercises. Training Protocol * Frequency: 3 sessions per week * Duration: Minimum of 30 minutes per session * Intervention period: 12 weeks * Warm-up (10 minutes): * 5 minutes cycling * 3 minutes stretching * 2 minutes rope jumping All sessions will be supervised to ensure safety and adherence. Outcome Measures Primary Outcome * Incidence of non-contact hamstring injuries Secondary Outcomes * Hamstring flexibility (Sit-and-Reach Test, Active Knee Extension Test) * Muscle performance (30-meter sprint time, vertical jump height) * Injury recurrence rates * Psychological satisfaction (Subjective Sports Performance Satisfaction Questionnaire) Assessments will be conducted at baseline, post-intervention (3 months), and at 6, 9, and 12 months. Safety and Risk Management All interventions will be supervised by trained professionals. Participants will be monitored for adverse events, including muscle soreness, strain, or joint discomfort. Pain levels will be recorded using the Visual Analogue Scale, and thigh circumference will be measured to detect swelling. Immediate medical care will be provided if required. Ethical Considerations Participation is voluntary, and players may withdraw at any time without penalty. Written informed consent will be obtained prior to participation. Confidentiality will be maintained through anonymized data storage and restricted access. Ethical approval will be obtained from relevant institutional and national ethics committees prior to study commencement. Expected Outcomes It is anticipated that eccentric hamstring strengthening will demonstrate superior effectiveness in reducing non-contact hamstring injuries, improving flexibility, enhancing sprint performance, and increasing psychological satisfaction compared to concentric or combined programs. The findings are expected to inform evidence-based injury prevention strategies in elite football. Dissemination of Results Results will be disseminated through: * PhD thesis submission * Publication in Scopus-indexed journals * National and international conferences * Reports shared with participating clubs and sports authorities Project Duration Total duration: 12 months, including baseline assessment, intervention phase, follow-up assessments, and data analysis.