تأثیر شش هفته تمرین نوردیک همسترینگ بر زاویه Q، نسبت قدرت همسترینگ به چهارسر ران و فعالیت الکتریکی این عضلات در فوتبالیست‌های نوجوان دارای زانوی پرانتزی

Background and Purpose
Hamstring strain injuries (HSIs) are among the most common musculoskeletal injuries in both professional and amateur sports, particularly those involving high-speed running and rapid changes in direction, such as soccer, rugby, and track and field. In elite soccer, HSIs account for approximately 12–16% of all injuries, with high recurrence rates of up to 30% within the same season. These injuries predominantly occur during eccentric muscle actions, especially in the terminal swing phase of sprinting, and pose significant challenges for athletes due to prolonged recovery periods and performance impairment. The Nordic hamstring exercise (NHE) is a widely recognized eccentric training method that strengthens the hamstring muscles and enhances neuromuscular control. It has been shown to reduce the incidence of HSIs by up to 51% in soccer players. Despite its efficacy, limited research has examined the effects of NHE on biomechanical and neuromuscular variables such as the Q angle, hamstring-to-quadriceps (H:Q) strength ratio, and muscle activation patterns in individuals with lower limb alignment abnormalities like genu varum (bow-leggedness). This condition alters knee biomechanics, increases medial knee stress, and may contribute to injury risk. This quasi-experimental study investigates the impact of a 6-week NHE intervention on the Q angle, H:Q strength ratio, and electromyographic (EMG) activity of the vastus lateralis, vastus medialis, semimembranosus, and biceps femoris muscles in adolescent male soccer players with genu varum. The findings may offer valuable insights for injury prevention and rehabilitation strategies in youth athletes with structural misalignments.
 Materials and Methods
This quasi-experimental study employed a pre-post design to assess the effects of a 6-week NHE intervention. Thirteen adolescent male soccer players (aged 15–18 years) with genu varum were recruited from soccer academies in Tehran, Iran, using purposive sampling. Genu varum was confirmed via clinical assessment. Participants were excluded if they had a history of lower limb surgery, recent musculoskeletal injuries, or neurological conditions affecting movement. Informed consent was provided by participants and their guardians. The study employed a repeated-measures design, with pre- and post-intervention assessments.
The Q angle was measured using a goniometer, while the H:Q strength ratio and electrical activity of the vastus lateralis, vastus medialis, semimembranosus, and biceps femoris muscles were assessed using an isokinetic dynamometer and electromyography (EMG), respectively. The Q angle was measured using a standardized goniometric protocol. Measurements were taken before and after a 6-week NHE program. Muscle strength and activation data were obtained using an isokinetic dynamometer at angular velocities of 60°/s and 180°/s. Surface EMG electrodes were placed following SENIAM guidelines. All participants completed the same testing protocol before and after the intervention period. Data were analyzed using SPSS. Normality was confirmed via the Shapiro-Wilk test. A dependent t-test with repeated measures was used to compare pre- and post-intervention outcomes, with a significance level of p<0.05.
 Results
The results revealed a significant reduction in the Q angle following the 6-week NHE program (p<0.001), indicating improved knee alignment. Significant improvements were also observed in the H:Q strength ratio during both extension and flexion modes at angular velocities of 60° (p<0.02 for extension, p<0.01 for flexion) and 180° per second (p<0.001 for extension, p<0.03 for flexion). These findings suggest that the NHE program effectively enhanced the balance between hamstring and quadriceps muscle strength, a key factor in injury prevention.
In terms of muscle electrical activity, significant increases were noted in the semimembranosus (p<0.01), biceps femoris (p<0.04), and vastus lateralis (p<0.02) muscles at 60° per second. At 180° per second, significant improvements were observed in the semimembranosus (p<0.01) and biceps femoris (p<0.001) muscles. However, no significant changes were detected in the vastus medialis muscle at either angular velocity (p>0.05) or in the vastus lateralis muscle at 180° per second (p>0.05). These results indicate that the NHE program was particularly effective in enhancing hamstring muscle activation, especially in the semimembranosus and biceps femoris, which are crucial for dynamic knee stabilization and injury prevention during high-speed athletic movements.
 Discussion
The results of this study support the efficacy of the Nordic hamstring exercise (NHE) as a neuromuscular training intervention capable of eliciting meaningful improvements in lower limb biomechanics among adolescent soccer players with genu varum. Over a 6-week period, the NHE program significantly improved the Q angle, hamstring-to-quadriceps (H:Q) strength ratio, and electromyographic (EMG) activity of key hamstring muscles, particularly the semimembranosus and biceps femoris. These adaptations suggest enhanced hamstring-mediated control of tibial rotation, improved eccentric strength, and greater neuromuscular recruitment during high-risk athletic movements such as sprinting and deceleration.
The observed reduction in Q angle may contribute to better alignment and reduced lateral patellar tracking, which is often exaggerated in individuals with genu varum. Similarly, the improved H:Q ratio addresses a well-known risk factor for hamstring strain injuries. The absence of significant changes in vastus medialis activation likely reflects the posterior-chain specificity of the NHE and may indicate a need for complementary quadriceps-focused exercises to ensure balanced development. These findings advocate for incorporating NHE into youth soccer training and rehabilitation programs not only as a preventive measure against hamstring injuries but also as a corrective strategy for lower limb malalignments. The exercise is cost-effective, evidence-based, and easily implemented, making it suitable for football academies and sports therapy settings.
 Conclusion
The findings of this study demonstrate that a 6-week Nordic hamstring exercise program can significantly improve knee alignment (Q angle), the hamstring-to-quadriceps strength ratio, and the neuromuscular activation of key hamstring muscles in adolescent soccer players with genu varum. These improvements are likely to contribute to better muscle coordination and joint control, both of which are important for preventing injuries during fast-paced actions like sprinting or changing direction. However, the exercise had little effect on one of the inner thigh muscles (vastus medialis), suggesting that additional exercises may be needed to ensure balanced development across all muscle groups. Overall, the findings highlight NHE as a practical, low-cost tool that could be added to training routines in youth soccer to enhance performance and reduce injury risk. Future research should examine the long-term sustainability of these adaptations, include more diverse populations, and explore the synergistic effects of combining NHE with other neuromuscular training modalities to optimize athletic development and injury prevention.
Article Message
This research explored how a six-week program of Nordic hamstring exercises could benefit young soccer players with bow-legged alignment, known as genu varum—a condition that may increase the risk of hamstring injuries. The results showed meaningful improvements: the alignment of the knee became more optimal, the strength ratio between hamstrings and quadriceps shifted toward a healthier balance, and the activation of key hamstring muscles increased. These changes suggest better muscle coordination and joint control, both of which are important for preventing injuries during fast-paced actions like sprinting or changing direction. However, the exercise had little effect on one of the inner thigh muscles, hinting that additional exercises might be needed to ensure balanced development across all muscle groups. Overall, the findings highlight NHE as a practical, low-cost tool that could be added to training routines in youth soccer to enhance performance and reduce injury risk. The study also encourages further investigation into how such programs perform over time and in a wider range of athletes.