Study Title

‍“Effects of flywheel leg curls on muscle structure and function in athletes with a history of hamstring strain injury: a randomized controlled trial” by Sampietro et al. (2025).  

Link to article:

‍https://journals.sagepub.com/doi/10.1177/19417381251355960

Purpose of the Study

Compare the effects of conventional leg curl and flywheel leg curl training on hamstring injury risk factors and reinjury rates in athletes with a history of hamstring injuries.  

Background

Hamstring strain injuries are common in running-based sports and have significant implications for both the athlete and the team they belong to. For example, about 1 in every 5 elite soccer players experience a hamstring injury during a season, and these injuries now account for about 20% of all injury absence days (6,7). Importantly, hamstring strain injuries have a high rate of reinjury within the first few months after the athlete’s return to play (5,7,8), highlighting the importance of return-to-play and post-injury strategies.  

Previous research has shown that hamstring strain injury risk and a history of hamstring injuries are associated with lower levels of knee flexor eccentric strength and shorter fascicles in the biceps femoris long head (4,10,12). So, methods that target these factors may help reduce the risk of hamstring strain injury and reinjury.  

Flywheel training has emerged as a promising method for this purpose. For example, programs with flywheel leg curls and stiff leg deadlifts can increase eccentric strength and biceps femoris long head fascicle length in well-trained athletes (1,3,11). And flywheel leg curl training has also shown promise as a pre-season training method, with findings of improved performance and reduced hamstring injury rates and severity (1,3).  

This study by Sampietro et al. expanded on this research, by comparing the effects of conventional leg curls and flywheel leg curls in athletes with a recent history of hamstring strain injury.  

Study Methods

Subjects

26 professional or semi-professional soccer and rugby athletes were randomly assigned to  groups that performed either flywheel leg curls or conventional leg curls as part of a lower body injury prevention program. All athletes had experienced a hamstring strain injury to one limb in the last 18 months but were currently participating in their sport.  

Training Programs

Both groups performed prone leg curls 2x/week for 8 weeks in addition to other lower body injury prevention exercises. The conventional group started with 3 sets of 12 reps and progressed to 4 sets of 8 reps by the final two weeks. The flywheel group had structured increases in inertial load every few weeks, and during the last two weeks used a strategy of performing the concentric phase with both legs and the eccentric phase with one leg to elicit eccentric overload.  

What They Measured

All athletes were assessed for eccentric and isometric strength, biceps femoris long head architecture (thickness, fascicle length, pennation angle), and passive and active flexibility before and after the 8-week training program. Additionally, the athletes were monitored for hamstring strain reinjury for 6-months after the program.  

Key Results

Here is a summary of key findings:

• The flywheel training group had superior gains in eccentric strength (+19.1% vs 6.2%), biceps femoris long head fascicle length (+8.9% vs +2.5%), isometric strength with hips and knees at 45 degrees (+27.7% vs +9.0%) and active knee flexor flexibility (+8.0% vs +0.7%).  

• The flywheel group decreased in pennation angle (-9.5%), while the conventional group did not.  

• Results were not statistically different for isometric strength at a hip/knee angle of 90 deg, muscle thickness, or passive flexibility.  

• During the 6-month follow-up period, 3/13 (23.0%) of athletes from the conventional group and 1/13 (7.7%) from the flywheel group experienced a hamstring strain reinjury.  

Discussion and Key Takeaways

Overall, this study found that flywheel leg curls resulted in greater improvements in hamstring injury risk factors like knee flexor eccentric strength and biceps femoris long head fascicle length compared to conventional leg curls. Additionally, the flywheel group had less cases of hamstring reinjury during the 6-month follow-up.  

This study provides additional support for the inclusion of flywheel hamstring exercises into training and return to play protocols for athletes that are at risk for hamstring strain injury. This is likely due to the eccentric loading and unique resistance offered by the method, which tends to provide a unique stimulus compared to traditional strength training methods (2).  

Overall, this study adds to a growing body of research on the benefits of flywheel hamstring training for preparing the hamstrings for the demands of high-intensity sport!

The Exerfly Advantage

Flywheel training requires high levels of eccentric muscle activation and effort to resist against the momentum built up during the concentric phase. By using high levels of effort during the concentric phase and rapidly applying braking forces to decelerate the flywheel’s momentum, you can apply a powerful eccentric training stimulus to the hamstrings.  

Exerfly Flywheel Training devices bring an extra dimension to your hamstring training, including:

• The versatility to perform a variety of exercises, including deadlift variations, leg curls, back extensions and more.  

• The option to stack up to 8 flywheels on each device, with a total loading capacity up to 0.8 kgm². This ensures you will always have the loading potential needed to challenge even the strongest of athletes.  

• The Exerfly Motorized Boost, which allows you to provide a pre-designated percentage of extra boost to the eccentric phase of each rep! This allows you to prescribe eccentric overload in precise and easy to progress increments without needing to use specialized technique to achieve extra eccentric loading.

Exercise Examples

1. Flywheel Hamstring Rollouts

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2. Hip Flexion with Glute Bridge

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3. Heel Elevated Deadlift with Exerfly Motorized Eccentric Overload

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4. Prone Leg Curl

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Want to check out more on how coaches and athletes are using Exerfly to build strong and resilient hamstrings? Check out some of our previous blogs!

• Bulletproof Your Hamstrings: 6 Exercises to Build Maximum Strength

• Eccentric Hamstring Exercises: Bulletproof Against Injury

References

1. Askling, C., Karlsson, J., & Thorstensson, A. (2003). Hamstring injury occurrence in elite soccer players after preseason strength training with eccentric overload. Scandinavian Journal of Medicine & Science in Sports, 13(4), 244-250.

2. Banks, N. F., Berry, A. C., Rogers, E. M., & Jenkins, N. D. (2024). Flywheel resistance training promotes unique muscle architectural and performance‐related adaptations in young adults. European Journal of Sport Science, 24(12), 1765-1778.

3. de Hoyo, M., Pozzo, M., Sañudo, B., Carrasco, L., Gonzalo-Skok, O., Domínguez-Cobo, S., & Morán-Camacho, E. (2015). Effects of a 10-week in-season eccentric-overload training program on muscle-injury prevention and performance in junior elite soccer players. International Journal of Sports Physiology and Performance, 10(1), 46-52.

4. de Lima-E-Silva, F. X., Oliveira, G. S., Medeiros, T. M., Dornelles, M. P., Ribeiro-Alvares, J. B. A., & Baroni, B. M. (2020). Flexibility, strength, and fascicle length of football players with and without history of hamstring strain injury in the prior season. Science and Medicine in Football, 4(4), 322-328.

5. Duhig, S. J., Bourne, M. N., Buhmann, R. L., Williams, M. D., Minett, G. M., Roberts, L. A., ... & Shield, A. J. (2019). Effect of concentric and eccentric hamstring training on sprint recovery, strength and muscle architecture in inexperienced athletes. Journal of Science and Medicine in Sport, 22(7), 769-774.

6. Ekstrand, J., Bengtsson, H., Waldén, M., Davison, M., Khan, K. M., & Hägglund, M. (2023). Hamstring injury rates have increased during recent seasons and now constitute 24% of all injuries in men’s professional football: the UEFA Elite Club Injury Study from 2001/02 to 2021/22. British Journal of Sports Medicine, 57(5), 292-298.

7. Ekstrand, J., Waldén, M., & Hägglund, M. (2016). Hamstring injuries have increased by 4% annually in men's professional football, since 2001: a 13-year longitudinal analysis of the UEFA Elite Club injury study. British Journal of Sports Medicine, 50(12), 731-737.

8. Opar DA, Williams MD, Shield AJ. Hamstring strain injuries: factors that lead to injury and re-injury. Sports Med. 2012 Mar 1;42(3):209-26.

9. Sampietro, M., Campana, V., Pereira Thiem, L., Albarenque, M., Ribeiro de Oliveira, R., & Baroni, B. M. (2025). Effects of Flywheel Leg Curls on Muscle Structure and Function in Athletes with a History of Hamstring Strain Injury: A Randomized Controlled Trial. Sports Health, 19417381251355960.

10. Timmins, R. G., Bourne, M. N., Shield, A. J., Williams, M. D., Lorenzen, C., & Opar, D. A. (2016). Short biceps femoris fascicles and eccentric knee flexor weakness increase the risk of hamstring injury in elite football (soccer): a prospective cohort study. British Journal of Sports Medicine, 50(24), 1524-1535.

11. Timmins, R. G., Filopoulos, D., Nguyen, V., Giannakis, J., Ruddy, J. D., Hickey, J. T., ... & Opar, D. A. (2021). Sprinting, strength, and architectural adaptations following hamstring training in Australian footballers. Scandinavian Journal of Medicine & Science in Sports, 31(6), 1276-1289.

12. Timmins, R., Shield, A., & Williams, M. (2015). Biceps femoris longhead architecture: a reliability and retrospective injury study. Medicine and Science in Sports and Exercise, 47(5), 905-913.