‍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.