Effects of Flywheel Leg Curls on Muscle Structure and Function in Athletes with a History of Hamstring Strain Injury: A Randomized Controlled Trial.

Background: Hamstring strain injuries (HSIs) have a high rate of recurrence, highlighting the need for effective tertiary prevention strategies. Flywheel resistance training appears effective in targeting muscular risk factors for HSI.

Hypothesis: Flywheel leg curls will result in greater improvements in eccentric knee flexor strength and biceps femoris long head (BF_LH) fascicle length compared with conventional leg curls.

Study design: Randomized controlled trial.

Level of evidence: Level 2.

Methods: Twenty-six athletes with a history of HSI within the past 18 months participated in an 8-week preventive training program. Athletes were assigned randomly to perform leg curls using either conventional (CON) or flywheel (FLY) equipment. Primary outcomes included eccentric knee flexor strength and BF_LH fascicle length. Secondary outcomes included isometric strength, maximal hip flexion active knee extension (MHFAKE), passive straight leg raise (PSLR), BF_LH muscle thickness, and pennation angle. Reinjury occurrence was monitored over a 6-month period.

Results: The FLY group showed significantly greater improvements in eccentric strength (19 ± 19% vs 6 ± 9%; P = 0.04) and BF_LH fascicle length (9 ± 8% vs 2 ± 3%; P = 0.01) compared with the CON group. The FLY group also exhibited superior gains in isometric strength and flexibility (MHFAKE), whereas the CON group showed a larger increase in BF_LH pennation angle. During follow-up, the CON group sustained 3 reinjuries, whereas the FLY group reported only 1 (reinjury risk ratio 3.0, 95% CI, 0.31-28.9).

Conclusion: Athletes with a history of HSI who participated in a preventive training program including flywheel leg curls showed greater improvements in knee flexor strength, fascicle length, and flexibility, as well as a lower reinjury occurrence compared with those trained with conventional leg curls.

Clinical relevance: Flywheel resistance training offers enhanced muscular adaptations that may reduce the risk of reinjury in athletes with a history of HSI.