Comparison of Medial and Lateral Hamstring Muscle Activation and Strength across Three Resistance Directions during Isometric Knee Flexion in Healthy Participants: A Pilot Study

Purpose: This study aimed to identify an effective hamstring exercise method that selectively activates the medial and lateral hamstrings.Methods: Five healthy adults were enrolled in this cross-sectional pilot laboratory study.Surface electromyography was recorded from the semitendinosus and biceps femoris of the dominant limb during prone isometric knee flexion at 90, with resistance applied at 0 (neutral), 45 (resistance applied from lateral to medial), and -45 (resistance applied from medial to lateral) in randomized order.Three 5-second maximal trials per direction were performed with 30-second rest intervals.Isometric knee flexion strength was measured using a Smart KEMA system (Koreatech Co., Ltd.), and the outcomes were descriptively compared.Results: The participants (three females and two males; 32.44.5 years) demonstrated the highest mean semitendinosus and biceps femoris activity at 0 (154.724.1 V and 100.225.1 V, respectively) and the lowest at 45 (105.35.6 V and 76.94.4 V) and -45 (129.312.1 V and 64.719.0 V).The medial-to-lateral activity ratio was highest at -45 (2.20.6), compared with 0 (1.60.3) and 45 (1.40.1).Strength was the greatest at 0 (7.50.5)compared with 45 (6.41.3) and -45 (5.51.3). Conclusion:In this pilot study, the medial-to-lateral electromyographic ratio tended to be highest when resistance was applied at -45, suggesting greater medial hamstring recruitment under this condition.These descriptive findings are preliminary and should be confirmed in adequately powered studies.Modifying the direction of resistance may be a feasible strategy to bias medial (-45) or lateral (45) hamstring recruitment depending on rehabilitation goals.