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; 32.4±4.5 years) demonstrated the highest mean semitendinosus and biceps femoris activity at 0° (154.7±24.1 μ V and 100.2±25.1 μ V, respectively) and the lowest at 45° (105.3±5.6 μ V and 76.9±4.4 μ V) and -45° (129.3±12.1 μ V and 64.7±19.0μ V).The medial-to-lateral activity ratio was highest at -45° (2.2±0.6), compared with 0° (1.6±0.3) and 45° (1.4±0.1).Strength was the greatest at 0° (7.5±0.5)compared with 45° (6.4±1.3) and -45° (5.5±1.3).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.