Impact of Hamstring Tightness on Muscle Activation in Healthy Young Adults.

BACKGROUND: Hamstring tightness is highly prevalent in young adults and may negatively affect neuromuscular performance. Despite growing interest in the biomechanical and neuromuscular consequences of reduced flexibility, few studies have examined its effect on muscle activation in healthy individuals. This study aimed to compare thigh muscle activation during functional tasks in healthy young males with and without hamstring tightness. METHODS: Thirty healthy male participants (18-26 years) were assigned to two groups based on the Active Knee Extension test: normal flexibility (<20°) and hamstring tightness (≥20°). Surface electromyography (sEMG) was used to assess the activation (%MVIC) of the biceps femoris and semitendinosus muscles during four functional exercises: unilateral standing knee flexion, unilateral bridge, elastic-band knee flexion, and Nordic curl. Independent t-tests were used to compare muscle activation between groups. RESULTS: Participants with hamstring tightness showed significantly lower activation of the semitendinosus during the unilateral bridge (p = 0.036) and Nordic curl (p = 0.024). Additionally, biceps femoris activation during the Nordic curl was reduced in the tightness group compared to the normal group (p = 0.044). No significant differences were observed in other exercises. CONCLUSIONS: Hamstring tightness is associated with reduced activation of key posterior thigh muscles during exercises that require high eccentric or isometric demands. These neuromuscular alterations may impair performance and increase the risk of injury in functional tasks. Clinically, assessing and addressing hamstring flexibility may support muscle recruitment efficiency and injury prevention strategies in young physically active populations.