Blood Flow Restriction During Nordic Hamstring Exercise in Elite Adolescent Athletes

Hamstring muscles play a crucial role in sprinting, jumping, change-of-direction movements, and other high-intensity athletic activities. Nordic Hamstring Exercise (NHE) is one of the most effective eccentric strengthening exercises for improving hamstring function and reducing the risk of hamstring strain injuries. Recently, blood flow restriction (BFR) training has gained increasing attention because it may enhance metabolic stress and alter muscle oxygenation responses during exercise. However, evidence regarding the acute physiological effects of combining BFR with Nordic Hamstring Exercise in elite adolescent athletes remains limited.

This study aims to compare hamstring muscle oxygenation responses during conventional Nordic Hamstring Exercise and Blood Flow Restriction Nordic Hamstring Exercise in elite adolescent athletes. A prospective crossover experimental design will be used, allowing each participant to complete both exercise conditions and serve as his or her own control. The order of the interventions will be randomized, and the two testing sessions will be separated by a 7-day washout period to minimize potential carryover effects.

Elite adolescent athletes aged 14-18 years who have participated in regular training for at least three years will be recruited. During each testing session, hamstring muscle oxygen saturation (SmO₂) and total hemoglobin (tHb) responses will be continuously monitored using wearable near-infrared spectroscopy (MOXY). Measurements will include baseline SmO₂, minimum SmO₂, delta SmO₂, recovery SmO₂, baseline total hemoglobin, peak total hemoglobin, and delta total hemoglobin. Blood lactate concentration, rating of perceived exertion using the Borg Rating of Perceived Exertion Scale, exercise performance variables, and post-exercise symptoms such as muscle soreness and fatigue will also be recorded.

Nordic Hamstring Exercise will be performed according to a standardized protocol under physiotherapist supervision. During the Blood Flow Restriction Nordic Hamstring Exercise condition, the same exercise protocol will be performed while individualized blood flow restriction is applied to the proximal lower extremity using a pneumatic cuff. The cuff pressure will be individualized based on participant tolerance and safety, and the intervention will be discontinued immediately if any adverse symptoms occur.

The primary objective is to determine whether blood flow restriction alters acute hamstring muscle oxygenation responses during Nordic Hamstring Exercise. Secondary objectives include comparing total hemoglobin responses, blood lactate concentration, perceived exertion, exercise performance, and post-exercise recovery between the two exercise conditions. The findings of this study are expected to improve understanding of the physiological mechanisms of blood flow restriction during eccentric hamstring exercise and provide evidence for its safe and effective use in adolescent athletic populations.

Inclusion criteria

• Elite adolescent athletes aged 14 to 18 years.
• Licensed athletes with at least 3 years of regular sports participation.
• Currently participating in a regular weekly training program.
• Physically able to complete the Nordic Hamstring Exercise and study testing protocol.
• Willing to participate in the study.
• Able to provide written informed consent.
• For participants younger than 18 years, written informed consent provided by a parent or legal guardian and assent provided by the participant.

Exclusion criteria

• Hamstring injury within the previous 6 months.
• Lower extremity surgery within the previous 6 months.
• Current lower extremity pain or musculoskeletal injury that may interfere with exercise performance.
• Cardiovascular, vascular, neurological, or other medical conditions that may contraindicate blood flow restriction exercise.
• Presence of skin lesions or conditions preventing placement of the blood flow restriction cuff or MOXY sensor.
• Inability to complete either exercise condition or follow the study protocol.