Many patients experience quadriceps inhibition after anterior cruciate ligament (ACL) reconstruction, delaying strength recovery, hindering return to sport, and potentially increasing the risk of re-injury. Two rehabilitation strategies-low-load blood flow restriction (BFR) training and surface electromyography (sEMG) biofeedback-aim to enhance neuromuscular activation and strength while limiting joint load. However, comparative and combined evidence in pragmatic, multicenter settings remains limited.
AMIRACL is a multicenter, prospective, randomized, controlled, parallel-group trial with four arms enrolling 200 adults (18-35 years) undergoing a first-time ACL reconstruction. Participants are randomized with center stratification; outcome assessors are blinded to allocation. The four groups are: (1) standard rehabilitation; (2) standard + early BFR; (3) standard + early sEMG biofeedback; and (4) standard + combined BFR and sEMG biofeedback. Interventions begin about 2 weeks postoperatively, are delivered over 6 weeks at three supervised sessions per week, and are integrated into contemporary ACL rehabilitation. BFR uses individualized, auto-regulated cuff pressure during low-load isometric and then dynamic exercises. sEMG biofeedback provides real-time visual and/or auditory feedback to optimize quadriceps recruitment during targeted tasks. The combined arm receives both modalities concurrently.
The primary objective is to compare quadriceps activation (sEMG) and maximal isometric knee extensor strength between groups at 3 and 6 months. Secondary objectives include return-to-sport readiness and patient-reported function (e.g., ACL-RSI, IKDC), broader knee outcomes (e.g., KOOS, Lysholm), adherence and adverse events across arms, and ACL re-injury (ipsilateral graft rupture or contralateral ACL injury) within 2 years. Longer-term patient-reported quality of life is explored up to 5 years.
Key eligibility criteria include age 18-35 years, first ACL reconstruction, and preinjury sport participation; major exclusions include revision ACL surgery, concomitant multi-ligament repair, neuromuscular disorders, and contraindications to BFR or sEMG. The planned sample size is 200 (50 per arm), powered to detect a clinically meaningful between-group difference in quadriceps activation. Analyses will follow the intention-to-treat principle using mixed-effects models for repeated measures. The study is conducted under Good Clinical Practice and applicable Swiss regulations; all participants provide written informed consent. Overall, AMIRACL will determine whether early BFR, sEMG biofeedback, or their combination meaningfully improves quadriceps activation, strength, and clinical recovery after ACL reconstruction compared with standard rehabilitation alone.
Adults aged 18-35 years.
Revision ACL surgery.
Parallel
Standard Rehabilitation
BehavioralStandardized postoperative ACL rehabilitation only. Begins \~2 weeks after surgery; supervised \~3 sessions/week for 6 weeks. Includes progressive range of motion, quadriceps activation/strengthening, functional drills, and return-to-activity progressions delivered per protocol.
Blood Flow Restriction (BFR) Training
BehavioralLow-load resistance exercises performed with individualized limb occlusion pressure (LOP) determined each session; target %LOP per protocol. Progresses from isometric to dynamic open/closed-chain tasks. Delivered under supervision \~3 sessions/week for 6 weeks; monitoring of tolerance and adverse events per safety LOP.
sEMG Biofeedback
BehavioralReal-time surface electromyography biofeedback to optimize quadriceps recruitment during targeted tasks (e.g., isometric contractions, straight-leg raise, sit-to-stand, mini-squat, gait drills). Visual and/or auditory feedback with progressive targets. Supervised \~3 sessions/week for 6 weeks; outcome assessors remain blinded.
Quadriceps neuromuscular activation (surface EMG)
Mean normalized surface electromyography (sEMG) activity of the quadriceps during standardized voluntary isometric knee extension; higher values indicate greater activation. Primary analysis compares groups at 3 and 6 months; baseline and post-intervention values are collected to describe trajectories.
Maximal isometric knee extensor strength (traction dynamometry)
Peak voluntary isometric knee extension strength recorded with a traction dynamometer under standardized positioning; primary analysis compares groups at 3 and 6 months; baseline and post-intervention values are collected to describe trajectories.
ACL-RSI (Return to Sport after Injury)
Patient-reported readiness and psychological response to return to sport using the ACL-RSI questionnaire; higher scores indicate greater readiness.
IKDC (International Knee Documentation Committee) subjective knee form
Patient-reported knee symptoms, function, and sports activity; higher scores indicate better status.
ACL re-injury/recurrence rate
Proportion of participants with ipsilateral graft rupture or contralateral ACL injury within 2 years, assessed via structured telephone interview and medical record verification when available.
KOOS (Knee injury and Osteoarthritis Outcome Score)
Patient-reported outcomes across KOOS subscales (Symptoms, Pain, ADL, Sport/Rec, QoL); higher scores indicate better status.
Quadriceps neuromuscular activation (sEMG) at end of intervention
Normalized sEMG activity measured immediately after the 6-week intervention phase to characterize early neuromuscular changes.
Maximal isometric knee extensor strength at end of intervention
Peak isometric knee extension strength measured with traction dynamometry after the 6-week intervention phase to characterize early strength changes.
Adverse events related to rehabilitation and/or BFR/sEMG procedures
Number and proportion of participants experiencing adverse events potentially related to the study interventions; categorized by severity (mild/moderate/severe) and seriousness per protocol definitions; events recorded at each supervised session and follow-up visit.
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