Introduction: ACL injury is very common among sport activities. The incidence is very high and causes an important disorder in the articular function. Some articles have been published in the recent years about the risks to suffer this injury and its consequences in the biomechanics and proprioception of the limb. Although most of them were performed in animals and cadaver specimens and a few of them were realized in patients for a dynamic evaluation.

Purpose: The objective was to analyze in a longitudinal and prospective manner the changes in proprioception (muscular latency in the muscles span the knee in front of a stimulus load) occurred in the joint in 25 patients undergone an ACL injury before and after ACL reconstruction using the contra lateral knee as control.

Material and Methods: The study was performed in an experimental task pre and 4 and 6 months post surgery. Clinical Tegner, Lysholm, IKDC and SF12 evaluation, motion analysis system (EliteR) and surface electromyography was performed in a synchronized manner during a single leg jump before and after a fatigue exercise of a 10 seconds repetitive single leg jump. The muscles studied were anterior rectus, lateral and medial vastus, semitendinous and femoral biceps of both knees. Statistical analysis was performed and a P value < 0.05 was considered significant.

Results: Statistical significant larger latency times in each muscle but lateral vastus was observed in the affected knee in the preoperative period with improvement in the first period of 4 months postoperatively reaching measures of the normal contralateral knee without further improvement at the 6 month period. Vastus medialis was the muscle more affected before surgery and semitendinous, although improved, never reached a normal reactivity. Extensor muscles of the normal knee presented in all the periods of the study similar results but flexor muscles showed significant better propioceptive function in the 4th and 6th month post surgery. Improvement in latency time of muscle reactivity correlated with better scores in the Lysholm, IKDC and SF12 scores, although the SF12 mental status didn’t change.

Conclusion: Operated knees improve their neuromuscular activity relatively fast during the first 4th months. Therefore, specifically talking about neuromuscular function normal physical activity may be can be allowed at the 4th month and that means two months before usually normal activity level is permitted. Preoperative rehabilitation would have to insist to get better medialis vastus function. The mental status didn’t correlate with the neuromuscular status that means that psychologic aspects must be treated simultaneously with the physical training. Synergy among neurological pathways would exist, appreciating improved response in flexor muscles in the contralateral knee during the rehabilitation period