Introduction:
The anterior cruciate ligament (ACL) is one of most important structures in the knee joint. It contains mechanoreceptors, that respond to mechanical pressure or distortion and enable one to feel the knee position and proprioreception. Injury of this structure can lead to changes in knee function. Issues which relate to knee translation and knee effusion are widely described in scientific literature but increasingly more attention is being paid to muscle function: strength and stiffness. ACL trauma is often accompanied by a sensation of instability and pain during knee load. This is accompanied by joint effusion, which gradually limits the knee range of motion, exacerbates the pain and secondarily has an impact on muscle function. Functional shortening of the hamstrings can be a symptom of inappropriate control in the knee joint.

Material and methods:
The study was carried out in a group of 46 men aged 18–46 years old, with a mean age of 25 years old (SD = 6) with an ACL injury, which was stated after 5–52 weeks from the injury, a mean of 20 weeks (SD = 15). The functional length of the hamstring and rectus femoris muscle were measured both passively and actively by a Saunders inclinometer which allows measurement with precision up to 1°.

Results:
Statistical analysis revealed a statistically significant negative correlation between functional shortening of the hamstring and rectus femoris muscle in the healthy extremity, in active and passive tests.

Conclusions:
With the increase in functional shortening of the hamstring, functional shortening of the rectus femoris muscle is diminished, both in active and passive tests in the healthy and ACL injured extremity.