It is known that individuals missing a functional anterior cruciate ligament (ACL) in one limb exhibit changes in the walking biomechanics in that limb during mid-stance (10-30% of the gait cycle). Specifically, they exhibit reduced activation of the quadriceps muscle group and increased activation of the hamstring muscle group, resulting in a decreased net knee joint extensor moment and increased knee joint flexion. These compensations have been called a "quadriceps avoidance" strategy. It is not known whether these compensations are the direct result of the injury, or whether compensations made early in the rehabilitation process play a role in these changes. The purpose of the present study was to investigate the lower limb biomechanics of ACL deficient individuals during a common rehabilitation exercise for this injury - stationary cycling.

Ten individuals with a unilateral ACL deficiency and ten age- and gender-matched controls performed six randomized bouts of stationary cycling for approximately one minute at intensities resulting from the combination of two cadences (60 and 90 rpm) and three power outputs (75, 125, and 175 W).

It was found that, similar to during walking, ACL deficient individuals exhibited decreases in the magnitude of the quadriceps muscle activation in the injured limb. When combined with no change in hamstrings muscle activation, this resulted in a decreased net knee joint extensor moment in the injured limb. However, in contrast to walking, where increases from the hip or ankle extensors compensate for the decreased output from the knee joint extensors, ACL deficient individuals in the present study decreased output from the entire injured limb, resulting in a "limb avoidance". This limb avoidance was manifested by decreases in the magnitude of muscle activation from the rectus femoris, vastus lateralis, and gluteus maximus, as well as decreases in the amount of force applied to the pedal.

It was concluded that these compensations occurred in order to reduce anterior tibial translation in the injured limb. These results may suggest that a "quadriceps avoidance" strategy may be due in part to a "limb avoidance" strategy learned early during rehabilitation.