In vitro study of patellar position during sitting, standing from squatting, and the stance phase of walking.

Muscle balance in quadricep muscles is an important factor for patellar instability. However, in vivo, the activity of the quadriceps varies during knee motion. To simulate this clinical condition, different loading conditions of the rectus femoris, vastus lateralis, and vastus medialis oblique at different knee flexion angles were studied, and patellar motion during knee extension was determined. Seven fresh-frozen adult cadaver knees were use. Patellar motion in terms of patellar shift, patellar tilt, and rotation were measured using a magnetic tracker system. The loads applied to each quadriceps muscle were calculated using clinical electromyographical data for three activities: 1) knee extension while sitting, 2) standing from squatting, and 3) stance phase of walking. The patellar tracking in these three loading patterns was compared with that under constant loading of each quadriceps muscle. Each activity demonstrated a neutral patellar shift and tilt, as did the traditional constant loading. However, the patella in walking showed internal rotation when the rectus femoris was much weaker than the other vasti.