Patellofemoral joint compressive forces in forward and backward running.

The use of backward running is becoming more common in the rehabilitation setting. In particular, backward running has been suggested as a treatment modality in patients experiencing patellofemoral pain syndrome. To date, no study has examined the loads at the patellofemoral joint during backward running. The purpose of this study was to compare patellofemoral joint compressive forces during forward and backward running. Ground reaction force and kinematic data were collected on five male joggers during free speed forward and backward running. A floor reaction force vector model was used to calculate the stance phase knee extension moments. The distance used for the extensor muscle lever arm was 4.9 cm. Patellar mechanism angle was calculated based on knee joint angle. There was a reduction in the peak patellofemoral joint compressive forces in backward compared with forward running (2277 +/- 192N vs. 4253 +/- 1292N; p < 0.05) at self-selected speeds. Peak patellofemoral joint compressive force occurred significantly later (p < 0.05) in the stance phase of backward running (52 +/- 4%) than in forward running (35 +/- 3%). The peak patellofemoral joint compressive force normalized to subject body weight was 5.6 +/- 1.3 body weight in forward running and 3.0 +/- 0.6 body weight in backward running. The results suggest that backward running at a self-selected speed may reduce patellofemoral joint compressive forces and, coupled with the quadriceps strengthening that has previously been reported, may be beneficial in the rehabilitation of patellofemoral pain syndrome in runners. However, constant speed comparisons or other models may yield different results.