Patellofemoral joint compression forces in backward and forward running

Abstract

Patellofemoral pain (PFP) is a common injury and increased patellofemoral joint compression forces (PFJCF) may aggravate symptoms. Backward running (BR) has been suggested for exercise with reduced PFJCF. The aims of this study were to (1) investigate if BR had reduced peak PFJCF compared to forward running (FR) at the same speed, and (2) if PFJCF was reduced in BR, to investigate which biomechanical parameters explained this. It was hypothesized that (1) PFJCF would be lower in BR, and (2) that this would coincide with a reduced peak knee moment caused by altered ground reaction forces (GRFs). Twenty healthy subjects ran in forward and backward directions at consistent speed. Kinematic and ground reaction force data were collected; inverse dynamic and PFJCF analyses were performed. PFJCF were higher in FR than BR (4.5±1.5; 3.4±1.4BW; p<0.01). The majority of this difference (93.1%) was predicted by increased knee moments in FR compared to BR (157±54; 124±51 Nm; p<0.01). 54.8% of differences in knee moments could be predicted by the magnitude of the GRF (2.3±0.3; 2.4±0.2BW), knee flexion angle (44±6; 41±7) and center of pressure location on the foot (25±11; 12±6%) at time of peak knee moment. Results were not consistent in all subjects. It was concluded that BR had reduced PFJCF compared to FR. This was caused by an increased knee moment, due to differences in magnitude and location of the GRF vector relative to the knee. BR can therefore be used to exercise with decreased PFJCF.

Fig. 1

Schematic overview of telescopic inverted pendulum (TIP) model for forward running (FR) on the left and backward running (BR) on the right, with the approach angle (θ_L), knee angle (θ_K), and length (L) of the contact leg.

Fig. 2

TIP model calculations with stance leg length (L) against θ_L. The blue lines are average data for backward running (BR) and the red lines for forward running (FR), with the thicker parts for the deceleration phase and the thinner parts for the push-off phase. FRPO and BRPO are push-off in FR and BR respectively, and the arrows indicate the walking directions. The stars indicate where M_k(max) occurred. In both FR and BR the leg shortened during the deceleration phase and extended during the push-off phase. The stars are very close together for FR and BR; the peak knee moment therefore occurred at similar body orientations for both running styles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Mean patellofemoral joint compressive force (PFJCF) with ±one standard deviation for each subject (S1–S17), with the blue bars for backward running (BR) and red bars for forward running (FR). The data of the subjects where PFJCF was higher or of similar magnitude in BR compared to FR are circled. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

COP location on the foot at foot strike (COP_locFS) versus patellofemoral joint compression force (PFJCF), with the blue circles for backward running (BR) and the red crosses for forward running (FR). There was a significant correlation between PFJCF and COP_locFS (R²=0.26 and p=0.008). Note the circled outlying trials with COP_locFS close to the forefoot and a relatively low PFJCF. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)