Knee muscle forces during walking and running in patellofemoral pain patients and pain-free controls

Abstract

One proposed mechanism of patellofemoral pain, increased stress in the joint, is dependent on forces generated by the quadriceps muscles. Describing causal relationships between muscle forces, tissue stresses, and pain is difficult due to the inability to directly measure these variables in vivo. The purpose of this study was to estimate quadriceps forces during walking and running in a group of male and female patients with patellofemoral pain (n = 27, 16 female; 11 male) and compare these to pain-free controls (n = 16, 8 female; 8 male). Subjects walked and ran at self-selected speeds in a gait laboratory. Lower limb kinematics and electromyography (EMG) data were input to an EMG-driven musculoskeletal model of the knee, which was scaled and calibrated to each individual to estimate forces in 10 muscles surrounding the joint. Compared to controls, the patellofemoral pain group had greater co-contraction of quadriceps and hamstrings (p = 0.025) and greater normalized muscle forces during walking, even though the net knee moment was similar between groups. Muscle forces during running were similar between groups, but the net knee extension moment was less in the patellofemoral pain group compared to controls. Females displayed 30-50% greater normalized hamstring and gastrocnemius muscle forces during both walking and running compared to males (p<0.05). These results suggest that some patellofemoral pain patients might experience greater joint contact forces and joint stresses than pain-free subjects. The muscle force data are available as supplementary material.

Figure 1

Mean knee flexion-extension moment (± one SD - shaded region) estimated using inverse dynamics during walking (a) and running (c) for Control and Patellofemoral Pain (PFP) subjects. Internal extension moment is positive. Peak extension moments are also shown for males and females during walking (b) and running (d). * indicates difference in peak moments between groups (p<0.05).

Figure 2

Mean muscle forces for pain-free Control (solid line) and Patellofemoral Pain subjects (PFP – dashed line) during stance phase of walking (shaded region = one SD). Forces are normalized by the maximum isometric muscle force (Fmax) for each muscle. * indicates a difference between PFP and pain-free control group in peak force (p < 0.05). † indicates a difference between PFP and pain-free control group at weight acceptance.

Figure 3

Mean muscle forces for pain-free Control (solid line) and Patellofemoral Pain subjects (PFP – dashed line) during stance phase of running (shaded region = one SD). Forces are normalized by the maximum isometric muscle force (Fmax) for each muscle. * indicates difference in peak force between PFP and pain-free control group (p < 0.05).

Figure 4

Relative co-contraction of quadriceps and hamstring muscles for pain-free Control and Patellofemoral Pain (PFP) subjects during walking and running. Co-contraction index was calculated as the product of the ratio of quadriceps-to-hamstrings muscle activity and the net activation of these muscles. Shaded regions show one standard deviation for each group. * indicates difference at heel strike between PFP and pain-free control group (p =0.025).

Figure 5

Relative contributions of the quadriceps muscles to the net quadriceps moment during walking and running for pain-free Controls and patients with Patellofemoral Pain.