Knee Joint Forces in Relation to Ground Surface Stiffness during Running

Abstract

Running shoes and surfaces have been developed to help enhance running efficiency and to reduce ground impacts by altering the total surface stiffness. However, to maintain running mechanics, an individual will increase leg joint stiffness while running across a more compliant ground surface and show an inverse effect when running across harder surfaces. Increasing leg stiffness causes landing impact forces to increase and may counteract the softer surface in terms of knee joint contact forces. Since the knee is an essential determinant for reducing impact forces and a primary site for changing leg stiffness, knowing more about knee joint forces while running on surfaces with different stiffnesses can be beneficial in developing injury prevention programs. It is our objective to determine the effect of surface stiffness on knee joint contact forces during running. We recruited 17 healthy recreational heel strike runners and ran across a 15m track at a consistent pace (3.46m/s + 5%) on 3 ground conditions (hard floor with embedded force plate and 1 and 2 layers of shock absorbing mat). The study protocol took place over 2 days. On day 1, participants were able to practice running over the various ground conditions at the test speed, on day 2, data were collected. Five successful trials per surface condition were gathered and analyzed with focus being on the knee joint and knee joint forces through musculoskeletal modeling. Data were statistically compared among surface conditions with a one way ANOVA, using three levels and alpha <0.05. Patello-femoral (PF) and tibio-femoral (TF) compressive forces were not significantly different between surface conditions. However, knee joint angular stiffness (P<0.01), the rate to the vertical ground reaction force (vGRF) impact peak (P = 0.02), anteroposterior breaking force magnitude (P <0.01), and TF shear force for both force magnitude (P <0.01) and rate (P = 0.03) to the maximum forces were found statistically different (P <0.05). All variables that were found to be significantly different decreased as the surface stiffness decreased. Our hypothesis was partially supported for knee joint compressive loads but not for the shear loads. As the participants ran across the increasingly dampened surfaces their knee joint angular stiffness decreased. This is contrary to existing literature that suggests an inverse effect between surface stiffness and leg stiffness, which is closely related to knee joint stiffness. In addition, the rate to the vGRF impact peak and the breaking force magnitude decreased with the surface stiffness. Our data supported the idea that running across differing surface stiffnesses does not statistically alter knee joint compressive forces but can reduce knee joint shear forces. Future research should determine if this strategy is beneficial to a broader range of individuals including those with fore- and mid-foot strike patterns.