Joint Stiffness On Different Inclines During Running

Abstract

The leg is modeled as a linear spring and lower limb joints as torsion springs for bounding (hopping, running) movements. Leg stiffness is associated with many general characteristics of level running (including stride frequency and reaction force), and knee stiffness appears to relate to leg stiffness. The effects of inclination on leg mechanics have been studied less than level conditions, but recent research has shown that peak perpendicular and braking forces are significantly larger when running on a negative incline than a level or positive incline (Gottschall & Kram 2004), perhaps indicative of increased leg (and joint) stiffness. PURPOSE The purpose of the current study was to determine the similarity of ankle and/or knee stiffness on level, positively inclined, and negatively inclined slopes. METHODS Four healthy, recreational runners (2 male and 2 female), ran on three inclined surfaces (+10%, 0%, −10%). Sagittal plane ground reaction forces and kinematics were recorded. Inverse dynamic analysis was used to combine kinetic and kinematic data to determine joint moments for the ankle and knee. Joint stiffness was determined by calculating the slope of the moment-joint angle relationship. An ANOVA with repeated measures was used to assess the presence of differences in ankle or knee stiffness between the three conditions (alpha = 0.05). RESULTS Knee stiffness was found to be different between the three conditions (p = 0.001). Post-hoc analysis revealed that knee stiffness to be significantly higher on the negative incline relative to both the level and positive incline conditions (p = 0.001; +10% = 5.9, 0% = 5.4, −10% = 12.9 Nm/kg/rad). There was no difference in knee stiffness between the level and positive incline conditions. There were no differences in ankle stiffness between the three incline conditions (p = 0.585; +10% = 2.4, 0% = 3.3, −10% = 3.1 Nm/kg/rad). On a decline, there was a large, net braking impulse in the horizontal direction (p <0.001; +10% = 0.02, 0% = −0.02, −10% = −0.69 Ns/kg). CONCLUSIONS Ankle stiffness appears to be unaffected by surface inclination, at least between −10 and +10%. However, knee stiffness is more than doubled on the negative incline relative to a level or positive slope. This might be related to large braking impulses observed when running on a negative incline (Gottschall & Kram, 2004), and is hypothesized to increase the risk of injury from running on negative slopes (downhill). Supported by NIH # S06 GM53933 to DFH & SJW.