The Contribution of Below Knee Wobbling Mass to the Estimation of Vertical Knee Joint Reaction Forces Following Impact with the Ground

Abstract

In human impacts involving high peak accelerations, the wobbling mass (skin, muscle, fat, and connective tissue) of the body will accelerate independently of the rigid mass (bone) . The purpose of this study was to quantify the effect that below knee wobbling mass has, if any, on the attenuation of peak forces transmitted through the leg to the knee joint, following impacts with the ground. Fifteen healthy subjects dropped vertically from heights of 5 and 10 cm, onto a force platform, with the ankle and knee joints of the support leg held rigidly. A uni-axial accelerometer was fixed, with skin bond cement, to the skin overlying the anterior upper tibia, and another to the posterior wobbling tissue of the support limb. Vertical ground reaction forces and accelerations were used in rigid only and rigid and wobbling link segment models of the leg, which resulted in estimates of vertical knee joint reaction forces (VKJRF). Mean peak VKJRFs resulting from rigid only calculations were 2.66±.55 x body weight (bw) and 3.53±.68 x bw, and from rigid and wobbling calculations were 2.64±.55 x bw and 3.52±.68 x bw, for the 5 and 10 cm heights, respectively. A two-way, repeated measures ANOVA revealed that there was no main effect for calculation method. Validation of the subject results was attempted indirectly by comparing them to the actual forces (load cell) at the knee of a manufactured model. The model was constructed in proportion to a human subject of mass 75 kg. When the model was dropped from the same heights as the subjects, the mean peak VKJRFs (22. 9 x bw) greatly overestimated the actual load cell values (8.2 x bw), and were unrealistic relative to subject values (approximately 3.0 x bw). Although mean peak VKJRFs were also overestimated when the wobbling mass accelerations were included, they were much closer to the actual values (9.9 x bw vs. 8.2 x bw).