Mechanical modeling of tibial axial accelerations following impulsive heelimpact

A fourth order mass/spring/damper (MSD) mechanical model with linear coeffi cients was used to estimate axial tibial accelerations following impulsive heel impacts. A generic heel pad with constant stiffness was modeled to imp rove the temporal characteristics of the model. Subjects (n = 14) dropped ( similar to 5 cm) onto a force platform (3 trials), landing on the right hee l pad with leg fully extended at the knee. A uni-axial accelerometer was mo unted over the skin on the anterior aspect of the medial tibial condyle inf erior to the tibial plateau using a Velcro(TM) strap (normal preload simila r to 45 N). Model coefficients for stiffness (k(1), k(2)) and damping (c(1) , c(2)) were varied systematically until the minimum difference in peak tib ial acceleration (%PTA(min)) plus maximum rate of tibial acceleration (%RTA (max)) between the estimated and measured curves was achieved for each tria l. Model responses to mean subject and mean group model coefficients were a lso determined. Subject PTA and RTA magnitudes were reproduced well by the model (%PTA(min) = 1.4 +/- 1.0 %, %RTA(min) = 2.2 +/- 2.7%). Model estimate s of PTA were fairly repeatable for a given subject despite generally high variability in the model coefficients, for subjects and for the group (coef ficients of variation: CVkl = 57; CVk2 = 59; CVc1 = 48; CVc2 = 85). Differe nces in estimated parameters increased progressively when subject and group mean coefficients (%PTA(sub)= 8.4 +/- 6.3%, %RTA(sub) = 18.9 +/- 18.6%, an d %PTA(grp) = 19.9 +/- 15.2 %, %RTA(grp) = 30.2 +/- 30.2%, respectively) we re utilized, suggesting that trial specific calibration of coefficients for each subject is required, Additional model refinement seems warranted in o rder to account for the large intra-subject variability in coefficients.