Estimation of trunk muscle parameters for a biomechanical model by age, height and weight.

To establish more accurate equations for estimating the moment arm length and cross-sectional area of the erector spinae and rectus abdominis muscles, the effects of height, weight and age on those muscles were analyzed by using a high-order polynomial equation. Data on the moment arm length and cross-sectional area at L3/4 were obtained from MRI images of 152 males and 98 females. The statistical model used in this study has any combination of up to third-order independent variables for age, height and weight. The effective independent variables were selected by the forward step method of multiple regression analyses. The results of multiple regression analyses showed that the polynomial equations for the moment arm length of erector spinae in both genders, and that for the rectus abdominis in males, contained all three variables of age, height and weight. That for the moment arm length of female rectus abdominis contained the variables of weight and age. The multiple correlation coefficients of the erector spinae and rectus abdominis were 0.355 and 0.650 for males, 0.364 and 0.411 for females, respectively. The equations for the cross-sectional area of the erector spinae in both genders, as well as that for male rectus abdominis contained only one variable (weight). The multiple correlation coefficients of the cross-sectional area of the erector spinae were 0.576 for males and 0.469 for females. The cross-sectional area of the female rectus abdominis had no effective variables.