From an ergonomic point of view, quantitative assessment of the feasibility of the task performance is an intricate process, which combines the multidimensional task demand profile with the individual’s multidimensional performance capacity profile. Trunk muscle strength is affected by the trunk posture but it is not clear how the variability of trunk muscle is affected by the trunk posture or exertion level. A validated triaxial dynamometer, Sharif-LIST, was used for to model the surface response of trunk muscle extension strength variability as a function of trunk posture in the sagittal, coronal and transverse planes in standing position. Fifteen healthy males with no history of low back pain in the previous year participated in this study. A full factorial method was used to design the experiments to fit a second-order model of response surface method (RSM) with block effect (BE). For the first time in the literature, the results of this study indicate that the quadratic RSM model is suitable to represent the trunk extension strength variability in terms of its coefficient of variation (CV) and standard deviation (SD) of maximum trunk torque as a function of trunk posture in coronal and sagittal planes (R2 = 0.6, p<0.5). These results showed that in the positions with higher trunk strength values, the higher standard deviation and lower coefficient of variation were observed which is consistent with previous findings.

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