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The research in this paper aims to correct evaluation tables of the modified flexibility sit-and-reach test, avoiding unreasonable great fluctuations in the scales. A human body model in the test, as a kinematic chain, including trunk and upper-extremity members connected with rotated joints, is proposed. An integral parameter of hip rotation and spine-bend flexibility is derived as an angle between the hip to shoulder straight line and horizon. The corresponding error of the angle in the frames of the simplified model appears to be nearly 1.5 %. The bend of trunk member is investigated, and its modeling with the integral parameter of flexibility causes an error of the fingertips displacement of nearly 0.4 cm, which is smaller than an error of the test score recorder. Two smooth evaluation scales are modeled with a linear function and with a logarithmic function. Corresponding tables for proportional evaluation and for rapidly increased mark values on the higher test scores are derived.