Overweight people commonly conduct a sedentary lifestyle because of difficulties in walking. To quantitatively monitor their gait and reduce soft tissue artefacts, optoelectronic systems are usually adopted, but also inertial measurement units (IMUs) have shown promising results. Considering overweight children, the long preparation time of optoelectronic systems could be a disadvantage. On the contrary, IMUs could simplify the walking acquisition protocol. In light of these considerations, a pilot study was conducted exploring the use of IMUs during the flex-extension of a polycentric knee model in presence and absence of soft tissue. In detail, the aims of the work were: (i) to evaluate the filtering effect on knee flex-extension movements calculated using IMUs, and (ii) to calculate knee flex-extension movements from linear acceleration and angular velocity of lower limb segments.