Spasticity is clinically assessed using goniometry to measure the joint angle of the catch (AOC) during fast passive muscle stretch. The precision and accuracy of the goniometric AOC measurements are questionable, because of the inevitable joint repositioning after occurrence of the catch. This study aims to evaluate the use of goniometry in estimating the AOC in spasticity assessment of the medial hamstrings, soleus and gastrocnemius in twenty children with Cerebral palsy (CP), using inertial sensors (IS) as reference system. The IS were initially validated with an optoelectronic system to measure 3d-orientation and proved to be accurate within 1 degree. To evaluate the precision and accuracy of the goniometry, the joint angle measured with the goniometer after repositioning was compared to the joint angle measured simultaneously with the IS, and to the true AOC, detected and measured with the IS during the fast muscle stretch. Results showed that goniometry is an imprecise method to measure the true AOC in spasticity assessment. The error is mainly due to joint repositioning after the fast muscle stretch. For spasticity assessment, it is advised to apply inertial sensors when a precise measurement of the angle of catch is required.