A protocol named Outwalk was developed to easily measure the thorax-pelvis and lower-limb 3D kinematics on children with cerebral palsy (CP) and amputees during gait in free-living conditions, by means of an Inertial and Magnetic Measurement System (IMMS). Outwalk defines the anatomical/functional coordinate systems (CS) for each body segment through three steps: (1) positioning the sensing units (SUs) of the IMMS on the subjects' thorax, pelvis, thighs, shanks and feet, following simple rules; (2) computing the orientation of the mean flexion-extension axis of the knees; (3) measuring the SUs' orientation while the subject's body is oriented in a predefined posture, either upright or supine. If the supine posture is chosen, e.g. when spasticity does not allow to maintain the upright posture, hips and knees static flexion angles must be measured through a standard goniometer and input into the equations that define Outwalk anatomical CSs. In order to test for the inter-rater measurement reliability of these angles, a study was carried out involving nine healthy children (7.9 +/- 2 years old) and two physical therapists as raters. Results showed RMS error of 1.4 degrees and 1.8 degrees and a negligible worst-case standard error of measurement of 2.0 degrees and 2.5 degrees for hip and knee angles, respectively. Results were thus smaller than those reported for the same measures when performed through an optoelectronic system with the CAST protocol and support the beginning of clinical trials of Outwalk with children with CP.