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‘Outwalk’: a protocol for clinical gait analysis based on inertial and magnetic sensors

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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° and 1.8° and a negligible worst-case standard error of measurement of 2.0° and 2.5° 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.

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Cerebral palsy


Coordinate system


Right drop-rise


Inertial and magnetic measurement system


Posterior–anterior tilting


Root mean square

SEMse :

Standard error of measurement considering systematic errors


Sensing unit of an IMMS


Joint representing the movements of the Pelvis relative to the Thorax

h :

Hip static flexion angle during Outwalk calibration in supine posture

k :

Knee static flexion angle during Outwalk calibration in supine posture

T 1, T 2 :

Physical therapists involved in the reliability study of h and k


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Correspondence to Andrea Giovanni Cutti.

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Xsens system. (a) Data-logger (Xbus Master) with two SUs connected; (b) an SU with its local CS, in the global (earth-based) CS (JPG 1554 kb)

Supplementary material 2 (DOC 59 kb)

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Cutti, A.G., Ferrari, A., Garofalo, P. et al. ‘Outwalk’: a protocol for clinical gait analysis based on inertial and magnetic sensors. Med Biol Eng Comput 48, 17–25 (2010).

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