Abstract: The present invention relates to a processing apparatus (10), system (1) and method (20) for determining an ambulation motion of a subject. Advantageously also a slow ambulation motion may be detected. The processing apparatus is configured to perform the steps of obtaining accelerometer data indicative of a trunk motion of the subject; extracting one or more orthogonal motion components from high-pass filtered accelerometer data; determining a gravity component from low-pass filtered accelerometer data; comparing a direction of at least one of said orthogonal motion components with a direction of the gravity component; and classifying a motion of the subject as ambulation if said motion component is orthogonal or parallel to said gravity component.