A system to recognize user activity includes a filter to form a gravity measurement from a three-dimensional (3D) accelerometer measurement of a user activity; a body component extractor to form a body measurement of the user activity by removing the gravity measurement from the 3D accelerometer measurement; a rotation matrix calculator to form a rotation matrix that is used to rotate the extracted gravity to align with a predetermined direction, and to rotate the 3D accelerometer measurements and the body measurement to be the input to features determiner module; and a features determiner to determine values of a predetermined number of features of the user activity based on the rotated 3D accelerometer measurement and the rotated body measurement in which the predetermined number of features are invariant under arbitrary rotation of the accelerometer measurement around the predetermined direction.