### What is the Plug-in Gait 'Progression Frame' and how is it used?

The 'Progression Direction' is defined in order to represent the general direction in which the subject walks in the global coordinate system. A coordinate system matrix (similar to a segment definition) is then defined and denoted the 'Progression Frame'. This allows the calculation by Plug-in Gait and Polygon of 'progression' related variables (HeadAngles, ThoraxAngles, PelvisAngles, FootProgressAngles, Step Width) in relation to this frame.

In Plug-in Gait, the lower body Progression Direction is found by looking at the first and last valid position in a trial of the LASI marker. If the distance between the first and last valid position of the LASI marker is greater than a threshold of 800 mm, the X displacement of LASI is compared to its Y displacement. If the X displacement is greater, the subject is deemed to have been walking along the X axis, either positively or negatively, depending on the sign of the X offset. If the Y displacement is greater, the subject is deemed to have been walking along the Y axis, either positively or negatively, depending on the sign of the Y offset.

If the distance between the first and last frame of the LASI marker is less than a threshold of 800 mm however, the Progression Direction is calculated using the direction the pelvis is facing during the middle of the trial. This direction is calculated as a mean over 10% of the frames of the complete trial. Within these frames, only those which have data for all the pelvis markers are used. For each such frame, the rear pelvis position is calculated from either the SACR marker directly, or the centre point of the LPSI and RPSI markers. The front of the pelvis is calculated as the centre point between the LASI and RASI markers. The pelvis direction is calculated as the direction vector from the rear position to the front. This direction is then used in place of the LASI displacement, as described above and compared to the laboratory X and Y axes to choose the Progression Direction.

Following this definition, the Progression Direction in which the subject walks is assumed to be one of four possibilities; Global axes positive X, Global axes positive Y, Global axes negative X or Global axes negative Y and not diagonally to any of these, for example.

In Plug-in Gait, the upper body Progression Direction is adopted as the same as the lower body's Progression Direction, if it has one. If no lower body Progression Direction has been calculated, an upper body Progression Direction is independently calculated in just the same way as for the lower body. C7 is tested first to determine if the subject moved a distance greater than the threshold. If not, the other thorax markers T10 CLAV and STRN are used to determine the general direction the thorax faces from a mean of 10% of the frames in the middle of the trial.

Once the Progression Direction along one of the four possible axes directions is determined, the Progression Frame is defined such that its X-axis is oriented positively along this Progression Direction. The Z axis is always assumed to be directed vertically upwards and the Progression Frame is defined following the right-hand rule. The diagram below shows this clearly for each of four circumstances where a subject walks along the different axis directions. The 'Progression Angles' of the head, thorax, pelvis and feet, calculated by Plug in Gait, are the YXZ Cardan angles calculated from the rotation transformation of the subject's Progression Frame for the trial, onto the orientation of each of these segments on a sample-by-sample basis.

The 'Step Length' calculated by Plug-in Gait, is the distance when the foot down event occurs, between the chosen marker (TOE by default) and the opposite foot's corresponding marker, ALONG the Progression Direction. For example, with the LTOE and RTOE markers chosen in the 'Gait Cycle Parameter Generation Options' for the 'Generate Gait Cycle Parameters' Workstation pipeline entry, the Left Step Length will be calculated as the distance between the LTOE marker and the RTOE marker along the Progression Direction.

The 'Stride Length' calculated by Plug-in Gait, is the distance moved by the chosen marker (TOE by default), ALONG the Progression Direction between one foot down event and the next (i.e. from the start to the end of the gait cycle). For example, with the LTOE and RTOE markers chosen in the 'Gait Cycle Parameter Generation Options' for the 'Generate Gait Cycle Parameters' Workstation pipeline entry, the Left Stride Length will be calculated as the distance between the LTOE marker at the occurrence of one foot down event and the LTOE marker at the occurrence of the next foot down event, along the Progression Direction.

The 'Step Width' calculated by Polygon, is the distance when the foot down event occurs, between the chosen marker (TOE by default) and the opposite foot's corresponding marker, NORMAL to the Progression Direction. For example, with the LTOE and RTOE markers chosen in the "Analysis" node's Properties, the Left Step Width will be calculated as the distance between the LTOE marker and the RTOE marker normal to the Progression Direction

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