How does Nexus (Plug-in Gait) and Polygon calculate Gait Cycle Parameters (Spatial and Temporal)?
In Nexus the Generate Gait Cycle Parameters Pipeline Operation can be used in conjunction with the Gait events to calculate standard Gait Cycle Spatial and Temporal Parameters. These Parameters and available units (the units can be change in the Generate Gait Cycle Parameters Options box) are:
Cadence - 1/s; 1/min; steps/s; steps/min; strides/s; strides/min
Walking speed - m/s; cm/s; mm/s; in/s
Step Time - s; %
Foot Off/Contact events - s; %
Single/Double Support - s; %
Stride/Step Length - m; cm; mm; in
The Distance Parameters are based in the marker position at the time, by default the toe marker (LTOE for left and RTOE for right) is used for the calculation. This can be changed in the Options box of the Generate Gait Cycle Parameters Pipeline Operation.
The Parameters are based on the first cycle for each side where all the necessary events are found.
Polygon can re-calculate the parameters and can define the parameters in the first cycle or the average of all defined cycles.
Cadence: number of strides per unit time (usually per minute). The left and right cadence are first calculated separately based on either a single stride or an average of the defined gait cycles (this is a preference setting in Polygon). The overall cadence is the average of the left and the right.
Stride time: time between successive ipsilateral foot strikes.
Step time: time between contralateral and the following ipsilateral foot contact, expressed in seconds or %GC.
Foot contact/off events are all expressed relative to the ipsilateral gait cycle, either as absolute time from ipsilateral foot contact or as %GC, as per the Polygon preference. Single and double support calculations are only valid for walking, i.e. when the contralateral foot off/contact events happen within the ipsilateral stance phase.
Foot off: time of ipsilateral foot off.
Opposite foot contact: time of contralateral foot contact.
Opposite foot off: time of contralateral foot off.
Single support: time from contralateral foot off to contralateral foot contact.
Double support: time from ipsilateral foot contact to contralateral foot off plus time from contralateral foot contact to ipsilateral foot off.
Limp index: the foot contact to foot off time of the ipsilateral foot is divided by the foot off to foot contact time plus the double support time. In other words, the limp index calculates the time the ipsilateral foot is on the ground and divides it by the time the contralateral foot is on the ground during the ipsilateral GC.
All distance and speed measurements use a reference marker on each foot, by default the LTOE/RTOE markers, but this can be changed in the preferences. The marker's position is evaluated in 3D at the time of the events.
Four 3D points are defined:
IP1 is the ipsilateral marker's position at the first ipsilateral foot contact.
IP2 is the ipsilateral marker's position at the second ipsilateral foot contact.
CP is the contralateral marker's position at the contralateral foot contact.
CPP is CP projected onto the IP1 to IP2 vector.
Stride length: is the distance from IP1 to IP2.
Step length: is the distance from CPP to IP2.
Step width: is the distance from CP to CPP.
Walking speed: is stride length divided by stride time.