Why are the stance phases in the Force/Time curves not starting at 0 kg (0 lbs)? Shouldn't the starting point be 0 kg (0 lbs)?

Question:Why are the stance phases in the Force/Time curves not starting at 0 kg (0 lbs)?  Shouldn't the starting point be 0 kg (0 lbs)?

Answer: When the foot is in contact with the floor or ground, there is pressure (or force greater than 0).

When the foot is not in contact with the floor or ground, there is no pressure (or force = 0).  With pressure/force measurement devices, the situation above is not always true.  There can be pressure or force on the device (and/or measured by the device) when the foot not on the measurement device.  Therefore, the device measures a force when the foot is off the mat; this effect is due to what we refer to as residual pressure or force.  The reason for this will not be explained here.  If there is residual force, there is therefore no frame or time when there is zero force.  In this situation, the software is unable to differentiate between a stance phase (foot in contact and force greater than 0) and a swing phase (foot not in contact and force = 0).  To counter this situation, the software needs a method to identify when foot is in contact vs. foot is not in contact.  To do this, an algorithm is applied to calculate a force threshold.  Below this force threshold, the software considers the forces as residual force, and sets the foot not in contact (not in stance phase).  Above this force threshold, the software considers the forces as part of the stance phase, and sets the foot in contact.

This algorithm is applied for all recordings, and is as follows:

Force Threshold = Minimum Force + (Maximum Force - Minimum Force)10%

 

As example:

Minimum Force = 0 kg (0 lbs)

Maximum Force = 100 kg (220 lbs)

Force Threshold = Minimum Force + (Maximum Force - Minimum Force)10%

                            = 0 + (100 - 0)10%

                            = 0 + (100)10%

                            = 0 + 10

Force Threshold = 10 kg

In lbs                   = 0 + (220 - 0)10%

                            = 0 + (220)10%

                            = 0 + 22

Force Threshold = 22 lbs

Other example:

Minimum Force = 5 kg (11 lbs)

Maximum Force = 100 kg (220 lbs)

Force Threshold = Minimum Force + (Maximum Force - Minimum Force)10%

                            = 5 + (100 - 5)10%

                            = 5 + (95)10%

                            = 5 + 9.5

Force Threshold = 14.5 kg

In lbs                   = 11 + (220 - 11)10%

                            = 11 + (209)10%

                            = 11 + 21

Force Threshold = 32 lbs

The software then uses this threshold to identify and set the start frame and end frame of the stance (foot in contact).

When using the Trigger feature, a force threshold is set to indentify when to start-stop a recording.

This is, however, different than the process to identify when the foot is considered to be in stance phase and in swing phase.

Note that the default Start Method is not with force = 0, but with force having a value greater than 0.

If Start Method is set with force = 1 kg (2.2 lbs), recording will start once 1 kg (2.2 lbs) is applied on the mat.

The recording is here started, BUT, to identify/differentiate between stance phase and start of curve in graph (foot considered on mat) and swing phase and end of curve in graph (foot not considered on mat), the threshold algorithm is still used.

The curve is set to start at the 1st (initial foot) contact frame.

Since these frames do not have a zero value, curve does not start at zero.