Customise Start of Movement Analysis
What is start of movement?
Start of movement (SoM) is the moment when ForceDecks detects that the rep being conducted has begun (“Rep Start” in the image below).
ForceDecks offers several methods for detecting start of movement. Different detection methods for start of movement can result in changes to some of the timing metrics calculated by ForceDecks (e.g., Eccentric Duration in a Countermovement Jump).
Selecting a start of movement method
Start of movement analysis settings can be customised in ForceDecks, ForceDecks Jump, and ForceDecks iOS. These settings will be applied to the following tests:
- Countermovement Jump
- Countermovement Jump - Loaded
- Squat Jump
- Squat Jump - Loaded
- Abalakov Jump
- Single Leg Jump
Start of movement methods
ForceDecks supports the following start of movement methods:
- Fixed threshold
- Standard deviations
- Yank-time
Force comes to within threshold of weight prior to movement (default)
This is the default method for detecting start of movement. Start of movement is defined as the moment when the total force moves ±20N from the measured bodyweight, working backwards from when the movement is known to have begun.
The following options are available to customise this method:
Start of Movement Threshold [N]
The amount that the total force needs to deviate from the measured bodyweight for start of movement to be detected. 20N by default.
Force (moving average) comes to within threshold of weight prior to movement
In this method start of movement is defined as the moment when a 50ms moving average of total force moves ±20N from the measured bodyweight, working backwards from when the movement is known to have begun.
The following options are available to customise this method:
Start of Movement Threshold [N]
The amount that the 50ms moving average of total force needs to deviate from the measured bodyweight for start of movement to be detected. 20N by default.
Force deviates from weight by threshold after start of trial
In this method start of movement is defined as the moment when the total force moves ±20N from the measured bodyweight, working forwards from the start of the trial.
The following options are available to customize this method:
Start of Movement Threshold [N]
The amount that the total force needs to deviate from the measured bodyweight for start of movement to be detected. 20N by default.
Force (moving average) deviates from weight after start of trial
In this method start of movement is defined as the moment when a 50ms moving average of total force moves ±20N from the measured bodyweight, working forwards from the start of the trial.
The following options are available to customize this method:
Start of Movement Threshold [N]
The amount that the 50ms moving average of total force needs to deviate from the measured bodyweight for start of movement to be detected. 20N by default.
Force comes to within Standard Deviation-based threshold of weight prior to movement
In this method for detecting start of movement, prior to each rep ForceDecks automatically detects a 1 second period of quiet standing, determines the standard deviation (SD) of total force during that period, and defines the start of movement as the moment when the total force moves ±(5*SD)N from the measured bodyweight, working backwards from when the movement is known to have begun.
Note: if a 1 second period of quiet standing cannot be detected prior to a rep, ForceDecks will revert to using the default “Force comes to within threshold of weight prior to movement” method.
Start of Movement Threshold StdDevs
The multiplier used with the measured standard deviation to determine the movement threshold. 5 by default.
Auto-detect Pre-Trial Quiet Period [s]
The amount of extra time that ForceDecks should review prior to each trial to determine the 1 second quiet period. The 1 second quiet period is the 1 second period of time when the standard deviation of total force is the lowest. 3 seconds by default.
The derivative of yank deviates from zero prior to movement
In this method yank (the derivative of force) is used to detect the start of movement. Start of movement is defined as the moment when the derivative of yank deviates from zero, working backwards from when the movement is known to have begun. Total force is smoothed using a 4th order butterworth filter prior to being differentiated. In addition, the yank curve is rectified prior to being differentiated.
Total force must be within 10% of the measured bodyweight for start of movement to be detected using this method, this check limits the occurrence of false positives in detecting start of movement when the total force curve is noisy prior to movement.