Force Plate CoP Metrics (MATLAB)

MATLAB functions for computing common center of pressure (CoP) metrics from force plate recordings, including:

• CoP Path Length (total)
• Sway Velocity (total; and optional AP/ML components)
• 95% CoP Ellipse Area
• Coefficient of Variation (CV) of CoP displacement (AP/ML/radial)

These scripts are designed for batch processing across many trials stored in a struct array (one element per trial).

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Author

Jeevan Jayasuriya
NeuroErgonomics Lab
University of Wisconsin–Madison

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What’s in this repo

Core metric functions

• CoP_PathLength.m
  Computes total CoP path length per trial and saves a CSV.

• CoP_SwayVelocity.m
  Computes total CoP path length and total sway velocity per trial and saves a CSV.

• CoP_vel_APandML.m
  Computes AP, ML, and total path length and sway velocity per trial and saves a CSV and .mat.

• CoP_EllipseArea.m
  Computes 95% ellipse area of CoP (AP–ML) per trial and saves a CSV.
  Includes optional plotting for a selected range of trials.

• CoP_CoefVatiation.m
  Computes CoP coefficient of variation (CV) using absolute displacement:

  • CV_AP = std(|AP|) / mean(|AP|)
  • CV_ML = std(|ML|) / mean(|ML|)
  • CV_R = std(r) / mean(r), where r = sqrt(AP^2 + ML^2)
    Saves a CSV.

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Input data format

All functions expect an input struct array named FFAllStruct (or equivalent), where each element is one trial and contains force plate channels:

Required fields per trial:

• Fx, Fy, Fz (forces)
• Mx, My (moments)

Optional (recommended) metadata fields:

• trialID, Subject, Activity, Stimulation, Session, Trial, etc.

The scripts attempt to carry through metadata when writing CSV outputs (when those fields exist).

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CoP computation used

The scripts compute CoP from forces and moments (in mm), using:

• CoP_ML = (-(My + Fx * az0) / Fz) * 1000
• CoP_AP = ((Mx - Fy * az0) / Fz) * 1000

Where:

• az0 is the vertical offset (default set to 0)
• near-zero Fz values are set to NaN to avoid division artifacts

CoP is then centered by subtracting its mean (omitnan) before metric calculation.

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Filtering and sampling assumptions

Most functions use the following defaults (can be edited inside each file):

• Sampling rate: 1000 Hz
• Low-pass filter: 4th-order Butterworth, cutoff 10 Hz
• applyFilter = true by default

If your force plate sampling rate differs, update samplingRate in each function accordingly.

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Outputs

Each function writes a CSV in the current working directory:

• CoP_length.csv
• CoP_and_Sway_Velocity.csv
• CoP_and_Sway_Velocity_with_AP_ML.csv (+ also saves CoP_and_Sway_Velocity_with_AP_ML.mat)
• CoP_ellipse_area.csv
• CoP_CV Measures.csv

In addition, functions return an updated struct (e.g., FFAllStructWithCoP) with computed fields appended.

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Quick start

1. Add this repo to your MATLAB path:

addpath(genpath(pwd));

2. Ensure your trial struct array exists in the workspace as FFAllStruct.

3. Run any metric:

Path length (total)

FFAllStructWithCoP = calculate_CoP_path_length(FFAllStruct);

Total sway velocity + total path length

FFAllStructWithCoP = calculate_CoP_path_length_and_sway_velocity(FFAllStruct);

AP/ML/Total path length + sway velocity (recommended)

FFAllStructWithCoP = calculate_CoP_path_length_and_sway_velocity(FFAllStruct);

Note: CoP_vel_APandML.m and CoP_SwayVelocity.m define the same function name but produce different outputs/files.
Recommendation: keep only one of them in your MATLAB path (or rename one function) to avoid name conflicts.

Ellipse area (with optional plotting)

plotRange = 1:10;  % plot trials 1–10 (optional)
FFAllStructWithCoP = calculate_CoP_ellipse_area_with_plot(FFAllStruct, plotRange);

CoP coefficient of variation (CV)

CoPstats = calculate_CoP_CV_allinone_abs(FFAllStruct);

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Common pitfalls

• Duplicate function name conflict:
  Both CoP_SwayVelocity.m and CoP_vel_APandML.m define
  calculate_CoP_path_length_and_sway_velocity. MATLAB will call whichever is first on the path.

  • Fix: rename one function (and file), or remove the one you don’t use.

• Incorrect sampling rate:
  Update samplingRate if your force plate is not 1000 Hz.

• Near-zero vertical force:
  The scripts set small Fz values to NaN. If your task includes flight phases or unloading, expect gaps.

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License

MIT License (see LICENSE).