Calculate whole-body center of mass from marker positions

Computes the whole-body center of mass (COM) at each time frame from 3D (or 2D) marker position data and body segment inertial parameters.

Usage

calculateCOM(pe, body_mass, skeleton = NULL, bsip = NULL, marker_map = NULL)

Arguments

pe

A PhysioExperiment object containing position_x, position_y, and optionally position_z assays.

body_mass

Numeric scalar, body mass in kilograms. Must be positive.

skeleton

Optional SkeletonModel object used for automatic marker mapping. If provided and marker_map is NULL, a default mapping is generated from the skeleton model.

bsip

Optional data.frame of body segment parameters (as returned by segmentParameters()). If NULL (default), uses segmentParameters("deLeva_male").

marker_map

Optional named list mapping each segment to its proximal and distal marker names. Each element should be a character vector of length 2: c(proximal_marker, distal_marker). Segment names must match those in the bsip table. Example: list(thigh_r = c("RHip", "RKnee"), thigh_l = c("LHip", "LKnee")).

Value

A PhysioExperiment object with additional assays com_x, com_y, and (if 3D) com_z. Each assay is a single-column matrix with column name "COM" and the same number of rows as the input.

Details

The algorithm proceeds as follows:

1. For each body segment, compute the segment COM position as: $$COM_{seg} = P_{prox} + f \times (P_{dist} - P_{prox})$$ where \(f\) is the COM proximal fraction from the BSIP table.

2. Compute the whole-body COM as the mass-weighted average of all segment COMs: $$COM_{body} = \frac{\sum_i m_i \times COM_i}{M}$$ where \(m_i = M \times f_i\) is the segment mass and \(M\) is the total body mass.

References

Winter DA (2009). "Biomechanics and Motor Control of Human Movement." 4th ed. John Wiley & Sons.

Zatsiorsky VM (2002). "Kinetics of Human Motion." Human Kinetics.

See also

segmentParameters() for body segment inertial parameters, calculateSegmentCOM() for individual segment center of mass, symmetryIndex() for bilateral symmetry assessment.

Examples

if (FALSE) { # \dontrun{
pe <- readTRC("markers.trc")
result <- calculateCOM(pe, body_mass = 75)
com_x <- SummarizedExperiment::assay(result, "com_x")
} # }