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

n <- 50
mk <- function(v) { m <- matrix(v, n, 3); colnames(m) <- c("A", "B", "C"); m }
pe <- PhysioExperiment(
  assays = S4Vectors::SimpleList(position_x = mk(1), position_y = mk(2),
                                 position_z = mk(0)),
  colData = S4Vectors::DataFrame(label = c("A", "B", "C"),
                                 type = rep("marker", 3)),
  samplingRate = 100)
bsip <- data.frame(segment = c("s1", "s2"), mass_fraction = c(50, 50),
  com_proximal_fraction = c(0.5, 0.5),
  proximal_marker = c("A", "B"), distal_marker = c("B", "C"),
  stringsAsFactors = FALSE)
pe_com <- calculateCOM(pe, body_mass = 70, bsip = bsip,
  marker_map = list(s1 = c("A", "B"), s2 = c("B", "C")))
SummarizedExperiment::assayNames(pe_com)
#> [1] "com_x" "com_y" "com_z"