Limits of Stability (LOS) reaction time, excursion and directional control

Quantifies a single limits-of-stability leaning trial toward a target direction from a CoP trace: reaction time, endpoint and maximum excursion (optionally as a percentage of a theoretical limit), and directional control.

Usage

limitsOfStability(
  cop,
  sampling_rate,
  target,
  ap = NULL,
  ml = NULL,
  onset_cue = 1L,
  onset_frac = 0.05,
  los_distance = NULL
)

Arguments

cop, ap, ml: CoP trace, as in swayMetrics() (a single leaning trial).
sampling_rate: Sampling rate in Hz.
target: Target direction: an angle in degrees (0 = +AP/forward, 90 = +ML/right, measured counter-clockwise from forward) or a length-2 numeric c(ap, ml) direction vector.
onset_cue: Sample index of the go cue (default 1).
onset_frac: Fraction of the maximum excursion defining movement onset (default 0.05).
los_distance: Optional theoretical limit-of-stability distance (CoP units) for the target; if supplied, excursions are also returned as a percentage of it.

Value

A los_result object with reaction_time, endpoint_excursion, max_excursion, optional endpoint_pct / max_pct, and directional_control.

Details

The CoP is projected onto the unit vector toward the target. Reaction time is the delay from the go cue to movement onset (projection first exceeding onset_frac of the maximum excursion). Endpoint excursion is the projection at the end of the initial movement (its first local maximum after onset); maximum excursion is the largest projection. Directional control is \(100 \times\) the path travelled toward the target divided by the total path length.

References

Nashner LM (1997). "Computerized dynamic posturography."

Examples

t <- seq(0, 3, by = 0.01)
lean <- pmin(t / 1.5, 1) * 5             # ramp forward to 5 units
cop <- data.frame(cop_x = rnorm(length(t), 0, 0.02), cop_y = lean)
limitsOfStability(cop, sampling_rate = 100, target = 0)
#> <los_result>
#>   reaction time: 0.080 s
#>   endpoint excursion: 5   max excursion: 5
#>   directional control: 50.0%