The intensity gradient summarises the distribution of physical-activity
intensity across the whole day: the log-log slope of time accumulated against
acceleration intensity (Rowlands et al. 2018). A less negative gradient means
relatively more time at higher intensities. Unlike a single MVPA cut-point it
uses the entire intensity range and is cut-point free.
Usage
intensityGradient(enmo_mg, epoch_sec, bin_width_mg = 25, max_mg = 4000)
Arguments
- enmo_mg
Per-epoch ENMO in milli-g.
- epoch_sec
Epoch length in seconds.
- bin_width_mg
Intensity bin width in mg (default 25).
- max_mg
Upper edge of the binned range in mg (default 4000).
Value
A list with gradient (slope), intercept, r_squared, and the
bins data frame (mid_mg, minutes) used in the regression.
References
Rowlands AV, et al. (2018). Med Sci Sports Exerc 50(6):1323-1332.
Examples
# a power-law intensity distribution: time proportional to intensity^-2
mids <- seq(12.5, 2000, by = 25)
minutes <- 1e5 * mids^(-2)
enmo <- rep(mids, round(minutes * 2)) # 30 s epochs -> 2 epochs/min
round(intensityGradient(enmo, epoch_sec = 30)$gradient, 2) # ~ -2
#> [1] -1.91