# Walking strides segmentation with adept

#### Marta Karas

#### Ciprian Crainiceanu

#### Jacek Urbanek

#### 2021-01-31

This vignette provides an example of segmentation of walking strides (two consecutive steps) in sub-second accelerometry data with `adept` package. The exemplary dataset is a part of the `adeptdata` package. We demonstrate that ADEPT1 can be used to perform automatic and precise walking stride segmentation from data collected during a combination of running, walking and resting exercises. We introduce how to segment data:

1. with the use of stride templates that were pre-computed based on data from an external study (attached to `adeptdata` package),
2. by deriving new stride templates in a semi-manual manner.

See Introduction to adept package 2 vignette for an introduction to the ADEPT and usage examples of the `segmentPattern` function which implements the ADEPT method.

# Raw accelerometry data sample

The `adeptdata` package contains `acc_running` - a sample of raw accelerometry data collected during 25 minutes of an outdoor run. Data were collected at the sampling frequency of 100 Hz with two ActiGraph GT9X Link sensors located at the left hip and left ankle.

### Running trial

mapmyrun mobile tracking application (link) was used during 25 minutes of running (Patterson Park area, Baltimore, MD) to collect `acc_running` accelerometry data set. Based on the mobile app, the distance covered is approximately 3.35 km. A ground elevation plot generated by the mobile app presents signature trial characteristics (see figure below). The timestamp in `acc_running` dataset matches the mobile app up to ~1-minute.

Screenshot taken from a personal profile of mapmyrun tracking application, accessed via https://www.mapmyrun.com.

### Sensor and accelerometry data

Data were collected with two ActiGraph GT9X Link physical activity monitors at the sampling frequency of 100 Hz. ActiGraph GT9X Link has 3-axis accelerometer collecting accelerometry data along three orthogonal axes. At a sampling frequency of 100 Hz, we collected 100 observations per second per axis (total of 300 observations per second).

### Sensor location

First sensor (denoted as “left_ankle”) was attached to the outer side of the left shoe with a slide-on clip, just below the ankle. A second sensor (denoted as “left_hip”) was attached on the left side of the elastic belt located hip (see image below). Both devices remained stable during the run-trial.

Wearable accelerometer devices location during the experiment. The devices were still covered with a protective plastic foil.

### Data set `acc_running`

To access the `acc_running` data, load `adeptdata` package.

    # devtools::install_github("martakarass/adept")
    library(adept)
    library(adeptdata)
    library(lubridate)
    options(digits.secs = 2)
    head(acc_running)
    #>     loc_id              date_time      x      y     z
    #> 1 left_hip 2018-10-25 17:57:30.00 -0.199 -1.012 0.133
    #> 2 left_hip 2018-10-25 17:57:30.00 -0.191 -1.027 0.133
    #> 3 left_hip 2018-10-25 17:57:30.01 -0.191 -1.035 0.121
    #> 4 left_hip 2018-10-25 17:57:30.02 -0.191 -1.027 0.109
    #> 5 left_hip 2018-10-25 17:57:30.03 -0.191 -1.016 0.109
    #> 6 left_hip 2018-10-25 17:57:30.04 -0.184 -1.012 0.121
    tz(acc_running$date_time)
    #> [1] "UTC"

`acc_running` consists of 300,000 observations of 5 variables:

- `loc_id` - sensor location label, one of: `"left_hip"`, `"left_ankle"`,
- `date_time` - timestamp, stored as `POSIXct`,
- `x` - accelerometry time-series collected on the \(x\)-axis of the sensor,
- `y` - accelerometry time-series collected on the \(y\)-axis of the sensor,
- `x` - accelerometry time-series collected on the \(z\)-axis of the sensor.

##### Note on `date_time` column values

- Values in the `date_time` column were generated via `seq(from = as.POSIXct("2018-10-25 17:57:30.00", tz = "UTC"), by = 0.01, length.out = 150000)` and its display exhibits, likely, the floating point arithmetic problem (see this SO question). Note `date_time` range is not affected and spans from `"2018-10-25 17:57:30.00 UTC"` to `"2018-10-25 18:22:29.99 UTC"`, inclusive.

# Accelerometry data visualization

### Sub-second level accelerometry data

One way to visualize raw accelerometry data is to plot it as a three-dimensional time-series \((x,y,z)\). Here, we plot data from three different time frames, each of 4 seconds length, simultaneously for data collected at left ankle and left hip.

    library(lubridate)
    library(dplyr)
    library(ggplot2)
    library(reshape2)
    library(gridExtra)
    library(magrittr)

    ## Define time frame start values for data subset
    t1 <- ymd_hms("2018-10-25 18:07:00", tz = "UTC")
    t2 <- ymd_hms("2018-10-25 18:20:30", tz = "UTC")
    t3 <- ymd_hms("2018-10-25 18:22:00", tz = "UTC")

    ## Mutate data: generate vector magnitude variable
    acc_running %<>% mutate(vm = sqrt(x^2 + y^2 + z^2))

    ## Subset data
    acc_running_sub <-
      acc_running %>%
      filter((date_time >= t1 & date_time < t1 + as.period(4, "seconds")) |
               (date_time >= t2 & date_time < t2 + as.period(4, "seconds")) |
               (date_time >= t3 & date_time < t3 + as.period(4, "seconds")) )

    ## Plot (x,y,z) values
    acc_running_sub %>%
      select(-vm) %>%
      melt(id.vars = c("date_time", "loc_id")) %>%
      mutate(dt_floor = paste0("time frame start: ",
                               floor_date(date_time, unit = "minutes"))) %>%
      ggplot(aes(x = date_time, y = value, color = variable)) +
      geom_line(size = 0.5) +
      facet_grid(loc_id ~ dt_floor, scales = "free_x") +
      theme_bw(base_size = 9) +
      labs(x = "Time [s]", y = "Acceleration [g]", color = "Accelerometer axis of measurement: ",
           title = "Raw accelerometry data (x,y,z)") +
      theme(legend.position = "top")

##### Note on sensors desynchronization

- The two sensors used in the experiment were set up to initialize data collection at the same time. However, as discussed in3, perfect synchronization on most of modern operating systems is impossible. Additionally, measurements might still get desynchronized across devices even after a few minutes of data collection. Because of that, sub-second level alignment of data cross devices cannot be expected.

### Vector magnitude

Vector magnitude \((vm)\) is often used to reduce the dimensionality of accelerometry time-series \((x,y,z)\). Vector magnitude is computed as \(vm = \sqrt{x^2 + y^2 + z^2}\) at each time point resulting in 1- instead of 3-dimensional time-series.

    ## Plot vector magnitude values
    acc_running_sub %>%
      mutate(dt_floor = paste0(
        "time frame start: ",  floor_date(date_time, unit = "minutes"))) %>%
      ggplot(aes(x = date_time, y = vm)) +
      geom_line(size = 0.5) +
      facet_grid(loc_id ~ dt_floor, scales = "free_x") +
      theme_bw(base_size = 9) +
      labs(x = "Time [s]", y = "Vector magnitue [g]",
           title = "Vector magnitude (vm) summary of raw accelerometry data")

Plots of\((x,y,z)\) and \((vm)\) show an asymmetric repetitive pattern corresponding to walking strides:

- On the \((vm)\) plot, the stride beginnings and ends can be identified by amplitude peaks corresponding to left foot heel-strikes; between these peaks, there are smaller peaks, corresponding to other stride-phases (toe-strikes, toe-offs of both feet and right foot heel-strike).

There are also visible differences in amplitudes and stride durations across the three vertical plot panels:

- The left vertical plot panel (`time frame start: 2018-10-25 18:07:00`) corresponds to a slow/moderate pace running uphill; \((vm)\) time-series is of a relatively high magnitude.

- The middle vertical plot panel (`time frame start: 2018-10-25 18:20:00`) corresponds to a relatively fast running. We can observe high amplitude of \((vm)\), as well as shorter stride durations.

- The right vertical plot panel (`time frame start: 2018-10-25 18:22:00`) corresponds to walking. We can observe a relatively low \((vm)\) amplitude and longer stride durations.

### Vector magnitude count

In reality, it is often challenging to make a plot of all data points collected at sampling frequency of 100 Hz even in 25 minutes-long time-series. A one way to summarize accelerometry data of such high density is to use a \((vmc)\) - vector magnitude count (also known as the mean amplitude deviation). For \(\overline{vm}(t,H)\) - average of \((vm)\) time-series over time window of length \(H\) starting at time \(t\), we define \[\mathrm { vmc } ( t, H ) = \frac { 1 } { H } \sum _ { h = 0 } ^ { H - 1 } \left| vm ( t + h ) - \overline{vm}(t,H) \right|.\]

    ## Function to compute vmc from a vm window vector
    vmc <- function(vm.win){
      mean(abs(vm.win - mean(vm.win)))
    }

    ## Compute vmc vector in 3-seconds windows
    vm <- acc_running$vm
    win.vl <- 100 * 3
    rn.seq <- seq(1, to = length(vm), by = win.vl)
    vmc.vec <- sapply(rn.seq, function(rn.i){
      vm.win.idx <- rn.i : (rn.i + win.vl - 1)
      vm.win <- vm[vm.win.idx]
      vmc(vm.win)
    })

    vmc.df <- data.frame(vmc = vmc.vec,
                         date_time = acc_running$date_time[rn.seq],
                         rn_seq = rn.seq,
                         loc_id = acc_running$loc_id[rn.seq])
    vmc.df.rest <-
      vmc.df %>%
      filter(loc_id == "left_ankle") %>%
      mutate(resting = ifelse(vmc < 0.4, 1, 0),
             vmc_tau_i = rn_seq - 150000) %>%
      select(date_time, resting, vmc_tau_i)
    vmc.df <- vmc.df %>% left_join(vmc.df.rest, by = "date_time")

    ## Plot vmc
    vmc.df %>%
      filter(date_time < max(acc_running$date_time) - 5,
             date_time > min(acc_running$date_time) + 5) %>%
      ggplot(aes(x = date_time, y = vmc)) +
      facet_grid(loc_id ~ .) +
      geom_tile(aes(fill = factor(resting)),  height = Inf, alpha = 0.1) +
      geom_line(size = 0.3) +
      theme_bw(base_size = 9) +
      labs(x = "Exercise time [min]", y = "Vector magnitue count",
           title = "Vector magnitude count (vmc) computed over 3 second-length windows of (vm)",
           fill = "Resting: ") +
      scale_fill_manual(values = c("white", "blue")) +
      theme(legend.position = "top",
            legend.background = element_rect(fill = "grey90"))

    ## Time percentage when (vmc) computed from left ankle is below 0.4
    (vmc.low.frac <- mean(vmc.df.rest$resting))
    #> [1] 0.186

- We can see that approximately \(19\)% of the time \((vmc)\) computed from left ankle data is below \(0.4\) value. Here, that part of data corresponds to standing with no walking/running legs movement involved. It is likely that the runner was i.e. waiting on the crossroad for a green light to cross. We use this arbitrarily selected threshold of \(0.4\) to partition data into time frames of resting (\((vmc)<0.4\); marked with light blue color) and non-resting (\((vmc)\geq 0.4\)).

- Analogously, high values of \((vmc)\) correspond to high volumes of movement; in this experiment, at approx. 18:21 (6:21 PM) a short period of a fast run started, resulting in high values of \((vmc)\) for both left ankle and left hip sensors.

# Walking strides segmentation

To segment strides from \((vm)\) time-series, we use stride accelerometry data templates. These templates are specific to a wearable sensor location (i.e. left ankle-specific template). We demonstrate two approaches:

1. Approach 1: Use stride templates derived from accelerometry data collected in a different experiment, with different participants. These templates are attached to the `adeptdata` package as `stride_template` object; data used to derive them are also attached to the `adeptdata` package as `acc_walking_IU` (see `?stride_template`, `?acc_walking_IU` for details).

2. Approach 2: Derive stride templates semi-manually from `acc_running` data set.

### Segmentation with Approach 1: use existing stride templates

##### Left ankle

We use pre-computed `stride_template {adeptdata}` to build a `template` object – a list of left ankle-specific stride templates.

    template <- list(stride_template$left_ankle[[2]][1, ],
                     stride_template$left_ankle[[2]][2, ])
    par(mfrow = c(1,2), cex = 0.7)
    plot(template[[1]], type = "l", xlab = "", ylab = "", main = "Left ankle: template 1")
    plot(template[[2]], type = "l", xlab = "", ylab = "", main = "Left ankle: template 2")

We use `segmentPattern {adept}` function to segment strides from \((vm)\) accelerometry time-series.

Explanation of function arguments used:

- `x` - A time-series to segment pattern occurrences from (here: \((vm)\) accelerometry time-series).
- `x.fs` - Data sampling frequency, expressed in a number of observations per second.
- `template` – Pattern template(s).
- `pattern.dur.seq` - A grid of potential stride durations, expressed in seconds.
- `similarity.measure` - Statistic used to quantify similarity between accelerometry time-series and template.
- `x.adept.ma.W` - Length of smoothing window used in accelerometry time-series processing before computing similarity between accelerometry time-series and templates, expressed in seconds.
- `finetune` - Procedure employed to fine-tune preliminarily identified beginnings and ends of a pattern in a accelerometry time-series.
- `finetune.maxima.ma.W` - Length of smoothing window used in `"maxima"` fine-tune procedure, expressed in seconds.
- `finetune.maxima.nbh.W` - Length of a local maxima search grid used in `"maxima"` fine-tune procedure, expressed in seconds.
- `compute.template.idx`- Whether or not to compute which of the (possibly multiple) templates best-matched accelerometry time-series.
  `run.parallel` - Whether or not to use parallel execution in the algorithm.

See `?segmentPattern` for a detailed explanation of all parameters. See Introduction to adept package vignette for explanation of `segmentPattern {adept}` parameters with simulated data examples.

    x.la <- acc_running$vm[acc_running$loc_id == "left_ankle"]
    out1.la <- segmentPattern(
      x = x.la,
      x.fs = 100,
      template = template,
      pattern.dur.seq = seq(0.5, 1.8, length.out = 50),
      similarity.measure = "cor",
      x.adept.ma.W = 0.15,
      finetune = "maxima",
      finetune.maxima.ma.W = 0.05,
      finetune.maxima.nbh.W = 0.2,
      compute.template.idx = TRUE,
      run.parallel = TRUE)

    head(out1.la)
    #>   tau_i T_i     sim_i template_i
    #> 1     1  65 0.3546074          1
    #> 2    65  50 0.3143345          2
    #> 3   133 124 0.7533280          1
    #> 4   256  76 0.8841799          1
    #> 5   371  81 0.9152038          1
    #> 6   451  78 0.9075649          1

The segmentation result is a data frame, where each row describes one identified pattern occurrence:

- `tau_i` - index of `x` where pattern starts,
- `T_i` - pattern duration, expressed in `x` vector length,
- `sim_i` - similarity between a pattern and `x`,
- `template_i` - index of a pattern template best matched to a pattern in the time-series `x`.

##### Left hip

We now segment strides from accelerometry data collected at left hip. We then compare the results with left ankle segmentation.

    template <- list(stride_template$left_hip[[1]][1, ],
                     stride_template$left_hip[[2]][2, ])
    par(mfrow = c(1,2), cex = 0.7)
    plot(template[[1]], type = "l", xlab = "", ylab = "", main = "Left hip: template 1")
    plot(template[[2]], type = "l", xlab = "", ylab = "", main = "Left hip: template 2")

    template <- list(stride_template$left_hip[[1]][1, ],
                     stride_template$left_hip[[2]][2, ])
    x.lh <- acc_running$vm[acc_running$loc_id == "left_hip"]
    out1.lh <- segmentPattern(x = x.lh,
                              x.fs = 100,
                              template = template,
                              pattern.dur.seq = seq(0.5, 1.8, length.out = 50),
                              similarity.measure = "cor",
                              x.adept.ma.W = 0.05,
                              finetune = "maxima",
                              finetune.maxima.ma.W = 0.15,
                              finetune.maxima.nbh.W = 0.2,
                              compute.template.idx = TRUE,
                              run.parallel = TRUE)

    head(out1.lh)
    #>   tau_i T_i     sim_i template_i
    #> 1     8  52 0.7124452          2
    #> 2    59  54 0.8113169          2
    #> 3   156 126 0.8446630          1
    #> 4   301  70 0.6299608          1
    #> 5   370  82 0.8684391          1
    #> 6   451  79 0.8231402          1

#### Results: estimated stride duration time

We estimate stride duration, expressed in seconds, as `T_i` divided by data sampling frequency (here: \(100\)). Similarly, we estimate stride start, expressed in seconds after run-recording began, as `tau_i` divided by data sampling frequency; to get estimated stride start given in minutes after run-recording began, we additionally divide by \(60\).

We use previously derived information of resting (\((vmc)<0.4\)) and non-resting (\((vmc)\geq 0.4\)) to filter out segmented data parts which likely do not correspond to running/walking. We then plot the estimated stride duration against estimated stride start. We use different background shade to mark the derived resting/non-resting labels for subsequent \(3\)-minute windows.

    ## Merge results from two sensor locations, add resting/non-resting info
    plt.df.la <-
      out1.la %>%
      merge(vmc.df.rest) %>%
      filter(tau_i >= vmc_tau_i, tau_i < vmc_tau_i + 3 * 100, resting == 0) %>%
      mutate(location = "left_ankle")
    plt.df.lh <-
      out1.lh %>%
      merge(vmc.df.rest) %>%
      filter(tau_i >= vmc_tau_i, tau_i < vmc_tau_i + 3 * 100, resting == 0) %>%
      mutate(location = "left_hip")
    plt.df <- rbind(plt.df.la, plt.df.lh)

    ggplot(plt.df,
        aes(x = tau_i / (100 * 60) , y = T_i / 100)) +
      geom_tile(data = vmc.df.rest,
                aes(x = vmc_tau_i / (100 * 60), y = 1, fill = factor(resting)),
                height = Inf, alpha = 0.1, inherit.aes = FALSE) +
      geom_point(alpha = 0.2) +
      facet_grid(location ~ .) +
      theme_bw(base_size = 10) +
      labs(x = "Exercise time [min]", y = "Estimated stride duration time [s]",
           fill = "Resting: ") +
      theme(legend.position = "top") +
      scale_fill_manual(values = c("white", "blue")) +
      theme(legend.position = "top",
            legend.background = element_rect(fill = "grey90")) +
      scale_y_continuous(limits = c(0.5, 1.8))

- We can see that the estimated stride duration (y-axis) is consistent between data collected at the left ankle and left hip. Some differences we observe may partially result from different smoothing parameters used in segmentation fine-tune procedure; left ankle data was smoothed less (`finetune.maxima.ma.W = 0.05`) than left hip data (`finetune.maxima.ma.W = 0.15`).

- We may suppose that around the resting periods (light blue background), strides of both relatively long and short duration may correspond to the runner walking (making long slow steps, or very short steps). They can be a mislabeled resting data, too.

#### Results: subject-specific stride pattern (left ankle)

We now use the estimated stride start (`tau_i`) and stride duration (`T_i`) to retrieve accelerometry \((vm)\) time-series segments corresponding to segmented strides. We further align them in phase and scale to better observe stride patterns.

    ## For data frame #1 (raw vm segments)
    stride.acc.vec.la <- numeric()
    stride.tau_i.vec.la <- numeric()
    stride.idx.vec.la <- numeric()
    ## For data frame #2 (scaled vm segments)
    stride_S.acc.vec.la <- numeric()
    stride_S.tau_i.vec.la <- numeric()
    stride_S.phase.vec.la <- numeric()

    for (i in 1:nrow(plt.df.la)){
      out.i <- plt.df.la[i, ]
      x.la.i <- x.la[out.i$tau_i : (out.i$tau_i + out.i$T_i - 1)]
      x.la.i.len <- length(x.la.i)
      if (var(x.la.i) < 1e-3) next
      ## For data frame #1
      stride.acc.vec.la   <- c(stride.acc.vec.la, x.la.i)
      stride.tau_i.vec.la <- c(stride.tau_i.vec.la, rep(out.i$tau_i, x.la.i.len))
      stride.idx.vec.la <- c(stride.idx.vec.la, 1:x.la.i.len)
      ## For data frame #2
      x.la.i_S <- approx(x = seq(0, 1, length.out = length(x.la.i)),
                         y = x.la.i,
                         xout = seq(0, 1, length.out = 200))$y
      x.la.i_S <- as.numeric(scale(x.la.i_S))
      stride_S.acc.vec.la <- c(stride_S.acc.vec.la, x.la.i_S)
      stride_S.tau_i.vec.la <- c(stride_S.tau_i.vec.la, rep(out.i$tau_i, 200))
      stride_S.phase.vec.la <- c(stride_S.phase.vec.la, seq(0, 1, length.out = 200))
    }

    ## data frame #1
    stride.df.la <- data.frame(acc = stride.acc.vec.la,
                               tau_i = stride.tau_i.vec.la,
                               idx = stride.idx.vec.la)
    ## data frame #2
    stride_S.df.la <- data.frame(acc = stride_S.acc.vec.la,
                                 tau_i = stride_S.tau_i.vec.la,
                                 phase = stride_S.phase.vec.la)

    ## Plot segmented walking strides
    plt1 <-
      stride.df.la %>%
      ggplot(aes(x = idx/100, y = acc, group = tau_i)) +
      geom_line(alpha = 0.1) +
      theme_bw(base_size = 8) +
      labs(x = "Stride pattern duration [s]", y = "Vector magnitude [g]",
           title = "Segmented walking strides")
    plt2 <-
      stride_S.df.la %>%
      ggplot(aes(x = phase, y = acc, group = tau_i)) +
      geom_line(alpha = 0.1) +
      theme_bw(base_size = 8) +
      labs(x = "Stride pattern phase", y = "Vector magnitude (scaled) [g]",
           title = "Segmented walking strides, aligned and scaled")
    grid.arrange(plt1, plt2, nrow = 1)

- Based on the plot of aligned and scaled strides (right plot above), it seems there are at least two distinct stride patterns pronounced in data - one with a “spike with a dip” in a middle of a stride pattern phase, and another, less frequent (fewer lines on the plot), with a spike at approx. \(2/3\) of a stride pattern phase.

##### Correlation clustering of segmented walking strides

We can further use correlation clustering to group segmented walking strides.

    ## Compute strides distance martrix
    stride_S.dfdc.la <- dcast(stride_S.df.la, phase ~ tau_i, value.var = "acc")[, -1]
    data.mat.tau_i <- as.numeric(colnames(stride_S.dfdc.la))
    data.mat <-  as.matrix(stride_S.dfdc.la)
    D.mat  <- dist(cor(data.mat))

    ## Get cluster medoids
    cluster.k <- 2
    medoids.idx <- round(seq(1, ncol(stride_S.dfdc.la), length.out = cluster.k + 2))
    medoids.idx <- medoids.idx[-c(1, medoids.idx + 2)]

    ## Cluster strides
    set.seed(1)
    pam.out <- cluster::pam(D.mat, cluster.k, diss = TRUE, medoids = medoids.idx)
    table(pam.out$clustering)
    #>
    #>    1    2
    #> 1420  120

    ## Put clustering results into data frame
    data.df <- as.data.frame(t(data.mat))
    colnames(data.df) <- seq(0, to = 1, length.out = 200)
    data.df$tau_i <- data.mat.tau_i
    data.df$cluster <- pam.out$clustering
    data.dfm <- melt(data.df, id.vars = c("tau_i", "cluster"))
    data.dfm$variable <- as.numeric(as.character(data.dfm$variable))
    data.dfm$cluster <- paste0("cluster ", data.dfm$cluster)

    data.dfm.agg <-
      data.dfm %>%
      group_by(variable, cluster) %>%
      summarise(value = mean(value))
    ggplot(data.dfm, aes(x = variable, y = value, group = tau_i)) +
      geom_line(alpha = 0.2) +
      geom_line(data = data.dfm.agg,  aes(x = variable, y = value, group = 1),
                color = "red", size = 1, inherit.aes = FALSE) +
      facet_grid(cluster ~ .) +
      theme_bw(base_size = 9) +
      labs(x = "Stride pattern phase", y = "Vector magnitude (scaled) [g]",
           title = "Segmented walking strides, aligned, scaled, clustered\nRed line: point-wise mean")

We further plot the estimated stride duration time over the course of the run-trial exercise. We mark the pattern cluster assignment with color.

    data.dfm %>%
      select(tau_i, cluster)  %>%
      distinct() %>%
      left_join(plt.df.la, by = "tau_i") %>%
      ggplot(aes(x = tau_i / (100 * 60) , y = T_i / 100, color = cluster)) +
      geom_tile(data = vmc.df.rest,
                aes(x = vmc_tau_i / (100 * 60), y = 1, fill = factor(resting)),
                height = Inf, alpha = 0.1, inherit.aes = FALSE) +
      geom_point(alpha = 0.4) +
      theme_bw(base_size = 9) +
      labs(x = "Exercise time [min]", y = "Estimated stride duration time [s]",
           color = "Stride assignment: ",
           fill = "Resting: ") +
      scale_fill_manual(values = c("white", "blue")) +
      theme(legend.position = "top",
            legend.background = element_rect(fill = "grey90"))  +
      scale_y_continuous(limits = c(0.5, 1.8))

- From the plot above we can note that pattern cluster assignment corresponds to performed activity: strides marked with red likely correspond to running, strides marked with green - walking.

### Segmentation with Approach 2: derive stride templates semi-manually

We now demonstrate a semi-manual way of deriving stride patterns from scratch.

1. Smooth accelerometry \((vm)\) time-series.

2. Select a short segment, or multiple segments, from the smoothed \((vm)\) time-series.

3. Within each selected \((vm)\) segment, use a function to automatically identify all local maxima.

4. Within each selected \((vm)\) segment, identify a subset of local maxima that corresponds to stride beginnings and ends via visual inspection (the “manual” part).

5. For each selected \((vm)\) segment, cut it at the points of identified local maxima subset from step 3., interpolate obtained parts to a common vector length, align, standardize to have mean 0 and variance 1, and compute their point-wise average. Standardize the average to have mean 0 and variance 1. For each \((vm)\) segment, the resulted point-wise average is a one newly created template.

Here, we select two \((vm)\) fragments in step 1., each of 6 seconds length, in a way that they correspond to a different pace of running/walking. We apply the above procedure separately for data collected at the left hip and left ankle. That way, for each sensor location, we arrive at two distinct stride templates that we further use in segmentation.

    ## Function to compute local maxima
    ## source: https://stackoverflow.com/questions/6836409/finding-local-maxima-and-minima
    localMaxima <- function(x) {
      y <- diff(c(-.Machine$integer.max, x)) > 0L
      y <- cumsum(rle(y)$lengths)
      y <- y[seq.int(1L, length(y), 2L)]
      if (x[[1]] == x[[2]]) {
        y <- y[-1]
      }
      y
    }

    ## Function which cut x vector at indices given by x.cut.idx,
    ## approximate cut parts into common length and average
    ## parts point-wise into one vector
    cut.and.avg <- function(x, x.cut.idx){
      x.cut.idx.l <- length(x.cut.idx)
      mat.out <- matrix(NA, nrow = x.cut.idx.l-1, ncol = 200)
      for (i in 1:(length(x.cut.idx)-1)){
        xp <- x[x.cut.idx[i]:x.cut.idx[i+1]]
        xpa <- approx(seq(0, 1, length.out = length(xp)), xp, seq(0, 1, length.out = 200))$y
        mat.out[i, ] <- as.numeric(scale(xpa, scale = TRUE, center = TRUE))
      }
      out <- apply(mat.out, 2, mean)
      as.numeric(scale(out, scale = TRUE, center = TRUE))
    }

    ## Smooth the accelerometry vm time-series
    acc_running$vm_smoothed1 <- windowSmooth(x = acc_running$vm, x.fs = 100, W = 0.05)

    ## Make subset of data which has data parts of different speed of running
    acc_running_sub2 <-
      acc_running %>%
      filter((date_time >= t2 & date_time < t2 + as.period(6, "seconds")) |
               (date_time >= t3 & date_time < t3 + as.period(6, "seconds")))  %>%
      mutate(dt_floor = floor_date(date_time, unit = "minutes"))
    ## Vector of signatures for data parts of different speed of running
    dt_floor.unique <- unique(acc_running_sub2$dt_floor)

##### Deriving stride patterns in a semi-manual way: left ankle

    ## Left ankle-specific subset of data
    sub.la <- acc_running_sub2[acc_running_sub2$loc_id == "left_ankle", ]
    par(mfrow = c(1,3), cex = 0.6)

    ## Left ankle: template 1
    x1 <- sub.la[sub.la$dt_floor == dt_floor.unique[1], "vm_smoothed1"]
    x1.locMax <- localMaxima(x1)
    plot(1:length(x1), x1, type = "l", main = "(vm) local maxima", xlab = "Index", ylab = "")
    abline(v = x1.locMax, col = "red")
    plot(1:length(x1), x1, type = "l", main = "(vm) local maxima subset", xlab = "Index", ylab = "")
    abline(v = x1.locMax[c(3, 6, 9, 13, 16, 19, 23, 26, 29)], col = "red")
    template.la.x1 <- cut.and.avg(x1, x1.locMax[c(3, 6, 9, 13, 16, 19, 23, 26, 29)])
    plot(1:length(template.la.x1), template.la.x1, type = "l",
         col = "red", main = "left ankle: template 1", xlab = "Index", ylab = "")

    ## Left ankle: template 2
    x2 <- sub.la[sub.la$dt_floor == dt_floor.unique[2], "vm_smoothed1"]
    x2.locMax <- localMaxima(x2)
    plot(1:length(x2), x2, type = "l", main = "(vm) local maxima", xlab = "Index", ylab = "")
    abline(v = x2.locMax, col = "red")
    plot(1:length(x2), x2, type = "l", main = "(vm) local maxima subset", xlab = "Index", ylab = "")
    abline(v = x2.locMax[c(8, 18, 26, 37, 44)], col = "red")
    template.la.x2 <- cut.and.avg(x2, x2.locMax[c(8, 18, 26, 37, 44)])
    plot(1:length(template.la.x2), template.la.x2, type = "l",
         col = "red", main = "left ankle: template 2", xlab = "Index", ylab = "")

    ## Template object
    template.la <- list(template.la.x1, template.la.x2)

##### Deriving stride patterns in a semi-manual way: left hip

    ## Left hip-specific subset of data
    sub.lh <- acc_running_sub2[acc_running_sub2$loc_id == "left_hip", ]
    par(mfrow = c(1,3), cex = 0.6)

    ## Left hip: template 1
    x1 <- sub.lh[sub.lh$dt_floor == dt_floor.unique[1], "vm_smoothed1"]
    x1.locMax <- localMaxima(x1)
    plot(1:length(x1), x1, type = "l", main = "(vm) local maxima", xlab = "Index", ylab = "")
    abline(v = x1.locMax, col = "red")
    plot(1:length(x1), x1, type = "l", main = "(vm) local maxima subset", xlab = "Index", ylab = "")
    abline(v = x1.locMax[c(3, 10, 17, 24, 30, 37, 43, 49, 56)], col = "red")
    template.lh.x1 <- cut.and.avg(x1, x1.locMax[c(3, 10, 17, 24, 30, 37, 43, 49, 56)])
    plot(1:length(template.lh.x1), template.lh.x1, type = "l",
         col = "red", main = "left ankle: template 1", xlab = "Index", ylab = "")

    ## Left hip: template 2
    x2 <- sub.lh[sub.lh$dt_floor == dt_floor.unique[2], "vm_smoothed1"]
    x2.locMax <- localMaxima(x2)
    plot(1:length(x2), x2, type = "l", main = "(vm) local maxima", xlab = "Index", ylab = "")
    abline(v = x2.locMax, col = "red")
    plot(1:length(x2), x2, type = "l", main = "(vm) local maxima subset", xlab = "Index", ylab = "")
    abline(v = x2.locMax[c(5, 13, 19, 24, 31)], col = "red")
    template.lh.x2 <- cut.and.avg(x2, x2.locMax[c(5, 13, 19, 24, 31)])
    plot(1:length(template.lh.x2), template.lh.x2, type = "l",
         col = "red", main = "left ankle: template 2", xlab = "Index", ylab = "")

    ## Template object
    template.lh <- list(template.lh.x1, template.lh.x2)

##### Segmentation

    out2.la <- segmentPattern(x = x.la,
                              x.fs = 100,
                              template = template.la,
                              pattern.dur.seq = seq(0.5, 1.8, length.out = 50),
                              similarity.measure = "cor",
                              x.adept.ma.W = 0.15,
                              finetune = "maxima",
                              finetune.maxima.ma.W = 0.05,
                              finetune.maxima.nbh.W = 0.2,
                              compute.template.idx = TRUE,
                              run.parallel = TRUE)

    out2.lh <- segmentPattern(x = x.lh,
                              x.fs = 100,
                              template = template.lh,
                              pattern.dur.seq = seq(0.5, 1.8, length.out = 50),
                              similarity.measure = "cor",
                              x.adept.ma.W = 0.05,
                              finetune = "maxima",
                              finetune.maxima.ma.W = 0.15,
                              finetune.maxima.nbh.W = 0.2,
                              compute.template.idx = TRUE,
                              run.parallel = TRUE)

#### Results: estimated stride duration time

    ## Merge results from two sensor locations, add resting/non-resting info
    plt.df.la.2 <-
      out2.la %>%
      merge(vmc.df.rest) %>%
      filter(tau_i >= vmc_tau_i, tau_i < vmc_tau_i + 3 * 100, resting == 0) %>%
      mutate(location = "left_ankle")
    plt.df.lh.2 <-
      out2.lh %>%
      merge(vmc.df.rest) %>%
      filter(tau_i >= vmc_tau_i, tau_i < vmc_tau_i + 3 * 100, resting == 0) %>%
      mutate(location = "left_hip")
    plt.df <- rbind(plt.df.la.2, plt.df.lh.2)

    ggplot(plt.df,
        aes(x = tau_i / (100 * 60) , y = T_i / 100)) +
      geom_tile(data = vmc.df.rest,
                aes(x = vmc_tau_i / (100 * 60), y = 1, fill = factor(resting)),
                height = Inf, alpha = 0.1, inherit.aes = FALSE) +
      geom_point(alpha = 0.2) +
      facet_grid(location ~ .) +
      theme_bw(base_size = 10) +
      labs(x = "Exercise time [min]", y = "Estimated stride duration time [s]",
           fill = "Resting: ") +
      theme(legend.position = "top") +
      scale_fill_manual(values = c("white", "blue")) +
      theme(legend.position = "top",
            legend.background = element_rect(fill = "grey90")) +
      scale_y_continuous(limits = c(0.5, 1.8))

- From the plot above we see that the estimated stride duration times are fairly consistent between the two sensor locations, and also consistent with the estimates we obtained based on external, pre-computed stride templates.

# References

1. Karas, M., Straczkiewicz, M., Fadel, W., Harezlak, J., Crainiceanu, C., Urbanek, J.K. Adaptive empirical pattern transformation (ADEPT) with application to walking stride segmentation, Submitted to Biostatistics, 2018.↩︎

2. Karas, M., Crainiceanu, C., Urbanek, J.: Introduction to adept package vignette to the ‘adept’ package.↩︎

3. Karas, M., Bai, J., Straczkiewicz, M., Harezlak, J., Glynn, N W., Harris, T., Zipunnikov, V., Crainiceanu, C., Urbanek, J.K. Accelerometry data in health research: challenges and opportunities. Review and examples, Statistics in Biosciences, 2018. (link).↩︎