Category Archives: Sports Science

The Nordic Hamstring Exercise, Hamstring Strains, & the Scientific Process

Doing science in lab is hard.

Doing science in the applied environment is hard — maybe even harder than the lab, at times, due to all of the variables you are unable to control for.

Reading and understanding science is also hard.

Let’s face it, science is tough! So tough, in fact, that scientists themselves have a difficult time with all of the above, and they do this stuff for a living! As such, to keep things in check, science applies a peer-review process to ensure that a certain level of standard is upheld.

Science has a very adversarial quality to it. One group of researchers formulate a hypothesis, conduct some research, and make a claim. Another group of researchers look at that claim and say, “Yeah, but…”, and then go to work trying to poke holes in it, looking to answer the question from a different angle, or trying to refute it altogether. The process continues until some type of consensus is agreed upon within the scientific community based on all of the available evidence.

This back-and-forth tennis match of science has a lot to teach those looking to improve their ability to read and understand research. Reading methodological papers and letters to the editor offer a glimpse into how other, more seasoned, scientists think and approach a problem. You get to see how they construct an argument, deal with a rebuttal, and discuss the limitations of both the work they are questioning and the work they are conducting themselves.

All of this brings me to a recent publication from Franco Impellizzeri, Alan McCall, and Maarten van Smeden, Why Methods Matter in a Meta-Analysis: A reappraisal showed inconclusive injury prevention effect of Nordic hamstring exercise.

The paper is directed at a prior meta-analysis which aggregated the findings of several studies with the aim of understanding the role of the Nordic hamstring exercise (NHE) in reducing the risk hamstring strain injuries in athletes. In a nutshell, Impellizzeri and colleagues felt like the conclusions and claims made from the original meta-analysis were too optimistic, as the title of the paper suggested that performing the NHE can halve the rate of hamstring injuries (Risk Ratio: 0.49, 95% CI: 0.32 to 0.74). Moreover, Impellizzeri et al, identified some methodological flaws with regard to how the meta-analysis was performed.

The reason I like this paper is because it literally steps you through the thought process of Impellizzeri and colleagues. First, it discusses the limitations that they feel are present in the previous meta-analysis. They then conduct their own meta-analysis by re-analyzing the data, however, they apply an inclusion criteria that was more strict and, therefore, only included 5 papers from the original study (they also identified and included a newer study that met their criteria). In analyzing the original five papers, they found prediction intervals ranging from 0.06 to 5.14. (Side Note: Another cool piece of this paper is the discussion and reporting of both confidence intervals and prediction intervals, the latter of which are rarely discussed in the sport science literature).

The paper is a nice read if you want to see the thought process around how scientists read research and go about the scientific process of challenging a claim.

Some general thoughts

  • Wide prediction intervals leave us with a lot of uncertainty around the effectiveness of NHE in reducing the risk of hamstring strain injuries. At the lower end of the interval NHE could be beneficial and protective for some while at the upper end, potentially harmful to others.
  • A possible reason for the large uncertainty in the relationship between NHE and hamstring strain injury is that we might be missing key information about the individual that could indicate whether the exercise would be helpful or harmful. For example, context around their previous injury history (hamstring strains in particular), their training age, or other variables within the training program, all might be useful information to help paint a clearer picture about who might benefit the most or the least.
  • Injury is highly complex and multi-faceted. Trying to pin a decrease in injury risk to a single exercise or a single type of intervention seems like a bit of a stretch.
  • No exercise is a panacea and we shouldn’t treat them as such. If you like doing NHE for yourself (I actually enjoy it!) then do it. If you don’t, then do something else. Let’s not believe that certain exercises have magical properties and let’s not fight athletes about what they are potentially missing from not including an exercise in their program when we don’t even have good certainty on whether it is beneficial or not.

Force Decks – Force Plate Shiny Dashboard

Last week, two of the data scientists at Vald Performance, Josh Ruddy and Nick Murray, put out a free online tutorial on how to create a force plate reports using R with data from their Force Decks software.

It was a nice tutorial to give an overview of some of the power behind ggplot2 and the suite of packages that come with tidyverse. Since they made the data available (in the link above), I decided to pull it down and put together a quick shiny app for those that might be interested in extending the report to an interactive web app.

This isn’t the first time I’ve build a shiny app for the blog using force plate data. Interested readers might want to check out my post from a year ago where I built a shiny interactive report for force-velocity profiling.

You can watch a short preview of the end product in the below video link and the screen shots below the link show a static view of what the final shiny App will look like.

A few key features:

  1. App always defaults to the most recent testing day on the testDay tab.
  2. The user can select the position group at the top and that position group will be maintained across all tabs. For example, if you select Forwards, when you switch between tabs one and two, forwards will always be there.
  3. The time series plots on the Player Time Series tab are done using plotly, so they are interactive, allowing the user to hover over each test session and see the change from week-to-week in the tool tip. When the change exceeds the meaningful change, the point turns red. Finally, because it is plotly, the user can slice out specific dates that they want to look at (as you can see me do in the video example), which comes in handy when there are a large number of tests over time.

All code and data s accessible through my GitHub page.


Loading and preparing the data

  • I load the data in using read.csv() and file.choose(), so navigate to wherever you have the data on your computer and select it.
  • There is some light cleaning to change the date in to a date variable. Additionally, there were no player positions in the original data set, so I just made some up and joined those in.

### packages ------------------------------------------------------------------


### load & clean data ---------------------------------------------------------
cmj <- read.csv(file.choose(), header = TRUE) %>%
  janitor::clean_names() %>%
  mutate(date = dmy(date))

player_positions <- data.frame(name = unique(cmj$name),
                               position = c(rep("Forwards", times = 15),
                                            rep("Mids", times = 15),
                                            rep("Backs", times = 15)))

# join position data with jump data
cmj <- cmj %>%


Determining Typical Error and Meaningful Change

  • In this example, I’ll just pretend as if the first 2 sessions represented our test-retest data and I’ll work from there.
  • Typical Error Measurement (TEM) was calculated as the standard deviation of differences between test 1 and 2 divided by the square root of 2.
  • For the meaningful change, instead of using 0.2 (the commonly used smallest worthwhile change multiplier) I decided to use a moderate change (0.6), since 0.2 is such a small fraction of the between subject SD.
  • For info on these two values, I covered them in a blog post last week using Python and a paper Anthony Turner and colleagues wrote.

change_standards <- cmj %>%
  group_by(name) %>%
  mutate(test_id = row_number()) %>%
  filter(test_id < 3) %>%
  select(name, test_id, rel_con_peak_power) %>%
  pivot_wider(names_from = test_id,
              names_prefix = "test_",
              values_from = rel_con_peak_power) %>%
  mutate(diff = test_2 - test_1) %>%
  ungroup() %>%
  summarize(TEM = sd(diff) / sqrt(2),
            moderate_change = 0.6 * sd(c(test_1, test_2)))

Building the Shiny App

  • In the user interface, I first create my sidebar panel, allowing the user to select the position group of interest. You’ll notice that this sidebar panel is not within the tab panels, which is why it stands alone and allows us to select a position group that will be retained across all tabs.
  • Next, I set up 2 tabs. Notice that in the first tab (testDay) I include a select input, to allow the user to select the date of interest. In the selected argument I tell shiny to always select the max(cmj$date) so that the most recent session is always shown to the user.
  • The server is pretty straight forward. I commented out where each tab data is built. Basically, it is just taking the user specified information and performing simple data filtering and then ggplot2 charts to provide us with the relevant information.
  • On the testDay plot, we use the meaningful change to shade the region around 0 in grey and we use the TEM around the athlete’s observed performance on a given day to specify the amount of error that we might expect for the test.
  • One the Player Time Series plot we have the athlete’s average line and ±1 SD lines to accompany their data, with points changing color when the week-to-week change exceeds out meaningful change.
### Shiny App -----------------------------------------------------------------------------

## Set up user interface

ui <- fluidPage(
  ## set title of the app
  titlePanel("Team CMJ Analysis"),
  ## create a selection bar for position group that works across all tabs
    selectInput(inputId = "position",
                label = "Select Position Group:",
                choices = unique(cmj$position),
                selected = "Backs",
                multiple = FALSE),
    width = 2
  ## set up 2 tabs: One for team daily analysis and one for player time series
    tabPanel(title = "testDay",
             selectInput(inputId = "date",
                         label = "Select Date:",
                         choices = unique(cmj$date)[-1],
                         selected = max(cmj$date),
                         multiple = FALSE),
             mainPanel(plotOutput(outputId = "day_plt", width = "100%", height = "650px"),
                       width = 12)),
    tabPanel(title = "Player Time Series",
             mainPanel(plotlyOutput(outputId = "player_plt", width = "100%", height = "700px"),
                       width = 12))

server <- function(input, output){
  ##### Day plot tab ####
  ## day plot data
  day_dat <- reactive({
    d <- cmj %>%
      group_by(name) %>%
      mutate(change_power = rel_con_peak_power - lag(rel_con_peak_power)) %>%
      filter(date == input$date,
             position == input$position)
  ## day plot
  output$day_plt <- renderPlot({ day_dat() %>%
      ggplot(aes(x = reorder(name, change_power), y = change_power)) +
      geom_rect(aes(ymin = -change_standards$moderate_change, ymax = change_standards$moderate_change),
                xmin = 0,
                xmax = Inf,
                fill = "light grey",
                alpha = 0.6) +
      geom_hline(yintercept = 0) +
      geom_point(size = 4) +
      geom_errorbar(aes(ymin = change_power - change_standards$TEM, ymax = change_power + change_standards$TEM),
                    width = 0.2,
                    size = 1.2) +
      theme(axis.text.x = element_text(angle = 60, vjust = 1, hjust = 1),
            axis.text = element_text(size = 16, face = "bold"),
            axis.title = element_text(size = 18, face = "bold"),
            plot.title = element_text(size = 22)) +
      labs(x = NULL,
           y = "Weekly Change",
           title = "Week-to-Week Change in Realtive Concentric Peak Power")
  ##### Player plot tab ####
  ## player plot data
  player_dat <- reactive({
    d <- cmj %>%
      group_by(name) %>%
      mutate(avg = mean(rel_con_peak_power),
             sd = sd(rel_con_peak_power),
             change = rel_con_peak_power - lag(rel_con_peak_power),
             change_flag = ifelse(change >= change_standards$moderate_change | change <= -change_standards$moderate_change, "Flag", "No Flag")) %>%
      filter(position == input$position)
  ## player plot
  output$player_plt <- renderPlotly({
    plt <- player_dat() %>%
      ggplot(aes(x = date, y = rel_con_peak_power, label = change)) +
      geom_rect(aes(ymin = avg - sd, ymax = avg + sd),
                xmin = 0,
                xmax = Inf,
                fill = "light grey",
                alpha = 0.6) +
      geom_hline(aes(yintercept = avg - sd),
                 color = "black",
                 linetype = "dashed",
                 size = 1.2) +
      geom_hline(aes(yintercept = avg + sd),
                 color = "black",
                 linetype = "dashed",
                 size = 1.2) +
      geom_hline(aes(yintercept = avg), size = 1) +
      geom_line(size = 1) +
      geom_point(shape = 21,
                 size = 3,
                 aes(fill = change_flag)) +
      facet_wrap(~name) +
      scale_fill_manual(values = c("red", "black", "black")) +
      theme(axis.text = element_text(size = 13, face = "bold"),
            axis.text.x = element_text(angle = 45, vjust = 1, hjust = 1),
            plot.title = element_text(size = 18),
            strip.background = element_rect(fill = "black"),
            strip.text = element_text(size = 13, face = "bold"),
            legend.position = "none") +
      labs(x = NULL,
           y = NULL,
           title = "Relative Concentric Peak Power")

shinyApp(ui, server)

Accessing Garmin Data For Building Your Own Running Report

In our most recent screen cast (TidyX 34) Ellis Hughes and I discussed how to create your own Garmin running report from the raw data collected on your watch.

You might be asking yourself, “Where did they get the raw data from?”

Therefore, I figured I’d put together a short tutorial on accessing raw Garmin data from your account.

  1. First log into your Garmin Connect account. Go HERE.
  2. Once you are signed in, you’ll see a dashboard
  3. Click the Activities drop down and select All Activities to see your workouts
  4. Click on the activity that you want to obtain raw data for
  5. Once inside the activity, click the gear in the upper right of the screen and select Export to TCX

That’s it! Now you have the raw data for that session and you are ready to create your own running reports!

Acute:Chronic Workload & Our Research

Some research that myself and a few colleagues have worked on for the past year (and discussed for far longer than that) regarding our critiques of the acute-chronic workload model for sports injury have been recently published.

It was a pleasure to collaborate with this group of researchers and I learned a lot throughout the process and hopefully others will learn a lot when they read our work.

Below are the papers that I’ve been a part of:

  1. Bornn L, Ward P, Norman D. (2019). Training schedule confounds the relationship between Acute:Chronic Workload Ratio and Injury. A causal analysis in professional soccer and American football. Sloan Sports Analytics Conference Paper.
  2. Impellizzeri F, Woodcock S, McCall A, Ward P, Coutts AJ. (2020). The acute-crhonic workload ratio-injury figure and its ‘sweet spot’ are flawed. SportRxiv Preprints.
  3. Impellizzeri FM, Ward P, Coutts AJ, Bornn L, McCall A. (2020). Training Load and Injury Part 1: The devil is in the detail – Challenges to applying the current research in the training load and injury field. Journal of Orthopedic and Sports Physical Therapy, 50(10): 577-584.
  4. Impellizzeri FM, Ward P, Coutts AJ, Bornn L, McCall A. (2020). Training Load and Injury Part 2: Questionable research practices hijack the truth and mislead well-intentioned clinicians. Journal of Orthopedic and Sports Physical Therapy, 50(10): 577-584.
  5. Impellizzeri FM, McCall A, Ward P, Bornn L, Coutts AC. (2020). Training load and its role in injury prevention, Part 2: Conceptual and Methodologic Pitfalls. Journal of Athletic Training, 55(9). 893-901.

Many will argue and say, “Who cares? What’s the big deal if there are issues with this research? People are using it and it is making them think about training load and it is increasing the conversations about training load within sports teams.”

I understand this argument to a point. Having been in pro sport for 7 years now, I can say that anything which increase conversation about training loads, how players are (or are not) adapting, and the potential role this all plays in non-contact injury and game day performance is incredibly useful. That being said, to make decisions we need to have good/accurate measures. Simply doing something for the sake of increasing the potential for conversation is silly to me. It is the same argument that gets made for wellness questionnaires (which I have also found little utility for in the practical environment).

When we measure something it means we are assigning a level of value to it. There is some amount of weighting we apply to that measurement within our decision making process. Even if we are under the belief that collecting the metric is solely for the purpose of increasing the opportunity to have a conversation with a player or coach. In the back of our minds we are still thinking, “Jeez, but his acute-chronic workload ratio was 2.2 today” or “Gosh, I don’t know. He did put an 8 down (out of 10) for soreness this morning”.

Of course challenging these ideas doesn’t mean we sit on our hands and do nothing. Taking simple training load measures (GPS, Accelerometers, Heart Rate, etc.) and applying basic logic about reasonably safe training load increases from week-to-week, doing some basic analysis to understand injury risks and rates within your sport (how they differ by position, age, etc), and identifying players that might be at higher risk of injury to begin with (IE, higher than the baseline risk) and having a more conservative approach with their progression in training can go a long way. Doing something simple like that, doing well, and creating an easy way to report said information to the coach and player can help increase the chance for more meaningful conversation without using measures that might otherwise give a flawed sense of certainty around injury risk.

Regardless of our work, people will use what they want to use and what they are (or have been) most comfortable with in practice. However, that shouldn’t deter us from challenging our processes, critiquing methodologies, and trying to better understand sport, training, physiology, and athlete health and wellness.

R Tips & Tricks: Building a Shiny Training Load Dashboard

In TidyX Episode 19 we discussed a way of building a dashboard with the {formattable} package. The dashboard included both a data table and a small visual using {sparkline}. Such a table is great when you need to make a report for a presentation but there are times when you might want to have something that is more interactive and flowing. In the sports science setting, this often comes in the form of evaluating athlete training loads. As such, I decided to spin up a quick {shiny} web app to show how easy it is to create whatever you want without sinking a ton of money into an athlete management system.

The code is available on my GITHUB page.

Packages & Custom Functions

Before doing anything, always start by loading the packages you will need for your work. In this tutorial, we will using the {tidyverse} and {shiny} packages, so be sure to install them if you haven’t already. I also like to set my plot theme to classic so that I get rid of grid lines for the {ggplot2} figures that I create.

Finally, I also wrote a custom function for calculating a z-score. This will come in handy when we go to visualize our data.

# custom function for calculating z-score
z_score <- function(x){
  z = (x - mean(x, na.rm = T)) / sd(x, na.rm = T)



Next we simulate a bunch of fake data for a basketball team so that we have something to build our {shiny} app against. We will simulate total weekly training loads for 10 athletes across three different positions (fwd, guard, center), for a 3 week pre-season and a 15 week in-season.


athlete <- rep(LETTERS[1:10], each = 15)
position <- rep(c("fwd", "fwd", "fwd", "fwd", "guard", "guard", "guard", "center", "center", "center"), each = 15)
week <- rep(c("pre_1", "pre_2", "pre_3", 1:12), times = 10)
training_load <- round(rnorm(n = length(athlete), mean = 1500, sd = 350), 1)

df <- data.frame(athlete, position, week, training_load)
df$flag <- factor(df$flag, levels = c("high", "normal", "low"))
df$week <- factor(df$week, levels = c("pre_1", "pre_2", "pre_3", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12")) 

df %>% head()

The first few rows of the data look like this:

Shiny App

There are two components to building a {shiny} app:

1) The user interface, which defines the input that the user specifies on the web app.
2) The server, which reacts to what the user does and then produces the output that the user sees.

The user interface I will define as tl_ui (training load user interface). I start by creating a fluid page (since the user is going to be defining what they want). I set the parameters of what the user can control. Here, I provide them with the ability to select the position group of interest and then the week(s) of training they would like to see in their table and plot.

The server, which I call tl_server (training load server), is a little more involved as we need to have it take an input from the user interface , get the correct data from our stored data frame, and then produce an output. This part can be a bit involved. Below you can see the code and I’ll try and articulate a few of the main things that are taking place.

1) First, I get the data for the visualization the user wants. I’m using our simulated data and I calculate the z-score for each individual athlete, using the custom function I wrote. After that I create my flag (±1 SD from the individual athlete’s mean) and then I finally filter the data specific to the inputs provided in the user interface. This last part is critical. If I filter before applying my z-score and flags, then the mean and SD used to calculate the z-score will only be relative to the data that has been filtered down. This probably isn’t what we want. Rather, we will want to calculate our z-score using all data, season-to-date. So we retain everything and perform our calculation and then we filter the data.

2) After getting the data, I build my plot using {ggplot2}.

3) Then I create another data set specific to the table weeks of training selected by the user. This time, we will retain the raw values (non-standardized) for each athlete as the user may want to see raw values in a table. Notice that I also pivot that data using the pivot_wider() function as the data is currently stored in a long format with each row representing a new week for the athlete. Instead, I’d like the user to see this data across their screen, with each week representing a new column. As the user selects new weeks of data to visualize our server will tell {shiny} to populate the next column for that athlete.

4) Once I have the data we need, I simply render the table.

Those 2 steps complete the building of our {shiny} app!

Finally, the last step is to run the {shiny} app using the shinyApp() function which takes two arguments, the name of your user interface (tl_ui) and the name of your server (tl_server). This will open up a shiny app in a new window within R Studio that you can then open in your browser.

Visually, you can see what the app looks like below. If you want to see it in action, aside from getting my code and running it yourself, I put together this short video >> R Tips & Tricks – Training Load Dashboard


As you can tell, it is pretty easy to build and customize your own {shiny} app. I’ll admit, the syntax for {shiny} can get a bit frustrating but once you get the hang of it, like most things, it isn’t that bad. Also, unless you are doing really complex tasks, most of the syntax for something as simple as a training load dashboard wont be too challenging. Finally, such a quick and easy method not only allows you to customize things exactly as you want them, but it also saves you money from having to purchase an expensive athlete management system.

As always, you can obtain the code for this tutorial on my GITHUB page.