The workloopR package also provides a function that can calculate the timing and magnitude of force during isometric experiments (twitch, tetanus) via the isometric_timing() function.

To demonstrate, we will first load workloopR and use example data provided with the package. We’ll also load a couple packages within the tidyverse as well as viridis to help with data wrangling and plotting.


We’ll now import the twitch.ddf file included with workloopR.

Let’s plot Force vs. Time to visualize the time course of force development and relaxation.

This plot reveals that the final row of the data has Force = 0 and is likely an artifact. We can also see that the most salient parts of the twitch occur between ~ 0.075 and ~ 0.2 seconds.

We’ll just re-plot the salient parts of the twitch by setting new limits on the axes via ggplot2::xlim() and ggplot2::ylim(). Please note that this will not change any analyses - we are simply doing it for ease of visualizing patterns.

Looks better!

Basics of isometric_timing()

Now we’re ready to use isometric_timing().

The function returns a new data.frame that provides information about the timing and magnitude of force at various intervals within the twitch. All returned values are absolute; in other words, time is measured from the beginning of the file and forces are returned in their actual magnitudes.

The first five columns of this data.frame are fixed. They will return (in this order): 1) the ID of the file, 2) the time at which stimulation occurs, 3) magnitude of force when stimulation occurs, 4) time at which peak force occurs, and 5) magnitude of peak force.

The function also provides data that help describe the rising and the relaxation phases of the twitch at certain “set points”. By default, in the rising phase the set points are at 10% and at 90% of peak force development. Timing and force magnitudes at these points are returned as columns in the data.frame. And for the relaxation phase, the time and magnitude of force when force has relaxed to 90% and 50% of peak force are given.

The user has some flexibility in specifying how data are grabbed from the rising and falling phases. There are two arguments: rising = c() and falling = c(). Each of these arguments can be filled with a vector of any length. Within the vector, each of these “set points” must be a vector between 0 and 100, signifying the % of peak force development that is to be described.

For example, if we’d like to describe the rising phase at six points (e.g. 5%, 10%, 25%, 50%, 75%, and 95% of peak force development):

Tetanus trials

The isometric_timing() function can also work on tetanus objects that have been imported via read_ddf(). Should a tetanus object be used, the set points for relaxing are automatically set to relaxing = c(), which excludes this argument from producing anything. Instead, the timing & magnitude of force at stimulation, peak force, and specified points of the rising phase are returned. The idea of ‘relaxation’ is simply ignored.

To demonstrate, we’ll use an example tetanus trial included in workloopR:

Computing intervals

The returned data.frame provides all timing and force magnitudes in absolute terms, i.e. time since the start of the file and actual force magnitudes. Often, we’d like to report characteristics of the twitch as intervals.

To calculate, e.g. the interval between stimulation and peak force (often reported as “time to peak force”):