### Summary

The following data set contains the raw MRI (T1-weighted) and DTI scans of the dominant lower leg

of 32 adults (males, females, young adults, older adults). These data were used to analyse the

regional variation in muscle fibre lengths in the medial and lateral gastrocnemius muscles. The T1-

weighted scans were used to segment the muscles and the DTI scans were used to reconstruct the

muscle fibre architecture. For the latter, a previously published pipeline was implemented (Bolsterlee

et al., 20191). The Matlab scripts that were then used to further process and assign the reconstructed

muscle fibres to specific muscle regions are also available with the data set. Below is additional

information that can be used complementary to the information provided in the manuscript. The first

section describes the implementation of the previously published pipeline for fibre tracking 1. While

this may provide some additional information to help describe the structure of the current data set,

we recommend the documentation originally published for this pipeline1. The second section

provides additional information to the reasoning and implementation of further analyses to assess

the regional muscle fibre lengths from the data. Please cite the appropriate sources when using any

of these analysis scripts or data.