Ultrasound-derived changes in thickness of human ankle plantar flexor muscles during walking and running are not homogeneous along the muscle mid-belly region

Skeletal muscle thickness is a valuable indicator of several aspects of a
muscle’s functional capabilities. We used computational analysis of
ultrasound images, recorded from 10 humans walking and running at a range
of speeds (0.7 – 5.0 m s-1), to quantify interactions in thickness change
between three ankle plantar flexor muscles (soleus, medial and lateral
gastrocnemius) and quantify thickness changes at multiple muscle sites
within each image. Statistical analysis of thickness change as a function
of stride cycle (1d statistical parametric mapping) revealed significant
differences between soleus and both gastrocnemii across the whole stride
cycle as they bulged within the shared anatomical space. Within each
muscle, changes in thickness differed between measurement sites but not
locomotor condition. For some of the stride, thickness measures taken from
the distal-mid image region represented the mean muscle thickness, which
may therefore be a reliable region for these measures. Assumptions that
muscle thickness is constant during a task, often made in musculoskeletal
models, do not hold for the muscles and locomotor conditions studied here
and researchers should not assume that a single thickness measure, from
one point of the stride cycle or a static image, represents muscle
thickness during dynamic movements.