Morphological changes in the long axis of the lower leg muscles during isometric contraction in the sitting position.
Ryo Miyachi, Toshiaki Yamazaki, Naoki Ohno and 1 others
PMID 33362354WHAT IT FOUND
During seated plantar flexion, muscle cross-sectional area shifts along the leg rather than swelling uniformly.
The soleus and medial gastrocnemius shrink in the mid-belly while the tibialis anterior expands. This means measuring thickness at one spot can miss what is actually happening.
Key findings
01The soleus muscle cross-sectional area decreased significantly in the middle portion during contraction compared to rest.
02The medial gastrocnemius cross-sectional area increased at the most proximal portion but decreased at the 1/4 distal portion during contraction.
03The tibialis anterior cross-sectional area increased significantly in the middle portion during plantar flexor contraction, likely due to antagonistic co-contraction.
STILL TO COME
How it was doneWhat they foundWhat it means for PTs
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What it does not show
The study included only healthy young women, so it is unclear if these morphological shifts occur in older adults or those with ankle injuries. Deep leg muscles were not measured, so their contribution to the observed changes is unknown. The study measured anatomical cross-sectional area, not physiological cross-sectional area, which ignores changes in pennation angle. Muscle activity was not measured with electromyography, so the exact level of activation driving these shape changes is inferred rather than recorded.
Declared interests
The work was supported by a JSPS KAKENHI grant. The authors declared no potential conflicts of interest.
The easy way to misread this
Do not interpret a decrease in muscle thickness at a standard ultrasound site as a sign of weakness or atrophy. In this study, the soleus and medial gastrocnemius actually got thinner in certain regions during contraction because the muscle belly shifted along the leg.