Kinugasa, R., Kawakami, Y., & Fukunaga, T. (2006). Load-specific distribution of muscle activity in human triceps surae muscles. Medicine and Science in Sports and Exercise, 38(5), Supplement abstract 2127.

"Motor units are arranged topographically within some muscles, and are activated in the order of their size, which enables individual regions to be activated in contraction. The triceps surae muscles are composed of the gastrocnemius and soleus muscles. The gastrocnemius contains a higher percentage of type II fibers than that in the soleus. [It was] hypothesized that these two muscles show different distribution characteristics of muscle activity".

This study compared the three-dimensional distribution of muscle activity in the medial gastrocnemius and soleus during plantar flexion exercises at two different loads. Males (N = 6) performed seven sets of ten repetitions of plantar flexion exercises with loads that were 25 or 75% of their 12 repetitions maximum.

The percentage area of activated muscle was significantly larger after exercise with a heavy load (~53%) than with a light load (~35%) in the gastrocnemius. The soleus did not show such differences.

Implication. The increase in load of repetitive plantar flexion exercises was accompanied by increased activations in the gastrocnemius, especially at around the region of proximal portion and at throughout in the transversal and longitudinal axes. The load-specific distribution characteristics of muscle activity were different between medial gastrocnemius and soleus. The layman's assumption that muscles act similarly in the same exercise is erroneous.

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