Electromyographic and Anatomical Evaluation of the Accessory Soleus Muscle and Its Clinical Implications

International Journal of Anatomy and Physiology | Vol. 15, No. 9, September 2026 | pp. 37–45

DOI: 10.46882/2026/IJAP/001005

Research Article

Title: Electromyographic and Anatomical Evaluation of the Accessory Soleus Muscle and Its Clinical Implications

Names of Authors: F. G. Santos¹, H. I. Takahashi², K. M. O’Connor³*

Authors’ Affiliations:
¹ Department of Morphology, Federal University of São Paulo, São Paulo, Brazil.
² Department of Anatomy, Osaka University Graduate School of Medicine, Osaka, Japan.
³ Department of Physical Therapy, Marquette University, Milwaukee, Wisconsin, USA.

Abstract:
The accessory soleus muscle is a rare anatomical variant located in the deep posterior compartment of the leg, which can present as a soft tissue mass in the posteromedial ankle region. It can cause chronic pain, swelling, or tarsal tunnel syndrome during physical activity. This study combined cadaveric dissection and surface electromyography (sEMG) to characterize the morphology and functional behavior of this muscle variant. Dissection of 80 formalin-fixed lower limbs revealed an accessory soleus muscle in 3 extremities (3.75%). All instances originated from the anterior surface of the soleus muscle and inserted into the medial surface of the calcaneus via a distinct tendon. In the clinical phase, five symptomatic athletes diagnosed with an accessory soleus via Magnetic Resonance Imaging (MRI) underwent sEMG evaluation during various ankle movements. Muscle activity was recorded during plantarflexion, dorsiflexion, inversion, and eversion, and compared against ten healthy controls. The sEMG findings demonstrated that the accessory soleus muscle displayed a high root-mean-square (RMS) voltage during sustained isometric plantarflexion, contributing significantly to joint torque but exhibiting premature fatigue compared to the normal soleus (p < 0.05). The co-activation of this anomalous muscle during forced ankle movements suggests that its hypertrophy and structural location can limit space in the posterior compartment, compressing the tibial nerve. Clinicians should consider this variant in cases of exertional ankle pain.

Keywords: Accessory soleus muscle, Anatomical variant, Dissection, Surface electromyography, Tarsal tunnel syndrome.

Manuscript Timeline: Received: June 03, 2026; Revised: July 15, 2026; Accepted: August 10, 2026; Published: September 19, 2026.

Citation: Santos FG, Takahashi HI, O’Connor KM. Electromyographic and Anatomical Evaluation of the Accessory Soleus Muscle and Its Clinical Implications. International Journal of Anatomy and Physiology. 2026; 15(9): 37–45. doi:10.46882/2026/IJAP/001005

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