Anomaly of anatomical origin of soleus muscle

In the laboratory mouse, the soleus muscle arises at the head of the fibula and inserts via the Achilles tendon on the tuber calcanei together with the gastrocnemius muscle. During routine dissection of mice from the BXD recombinant inbred (RI) strains, we found that the soleus often originated from the lateral epicondyle of the femur instead of the head of the fibula. This soleus femoral attachment anomaly (SFAA) changes the soleus from being a single-joint to a two-joint muscle. The incidence of SFAA was 45% in the BXD38 RI strain. Bilateral inspection indicated that SFAA may be present unilaterally or bilaterally within an individual mouse. We explored the effect of SFAA on muscle weight in mice with unilateral expression. The weight of SFAA soleii was significantly less (P <0.01) than that of the soleii with normal attachment by 6% (females) and 14% (males). Similar anatomical anomalies of the soleus muscle have been noted in humans. The mouse model will provide the means to explore the physiological consequences and genetic basis for such anomalies.