Based on a 2003 JEP paper by SUKHO LEE AND ROGER P. FARRAR it was the flexor hallucis longus (FHL). RESISTANCE TRAINING INDUCES MUSCLE-SPECIFIC CHANGES IN MUSCLE MASS AND FUNCTION IN RAT. Sukho Lee, Roger P. Farrar. JEPonline. 2003;6(2):80-87
I have worked with mice under exercise. Maybe this article published by my research group can help you for your purpose:
Light-emitting diode therapy in exercise-trained mice increases muscle performance, cytochrome c oxidase activity, ATP and cell proliferation. Journal of Biophotonics. 11/2014; 9999. DOI: 10.1002/jbio.201400087