Myosin has been a focus in the muscle field for a long time, yet there are still many major unanswered questions about this motor protein. One emerging focus is on the super-relaxed state (SRX) of myosin. It has been previously reported that the SRX is involved in resting skeletal muscle thermogenesis and in cardiac muscle contractility. Furthermore, there is evidence that estrogen affects these systems as well. I, therefore, am interested in establishing the structural and functional regulations of the SRX myosin with respect to estrogen. Using quantitative epifluorescence microscopy of fluorescent ATP, I examine the age and sex dependence of myosin ATPase activity in mice. I am also designing site-directed spectroscopic probes on myosin and other myofilament proteins in SRX fibers to help determine their changes in protein structures and dynamics in response to estrogen signaling. While SRX myosin has been elusive from recent attempts to characterize it, I hope to make significant progress in the field which will undoubtedly provide more insights to further studies of muscle diseases.