In skeletal, cardiac, and smooth muscle fibers, we are also examining the relationships between contractile protein expression and both mechanical and energetic properties. In particular, we are interested in the expression of different isoforms of myosin heavy chain which form cross-bridges with actin during force development and contraction. The expression of different myosin heavy chain isoforms also responds to conditions of altered use and thereby affects the mechanical and energetic properties of muscle fibers. Calcium binding to troponin C and the regulation of cross-bridge formation is also an area of interest within our laboratory.