Enrolled participants will begin dosing of tirasemtiv 125 mg twice daily (250 mg/day) for a period of 4 weeks and will titrate to their tolerated dose, the maximum dose being 250 mg twice daily (500 mg/day). This study will also compare the clinical course of patients who completed treatment with tirasemtiv in CY 4031 with those who completed treatment with placebo in CY 4031 during continued treatment of both groups with tirasemtiv during CY 4033, compare the clinical course of patients who completed treatment with tirasemtiv in CY 4031 during that study with their clinical course during continued treatment with tirasemtiv during CY 4033, and compare the clinical course of patients who completed treatment with placebo in CY 4031 during that study with their clinical course during treatment with tirasemtiv during CY 4033.