A cardiac sarcomere activator (CSA) for use in a method of treating a subject with heart failure (HF), wherein the CSA is omecamtiv mecarbil (OM), or a pharmaceutically acceptable salt thereof, wherein the method comprises: (a) administering to the subject an initial dose of the CSA for an initial time period, wherein the initial time period is at least about 1 week, or at least about 2 weeks, and the initial dose is 25 mg, administered to the subject twice daily; and (b) subsequently administering to the subject twice daily (i) about 25 mg of the CSA, when the subject's plasma concentration of the CSA measured or determined during or at the end of the initial time period is greater than or about 300 ng/mL, (ii) about 37.5 mg of the CSA, when the subject's plasma concentration of the CSA measured or determined during or at the end of the initial time period is greater than or about 200 ng/mL but less than 300 ng/mL, or (iii) about 50 mg, when the subject's plasma concentration of the CSA measured or determined during or at the end of the initial time period is less than 200 ng/mL. wherein the CSA is omecamtiv mecarbil (OM), or a pharmaceutically acceptable salt thereof, wherein the method comprises: (a) administering to the subject an initial dose of the CSA for an initial time period, wherein the initial time period is at least about 1 week, or at least about 2 weeks, and the initial dose is 25 mg, administered to the subject twice daily; and (b) subsequently administering to the subject twice daily (i) about 25 mg of the CSA, when the subject's plasma concentration of the CSA measured or determined during or at the end of the initial time period is greater than or about 300 ng/mL, (ii) about 37.5 mg of the CSA, when the subject's plasma concentration of the CSA measured or determined during or at the end of the initial time period is greater than or about 200 ng/mL but less than 300 ng/mL, or (iii) about 50 mg, when the subject's plasma concentration of the CSA measured or determined during or at the end of the initial time period is less than 200 ng/mL. (a) administering to the subject an initial dose of the CSA for an initial time period, wherein the initial time period is at least about 1 week, or at least about 2 weeks, and the initial dose is 25 mg, administered to the subject twice daily; and (b) subsequently administering to the subject twice daily (i) about 25 mg of the CSA, when the subject's plasma concentration of the CSA measured or determined during or at the end of the initial time period is greater than or about 300 ng/mL, (ii) about 37.5 mg of the CSA, when the subject's plasma concentration of the CSA measured or determined during or at the end of the initial time period is greater than or about 200 ng/mL but less than 300 ng/mL, or (iii) about 50 mg, when the subject's plasma concentration of the CSA measured or determined during or at the end of the initial time period is less than 200 ng/mL. (i) about 25 mg of the CSA, when the subject's plasma concentration of the CSA measured or determined during or at the end of the initial time period is greater than or about 300 ng/mL, (ii) about 37.5 mg of the CSA, when the subject's plasma concentration of the CSA measured or determined during or at the end of the initial time period is greater than or about 200 ng/mL but less than 300 ng/mL, or (iii) about 50 mg, when the subject's plasma concentration of the CSA measured or determined during or at the end of the initial time period is less than 200 ng/mL. <EOS>