A clinical trial of exercise-similar heart rate acceleration delivered via cardiac pacing vs. sham intervention in subjects at rest will be performed. The study population comprises subjects with guideline-directed medically managed left ventricular dysfunction due to ischemic or non-ischemic cardiomyopathy and an existing implantable cardioverter defibrillator or biventricular implantable cardioverter defibrillator. The purpose of the study is to understand how the heart rate pattern of exercise contributes to the considerable cardiac conditioning effects of exercise and estimate whether the pacing approach may have translational clinical applicability. Fifty-two subjects will be randomized, single-blinded, to either the pacing intervention or a sham intervention which they will receive once daily, 3 days/week for 6 weeks. Baseline symptoms and clinical test results will be compared to the same measures at 2 weeks, 4 weeks and 6 weeks of intervention/sham and at 3 months and one-year post-intervention. The primary endpoint will be the change in left ventricular ejection fraction from baseline in intervention vs. sham groups (mixed effects linear regression with time and treatment arm as fixed effects and pre-specified covariates of sex and cardiomyopathy type as random effects). Secondary endpoints will include changes in quality of life, 6-minute walk distance, cardiopulmonary exercise test (CPET) measures, daily activity and major adverse cardiac events (MACE) at 3 and 12 months between pacing and sham groups. A dose-response analysis of outcomes at 2, 4, and 6 weeks of the intervention vs. sham compared with baseline will be performed.