As frailty in older patients with acute heart failure (AHF) has an adverse effect on clinical outcomes, the addition of electrical muscle stimulation (EMS) to exercise-based early rehabilitation may improve the effects of treatment. Post hoc analysis was performed on a randomized controlled study for clinical outcomes and prespecified subgroups (ACTIVE-EMS: UMIN000019551). In this trial, 31 AHF patients aged ≥ 75 years with frailty (Short Physical Performance Battery [SPPB] score 4–9) were randomized 1:1 to receive treatment with an early rehabilitation program only (n = 16) or early rehabilitation with add-on EMS therapy (n = 15) for 2 weeks. Changes in physical function and cognitive function between baseline and after two weeks of treatment were assessed. There were no adverse events during the EMS period. The EMS group showed significantly greater changes in quadriceps’ isometric strength and SPPB compared to the control group, and EMS therapy showed uniform effects in the prespecified subgroups. There were no significant differences in the changes in other indexes of physical function and cognitive function between groups. There was no significant difference in the rate of heart failure hospitalization at 90 days between groups. In conclusion, older AHF patients with frailty showed greater improvement in lower extremity function with the addition of EMS therapy to early rehabilitation without adverse events.