The objective of this randomized clinical trial is to compare the effects of a standardized exercise program alone versus the same program combined with neuromuscular electrical stimulation in patients undergoing heart failure . The main questions it aims to answer are:

• Assessing the ultrasonographic parameters: echo intensity (echogenicity), cross-sectional area, thickness, and pennation angle of the rectus femoris muscle in both lower limbs.
• Evaluating the strength of the femoral quadriceps muscle
• Evaluating the changes in the chronaxie of the rectus femoris muscle in both lower limbs.

The protocol will have a total duration of 32 days, with an initial intervention period of 18 days, followed by a 14-day follow-up period.

- Introduction: Patients with refractory heart failure who develop cardiogenic shock may require the use of an intra aortic balloon pump. This device, when inserted through the femoral artery, necessitates bed rest and increases the incidence of muscle mass loss. Rehabilitation programs in the intensive care unit that include resistive training bring benefits such as improving muscle architecture, strength, and function. Neuromuscular electrical stimulation (NMES) can be an important tool to assess excitability and improve muscle strength through muscle contractions generated by motor axon depolarization. Muscle ultrasonography is currently used as a tool for evaluating and monitoring muscle architecture (echointensity) and mass.

- Objectives: To evaluate the effect of combining NMES with a standardized exercise program on the echointensity (echogenicity) of the rectus femoris muscle in hemodynamically stable patients using an intra aortic balloon pump.

- Method: This is a randomized controlled clinical trial comparing two groups: one group will perform resisted knee extension exercises in both lower limbs, while the other group will perform the same exercises synchronously with NMES of the femoral quadriceps muscle. The resistance load will be determined by the one-repetition maximum test and adjusted if necessary. The intervention period will be 21 days (5 days/week) with a follow-up of 14 days. Muscle architecture and mass (ultrasonography), neuromuscular excitability (using the same NMES device), and knee extension strength (dynamometry) will be assessed at baseline, after 10 and 15 days of intervention and after 14 days from the last intervention day.

Inclusion Criteria:

• Age ≥ 18 years
• Intra-aortic balloon pump (IABP) for more than 48 hours
• Dobutamine ≤ 20 mcg/kg/min
• Norepinephrine ≤ 0.2 mcg/kg/min (35)
• Absence of device failures or bleeding in the last 24 hours
• Mean arterial pressure (MAP) ≥ 60 mmHg and ≤ 120 mmHg
• Heart rate (HR) ≥ 60 bpm and ≤ 120 bpm
• Absence of neurological event with previous cognitive or motor deficit
• Presence of untreated deep venous thrombosis
• Absence of previous autoimmune diseases
• Absence of previous rheumatic diseases

Protocol Discontinuation Criteria:

• Need for norepinephrine > 0.2 mcg/kg/min
• Acute arrhythmia of any etiology with hemodynamic instability
• Hemodynamic instability: MAP < 60 mmHg or >120 mmHg or HR < 60 bpm or > 120 bpm
• Occurrence of neurological event with cognitive or motor deficit

Exclusion Criteria:

• Need for invasive mechanical ventilatory support
• If the patient or responsible family member fails to sign or withdraws the informed consent

The researcher responsible for conducting the ultrasonographic measurements will be blinded regarding the randomization and allocation groups.