Neuromuscular electrical stimulation (NMES) implemented during the intensive care unit(ICU) stay may improve exercise capacity and prevent muscular weakness in critically ill patients with respiratory disease. Main objective: To evaluate, in respiratory critically ill patients, the effects of a conventional physiotherapy program with or without additional NMES applied only during the ICU stay on exercise capacity. Design: Randomized, controlled, double-blind clinical trial on patients (>18yo) with diagnosis on admission of acute or acute on chronic respiratory disease, on mechanical ventilation (MV) > 72h and expected MV > 24h. Intervention: Conventional physiotherapy with or without 30-minutes (5days/week) on quadriceps femoris and gluteus maximus. Patients in the control group will follow the same protocol but the device will not be activated. Measures: Demographic data, body composition through bioelectrical impedance analysis (BIA), and functional capacity before admission through Barthel scale will be registered upon inclusion. Exercise capacity through test sit-to stand, muscular strength through Medical Research Council (MRC) score and dynamometry, body composition through BIA, and functional capacity through Barthel scale will be obtained at ICU and hospital discharge.

Inclusion Criteria:

• Diagnosis at admission: acute or acute on chronic respiratory disease
• Mechanical ventilation > 72h at inclusion
• Expected mechanical ventilation > 24h
• Informed consent

Exclusion Criteria:

• Patients re-admitted to ICU (no hospital discharge)
• Exitus or transfer to another service/hospital
• Respiratory instability [Inspired fraction of oxygen (FIO2) >60% or positive end-expiratory pressure (PEEP) >10 cmH2O] and/or hemodynamic instability [Mean arterial pressure ( MAP)<60mmHg although vasoactive drugs] during > 24h.
• Pregnancy
• Neuromuscular disease before admission
• Skin lesions
• Limitation of therapeutic effort
• Mental and/or physical limitation to understand/realize tests for evaluation