Up to 60% of patients admitted to the Intensive Care Unit (ICU) with a prolonged stay in the ICU develop complications such as intensive care unit acquired weakness (ICUAW) characterized by limb and respiratory muscle weakness. ICUAW is associated with worse prognosis, longer ICU stay and increased morbidity and mortality.

Physical therapy (PT) interventions in the intensive care unit (ICU), can improve patients' outcomes.

However, improvements in muscle function achieved with standard physical activity interventions aiming at early mobilization are highly variable due to lack of consistency in definition of the interventions, lack of consideration for the complexity of exercise dose and/or insufficient stimulation of muscles during interventions. It has been suggested that modifying early mobilization and exercise protocols towards shorter intervals consisting of higher intensity exercises might result in more optimal stimulation of muscles.

In the present study the researchers therefore aim to simultaneously assess (by non-invasive technologies) locomotor muscle oxygenation and activation along with the measurements of the load imposed on respiration and circulation during two different training modalities i.e., moderate intensity continuous bed-cycling (endurance training) vs high-intensity alternated by lower intensity periods of bed-cycling (interval training).

Critical illness is related to high morbidity and mortality rates, and health-care costs. Up to 60% of patients admitted to the Intensive Care Unit (ICU) with a prolonged stay in the ICU develop complications such as intensive care unit acquired weakness (ICUAW) characterized by limb and respiratory muscle weakness. These abnormalities develop already within the first days to weeks after intensive care unit (ICU) admission and are related to immobility, sepsis, inflammatory response syndrome (SIRS), prolonged mechanical ventilation, multiple organ failure, and the use of corticosteroids. ICUAW is associated with worse prognosis, longer ICU stay and increased morbidity and mortality. Survivors of critical illness frequently report long-term physical impairments persisting up to 5 years after discharge.

Physical therapy (PT) interventions in the intensive care unit (ICU), can improve patients' outcomes. A systematic review of randomized controlled trials (RCTs) of strategies to improve physical functioning of ICU survivors identified the importance of PT interventions in the ICU. Early rehabilitation during ICU admission has the potential to result in important clinical benefits for patients. These findings highlight the importance of aiming to apply mobilization strategies early during ICU stay to maintain and improve physical functioning as good as possible.

With a projected increase in the number of critically ill patients, requiring rehabilitation in the ICU effective and efficient rehabilitation interventions are warranted. However, improvements in muscle function achieved with standard physical activity interventions aiming at early mobilization are highly variable. Therefore, there is a need for implementing more evidence-based PT interventions, as part of routine clinical practice. Variable results of current interventions may be due to lack of consistency in definition of the interventions, lack of consideration for the complexity of exercise dose and/or insufficient stimulation of muscles during interventions. It has been suggested that modifying early mobilization and exercise protocols towards shorter intervals consisting of higher intensity exercises might result in more optimal stimulation of muscles.

A recent study evaluating a cohort of 181 consecutive patients receiving 541 in-bed cycling sessions as part of routine PT interventions in ICU showed that constant-load bed-cycling appears to be both feasible and safe. In addition, recent evidence in patients with chronic lung disease shows that acute alteration of intense and less intense periods of exercise induced partial restoration of local muscle oxygen stores during the less intense periods of exercise facilitating the muscles to achieve higher exercise intensities during the intense periods, compared to constant-load submaximal exercise. Hence, in patients with chronic lung diseases, alternating intense with less intense loads during interval exercise may be physiologically more effective than constant submaximal workloads maintained during endurance type training for achieving a higher stimulation of locomotor muscles. This has not been investigated so far in intensive care unit patients.

Inclusion Criteria:

• Full cooperatively adult patients indicated by the Adequacy Score of standardized 5 questions (SQ5) = 5/5
• Patients mechanically ventilated for longer than 48 hours during the same ICU admission
• Patients are expected to remain in the ICU for more than an additional 48 hours starting from study enrollment
• Patients able to perform active cycling for > 10 consecutive minutes

Exclusion Criteria:

• Pre-existing functional limitations
• Low limb injuries or conditions that would preclude in-bed cycling such as a body habitus unable to fit the bike
• Extreme obesity (body mass index >35 kg/m2)
• Neurologically unstable
• Acute surgery
• Palliative goals of care
• Temperature > 40 °C
• An anticipated fatal outcome
• Evidence of coronary ischaemia, for example, chest pain or electrocardiogram changes
• Resting heart rate <40 or >120 beats per minute
• Mean arterial pressure <60 or >120 mmHg
• Peripheral capillary oxygen saturation < 90%
• Wounds, trauma or surgery of leg precluding cycle ergometry
• Wounds, trauma or surgery of pelvis precluding cycle ergometry
• Wounds, trauma or surgery of lumbar spine precluding cycle ergometry
• Coagulation disorder (international normalised ratio > 1.8, or platelets < 50,000 mcL)
• Intracranial pressure >20 mm Hg
• Femoral access other than femoral central line
• Acute deep vein thrombosis
• Pulmonary embolism
• >20 mcg/min of noradrenaline
• inotropic or vasopressor support comparable to a dose of noradrenaline >20mcg/min
• Fraction of inspired oxygen > 55%
• Arterial partial pressure of oxygen (PaO2) <65 torr (<8.66 kPa)
• Positive end-expiratory pressure > 10 cmH2O
• Respiratory rate > 30 breaths per minutes with adequate ventilatory support
• Minute ventilation >150 mL/kg body weight