Background: Patients in the intensive care unit (ICU) are the sickest in hospital, and need advanced life-support. Survivors of critical illness are very vulnerable to weakness and disability. Up to 1 in 4 have severe leg weakness impairing their quality of life for as long as 5 years after ICU discharge. In-bed cycling employs special equipment that attaches to a patient's hospital bed, allowing them gentle leg exercise while in the ICU. The investigators will offer patients in-bed cycling while they are needing a breathing machine, to help them recover from weakness as fast as possible. Before doing a larger trial testing if in-bed cycling reduces disability, data on safety (Is it safe?) and feasibility (Can the investigators do it?) are needed.