Quadriceps Muscle Activation and Oxygenation Responses During Usual Care Compared to Individualized Interval and Constant Work Rate Bed-cycling. An Experimental Study.

\### Study Summary - AcuteBike-ICU Bed-cycling is commonly used as part of early rehabilitation in critically ill patients in the intensive care unit (ICU). However, the extent to which usual care bed-cycling activates the quadriceps muscles remains unclear, as does whether individualized exercise protocols can increase muscle stimulation. The AcuteBike-ICU study is an experimental study aiming to compare the acute physiological responses to three bed-cycling protocols in critically ill patients: usual care constant work-rate cycling (u-CWR), individualized constant work-rate cycling (i-CWR), and individualized interval training (i-IT). A total of 29 critically ill patients admitted to medical or surgical ICUs will be included. Participants will undergo an incremental loading test to determine their individual exercise capacity, followed by the different cycling protocols. The individualized continuous protocol involves cycling against a moderate resistance, whereas interval training alternates short periods of high-intensity cycling with passive cycling. The primary outcome is quadriceps muscle activation, assessed using surface electromyography (sEMG) across four quadriceps muscle regions. Secondary outcomes include muscle fatigue, activation heterogeneity, muscle oxygenation and metabolism assessed using near-infrared spectroscopy (NIRS), as well as safety, feasibility, and patient-reported symptoms. Retrospective data from routine bed-cycling sessions will also be analyzed to characterize current clinical practice. Each participant will complete the study within a maximum of five days. By identifying which cycling protocol elicits the greatest quadriceps muscle stimulation, this study aims to support the development of more individualized and potentially more effective early rehabilitation strategies for critically ill patients.

Inclusion criteria * Voluntary written informed consent of the participant has been obtained prior to the initiation of any study-related measurements * Adults (≥18 years) * Mechanically ventilated for ≥ 48 hours consecutively * Fully cooperative, as assessed daily during routine clinical care by ICU physiotherapists using five standardized commands ("open/close your eyes", "look at me", "open your mouth and stick out your tongue", "nod your head", and "raise your eyebrows when I have counted to 5"). * Able to perform active cycling for \>10 consecutive minutes * Expected to remain in the ICU for more than an additional 4 days starting from study enrolment. Exclusion criteria MEDICAL STABILITY A. Hemodynamic instability * Evidence of acute coronary ischemia * Heart rate \120 beats per minute * Mean arterial pressure \110 mmHg * Noradrenaline or comparable inotropic or vasopressor support (norepinephrine \>0.2ug/kg/min, dobutamine \>5ug/kg/min, corotrope \>0.2 ug/kg/min) B. Respiratory instability * Fraction of inspired oxygen ≥ 60% * PaO₂/FiO₂ ratio \< 200 * Positive end-expiratory pressure \> 10 cmH2O * Respiratory rate \> 30 breaths·min-¹ C. Surgery within the past 24h D. Body temperature \> 40 oC CONDITIONS IMPAIRING CYCLING MOVEMENT * Trauma or surgery to lower limb, pelvis lumbar spine * Extreme obesity (body mass index \>35 kg/m2) * Extensive abdominal wounds or pressure ulcers * Continuous renal replacement therapy by CVVH-femoral catheter PALIATIVE CARE