The investigators have designed a 2-center, pilot feasibility, randomized controlled trial (PROXIMUS) to determine the feasibility and safety of a larger multi center, randomized open-label trial comparing high protein combined with individualized exercise vs. standard management during the acute phase of critical illness in children. The investigators aim to determine the impact of the intervention on preservation of muscle mass; and functional status at 1 month and 6 months after randomization.

Mortality in U.S. pediatric intensive care units (PICUs) is improving, resulting in an increased focus on the quality of life of survivors who often have persistent physical, cognitive, and psychological impairments. Mechanically ventilated children in particular experience immobilization and poor nutrient intake, which results in muscle loss during acute illness. Early mobility and optimal nutrition during acute critical illness have been associated with muscle mass preservation and decreased duration of mechanical ventilation. These interventions may have the potential to decrease preventable PICU-acquired morbidities. However, the optimal doses and long-term benefits of these interventions have not been shown in randomized studies of critically ill PICU patients.

Thus, the investigators will test the first pediatric-specific, inter-professional intervention that integrates high protein and individualized exercise. This intervention was developed by a team of experts in the fields of nutrition, early mobility, rehabilitative exercise, muscle ultrasound (US), and functional outcomes.

The investigators propose a 2-center, pilot feasibility, randomized controlled trial (PROXIMUS) with the following Specific Aims:

1. To determine the feasibility and safety of a randomized open-label trial comparing high protein combined with individualized exercise vs. standard management during the acute phase of critical illness in children;
2. To determine the impact of high protein and individualized exercise on preservation of muscle mass; and
3. To determine the impact of high protein and individualized exercise on functional status at 1 month and 6 months after randomization.

Outcomes: The investigators will assess feasibility with a composite measure that combines predetermined thresholds for screening, enrollment, allocation, and compliance/adherence with significant separation in doses of protein and exercise between the two arms. Safety will be defined as absence of difference in rates of new renal injury and exercise related adverse events.

Preservation of muscle mass will be assessed by percent daily change in US-measured quadriceps femoris muscle thickness (QFMT) during the PICU stay. The investigators will use a validated imaging protocol with high reliability and reproducibility. The investigators will also examine the difference in the normative standard scores (T-scores) between the two arms for the Pediatric Evaluation of Disability Inventory - Computer Adaptive Test (PEDI-CAT) assessment. Other secondary outcomes are Functional Status Scale, PedsQL, and muscle quality by US.

The investigators hypothesize that a larger trial will be feasible and safe. Patients randomized to the combined intervention will have lower decline in QFMT and better scores on PEDI-CAT assessment.

If proven feasible and safe, the PROXIMUS intervention has potential to profoundly change medical care in the PICU and substantially impact public health by improving long-term outcomes for the growing number of pediatric survivors of critical illness.

Inclusion Criteria:

1. ICU patients aged 1 year (corrected) to <18 years
2. Require mechanical ventilation (endotracheal intubation or tracheostomy, or initiation of noninvasive ventilation) in the first 72 hours of PICU admission.
3. Able to consent to participate within 72 hours of initiation of mechanical ventilation initiation.

Exclusion Criteria:

• Patients unable to receive EN, PN, or who are on a specialized diet incompatible with the study intervention
• Fulminant liver failure
• Kidney failure (≥KDIGO Stage 3) without replacement therapy
• Functional Status Scale score at PICU admission <9
• End of life/redirection of care
• ECMO therapy
• Continuous neuromuscular blockade and/or bedrest is medically or surgically necessitated Participation in a conflicting interventional trial
• High risk of refeeding syndrome
• Inborn errors of metabolism
• High BSA burns.