Muscle comprises the largest protein pool in the body. Critical illness is characterized by substantial hormone- and cytokine-mediated protein metabolism changes in various organs, leading to increased breakdown and decreased synthesis rates. Medical therapeutic measures such as long-term sedation and mechanical ventilation during ICU stay can further enhance this muscle degradation (up to 2 % muscle mass per day. Each gram of nitrogen lost correlates to the loss of 30 g of lean tissue). The goal of the nutritional plan is to provide the necessary nutrient substrates to critically ill patients to prevent muscle atrophy and enhance patient's recovery. Muscle atrophy or ICU-acquired weakness (ICU-AW), which is considered a bilateral symmetrical muscle weakness in critically ill patients without prior neuromuscular disorders. These effects can be evident both directly from the harmful impact of drugs and from the disruption of microcirculation and also indirectly due to muscle atrophy from prolonged immobilization. Point-of-care ultrasound (POCUS) is rapid, accurate, repeatable, non-expensive, noninvasive, and without the risk of radiation. Diaphragm and skeletal muscle ultrasound provides a real-time image and a permanent record of function. This review aims to illustrate the role of ultrasound employment in detecting the changes of muscle mass and function during critical illness.