Abstract Introduction One of the most important consequences of hospitalisation in older patients is sarcopenia. This study aims to determine the impact of hospitalisation on the muscle mass, functional status, nutritional status, and short-term clinical outcomes. Methods A prospective study of patients admitted to an Acute Geriatric Ward between 1st November and 30th December 2022. Muscle ultrasound, utilising Point of Care Ultrasound (POCUS) at bedside, was employed to estimate rectus femoris muscle thickness (MT), area (Ar), pennation angle (PA), and fascicle length (FL) at the time of hospital admission, 3 days post-admission, and at hospital discharge. Results 30 patients included, with a median age of 84 years (SD 72–93), 63.3% male, and 70% Clinical Frailty Scale score ≥ 4. Barthel Index and Functional Ambulation Category revealed median values of 72.33 and 3.87 respectively. Global Deterioration Scale median was 2.47. Mini Nutritional Assessment Short-Form (MNA) and total serum protein showed median values of 7.40 and 6.35 respectively. The median length of hospital stay was 5.79 days, with inpatient mortality rate of 10% and 53.3% incidence of delirium. Ultrasound showed a decrease in PA by 36.31%, Ar by 34.30%, and MT by 24.50%, and increase in FL by 10.47%. Sarcopenia classification at admission and discharge revealed an increase in mean index from 5.04 to 7.74. Conclusions In our cohort of patients admitted to an acute geriatric unit, POCUS identified real-time decreases in MT, Ar, and PA at the muscular level before these manifested as functional changes. It demonstrated an inverse relationship between frailty and muscle morphology as living with frailty associated further decreases in muscle mass at discharge. The study also established a direct relationship between MNA, muscle thickness, PA, and fascicles length at discharge. POCUS assessment of muscle mass could indirectly predict outcomes and guide decisions to address muscle mass abnormalities.