Sarcopenia describes age related muscle loss, and closely associates with post-operative morbidity and long-term survival. Our understanding of sarcopenia as a predictor of operative risk is developing, and will become useful to inform surgical decision making. Clinical implementation is impeded by lack of clarity on how best to measure sarcopenia. This narrative review examines the measurement of muscle mass across malnutrition, frailty and sarcopenia research. Studley first demonstrated in 1936 the correlation between body composition and surgical outcomes, reporting that pre-operative weight loss was a prognostic indicator. Whilst this was subsequently disproven (unless associated with physiological impairment), this lead to research on body composition and surgical outcomes, with later research demonstrating the importance of arm muscle circumference and grip strength. Frailty is a marker of biological age. Some meta-analyses demonstrate no relation between age and surgical outcomes, once the effects of frailty were removed. Whilst frailty is a strong predictor of surgical mortality, its measurement is complex. Muscle mass and strength is one domain of frailty – some research suggests this may be the most important domain in estimating operative risk. Sarcopenia was proposed by Rosenberg, building on earlier work by Shock examining changes to the human body with age. Rosenberg recognised that no bodily decline was as dramatic as muscle mass, nor as consequential. The impact of sarcopenia on broad medical and surgical outcomes has been established. Despite muscle mass being the bodies largest component, it’s measurement has always been challenging - this impairs the implementation of sarcopenia as a marker of operative risk. In this narrative review, we examine the measurement of muscle as a marker of operative risk across frailty, malnutrition and sarcopenia research, to gain insights on how best to measure sarcopenia in research and clinical practice.