Skeletal muscle is a highly plastic tissue capable of modifying its phenotype (i.e., structural, contractile, and metabolic properties) in response to alterations in mechanical loading. Mechanistically underpinning skeletal muscle plasticity are changes in skeletal muscle protein turnover. Skeletal muscle size is dictated by changes in rates of muscle protein synthesis (MPS) and rates of muscle protein breakdown (MPB) with changes in rates of MPS being the primary determinant of human skeletal muscle mass. Both MPS and MPB are highly sensitive to contractile and nutritional cues. In response to EAA ingestion, there is a rise in rates of MPS and a mild suppression of MPB rates resulting in a positive state of protein balance. Similarly, when an individual performs a bout of resistance exercise, there is an increase in rates of MPS that is potentiated by EAA feeding; It is for this reason that when repeated bouts of resistance exercise are coupled with EAA intake over time, there is a gradual increase in skeletal muscle mass termed hypertrophy. In contrast, when an individual undergoes a reduction in levels of contractile activity (e.g., immobilization due to injury or surgery), there is a reduction in both fed and fasted rates of MPS leading to the loss of skeletal muscle mass and size termed muscle atrophy.

Although it is well known that both nutrition and contractile activity affect rates of muscle protein turnover and skeletal muscle mass, our current knowledge is limited by most studies reporting rates of MPS and MPB that are averages of thousands of proteins in the whole muscle, or subcellular protein fractions, such as myofibrillar, sarcoplasmic, and mitochondrial. Further, individual protein MPS and MPB rates might span a broad range and there may be selective changes to the turnover of individual proteins under different skeletal muscle loading scenarios. Dynamic proteomic profiling (DPP) is an emerging methodology that combines quantitative proteomic abundance measurements with individual protein MPS and MPB rates, to deliver unprecedented insight into the molecular regulation of individual protein turnover. Another major consideration is that nearly all studies in this field have been conducted in males, with limited data in females. The lack of data in females is a major knowledge gap and of major concern particularly given there is evidence that women may display different molecular responses to exercise, nutrition, and disuse compared to men.

The purpose of this investigation is to gain a better understanding of the acute and short-term effects of an EAA supplement and an acute bout of resistance exercise on rates of muscle protein turnover. Further, the investigators aim to measure the dynamic proteome during 10 days of unilateral leg immobilization, and following several bouts of resistance exercise in the contralateral leg, in young healthy women. The present investigation will characterize skeletal muscle mass, strength, protein expression, and protein synthesis rates (individual \[i.e., DPP\] and average). The study may inform potential future novel interventions to attenuate losses in skeletal muscle mass owing to disuse, aging, or injury.