Patients in rehabilitation may undergo periods of prolonged limb immobilization in response to injury, surgery, or illness. Due to disuse, the size and strength of muscles controlling the affected limb can decrease significantly, resulting in physical impairment or lower quality of life during the recovery phase. When measured as maximal isometric contraction, muscle strength in the immobilized limb decreases at a dramatic rate, around 1.0-1.2%/day. These declines in muscle size and strength may vary between sexes - this remains unclear. Muscle atrophy is thought to be caused by a significant decline in muscle protein synthesis (the main pathway attributing to muscle growth/ hypertrophy). There are also various pathways by which muscle is broken down. It is the sex-based differences in this muscle protein synthesis rate of decline and the pathway by which muscle is broken down that we would like to investigate for their contribution to muscle size and strength decline after a period of immobilization.

The purpose of the proposed research is to define whether there are differences between females and males (i.e. sex-based differences) in the metabolic and mechanistic regulation of disuse-induced muscle atrophy in vivo in humans.

On the participant's first visit, prior to beginning the study (Day 0), they will be instructed to fill out a health-related questionnaire, screened for all anthropometric characteristics (height, weight, age, sex) and undergo a DXA scan for body composition details. They will be familiarized with all study procedures including the muscle biopsies, strength tests, D2O, diet and exercise recording. They will also be fitted for the immobilization brace. The leg that will be immobilized will be randomized in each participant. This first visit will last approximately 2 hours. When participants return on Day 1 of the study, they will first have a lower-body MRI scan done. Following this, participants will perform single-leg strength tests on the Biodex. Thereafter on Day 2, venous blood draws and saliva samples will be taken. Participants will be administered their loading dose of D2O (5mL·kg body mass-1 of 70% D2O) and provided with daily maintenance doses of 50mL of D2O to continue to take each day for the duration of the experiment. Upon returning for Day 3, participants will undergo a single skeletal muscle biopsy from the vastus lateralis of one leg. Participants will repeat their blood and saliva sample on this day. Participants will then be outfitted with the knee brace and crutches prior to leaving the facility. Participants will also be outfitted with an ActiGraph activity monitor to record physical activity and energy expenditure while they are immobilized for 7 days. This third visit will last approximately 3 hours. Participants will monitor their own dietary intake via diet recording, which they will be instructed to carry out for 2 weekdays (i.e. Monday - Friday) and one weekend day (i.e. Saturday, Sunday). During the 7-day period of immobilization, participants will provide a saliva sample every day for measurement of 2H enrichment in body water. The saliva samples should be taken in the morning right after the participants wake up and before any food intake. Participants should drink the 1 × 50 mL maintenance dose of D2O per day provided right after collecting their saliva samples. On Day 8 participants will return to the facility to undergo post-immobilization testing which will include: a DXA scan, a lower-body MRI, two skeletal muscle biopsies (one in each leg), a blood sample, a saliva sample, and single-leg strength tests on both legs. This final visit will last approximately 4 hours.