Participants will be assigned to either single-bout (SINGLE; n=10) or multiple-bouts (MULTI; n=10) of resistance exercise prehabilitation prior to 5 consecutive days of bed rest. Groups will be matched for age, anthropometric characteristics and activity levels. Only males will be studied due to potential gender differences in muscle protein turnover. A single-leg resistance exercise prehabilitation model will allow for a direct within-subject comparison against the non-exercised control leg.
Preliminary Assessments
Following an induction meeting and obtainment of informed consent, participants will report to The National Institute of Health/Wellcome Trust Clinical Research Facility (CRF) at University Hospital Birmingham (UHB) at 0800h having fasted from 2100h the previous night. After measuring height and weight, the following parameters will be determined in the order outlined:
1. Muscle biopsy, blood and saliva sampling: a muscle biopsy sample will be obtained from a specific thigh muscle under local anaesthetic. A blood sample will then be obtained from a forearm vein. Participants will also provide a saliva sample. This will be obtained prior to the following measurements.
2. Lower-limb function: The short physical performance battery (SPPB) test will be used.
3. Body composition: bioelectrical impedance analysis will be used to determine fat/fat-free mass on a whole-body and anatomical regional basis (CRF nurse).
4. Muscle size/architecture: The thickness of the thigh muscles will be determined via non-invasive ultrasound imaging.
5. Leg strength and muscle activation: Estimated maximal leg strength will be measured firstly on a standard knee extension exercise machine.
6. Stable isotope procedure: Immediately following muscle biopsy, blood and saliva sampling, participants will orally consume a small bolus of 'heavy water' tracer to label the body water pool with a small amount of this isotope. Participants will provide a daily saliva sample to monitor body water tracer enrichment. This technique will allow us to quantify muscle protein synthesis over the course of prehabilitation and bed-rest.
7. Participants will be fitted with an accelerometer and pedometer to asses daily activity levels.
8. Participants are given a food diary to record daily nutritional intake over a 3-day period.
Exercise Prehabilitation (Days 0-7) Participants will be randomly assigned to undergo 4 resistance exercise bouts the week before (Day 1, 2, 5 and 7), or 1 resistance exercise bout on the evening prior to bed rest (i.e. Day 7). Exercise bouts will consist of 8 sets of single-leg knee extension exercise and 8 sets of single-leg leg curls. The non-exercised control leg will remain passive throughout training. Resistance exercise will be performed at 70% of previously determined maximal strength (10-15 repetitions).
Intervention - 5-days of bed-rest (Days 8-13) Participants will report to the CRF at 0700h the morning immediately after exercise prehabilitation phase. At 0800h, muscle biopsies will be obtained from both legs to assess changes in muscle metabolism over the 7 days of prehabilitation. Participants will then be transported for dual energy x-ray absorptiometry (DXA), Magnetic Resonance Imaging (MRI) and ultrasound assessment of body/muscle composition, after which the 5 consecutive days of strict bed rest will begin. The bed rest model will mimic a traditional inpatient hospital stay and reflect the level of muscle unloading that occurs in older individuals following acute illness. Participants will spend the majority of time in bed and will be allowed to adjust the hospital bed head height for reading, eating and watching television, but will otherwise be instructed to lie flat in bed. Bathing/hygiene will be performed in a wheelchair at a sink. The bathroom accessed using a wheelchair. Adherence to bed rest will be monitored by nursing staff and through daily analysis of accelerometry data.
Post Intervention Assessments (Day 13) On the morning after completion of the 5-day bed rest phase (i.e. Day 13) participants will awake at 0700h in the CRF and remain in bed for assessment of muscle protein synthesis in trained and untrained control legs. A cannula will be inserted into a vein of both forearms for frequent blood sampling and a stable isotope tracer infusion. During the tracer infusion, muscle biopsies will be obtained from both trained and untrained legs before and after consumption of a milk protein drink, to assess muscle metabolism. Participants will then be transported by wheelchair for repeat bioelectrical impedance analysis, MRI and ultrasound scans to assess body/muscle composition. Leg strength will be reassessed in trained and untrained legs. A physiotherapist will perform a functional assessment of participants before they are discharged.
Rehabilitation Training Participants will be given the opportunity to complete a 6-week rehabilitation programme, consisting of a progressive 3 x weekly leg resistance training with protein supplementation, designed to fully restore any loss in muscle mass and strength from short-term bed rest. Resistance training rehabilitation will not form a specific study objective, but muscle size and strength will be assessed to ensure restoration of any muscle decline from bed rest.
