Exercise is not the only factor that determines the adaptations observed in skeletal muscle during a period of disuse. Nutrient intake provides a potent metabolic stimulus independent of exercise and these factors can work synergistically. In this capacity, there is growing evidence to support nutrient timing (ingesting protein or amino acids and carbohydrate within approximately 30 minutes before or after a resistance training session) as an effective strategy to promote muscle growth. Previous studies in ambulatory subjects have shown that consuming a combination of amino acids and carbohydrates immediately before or after an exercise session stimulates muscle protein synthesis to a greater extent than when performing the same exercise when fasted. In studies focusing on disuse, timing essential amino acids with carbohydrate before (less than five minutes) or after (three hours) resistance training has recently shown to attenuate the loss of muscle mass and strength during 28 days of bed rest even when the body is in an overall energy deficit . Thus, there appears to be strong support for consuming a protein-carbohydrate source that is rich in essential amino acids before or after each resistance training session in order to optimize training adaptations and this strategy is effective even during energy restriction. Combining milk intake with resistance exercise training has also shown to increase muscle fiber size compared to fat-free soy milk and an isoenergetic carbohydrate supplement.
1. To determine the morphological and neurological alterations that occur in skeletal muscle following combined exercise and nutritional countermeasures during disuse (unloaded-left limb) and use (loaded-right limb).
2. To characterize local (calf and thigh) blood flow, tissue oxygen saturation, tissue pH, limb circumference, heart rate, systemic blood pressure, and stroke volume during low load, low load with low blood flow restriction (BFR), low load with moderate BFR, and high load resistance training.
3. To compare the effects of 30 days of traditional high load and low load with moderate BFR resistance training in the weight-bearing and non-weight-bearing limb on popliteal and femoral blood flow and diameter, and tissue oxygen saturation and pH at the calf and thigh.
Subjects must self-report that they are at least recreationally active and pass the standard physical (Air Force Class 3 equivalent). This study was divided into two phases: first, a characterization phase and second, a four week crutch walking paired with resistance training phase. During the characterization phase participants practiced crutch walking and resistance exercise and were educated on documenting serving sizes and recording dietary intake by a member of the research team. Each participant was provided with a food scale, a dietary log, and had communication ability with a research assistant to answer any questions pertaining to food content. With these resources, participants were asked to consume and document three to five days per week a euenergetic diet that is estimated from World Health Organization equations for age and gender. Additionally, participants worked with researchers on maintaining a macronutrient profile that is representative of the NASA requirements. During the characterization phase subjects also performed four resistance exercise sessions under four different conditions: low load, low load with low BFR, low load with moderate BRF, and high load resistance exercise. During these sessions several cardiovascular variables were measured rest and immediately after each set of exercise including, blood flow through the popliteal and femoral blood vessels (ultrasound), stroke volume (ultrasound), blood pressure (Finapres), heart rate (ECG), tissue oxygenation and pH (NIRS).
To characterize the cellular mechanism related to basal mTORC1 signaling, muscle biopsies were obtained on a subset of participants from vastus lateralis and lateral gastrocnemius in both unloaded and weight-bearing limbs. Muscle cross-sectional area (CSA), strength, central activation capacity, rate of force development, force steadiness, muscle endurance, and biopsies were obtained the week prior to starting unilateral lower-limb suspension (ULLS). Post-ULLS, muscle CSA was evaluated on either day 24 or 25 of ULLS. Strength, central activation capacity, rate of force development, force steadiness, muscle endurance were evaluated in a muscle function test battery performed on either day 25 or 26 of ULLS. Muscle biopsies were obtained on either day 27 or 28 of ULLS.
In the second phase of the study, subjects participated in a battery of muscle function, size, and strength tests that are indicators of muscle atrophy/hypertrophy before and after four weeks of ULLS combined with resistance training. These tests include MRI, 1 repetition maximum (1-RM), maximal power, interpolated twitch, isokinetic, isometric, and muscle biopsies. In addition, the previously mentioned cardiovascular parameters were measured pre, mid, and post ULLS.
This investigation measured local vascular responses, tissue oxygen saturation (StO2), and cardiovascular responses during supine unilateral leg press and heel raise exercise in four conditions: high load with no occlusion cuff (HL), low load with no occlusion cuff (LL), and low load with occlusion cuff pressure set at 1.3 times resting diastolic blood pressure (BFRDBP) or at 1.3 times resting systolic blood pressure (BFRSBP). Subjects (N=13) performed three sets of leg press and heel raise to fatigue with 90-seconds rest. Artery diameter, velocity time integral (VTI), and stroke volume (SV) were measured using Doppler ultrasound at rest and immediately after exercise. Heart rate (HR) was monitored using a 3-lead ECG. Finger blood pressure (BP) was acquired by photoplethysmography. StO2 was measured using near-infrared spectroscopy (NIRS).
Blood flow restricted resistance exercise resulted in exercise blood flow and tissue oxygen saturation compared to exercise without restricted blood flow. Systemic cardiovascular responses (i.e. HR, BP, SV, and cardiac output) were also altered with blood flow restricted exercise. We also demonstrated that 4 weeks of blood flow restricted exercise training maintained muscle function during normal ambulation similarly to high load resistance training. However, blood flow restricted exercise was not fully protective in an unloaded condition.
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