A. Experimental Approach
To test these hypotheses, we are proposing four phases of study. Phase I is focused on conducting a series of biomechanical studies that will provide the foundation for the proposed training studies necessary to demonstrate the efficacy of SHRT. The Space Cycle has been modified such that one of the centrifuge arms can be used to perform leg squat resistance training under different hypergravity conditions. The biomechanical studies will be led by a world-renowned biomechanist, Dr. Peter Cavanagh, and Dr. Mark Pierre (a postdoctoral fellow). The objective of these studies will be to identify hypergravity conditions that provide ground reaction forces (or foot forces) without the use of external weights that emulate those seen during 10RM SRT performed at 1 G.
Figure 1. Stop-frame movie with rider on Space Cycle. The Space Cycle has two centrifuge arms, and each has a pivot. The centrifuge arm initially begins with the rider(s) in a vertical position. Cycling causes the centrifuge to rotate. As the angular velocity increases, the centrifuge arm begins to pivot such that the Gz axis of the rider moves toward the horizontal plane. The angle (net vector) of the centrifuge arm is the resultant of the vertical (i.e., gravity) and horizontal forces. Note that in this sequence, the angle of the pivot relative to the vertical plane is ~50o, and the rider is experiencing approximately 3 Gz at the feet. In space, there would be no need for a pivot, and the centrifuge would simply have a rigid arm.
Once the appropriate hypergravity conditions have been defined in Phase I, then the research will transition into three additional phases that are designed to evaluate the efficacy of SHRT as a countermeasure to microgravity. These studies will involve both the muscle biology team at U.C. Irvine and Dr. Cavanagh’s biomechanical team at the Cleveland Clinic. In Phase II, subjects will engage in a 10-week SHRT program. The SHRT program will be designed such that the subjects will train under hypergravity loading conditions that emulate a 1 G SRT program. Pre and post training tests will include measures of muscle mass, function, and cellular/molecular markers of growth/atrophy. These results will be compared with those obtained from subjects participating in a 1 G SRT program. Our working hypothesis is that if a SHRT program emulates foot forces similar to those seen in a 1 G SRT program, then the SHRT program will produce results comparable to those of SRT.
The goal of Phase III is to employ the optimal SHRT program developed in Phase II as a countermeasure to the unloading of skeletal muscle produced by unilateral lower limb suspension (ULLS). The pre and post training analyses will be identical to those used in Phase II. The underlying rationale of this phase is to use ULLS as an inexpensive mode for inducing muscle unloading and atrophy, and to gain some initial insight into the efficacy of the SHRT program developed in Phase II.
The goal of Phase IV is to extend the duration of muscle unloading using either ULLS or bed rest. In this phase, subjects will undergo 6 wks of ULLS or bed rest. Subjects will participate in the same SHRT as used in Phase III, and the analyses will be identical to those used in Phases II-III.
The paradigm that will be used throughout Phases II-IV to evaluate the effectiveness of SHRT as a countermeasure is shown in Figure 2 below. This reductionist pyramid represents an integrated approach designed to provide valuable information at the: i) functional; ii) cellular; and iii) molecular levels. Muscle function will be assessed by several means. The first will simply be done by determining the maximal weight that can be lifted during 10 squats (i.e., a 10 RM test). The second is to make measurements of the in vivo torque-velocity relationships and power of the hip, knee, and ankle extensors. Magnetic resonance imaging will be used to measure the cross-sectional area of the vastus lateralis, gastrocnemius, and soleus muscles. Muscle biopsy samples will be obtained from the vastus lateralis muscle so that analyses can be performed on: i) muscle fiber size; ii) myofibrillar protein concentration; iii) myosin heavy chain isoform composition; and iv) and markers of muscle growth/atrophy.
All phases (i.e., Phases II-IV) will be conducted under the auspices of U.C. Irvine’s NIH funded GCRC. The advantages of this are as follows: i) the Space Cycle is located within the GCRC; ii) a Space Cycle Research Team involving the GCRC has already been established; iii) one of the core facilities of the GCRC is the Applied Physiology-Human Performance Laboratory(Dr. Caiozzo is the director of this laboratory) that contains key equipment that will be used for some of the testing procedures; iv) the GCRC will provide key personnel (exercise physiologist, research nurse, receptionist) at no cost; v) the GCRC will defray some of the costs associated with MR imaging; and vi) the GCRC provides oversight in addition to that of the IRB.