Space-induced muscle atrophy is thought to create an imbalance between muscles and tendons, increasing the risk of strain injuries and motor function errors. Dr. Shantanu Sinha is examining changes in the muscle’s ability to endure strain during and after spaceflight in order to understand the risk of strain injury and to optimize the best strategy for rehabilitation after atrophy.
Overview
In Vivo Stress-Strain Dynamics in Human Muscle
Principal Investigator:
Shantanu Sinha, Ph.D.
Organization:
University of California, Los Angeles
Technical Summary
Muscle atrophy refers to the wasting or loss of muscle tissue resulting from lack of activity due to several reasons, including prolonged bed rest, limb suspension and immobilization due to disease and trauma, aging and spaceflight. Regarding a space mission in the microgravity environment, muscle atrophy and the accompanying neuromuscular disorders have been recognized as serious obstacles to be overcome for long-duration space exploration. Although many research projects have well documented morphological, biological and physiological changes in atrophied muscles using spaceflight and ground-based experimental models, it is virtually unknown what functional implications the changes in mechanical properties and structure of intramuscular tendinous tissues have. In addition, in order to investigate this issue in human muscles in vivo under the environment closer to physiological condition, it is required to develop a noninvasive measurement technique.
For the last three years, our laboratory has developed, verified and perfected a completely noninvasive measurement technique using velocity-encoded, phase-contrast MR imaging combined with a custom-built, MR-compatible muscle strength-testing apparatus. This method enabled us to accurately quantify muscle volume and strength and strain dynamics of muscular-tendinous structures in the human lower leg. Using this method, we examined the effect of a period of decreased mechanical load/suspension and the changes that occur upon recovery of the muscle to normal loading condition.
For 13 subjects, atrophy was induced using a four-week unilateral limb suspension (ULLS) model followed by six-week rehabilitation. The ULLS model used was very effective for inducing localized atrophy, completely reversible and well-tolerated by the normal volunteers. Preliminary results of the previous ULLS study (performed in the prior years of the grant) showed that the decrease in muscle volumes following ULLS was proportionate to similar changes in relative strain distribution of the tendinous structures, suggesting that the structure and function are tightly coupled. Moreover, preliminary results of the most recent ULLS study (performed in 2004) demonstrated that absolute strain distribution (assessed with ten and 20 percent Pre-ULLS MVC) of tendinous structures changed following ULLS, possibly due to different motor unit recruitment strategy and altered mechanical properties of the structures. In addition, a different rehabilitation paradigm with moderate resistance training protocol (using loading condition less than subjects body weight) was tried out and found to be very effective.
For the coming years, we would like to pursue better understanding of functional implications of architecture and mechanical linkages in atrophied muscles using MR Tractography on the top of current method. We strongly believe that future research study will provide valuable information to understand the structure and functional properties of normal and atrophied muscle and to develop more effective exercise regimen for astronauts.
Earth Applications
The method developed herein, namely, using a noninvasive method of monitoring structural and functional differences between normal and atrophied muscles has a tremendous potential for Earth-based clinical use. Atrophy can result not only from microgravity as in the case of astronauts in spaceflights, but also from prolonged bed rest or immobilization from casting. In these cases, the extent to which the muscle normalizes subsequent to rehabilitation is a very important clinical question, and can be addressed quantitatively by the method developed herein.
This project's funding ended in 2005