Electrical impedance alterations in the rat hind limb with unloading

Objectives: Methods are needed for quantifying muscle deconditioning due to immobilization, aging, or spaceflight. Electrical impedance myography (EIM) is one technique that may offer easy-to-follow metrics. Here, we evaluate the time course and character of the change in single- and multi-frequency EIM parameters in the hind-limb suspension model of muscle deconditioning in rats. Methods: Sixty-two rats were studied with EIM during a two-week period of hind limb unloading followed by a two-week recovery period. Random subsets of animals were sacrificed at one-week time intervals to measure muscle fiber size. Results: Significant alterations were observed in nearly all impedance parameters. The 50 kHz phase and multi-frequency phase-slope, created by taking the slope of a line fitted to the impedance values between 100-500 kHz, appeared most sensitive to disuse atrophy, the latter decreasing by over 33.0 +/- 6.6% (p < 0.001), a change similar to the maximum reduction in muscle fiber size. Impedance alterations, however, lagged changes in muscle fiber size. Conclusions: EIM is sensitive to disuse change in the rat, albeit with a delay relative to alterations in muscle fiber size. Given the rapidity and simplicity of EIM measurements, the technique could prove useful in providing a non-invasive approach to measuring disuse change in animal models and human subjects.