Short communicationDensity and hydration of fresh and fixed human skeletal muscle
Introduction
Muscle function is often inferred from its architecture. One key parameter of muscle function is maximum tension (P0) or maximum force producing capacity. Given the difficulty of directly measuring this value in humans, muscle architecture is often used to estimate P0. Specifically, physiological cross-sectional area (PCSA) has been shown to be an excellent predictor of P0 and is calculated using the following equation (Powell et al., 1984; Sacks and Roy, 1982).where muscle density (ρ) is 1.0597 g/cm3 (Mendez and Keys, 1960) and θ is fiber pennation angle.
Current biomechanical modeling techniques rely on PCSA to estimate peak muscle force production during a task (Anderson and Pandy, 2003; Buchanan and Shreeve, 1996). However, Brand and colleagues (Brand et al., 1986) demonstrated that muscle force predictions are highly sensitive to changes in PCSA, therefore, it seems apparent that the accuracy of measured values used to compute PCSA is important. Muscle architecture reports typically do not directly measure muscle density (Lieber et al., 1992; Wickiewicz et al., 1983). Instead, most studies use the value 1.0597 g/cm3, which was derived from unfixed rabbit and canine muscle tissue (Mendez and Keys, 1960).
Given the fact that human muscle architecture is often characterized in formaldehyde-fixed tissue, this previously defined value may be inaccurate for several reasons. First, a species effect may exist so that rabbit or canine muscle density may differ from human muscle density. Second, the method and duration of fixation may cause shrinkage and thus dehydration, which may alter muscle density. Finally, the time in which stored muscle samples hydrate in buffered saline may affect volume and thus density. If these variables affect density either separately or in combination, current estimates of muscle PCSA and thus predictions of muscle force may be inaccurate. Thus, the purpose of this study was to measure muscle density directly as a function of fixation method and hydration time in human skeletal muscle.
Section snippets
Methods
Muscle samples from three living subjects (four samples), three immersion-fixed cadavers (54 samples) and three perfusion-fixed cadavers (54 samples) were obtained for this investigation. Subject groups were not significantly different in terms of age, however, living subjects were younger on average (59±14 years) compared to immersion-fixed cadaver specimens (76±8 years) and perfusion-fixed cadaver specimens (79±9 years). Causes of death in the cadaveric specimens included respiratory arrest,
Results and discussion
For water content, there was a significant main effect for fixation method (), hydration time (), and fixation method by hydration time interaction () (Fig. 2). Post hoc testing demonstrated that 4% samples had significantly greater water content than 37% samples at each hydration time point (Fig. 2). The largest difference being before hydration (10%, ) and the smallest difference after 24 h of hydration (5%, ).
Post hoc comparisons also demonstrated water
Summary
Living muscle water content averaged 77% in this study, which agrees with the 76% value reported by Hargens et al. (1983). When immersed in PBS, fixed muscle-water content increased over time regardless of fixation method. Although 4% formaldehyde-fixed muscle had greater water content than 37% formaldehyde-fixed muscle at each time point, they had similar water content to living muscle after 24 h of hydration. The fact that 37% formaldehyde-fixed muscle had greater hydration rates than 4%
Acknowledgements
The authors gratefully acknowledge Dr. Lawrence Frank for his contributions of time and expertise with MR imaging.
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