A comparison of fiber types and measurement techniques in the medial gastrocnemius and soleus muscles of the rabbit.

Medial gastrocnemius (MG) and soleus (S) histochemical fiber type profiles were determined in female albino rabbits (n = 5). The fiber composition of the central S was 93.5 +/- 12.7 S.D.% slow-twitch oxidative (STO), and 6.5 +/- 12.7% fast-twitch oxidative glycolytic (FTOG). No fast-twitch glycolytic (FTG) fibers were found in any of the S muscles. The S FTOG fiber percentage varied from a minimum of 0% to a maximum of 29% in different rabbits. The composition of the central MG was 22.1 +/- 2.0% STO, 28.8 +/- 4.6% FTOG, 43.9 +- 3.5% FTG, and 5.2 +/- 2.5% unidentified fibers. In the MG the STO fiber percentage increased anteriorly, while the FTG fiber percentage increased posteriorly. Two methods were employed to determine fiber diameters. Using the mean of orthogonal diameters method, we found central S diameters to be 74.6 +/- 5.1 S.D. micrometers for STO, ad 67.7 +/- 7.9 micrometers for FTOG fibers. Diameters for the central MG were 62.1 +/- 7.2 micrometers for STO, 66.1 +/- 9.4 micrometers for FTOG, 95.9 +/- 9.3 micrometers for FTG, and 83.3 +/- 9.1 micrometers for unidentified fibers. Fiber diameters calculated by the mean of orthogonal diameters method were for the S, 1.16 +/- 0.01, and for the MG, 1.21 +/- 0.01, times larger than those obtained using the "smallest diameter" (maximum minor diameter) technique. The relationship between these measurement parameters varied in the different muscles due to fiber shape variation. In the central S, STO fibers occupied 94.6%, and FTOG, 5.4% of the cross sectional area, using areas derived from the mean of orthogonal diameters measurements. In the central MG, the relative areas were 13.2% for STO, 19.6% for FTOG, 61.6% for FTG, and 5.6% for unidentified fibers. Relative area calculations were comparable using both of the measurement methods. Therefore, relative area comparisons tend to discount variations introduced by different measurement techniques.