Samples were prepared as described by Kohn et al. (2007c), with modifications. A small piece of frozen tissue was weighed and 100 mM potassium phosphate buffer, pH 7.30, added to a ratio of 1:19. The tissue was homogenized on ice using a Teflon tip, after which it was sonicated twice for 10 s at 6 W using a micro sonication probe (Virtis Virsonic Ultrasonic Cell Disrupter 100) and centrifuged at 1700 g for 5 min (4 °C). Enzyme assays were performed using the supernatant, whereas a small part of the pellet was diluted with sample buffer (5% β-mercaptoethanol, 2.5% SDS, 10% glycerol, 62.5 mM Tris, pH 6.8 and 0.1% bromophenol blue). These latter samples were heated to 95 °C for 3 min and used for determining the MHC isoform content and Western blot analyses. A rat muscle sample obtained from a previous published study was included to serve as control for the MHC IIb isoform on Western blots (Smith et al., 2008).