The authors concluded that the intensity during a single bout of exercise regulates PGC-1�� mRNA abundance by activating selected upstream signalling pathways in human skeletal muscle with an intensity-dependent response. Human skeletal muscles are heterogeneous and consist of two main fibre types, slow (type I or oxidative) and fast (type IIa and IIx, or glycolytic with varying range of oxidative potential) twitch fibres. These fibres differ in their contractile speed, metabolic profile and fatigue resistance (Coyle, 2000). As described by Ryan et al. (2006), in sedentary people with similar age as the subjects in the study GDC-941
by Egan et al., the vastus lateralis generally contains 40% type I fibre, 35% type IIa fibre and 25% type IIx fibre. Their recruitment during exercise depends on both intensity and duration: type I fibres are mainly recruited at low-intensity exercise (<40% of ) while increasing intensity leads to greater type II fibre recruitment (Sale, 1987). Egan et al. compared two different exercise <a href="http://en.wikipedia.org/wiki/Diglyceride
">diglyceride intensities (40%vs. 80% of ) and we could speculate that the fibre type recruitment was different between the two exercise bouts: the 40% exercise would be associated with mainly type I fibre recruitment while, the 80% exercise would involve a greater proportion of type II fibre. This may explain the differences reported between the two exercise intensities used. Several studies have focused on specific fibre type characteristics and response to exercise. The expression of PGC-1��, in response to exercise in human vastus lateralis differs between fibre types. Russell et al. (2003) observed more than a 3-fold higher PGC-1�� protein content in type IIa fibres than in type I fibres after 6 weeks of interval training consisting of 5 to 6 intervals of 1�C3 min at 70�C80% of with 1 min of recovery at 50% of . The training intensity was similar to the 80% that was used during the high-intensity exercise in the study by Egan et al. suggesting a higher PGC-1�� gene activation would be observed when type II fibres are stimulated such as during high-intensity exercise. AMPK, one of the activators of PGC-1�� that were investigated in the study of Egan et al., was also shown to differ between fibre types. In young AZD0530 cost
untrained humans, Lee-Young et al. (2009) reported a higher baseline AMPK phosphorylation indicating activation in type II fibres as well as an increase in AMPK phosphorylation which was more pronounced in type II fibres than in type I fibres after an acute exercise bout at 65% of . These results support a fibre type-specific regulation of PGC-1��. Egan et al. proposed that CaMKII was also regulated by exercise intensity. To our knowledge, the fibre type specificity of CaMKII activity has still not been explored.