Figure 33.16: A Regulator of Muscle Fiber Type In response to training with endurance exercise, skeletal muscles undergo changes that make the muscles more resistant to fatigue. How are these responses regulated? Some evidence indicated that a transcription factor called PPARδ (“PPAR delta”), which is present in skeletal muscle and is activated in response to endurance exercise, might stimulate the formation of slow oxidative cells, a key response to endurance training. Yong-Xu Wang and colleagues used transgenic mice to investigate whether artificially increasing the amount of active PPARδ would initiate exercise-associated responses in muscles even in the absence of endurance training.a