So, you've been running for 4...6...10...18...24...hours. Yes, your legs are tired. But, what about your respiratory muscles? Are they suited to breathing heavily for that long? Do they fatigue? Does greater effort to breath in a long ultra translate into higher ratings of perceived exertion? Check out my interview with Darlene Reid, PhD to learn the answers to those questions and how you can train your respiratory muscle to become your ultra best!