Today's cardiac output session, semi - variable work and rest, HR roughly between 130 and 150 bpm. For a great explanation of the cardiac output method consult Joel Jamison's #ultimatemmaconditioning. Basically there's three ways to program interval sessions with regards to work - to - rest ratios. The most basic form is a fixed ratio, e.g., the famous and often misunderstood #Tabata protocol. Alternatively, either rest our work (rare) can be chosen in a variable manner. Doing 400m sprints and recovering until HR drops under 130 bpm would be an example. Think @themartinrooney and his #tfw #hurricane training. Finally, both work and rest can be chosen variable, i.e., work up to an upper threshold HR and recover back to a certain lower threshold before starting the next interval. @alwyncosgrove and his team at #resultsfitness seem to use this type of programming a lot with great success. Each of those methods has its place. I'm using a semi - variable approach to determi...