Our favorite running calculator will always be the McMillan Calculator, but Running Writings just published a new one worth bookmarking. It’s built around a metric called critical speed (CS) – the boundary that separates metabolically sustainable running from metabolically unsustainable running. Below your CS, key physiological markers like oxygen consumption, blood lactate, and intramuscular acidity stay stable. Above it, they all climb until you’re cooked. The “Critical Speed Calculator” from Running Writings estimates your CS using at least two recent race performances lasting between roughly 2 and 25 minutes, then plots them against a hyperbolic curve to identify where that threshold sits. With three or more performances, it also generates a 90% uncertainty range: going on the low end of your CS is useful for threshold workouts, where you want to stay just inside metabolic steady-state. CS+ sets the floor for VO2max-style intervals, where the whole point is to push into unsustainable territory. Once you know your CS, there’s a low-cost way to nudge your speed upward at any intensity: VKTRY insoles. In Brian Metzler’s review “Can VKTRY Performance Insoles Transform Any Training Shoe into A Supershoe?,” he noted they added measurable propulsion to several training shoes, logging lower heart rates during 800-meter repeats while wearing them. They’re reusable across shoe rotations too, making them a practical alternative to cycling through $250+ super shoes every 200 miles.

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