Glossary

The running glossary

Your watch shows you perhaps thirty numbers. Most runners could confidently define four of them. That is not a failure of attention — it is that the definitions available are either a vendor's marketing sentence or a forum argument, and neither tells you what moves the number or when to ignore it.

This glossary fixes that one term at a time. Each entry opens with a definition short enough to quote and specific enough to be checkable, usually under thirty words. Then four things, in the same order every time.

What it is. The plain description, with the units and the range you would actually see.

What it is not. This section exists because most metric confusion is a collision between the name and the meaning. A fitness score is not fitness. A training load is not effort. Relative Effort is not how hard the run felt to you. Naming the confusion directly, in the words runners use for it, resolves more than another paragraph of correct definition would.

What moves it. Three to five bullets with numbers attached, so you can predict which way the metric will go after a hard week, a hot week, or ten days off.

When to ignore it. Every one of these numbers has conditions under which it is noise, and saying so is not undermining the metric — it is the difference between a number you can use and a number you obey.

Where a term is something Inertial itself computes, the entry links to the method page that gives the formula, the inputs and the window. You should be able to check our arithmetic; that is the point of publishing it.

Terms are grouped below by what they belong to — load and fitness, heart rate, pace and speed, physiology, racing, and devices and data. Every entry links to the explainer that goes deeper and, where one exists, to the calculator that does the sum for you. If a term you looked for is missing, it is missing because it has not been written properly yet rather than because it does not matter.