The Inertia indicator takes its name from the realm of physics. Used to describe the tendency of a body in motion to stay in motion until acted upon by an outside force, here it is used to measure the momentum of a stock based upon its volatility. An outgrowth of Donald Dorsey's Relative Volatility Index, Inertia is simply a smoothed RVI. Dorsey introduced the idea of Inertia in the September 1995 issue of Technical Analysis of Stocks and Commodities.

Inertia is measured on a scale from 0 to 100. Negative inertia is seen if the indicator is below 50. If the indicator is above 50, it is said to have positive inertia. Signs of positive inertia are indicative of a long-term upward trend. Signs of negative inertia illustrate long-term downtrends.