Runners have more high-density lipoprotein (HDL) cholesterol per unit of blood than sedentary men and women. High HDL-cholesterol (60 mg/dL and over) is considered to provide some protection from heart attack risk while low HDL-cholesterol (under 40 mg/dL) places an individual at increased risk of an attack. The HDL-cholesterol measurement is actually the sum of all cholesterol carried on multiple groups (subclasses) of HDL particles, which can be separated by density, size and buoyancy. Not all HDL-subclasses are equally protective. The protection is thought to primarily come from the larger, more-buoyant particles. The technique of "analytic ultracentrifugation" is used to separate the HDL particles by buoyancy into flotation intervals. The technique requires the use of analytic ultracentrifuge machines that were built over 50 years ago. Lipoproteins are placed into a plastic cell with a saline solution of greater density, and then spun in the machine. Photographs are taken during centrifugation using special "Schlieren" optics of the same vintage. The film records the distribution of the HDL particles as they float from the bottom to the top of the cell in accordance with each particle's density. The distribution is divided into 15 flotation intervals (F1.20) which lie within either the HDL3 (F1.20 0-3.5) or HDL2 (F1.20 3.5-9.0) region.