CNN According to Lance October 21, 2013 05:31:00
Your red blood cells carry oxygen to your muscles. so basically the higher your red blood cell percentage the better your muscles will operate under stress. in layman's terms, the more red blood cells you have the faster you're going to ride a bike. >> so what was the doctor's solution to raising those levels? >> yeah, a couple months before, maybe a month and a half before my first tour de france, it was epo. >> under uci rules at the time, riders were allowed a hematocrit level of 50% but no higher. tyler hamilton said doctors would tell riders what their glow time was with different drugs. ...
Source: archive.org