The Beep Test, also known as the PACERtest or PACER (Progressive Aerobic muscular Run), the 20 m Shuttle Run Test (20 m SRT), or the multi-stage fitness test, is a running test used to estimate an athlete's aerobic capacity (VO2 max). In the test, athletes must run from one line to another (20m apart) before a timed beep. Athletes must continue running back and forth, each time reaching the line before the next beep. Once one can no longer run, the test is over and the number of laps is recorded. As the test continues, the time between beeps gets shorter.