The yo-yo intermittent recovery test: physiological response, reliability, and validity.

Purpose: To examine the physiological response and reproducibility of the Yo-Yo intermittent recovery test and its application to elite soccer.

Methods: Heart rate was measured, and metabolites were determined in blood and muscle biopsies obtained before, during, and after the Yo-Yo test in 17 males. Physiological measurements were also performed during a Yo-Yo retest and an exhaustive incremental treadmill test (ITT).