dd8f1431_How_Heart_Rate_Zones_Work__Description
[Heart Rate Zone] Zone 4 Goals: Muscular Endurance, Lactic Acid Tolerance, Low-End SpeedEnergy System: Oxidative, Glycolytic 96-103% [Description] 
Lactate threshold (LT), also called anaerobic threshold (AT), is the highest intensity at which your body can recycle lactic acid as quickly as it is produced. At this intensity, you are working very hard, but can still maintain your maximum sustainable pace and relatively good form because lactic acid levels in the blood and muscles are steady, not increasing.Increasing the intensity above this zone or towards the high end of this zone can cause lactic acid to more rapidly accumulate and bring premature fatigue and delayed recovery due to acidic hydrogen ion build-up and more rapid carbohydrate depletion. Performing interval training sessions near lactate threshold can teach your body to decrease the amount of lactic acid being produced and increase lactate removal at any given intensity.At this intensity, the fast-twitch fibers can be trained to produce less lactic acid and the slow twitch fibers can be trained to burn more lactic acid, both of which raise the lactic acid threshold and allow you to work harder at a higher intensity. Since you’re not at an all-out, high-impact pace as you would be in higher, fast-twitch muscle utilizing zones, recovery from Zone 4 training can occur quicker than recovery from other high-intensity training zones, and from an interval training standpoint, zone 4 training gives you a lot of bang for your buck.When you experience “rubbery leg” syndrome, a marked increase in breathing difficulty, or an inability to maintain good form, you have reached the point where lactic acid is accumulating at a faster rate than it can be removed, which can significantly decrease your ability to maintain a steady effort and also has significant recovery implications.Good for 1 to 7 minute intervals with about a 2:1 to 3:1 work:rest ratios.
How Heart Rate Zones Work
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