Your hard days should be hard and your easy days easy. Simple training advice that holds some truth but doesn’t give us the guidance we need to reach our performance goals. In comes the lactate threshold (LT) test, which can define your hard and easy days by heart rate, pace and/or watts.
Lactate is a biomarker in our bodies that increases as exercise intensity increases. At low intensities, the exercise we do is primarily aerobic which means we are relying on oxygen to produce energy for a long time and any lactate we produce, is cleared. At high intensities, the oxygen delivery system takes too long! To get more energy faster, our bodies transition to an anaerobic state which does not depend on oxygen but does produce lactate.We all have certain paces or power outputs that cause lactate to increase. That individual pace or power where your lactate makes a dramatic shift up, is your threshold. When properly performed, a lactate threshold test can identify a lower and upper threshold, corresponding to your aerobic and anaerobic thresholds. The heart rate, pace, and power at the lower/aerobic threshold defines your zone for easy days while the upper/anaerobic threshold defines your hard days. When this data is applied to training, the goal is to reduce your lactate at each threshold, allowing you to go faster.Functional threshold power or FTP testing aims to replicate the anaerobic threshold. While widely used by cyclists, the relationship of FTP and LT has not been well studied. Borszcz et al found that in the context of group data, FTP and LT correlated but when individual data was plotted, HR and watts were off by as much as 20 bpm and 50 watts respectively. Thus, the authors caution against individuals solely using FTP to determine hard and easy training zones.
Borszcz et al (2018). Functional Threshold Power in Cyclists: Validity of the Concept and Physiological Response. Int. J. Sports Med., 39: 737-742.Michelle Slawinski is an American College of Sports Medicine Certified Exercise Physiologist and co-owner of Summit Multisport, LLC. She works as a clinical exercise physiologist performing research VO2 testing. With Summit Multisport, she provides cycling and running lactate threshold tests as well as body composition evaluations. She is an avid endurance athlete competing in marathons, triathlons, and the daily endurance event of keeping up with a toddler!