Abstract:

Assessment and monitoring of lactate threshold (LT) are important factors to monitor to prioritize which metabolic system is being stressed.  LT training is a popular form of training due to its ability improve anaerobic performance.  Furthermore, training below this intensity can be extremely advantageous for producing adaptations to aid in aerobic performance.  However, accumulation of fatigue has been shown to precipitate changes to muscle recruitment strategies and metabolic pathways.  These changes could then lower the intensity at which lactate begins to accumulate.  The purpose of this study was to evaluate the reliability of LT threshold following a bout of exhaustive exercise. 
Methods: Eleven healthy, asymptomatic adults completed two graded exercises tests (GXT) interspersed with a 30-min fatiguing cycling session.  The GXT started at 80 watts followed by increases of 40 W every three minutes at a cadence of 80 rpm.  The GXT was ended upon volitional exhaustion or if cadence dropped below 75 rpm.  Oxygen consumption was recorded throughout the GXT trials, while lactate was recorded every minute.  The thirty minute fatiguing trial was completed at one stage above LT. Raw lactate values from the GXT tests were then input into a Dmax calculator in order to obtain threshold values and the corresponding stages in which they occurred. 
Results: A paired samples T-test found revealed significant differences in baseline lactate values from when comparing the pre-fatigue trial (1.55 ± 0.49 mmol^.L^-1) to the post fatigue trial (3.44 ± 2.04 mmol^.L^-1; p = 0.09, Cohen’s d = 1.27).  No significant mean differences were seen in lactate values threshold during the pre-fatigue trial (3.56 ± 0.84 mmol^.L^-1) versus the post fatigue trial (3.50 ± 1.99 mmol^.L^-1).  However, when expressed as a wattage, LT occurred at a significantly higher wattage in the pre-fatigue trial than the post fatigue trial (128 vs. 104 watts; p = 0.03).  Half of the participants reached LT during the same stage of the GXT, while four participants had an LT that occurred one stage lower in the post fatigue trial than the pre-fatigue trial.  When expressed as %VO_2peak, there were no significant mean differences between pre-fatigue LT compared to post fatigue LT (p = 0.19; Cohen’s d = 0.75).  LT with poor reliability (ICC = -0.36; 95% CI = -3.32 to 0.637; p = 0.69).  Furthermore, there were no significant mean differences in LT when data was expressed as %HR_max (p = 0.74, Cohen’s d = 0.17). 
Conclusions: A fatiguing bout of exercise does not mediate LT when examining blood lactate levels, or when LT is represented as a percentage of maximal heart rate and peak oxygen consumption.  However, it appears that the power output needed to achieve LT is significantly lower after fatigue has set in.  Practical Application: Lactate threshold is an important metric for prescribing exercise intensity for optimal athletic performance.  Practitioners need to confirm that the athlete is adequately recovered before measuring LT to ensure an accuracy of the work output measure.  Furthermore, LT training should be prescribed as a %HR_max or %VO_2peak rather than a raw power to help control for the effect of fatigue on the measure.