The Effect of Fatigue on Rate of Change of Physiological Markers on Exertion During Graded Exercise

Document Type

Conference Proceeding

Publication Date


Publication Title

Proceedings of the National Strength and Conditioning Association National Conference


Background: The accumulation of fatigue during exercise can severely impact performance, physiological systems, and perceptual responses causing a disassociation between external work and physiological variables. Often times variables such as heart rate (HR) and rating of perceived exertion (RPE) are used to prescribe exercise intensity. It may be that in a fatigued state, these variables disassociate from external work, thus causing the athlete to either overestimate or underestimate their performance. This could lead to either underperformance due to training and a decreased intensity or overtraining which again leads to detrimental performance. Thus, the purpose of this study was to evaluate the role that acute fatigue plays in measures of physiological markers of exertion.

Methods: Ten healthy, college-aged subjects participated in two graded exercise tests (GXT), interspersed with a 30-min, steady state fatiguing protocol. Both GXT’s started with an initial workload of 80 watts with 40 watt increases in workload every three minutes and subjects were asked to maintain a cadence of 80 ± 5 revolutions per minute. The test was terminated upon volitional exhaustion or the inability to maintain a cadence of at least 75 revolutions per minute. Subjects were then given 15 minutes to recover before beginning the fatiguing protocol, which intensity was set at one stage below lactate threshold, which was measured during the initial GXT. Immediately following the 30 minute fatiguing session, subjects were asked to complete another GXT identical to the first one. Heart rate, relative VO2, lactate, and rating of perceived exertion were recorded within the last minute of each stage of the tests. Hierarchical linear growth modeling (HLGM) was used to control for variance across trial (pre-fatigue vs. post-fatigue) and stage, as well as allow for missing data from cessation of exercise at different stages.

Results: The analysis revealed no main effect for trial (p = 0.27) or interaction effect for VO2 (p = 0.49). However, there was a significant main effect for trial on heart rate where subjects in the post fatigue session exhibited a significantly higher heart rate than the pre-fatigue trial despite equivalent intensities (91 bpm vs. 104 bpm; p < 0.001). As to be expected, rate of HR increase was significant with HR increasing 18 bpm per stage. Lastly, there was a significant trial effect for rating of perceived exertion where participants reported an RPE 1.7 points higher in the post fatigue trial (p = 0.002). A significant time effect was also observed where RPE increased linearly from at a rate of 3.02 points per stage.

Conclusions: It appears that fatigue severely impacts measures of exertion at equivalent exercise intensities. Both rate of HR increase and RPE were found to be significantly higher in the post fatigue trials. If athletes were asked to control performance by RPE or HR in a fatigued state then this would lead to under performance.

Practical Application: Practitioners need to be weary of prescribing exercise based upon HR or RPE during periods of high intensity training. It is suggested that measures of external work (i.e., %1-RM or km/h) are used to help alleviate this discrepancies brought on by accumulated fatigue. That notwithstanding, practitioners should not be dismissive of these inflated perceptual and physiological measures, as they may be signs of impending overtraining.