Purpose: This study investigated the relationship between training-load (TL) metrics and the acute performance decrement (APD) measured immediately after 4 training sessions performed by well-trained runners. Methods: On a treadmill, 12 well-trained runners (10 men and 2 women) performed an incremental test, a baseline time-to-exhaustion (TTE) test at maximal aerobic speed, and 4 randomized training sessions followed by a TTE test to measure APD. The training sessions were matched for external load (60 arbitrary units) but differed in the time spent in the 3 intensity domains. The TL metrics used were based on training impulse, heart-rate variability, ratings of perceived exertion, and the NASA Task Load Index (NASA-TLX) rating scale. Results: TTE was significantly shorter after all the training sessions compared with baseline (P < .001). While APD was higher (+16%, P = .035) for long-duration high-intensity training (HITlong) compared with low-intensity training (LIT), most TL metrics showed higher values (P < .001) in LIT than in HITlong. Conversely, NASA-TLX values were higher (P < .001) in HITlong than in LIT and were significantly associated with APD values (P < .001, ß = 0.54). Physiological parameters showed that less time was spent above 90% of VO2max during LIT compared with the other training sessions (P < .01), while average respiratory frequency and mean respiratory exchange ratio were higher during HITlong compared to LIT (P < .01). Conclusion: APD was observed after the 4 running training sessions, and it was not associated with most of the TL metrics. Only NASA-TLX was associated with APD, suggesting that this TL metric could be leveraged for training monitoring.
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