Dynamic Changes of Performance Fatigability and Muscular O₂ Saturation in a 4-km Cycling Time Trial

Exercise intensity variations throughout a cycling time trial (TT) might be influenced by subject's functional state. Purpose: The current study characterized the performance fatigability etiology, immediately after exercise cessation, and its relation to the dynamic changes in muscle O₂ saturation (SmO₂) at different TT phases. Methods: Twelve males performed three separated TT of different distances, in a crossover counterbalanced design, until the end of the fast-start (FS, 827 ± 135 m), even-pace (EP, 3590 ± 66 m), or end-spurt (ES, 4000 m) TT phases. Performance fatigability was characterized by using isometric maximal voluntary contractions (IMVC), whereas the maximal voluntary activation (VA) and contractile function of knee extensors (e.g., peak torque of potentiated twitches [TwPt]) were evaluated using electrically evoked contractions performed before and immediately after each exercise bouts. SmO₂, power output (PO), and EMG were also recorded. Results: Immediately after the FS phase, there were lower values for IMVC (−23%), VA (−8%), and TwPt (−43%) (all P < 0.001), but no further changes were measured after EP (IMVC, −28%; VA, −8%; TwPt, −38%). After the ES phase, IMVC (−34%) and TwPt (−59%) further decreased compared with the previous phases (P < 0.05). There were lower SmO₂ and higher EMG/PO values during FS and ES compared with EP phase. Conclusion: FS and EP phases had similar performance fatigability etiology, but ES showed further impairments in contractile function. This later finding might be due to the abrupt changes in SmO₂ and EMG/PO because of the high exercise intensity during the ES, which elicited maximal decline in contractile function at the finish line.