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

Authors: Rafael de Almeida Azevedo, Fabio Milioni, Juan M Murias, Romulo Bertuzzi, Guillaume Y Millet.

Published: 18 August 2020 - Med Sci Sports Exerc.

ABSTRACT

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 at different TT phases.

Methods: Twelve males performed three separated TTs of different distances, in a crossover counterbalanced design, until the end of the fast-start (FS, 827±135m), even-pace (EP, 3590±66m) or end-spurt (ES, 4000m) TT phases. Performance fatigability was characterized by using isometric maximal voluntary contractions (IMVC), while 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. Muscle O2 saturation (SmO2), power output (PO) and electromyographic (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, the IMVC (-34%) and TwPt (-59%) further decreased compared to the previous phases (p<0.05). There were lower SmO2 and higher EMG/PO values during FS and ES compared to 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 end-spurt, which elicited maximal decline in contractile function at the finish line.

Keywords:: Central fatigue; Exercise performance; NIRS; Oxygen availability; Peripheral fatigue.

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