Increased variability in the movement control strategy for power production at the start of maximal cycling performed following pre-fatiguing knee extension exercise, was supported by increased inter-individual variability in the application of the crank forces (Figure 4.11). During the extension phase, this likely evolved from increased inter- individual variability in VAS EMG. This result supports the suggestion that some participants altered the movement strategy for power production during the extension phase, thereby increasing inter-individual variability in the demands for the knee extensors to generate effective crank forces. Alternatively, increased inter-individual variability in VAS EMG may have evolved from the large spread of the reduction in VA following pre-fatiguing exercise, thereby leading to large differences in the capacity to maximise the discharge number and rate of vastii motor units (Gandevia 2001). It is also possible that the large spread in the change in GMAX EMG (range = -26% to +20%) and co-activation for all muscle pairs (especially VAS/APF, range = -20% to +20%) influenced the intra-individual correlations between knee extensor IMVF and extension power. Unequal reductions in GMAX EMG within the participants (n = 8), may have evolved from differences in the level of knee extensor fatigue and associated group III and IV afferent feedback to the GMAX motoneuron pool. Indeed, the inhibitory influence of group III and IV afferent feedback is not specific to the fatigued muscles involved in a locomotor task such as cycling (Sidhu et al. 2014), and inhibition of GMAX is likely as it is part of the same muscle synergy as VAS muscles (Sacco et al. 1997; Ciubotariu et al. 2004; Hug et al. 2010; Raasch and Zajac 1999). The reduction in GMAX EMG may indicate non- local fatigue development in this muscle due to high levels of central fatigue in the knee extensors (Halperin et al. 2015). Unequal reductions in VAS/APF co-activation (n = 8) revealed that the overlap of individual EMG bursts was reduced for some participants more than others. This change in co-activation likely influenced the ability of the ankle plantar flexors to effectively transfer knee extension power to the crank during the first part of the extension phase (crank angle ~0-90°) (Zajac et al. 2002), thereby leading to suboptimal inter-muscular coordination and increasing inter-individual variability in the effective crank forces.
Evidence for an alteration in the movement strategy during the flexion phase was supported by increased inter-individual variability in HAM, TA and RF EMG (Figure
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4.11). These muscles have previously been shown to exhibit large inter-individual variability due to their role in transferring power amongst the segments and orientating crank forces (Hug et al. 2004; Hug et al. 2008; Hug et al. 2010; Ryan and Gregor 1992). Indeed, the knee extensors generate the most amount of work during maximal cycling (Raasch et al. 1997), and therefore inter-individual differences in knee extensor fatigue are likely to lead to unequal requirements to distribute and transfer leg energy to the crank. It is also possible that increased inter-individual variability in activation patterns of flexion muscles occurred due to some participants focusing on pulling up on the pedals during the flexion phase. HAM muscles have the capacity to generate large tangential crank forces during the early flexion phase (Zajac et al. 2002; Elmer et al. 2011), and a similar strategy has previously been reported during submaximal cycling and in the presence of unilateral knee extensor fatigue (Brochner Nielsen et al. 2016). Due to the bilateral nature of the cycling exercise and the mechanical coupling of the cranks, this strategy would also assist in accelerating the contralateral crank during the extension phase. Hence, evidence for large inter-individual variability and a change to the movement strategy during the flexion phase, may have influenced the correlations between knee extensor IMVF and power production during the flexion and extension phases. Intra-individual correlations between knee extensor IMVF and flexion power were also likely influenced by the large spread in the reduction in HAM EMG (range = - 0.5% to -34%), TA EMG (range = +1% to -49%) and RF EMG (range = -2% to -29%). As the contractile properties of HAM and TA muscles were presumably unchanged following pre-fatiguing exercise, it is possible that intra-individual differences in knee extensor fatigue and group III and IV afferent feedback led to variable degrees of non-local fatigue in these muscles (Sidhu et al. 2014; Halperin et al. 2015). At the end of the maximal cycling exercise, large inter-individual variability in the activation patterns of HAM, RF and TA muscles occurred without increased variability in the application of the crank forces. This finding supports previous results reported during submaximal cycling exercise (Hug et al. 2008) and illustrates the redundancy of the motor system during fatiguing maximal cycling exercise.
4.4.4 Limitations
The associations between knee extensor IMVF and power production during maximal cycling may have been blurred by inter-individual differences in the rate of fatigue recovery during the 1-min time delay between fatigue assessment and the start of maximal cycling (Froyd et al. 2013; Hamada et al. 2003; Cheng and Rice 2005). This time delay was unavoidable due to the requirement to transfer participants from the pre-
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fatiguing exercise equipment to the cycle ergometer. From an anatomical perspective, it is also possible that the stimulating electrode configuration (i.e. anode in the gluteal fold) may have evoked some co-activation in the antagonist hamstring muscles and influenced the knee extensor twitch force response.
4.5 Conclusion
The amount of knee extensor fatigue developed during isolated pre-fatiguing exercise does not determine the reduction in power output during the extension or flexion phases of maximal cycling exercise. This is likely due to changes in motor command, including increased variability in the movement control strategy and non-local fatigue development in lower-limb muscle groups. More research is required to elucidate the movement strategies that influence the relationship between knee extensor fatigue and crank power during maximal cycling, and to determine the rate of recovery between fatigue and crank power measurements.
Author contributions
SJ O’bryan was responsible for conception and design of experiments, data collection and analysis, preparation of figures, drafting the chapter and revising the chapter for important intellectual content. JL Taylor was responsible for reviewing the chapter and providing feedback. DM Rouffet was responsible for conception and design of experiments, data collection and analysis, preparation of figures and reviewing the chapter and providing feedback. R Bourke assisted with data collection.
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