The development and origin of muscle fatigue after hexagonal-barbell deadlift exercise of different loads and velocities: contributing to a knowledge-base for coaches to make evidence-informed decisions regarding athletic training programmes

Athletes perform bouts of resistance exercise to develop athletic performance and reduce the risk of injury. Fatigue in response to exercise can be associated with alterations to voluntary muscle activation or contractile function. Recent literature has proposed terms of performance fatigability and perceived fatigability to better describe the broad interpretation of fatigue during human performance. Accordingly, performance fatigability depends on the capabilities of the contractile machinery and nervous system to provide adequate activation signal to maintain the task. The rationale for the experimental chapters in this thesis was established after conducting a review of the literature relating to fatigue and human performance (chapter 2). This review highlighted that studies of performance fatigability have generally focused on locomotor exercise such as running or cycling. However, very little work has examined the influence of resistance exercise on performance fatigability. Specifically, it was identified that performance fatigability during multi-joint resistance exercise performed with different loads until mechanical failure and modification of lifting tempo required further research. For these reasons, two experimental studies were conducted to address these gaps in knowledge. The first study (chapter 3) examined the influence of an exhaustive bout of high-load and moderate-load hexagonal-barbell deadlift (HBD) resistance exercise on acute changes and recovery of contractile function and voluntary activation in resistance-trained males. The results indicate that both voluntary activation and contractile function were reduced after moderate-load HBD exercise, but not high-load exercise. Additionally, after a 24-h recovery period, both voluntary activation and contractile function were impaired. Interestingly, the partial recovery was due to a near return to before exercise values of contractile function and therefore the incomplete recovery was due to reduced voluntary activation. In chapter four, the influence of a structured bout of volume load-equated HBD exercise with manipulation of lifting tempo on changes to contractile function and voluntary activation was examined. The main finding was that slow tempo and fast tempo HBD exercise resulted in similar reductions to both voluntary activation and contractile function. However, it is unknown if this remains the case when greater resistance exercise volumes are performed. Additionally, further research is required to understand the mechanisms of reduced voluntary activation during fast tempo resistance exercise. Finally, the findings from the experimental chapters are summarised and practical recommendations for the prescription of resistance exercise within athletic training programmes are presented. In conclusion, because of the lowered reductions of contractile function and voluntary activation observed after high-load resistance exercise, it may be a preferable training modality for coaches to employ during the in-season period or during times of intense concurrent training. Additionally, changes to contractile function and voluntary activation associated with fast and slow tempo volume load-equated exercise is similar. However, more work is required to see if this remains the case when larger volumes of work are completed.