oa Does supraspinal fatigue limit repeated sprint ability?

Background and Objectives: Most previous explanations of fatigue during repeated sprint exercise have focused on muscular factors associated with cellular mechanisms. At present, the nature of the neural adjustments and the extent to which corticospinal responsiveness is altered after the completion of repeated sprint exercise are yet to be elucidated. This study was undertaken to test the hypothesis that, in addition to the large peripheral fatigue, supraspinal factors would contribute substantially to neuromuscular adjustments of the quadriceps muscle in response to a repeated sprint cycling exercise. Methods: Twelve active individuals performed 10 x 6-s 'all out' sprints on a cycle ergometer (recovery= 30 s), followed 6 min latter by 5 x 6-s sprints (recovery= 30 s). Surface electromyographic activity was continuously recorded during the exercise. In addition, transcranial magnetic and electrical femoral nerve stimulations during brief (5-s) and sustained (30-s) isometric contractions of the knee extensors were performed before and 3 min post-exercise. Results: Peak power output and vastus lateralis root mean square activity decreased across sprint repetitions (P <0.001). These decrements were significantly (r= 0.89; P <0.001) correlated. Maximal strength of the knee extensors decreased during brief and sustained contractions (∼11% and 9%, respectively; P <0.001). Cortical voluntary activation values (∼90%), motor evoked potential amplitude and silent period duration responses measured during briefs contractions were unaltered. While cortical voluntary activation declined (P <0.01) during the sustained maximal contraction in both test sessions, larger reductions occurred (P <0.05) after exercise. Lastly, resting twitch amplitude in response to both femoral nerve and cortical stimulation was largely reduced (>40%, P <0.001) following exercise. Conclusion: The novel findings provided by motor cortex stimulation are that exercise induced reductions in cortical voluntary activation were seen in sustained, but not brief, maximal isometric contractions of the knee extensors. This, along with the lack of corticospinal responsiveness impairment from pre- to post-exercise, indicates that supraspinal processes do not necessarily limit repeated sprint ability. Rather, fatigue exhibited by the quadriceps when cycling sprints are repeated resides within the muscle itself, due to limitations in energy supply and/or the intramuscular accumulation of metabolic by-products.