Neural influences on sprint running - Training adaptations and acute responses

Performance in sprint exercise is determined by the ability to accelerate, the magnitude of maximal velocity and the ability to maintain velocity agai nst the onset of fatigue. These factors are strongly influenced by metaboli c and anthropometric components. Improved temporal sequencing of muscle act ivation and/or improved fast twitch fibre recruitment may contribute to sup erior sprint performance. Speed of impulse transmission along the motor axo n may also have implications on sprint performance. Nerve conduction veloci ty (NCV) has been shown to increase in response to a period of sprint train ing. However, it is difficult to determine if increased NCV is likely to co ntribute to improved sprint performance. An increase in motoneuron excitabi lity, as measured by the Hoffman reflex (H-reflex), has been reported to pr oduce a more powerful muscular contraction, hence maximising motoneuron exc itability would be expected to benefit sprint performance. Motoneuron excit ability can be raised acutely by an appropriate stimulus with obvious impli cations for sprint performance. However, at rest reflex has been reported t o be lower in athletes trained for explosive events compared with endurance -trained athletes. This may be caused by the relatively high, fast twitch f ibre percentage and the consequent high activation thresholds of such motor units in power-trained populations. In contrast, stretch reflexes appear t o be enhanced in sprint athletes possibly because of increased muscle spind le sensitivity as a result of sprint training. With muscle in a contracted state, however, there is evidence to suggest greater reflex potentiation am ong both sprint and resistance-trained populations compared with controls. Again this may be indicative of the predominant types of motor units in the se populations, but may also mean an enhanced reflex contribution to force production during running in sprint-trained athletes. Fatigue of neural origin both during and following sprint exercise has impl ications with respect to optimising training frequency and volume. Research suggests athletes are unable to maintain maximal firing frequencies for th e full duration of, for example, a 100m sprint. Fatigue after a single trai ning session may also have a neural manifestation with some athletes unable to voluntarily fully activate muscle or experiencing stretch reflex inhibi tion after heavy training. This may occur in conjunction with muscle damage . Research investigating the neural influences on sprint performance is limit ed. Further longitudinal research is necessary to improve our understanding of neural factors that contribute to training-induced improvements in spri nt performance.