Neuromuscular performance of the plantar flexors in sprinters and the contribution to the sprint start response time sequence

The sprint start is an important phase of maximal sprinting, with various muscle-tendon units  (MTU) and joints contributing toward performance in this phase. The ankle joint and plantar flexor MTU are of particular importance. Ankle joint and plantar flexor MTU performance in explosive  tasks is dependent on neural, morphological, and mechanical factors and may contribute to  mechanical delays in sprint start response time (SSRT). Despite the importance of the ankle joint  and plantar flexor MTU, their precise contribution to sprint start performance remains unclear.  The aim of this thesis is to investigate the influence of ankle joint mechanical delays and plantar  flexor musculoskeletal factors on sprint start performance. This research has been implemented  by conducting a literature review, and a series of experiments examining: i) the relationship  between plantar flexor mechanical delays and SSRT, ii) the accuracy of EMG onset detection  methods, iii) the reliability of neuromuscular measures of plantar flexor function across ankle joint  angles, muscle activation modes, and sex, iv) the differences in plantar flexor neuromuscular and  mechanical function due to training background, and v) the influence of the neuromuscular system  on sprint start performance variables and SSRT. Examination of sprinters’ electromechanical  delay (EMD) during a heel-lift experiment, and SSRT during competitive sprint starts found the  combined influences of signal processing time (SPT) and EMD accounted for a significant  proportion of the variability in an athlete’s SSRT (37%). This provided initial suggestions that  certain athletes may gain a performance advantage due to reduced EMD. The accuracy of  automatic detection methods compared to visual detection in EMG onset detection was examined  across various contraction types. To attain the accuracy needed, visual detection is recommended.  The inclusion of the Teager-Kaiser energy operator (TKEO) as a conditioning step prior to visual detection improved inter and intra-rater reliability. A reliability analysis highlighted that maximal  voluntary torque (MVT), early-phase rate of torque development (RTD) measures (involuntary,  and voluntary explosive), late-phase RTD measures (involuntary only), involuntary force  development time (FDT), and tendon stiffness (DF angle only) can be reliably interpreted across  the measured ankle angles when assessing between-group differences in these parameters. A  comparison of the neuromuscular function of the plantar flexor MTU of sprinters and physically  active individuals revealed explosive RTD production was greater in sprinters across ankle angles  (10° plantarflexion (PF), 0°(anatomical zero (AZ)), 10° dorsiflexion (DF)) for early-phase RTD  and at 10° DF only for late-phase RTD. Improved synchrony in neural activation of the  predominantly fast-twitch gastrocnemii muscles and the intrinsic contractile properties of the  sprinters appears to explain the greater early-phase RTD compared to physically active  individuals. Sprinters’ higher late-phase RTD at the DF angle is consistent with increased plantar  flexor neuromuscular activation and relative MVT at this angle, which are key determinants of  late-phase RTD. This work has implications for practitioners, suggesting that practice of explosive  movements can improve early and late-phase RTD of the plantar flexor MTU, mainly through  neural adaptations. During the sprint start, ankle joint muscle activation onsets and mechanical  delay parameters were related to sprint start performance variables of block clearance time (BCT)  and center of mass velocity (COMV) at block exit as well as SSRT. Improved rapid muscle  activation appears to be more important for sprint start performance than maximal muscular  strength or tendon stiffness, which aligns with previous research on the determinants of explosive  force production. Furthermore, reduced mechanical delay periods of the ankle joint were related  to a decreased SSRT.