Neuromuscular performance of the plantar flexors in sprinters and the contribution to the sprint start response time sequence
Creators: name: Crotty, Evan David
Publisher: name: University of Limerick
Year: 2022
Subjects: Sports science and exercise

description: 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.