Neuromuscular Plantar Flexor Performance of Sprinters versus Physically Active Individuals.

INTRODUCTION: Comparison of the neuromuscular performance of different athlete types may give insight into the in vivo variability of these measures and their underpinning mechanisms. The study aims to compare the neuromuscular function of the plantar flexors of sprinters and physically active individuals to assess any differences in explosive force performance. METHODS: Neuromuscular performance of a group of sprinters (highly trained/national level, n = 12; elite/international level, n = 2) and physically active individuals ( n = 14) were assessed during involuntary, explosive, and maximum voluntary isometric plantar flexions, across different muscle-tendon unit (MTU) lengths (10° plantarflexion, 0° (anatomical zero/neutral), and 10° dorsiflexion). Plantarflexion rate of torque development (RTD) was measured in three 50-ms time windows from their onset. The synchronous activation of the plantar flexor agonist muscles was calculated as the time difference between 1) the first and last muscle onset and 2) the onsets of the two gastrocnemii muscles. Muscle size and MTU stiffness were assessed using sonograms of the medial gastrocnemius and myotendinous junction. RESULTS: Sprinters exhibited greater involuntary RTD across time points (0-50 ms, 50-100 ms) and MTU lengths. In addition, sprinters demonstrated greater early phase voluntary RTD (0-50 ms, 50-100 ms) across MTU lengths. Sprinters also demonstrated greater late-phase RTD (100-150 ms), and relative maximal voluntary torque at the DF angle only. The sprinters demonstrated a more synchronous activation of the gastrocnemii muscles. There were no observable differences in muscle size and MTU stiffness between groups. CONCLUSIONS: These findings suggest sprint-specific training could be a contributing factor toward improved explosive performance of the plantar flexors, particularly in the early phase of muscular contraction, evidenced by the greater explosive torque producing capabilities of sprinters.

Ankle and Plantar Flexor Muscle-Tendon Unit Function in Sprinters: A Narrative Review.

Maximal sprinting in humans requires the contribution of various muscle-tendon units (MTUs) and joints to maximize performance. The plantar flexor MTU and ankle joint are of particular importance due to their role in applying force to the ground. This narrative review examines the contribution of the ankle joint and plantar flexor MTUs across the phases of sprinting (start, acceleration, and maximum velocity), alongside the musculotendinous properties that contribute to improved plantar flexor MTU performance. For the sprint start, the rear leg ankle joint appears to be a particularly important contributor to sprint start performance, alongside the stretch-shortening cycle (SSC) action of the plantar flexor MTU. Comparing elite and sub-elite sprinters revealed that elite sprinters had a higher rate of force development (RFD) and normalized average horizontal block power, which was transferred via the ankle joint to the block. For the acceleration phase, the ankle joint and plantar flexor MTU appear to be the most critical of the major lower limb joints/MTUs. The contribution of the ankle joint to power generation and positive work is minimal during the first stance, but an increased contribution is observed during the second stance, mid-acceleration, and late-acceleration. In terms of muscular contributions, the gastrocnemius and soleus have distinct roles. The soleus acts mainly as a supporter, generating large portions of the upward impulse, whereas the gastrocnemius acts as both an accelerator and a supporter, contributing significantly to propulsive and upward impulses. During maximum velocity sprinting the ankle joint is a net dissipater of energy, potentially due to the greater vertical loading placed on the plantar flexors. However, the ankle joint is critical for energy transfer from proximal joints to ground force application to maintain velocity. In terms of the contribution of musculoskeletal factors to ankle joint and plantar flexor performance, an optimal plantar flexor MTU profile potentially exists, which is possibly a combination of several musculoskeletal factors, alongside factors such as footwear and technique.

Reliability of mechanical properties of the plantar flexor muscle tendon unit with consideration to joint angle and sex.

The reliability of mechanical measures can be impacted by the protocol used, including factors such as joint angle and the sex of participants. This study aimed to determine the inter-day reliability of plantar flexor mechanical measures across ankle joint angles and contraction types and consider potential sex-specific effects. 14 physically-active individuals participated in two identical measurement sessions involving involuntary and voluntary plantar flexor contractions, at three ankle angles (10° plantarflexion (PF), 0° (anatomical zero (AZ)), and 10° dorsiflexion (DF)), while torque and surface EMG were recorded. The reliability of mechanical parameters of maximal voluntary torque (MVT), rate of torque development (RTD), electromechanical delay, and tendon stiffness were assessed using absolute and relative reliability measures. MVT measures were reliable across ankle angles. RTD measures showed good group level reliability and moderate reliability for an individual during the early phase of contraction across ankle angles. Explosive voluntary torque measures tended to be less reliable from 50 ms onward, with varied reliability across angles for late-phase RTD. Tendon stiffness demonstrated the best reliability at the DF angle. Sex-based differences in the reliability of tendon measures found that females had significantly different initial tendon length between testing sessions. Despite this, tendon excursion, force, and stiffness measures demonstrated similar reliability compared to males. Ankle angle changes influence the reliability of plantar flexor mechanical measurements across contraction types, particularly for voluntary contractions. These results highlight the importance of establishing potential protocol effects on measurement reliability prior to quantifying plantar flexor mechanical measures.

Sprint start performance: the potential influence of triceps surae electromechanical delay.

In the sprint start, a defined sequence of distinct response delays occurs before the athlete produces a movement response. Excitation of lower limb muscles occurs prior to force production against the blocks, culminating in a movement response. The time delay between muscle excitation and movement, electromechanical delay (EMD), is considered to influence sprint start response time (SSRT). This study examined the delay in sprint start performance from EMD of the triceps surae muscle and examined whether certain sprinters gain an advantage in SSRT. Nineteen experienced sprinters performed sprint starts from blocks, with SSRT measured by an International Association of Athletics Federations (IAAF)-approved starting block system. EMD times were detected during a heel-lift experiment. Using revised SSRT limits, based on concerns over the validity of the IAAF 100 ms false start limit, EMD produced a significant moderate correlation with SSRT (r = 0.572, p = 0.011). Regression analysis determined that together, EMD and signal processing time (the delay between the auditory signal and muscle excitation) accounted for 37% of the variance in SSRT. Initial results suggest EMD is part of the response time process and that certain athletes may gain a performance advantage due to reduced EMD.

Onset detection in surface electromyographic signals across isometric explosive and ramped contractions: a comparison of computer-based methods.

Objective. Accurate identification of surface electromyography (EMG) muscle onset is vital when examining short temporal parameters such as electromechanical delay. The visual method is considered the 'gold standard' in onset detection. Automatic detection methods are commonly employed to increase objectivity and reduce analysis time, but it is unclear if they are sensitive enough to accurately detect EMG onset when relating them to short-duration motor events.Approach. This study aimed to determine: (1) if automatic detection methods could be used interchangeably with visual methods in detecting EMG onsets (2) if the Teager-Kaiser energy operator (TKEO) as a conditioning step would improve the accuracy of popular EMG onset detection methods. The accuracy of three automatic onset detection methods: approximated generalized likelihood ratio (AGLR), TKEO, and threshold-based method were examined against the visual method. EMG signals from fast, explosive, and slow, ramped isometric plantarflexor contractions were evaluated using each technique.Main results. For fast, explosive contractions, the TKEO was the best-performing automatic detection method, with a low bias level (4.7 ± 5.6 ms) and excellent intraclass correlation coefficient (ICC) of 0.993, however with wide limits of agreement (LoA) (-6.2 to +15.7 ms). For slow, ramped contractions, the AGLR with TKEO conditioning was the best-performing automatic detection method with the smallest bias (11.3 ± 32.9 ms) and excellent ICC (0.983) but produced wide LoA (-53.2 to +75.8 ms). For visual detection, the inclusion of TKEO conditioning improved inter-rater and intra-rater reliability across contraction types compared with visual detection without TKEO conditioning.Significance. In conclusion, the examined automatic detection methods are not sensitive enough to be applied when relating EMG onset to a motor event of short duration. To attain the accuracy needed, visual detection is recommended. The inclusion of TKEO as a conditioning step before visual detection of EMG onsets is recommended to improve visual detection reliability.