Author Identifiers

Cody Wilson

https://orcid.org/0000-0001-6310-8471

Date of Award

2020

Degree Type

Thesis

Degree Name

Master of Science (Sports Science)

School

School of Medical and Health Sciences

First Advisor

Anthony Blazevich

Second Advisor

Nicolas Babault

Abstract

The present research explored the mechanisms underpinning the enhancement of voluntary knee extensor torque after (1) extensive task-specific practice, and then (2) brief, intense warm-up exercise (conditioning activity; CA) as part of a complete warm-up routine. The same warm-up was completed in two Experiments (detailed below). In Experiment 1, voluntary (180⁰·s-1; T180) and electrically-evoked (isometric) knee extensor torques and electromyogram (EMG) and muscle temperature (Tm) data were recorded before and after (1) extensive task-specific practice where peak voluntary knee extensor performance was achieved, and (2) two isokinetic CAs matched for total concentric contraction time (CA60: 5 repetitions at 60⁰·s-1 vs. CA300: 25 repetitions at 300⁰·s-1; 7.5 s).

In Experiment 1, nineteen healthy adults (13 male, 6 female) completed the study. T180 increased after task-practice in both conditions (CA60: 6.2 ± 8.5% vs. CA300: 8.4 ± 8.0%). After the CA, T180 decreased at 1 min (-7.0 ± 8.6%) but then returned to post-TP values at 9 min post-CA (0.2 ± 9.8%) in CA60, while T180 did not change at any post-CA time point compared to immediately post-TP (1-3%) but remained significantly elevated (~9-12%) compared to baseline in CA300. Tm increased from baseline to post-TP (CA60: 0.6 ± 0.3°C vs. CA300: 0.4 ± 0.2°C) in both conditions and then increased further from post-TP (by 0.5 ± 0.2°C in CA60 and 0.8 ± 0.3°C in CA300). The 20 Hz torque measured with versus without shortinterval “double pulse” at stimulation onset (i.e. VFT:20 ratio) was decreased (-50-90%) after the CA. Further, peak twitch torque (Ttw,peak) increased from baseline to post-TP in both conditions (CA60: 13.5 ± 9.1% vs. CA300: 16.0 ± 7.2%), but did not increase further after the CAs ( < 2%) in either condition (9-14% above baseline), indicating that calcium sensitivity of the acto-myosin complex may have been enhanced. M-wave normalised VL EMG (EMGVL,40ms/M) increased from baseline to post-TP (CA60: 68.9 ± 105.7% vs. CA300: 116.4 ± 170.5%) then decreased from post-TP to 1 min post-CA (-22.0 ± 48.2%) in CA60, but remained elevated at all post-CA tests in CA300. Ttw,peak (r = 0.53), Tm (r = 0.63), VFT:20 ratio (r = 0.32) and EMGVL,40ms/M (r = 0.30) were all moderately-strongly correlated with T180 in CA300, but not CA60 ( < r = 0.15). EMG was the only variable that was associated with T180 in both conditions (r = 0.3-0.5). Therefore, EMG was included in a model with Tm (i.e. ‘EMGVL,T180/M + Tm’) and observed a moderate correlation with T180 in CA60 (r = 0.38) and a strong correlation in CA300 (r = 0.60). This suggests that muscle activation capacity mediates the effect of muscle temperature on voluntary muscle performance.

In Experiment 2, exercise-induced muscle fluid shifts after warm-up (identical to Experiment 1) were estimated on 19 healthy adults (13 male, 6 female) through assessments of muscle blood content (total haemoglobin concentration; THb), size (VL muscle thickness) and stiffness (VL passive stiffness). THb increased at post-TP (CA60: 55.5 ± 133.7% vs. CA300: 156.2 ± 267.5%) and continued to increase in both conditions to 9 min (CA60: 261.0 ± 226.5% vs. CA300: 759.8 ± 1141.8%). VL muscle thickness increased statistically from post-TP to all post-CA tests (1.8-2.4%) in CA60 but not CA300 (0.6-1.2%), while passive muscle stiffness increased from baseline to 1 min post-CA (3.8 ± 5.0%) in CA300 but not CA60 (~1%). THb was moderately correlated with VL muscle thickness (CA60: r = 0.31 vs. CA300: r = 0.34) and passive muscle stiffness (CA300: r = 0.48). Further, VL muscle thickness showed a small correlation with passive muscle stiffness (r = 0.28) in CA300 only.

Collectively, the present data support the hypothesis that significant enhancement of voluntary muscle performance can be achieved with sufficient task-specific practice, and that this enhancement was associated with enhanced muscle activation capacity (i.e. increased early-burst EMG), possibly indicating motor pattern optimisation as well as increased Tm (and other related variables). Furthermore, including brief, intense warmup exercise (CA) did not enhance voluntary muscle performance beyond the level achieved by task-specific practice, despite inducing a greater increase in numerous physiological markers (i.e. Tm, muscle water/blood, Ca2+ sensitivity, etc.) that could influence voluntary muscle performance. However, the most intriguing finding was that the performance of an under-speed (high force) CA (commonly utilised in the literature) resulted in a decrease in voluntary muscle performance after the task practice period (i.e. back to baseline levels) and that this was associated with decreased muscle activity (peak and early EMG). Therefore, voluntary muscle performance after warm-up is strongly associated with changes in both peak and early EMG; muscle activation capacity may therefore be a mediating variable that affects the normal relationship between changes in Tm and changes in dynamic muscle function.

Access Note

Some images are not available in this version of the thesis due to copyright considerations.

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