The Hoffmann reflex (H-reflex) is used to investigate the influence of Ia afferent projection on the spinal motoneuron activities. It has been suggested that the H-reflex is task-dependent and demonstrates adaptations to exercise training. Much of the previous research on the H-reflex was based on young populations. Little information is available on the H-reflex modulation in response to exercise and training in older populations. The objective of the research work presented in this thesis was to expand our knowledge on the effects of ageing and Tai Chi (TC) training on the soleus (SOL) H-reflex modulations in older adults. Four related studies have been conducted. Study I. The aim of this study was to determine the test-retest reliability of SOL Hreflex assessment during isometric muscle contraction at various intensities and ankle joint positions. The H-reflex was assessed when the ankle joint was placed at neutral (0°), plantarflexion 20°, and dosiflexion 20° positions and during isometric plantarflexion at 10%, 30% and 50% of the maximal voluntary contraction (MVC) levels, on two separate days, in a group of young adults (5 males and 5 females, age 24.9 ± 5 years). The results showed a high level of test-retested reliability (Intraclass Correlation Coefficients, ICC) for the SOL H-reflex tested at the neutral (ICC = 0.96) and plantarflexion (ICC = 0.92) positions, and a moderate level of ICC at the dorsiflexion position (ICC = 0.75) during rest. The results also demonstrated that assessing the SOL H-reflex during low intensity (10% MVC) of isometric muscle contractions yielded more reliable test-retest outcomes (ICC = 0.92 – 0.95) than that during contractions at higher intensities (30% and 50% MVC, ICC = 0.62 – 0.97), regardless of ankle joint positions. Study II. The aim of this study was to use a cross-sectional research design to compare the effects of ankle joint position and muscle contraction intensity on SOL H-reflex gain, latency and duration between young (10 males and 10 females, age 25.1 ± 4.0 years) and older (10 males and 10 females, age 74.2 ± 5.1 years) adults. The results showed that there were significant differences of the SOL H-reflex parameters between young and older adults under all testing conditions. However, when contraction intensity was progressively increased, a similar down-regulation of the SOL H-reflex gain was observed in both aged groups. This result may indicate a possible reservation of motor function regulated by the supraspinal mechanisms in older adults. Study III. The aim of this study was to investigate the effect of ageing on soleus (SOL) H-reflex modulation during shortening and lengthening muscle actions. Cross-sectional comparisons of the maximal amplitude of H-reflex (Hmax) and maximal amplitude of Mwave (Mmax) ratio were made between young (10 males and 10 females, age 24.4 ± 4.0 years) and older (10 males and 10 females, age 73.3 ± 5.0 years) adults during passive movements and voluntary contractions of the muscles around the ankle joint. The Hreflex modulation during upright standing under eyes open/closed and on stable/unstable surface conditions were also investigated in this study. The correlations of SOL Hmax between these dynamic muscle actions and postural tasks were evaluated. The results indicated that there were significant age-related differences of the SOL Hreflex modulation during passive and active shortening and lengthening muscle actions. Pearson correlation analysis revealed that the SOL Hmax during both the passive and active shortening and lengthening plantarflexions was significantly correlated with that during the postural tasks in young adults. However, older adults only demonstrated a significant correlation of the SOL Hmax between the passive shortening and lengthening actions and postural tasks. Study IV. The aim of this study was to investigate the effects of 12 weeks of TC training on the SOL H-reflex during upright standing under different visual and somatosensory conditions in older adults. Thirty-four volunteers (17 males and 17 females, age 72.9 ± 5.9 years) were assigned into a TC and a control group. The results demonstrated that the SOL Hmax/Mmax ratios during upright standing under eyes open/closed and stable/unstable surface conditions were significantly increased after the TC training in older adults. However, the mean displacements of centre of pressure (COP) in anteriorposterior and medial-lateral directions were not significantly changed after training. The results suggested that adaptive change of the SOL H-reflex is not related to control of static posture after 12 weeks TC training in older adults. This thesis demonstrated the age-related differences in the SOL H-reflex modulations during different ankle joint positions, isometric and dynamic ankle muscle actions and static upright standing. The adaptive change of the SOL H-reflex to 12 weeks of TC training may provide insight into the understanding of the neural adaptation to TC training in older adults
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