Victor S Selvanayagam (2012). EARLY NEURAL ADAPTATION TO STRENGTH TRAINING PhD Thesis, School of Human Movement Studies, The University of Queensland.

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Author Victor S Selvanayagam
School, Centre or Institute School of Human Movement Studies
Institution The University of Queensland
Publication date 2012-01
Thesis type PhD Thesis
Supervisor Associate Professor Dr Timothy J Carroll
Associate Professor Dr Stephan Riek
Total pages 137
Total colour pages 3
Total black and white pages 134
Language eng
Subjects 110603 Motor Control
110602 Exercise Physiology
110601 Biomechanics
Abstract/Summary The purpose of this thesis was to advance our understanding in the area of neural adaptation to strength training. Current evidence has yet to establish the precise nature of this adaptation. The thesis comprises a series of reviews and experimental chapters that consider the idea that neural adaptation to strength training may share similar mechanisms to some forms of motor learning (i.e. use-dependent learning). For strength to increase by neural mechanisms, the nervous system must act by increasing neural drive to the agonist and/or synergist muscles that contribute to torque and/or inhibit neural drive to antagonist muscles that oppose torque in the desired direction. This process is similar to some forms of motor learning whereby neural connectivity for the desired movement is strengthened. A wealth of information about neural responses to single-session training paradigms is available in the motor learning literature. Chapter 2 therefore provides a brief review of motor learning paradigms that could be incorporated into research on strength training, including the mechanisms identified. This section concludes by providing a conceptual framework for early neural responses to training, highlighting the types of adaptation that are likely to occur and how this should contribute to long term adaptation and ultimately strength gains. The first experimental chapter (chapter 3) addresses a methodological issue (cross-talk) that could affect the findings of studies subsequently reported in the thesis and other studies using non-invasive stimulation techniques (i.e. transcranial magnetic stimulation (TMS) and reflex studies). Cross-talk occurs when electromyographic signals generated by non-target muscles are recorded from electrodes intended to pick up signals from a specific target muscle. This chapter revealed that cross-talk can be large between muscles of the forearm and hand and provided a solution by designing a systematic method that can be used to quantify cross-talk during standard recording conditions. This method can also be used to identify cross-nerve stimulation. Chapter 4 examined whether strength training causes rapid adaptation to corticospinal responsiveness by assessing changes to the direction of twitches evoked by TMS, which are the result of the overall output of force produced by multiple muscles. Following the detection of baseline TMS-evoked twitch direction, a single session of strength training was conducted involving maximal force production 90o away from the baseline direction. After training there was a shift in TMS-evoked twitches towards the training direction. This outcome occurred for all three training conditions that required maximal neural drive but varied in the rate of force development and contraction duration [i.e. rapid (ballistic) or ballistic sustained or slow, sustained contractions]. The strongest effect was found when the training program incorporated both ballistic and sustained contractions. Since no change occurred at the muscle level, this change was attributed to the adaptation of the nervous system that strengthens the neural networks in favour of the training direction. This change was also similar in characteristics to that previously found in ballistic motor learning, suggesting the possible involvement of similar neural mechanisms. Chapter 5 revealed that strength training (ballistic sustained contraction) also produced a shift in the direction of voluntary force production towards the training direction in a low-force isometric aiming task. The shift occurred even for targets that were distant (at least + 90o) from the training direction. This suggests that early neural adaptation to strength training causes an aiming bias that generalises broadly to affect movements for a wide range of directions. A second part of this study showed that the aiming bias occurred towards the training direction defined in extrinsic space rather than muscle space. This information was revealed by performing strength training in a 90o pronated forearm posture, whereas the low-force aiming task was performed in a neutral forearm posture. The shift in aiming was towards the training direction defined in extrinsic space, even for targets where this resulted in errors away from the pulling direction of the trained muscles in the alternative posture. It was concluded that a strengthening of neural networks had occurred for neurons that code movement direction in extrinsic space. Finally, the experiment also showed that strong neural drive was necessary to induce this change since low force contractions showed no bias towards the training direction as those found with high force contractions. In summary, the work described in this thesis provides evidence of neural adaptation to strength training as early as after a single session of training. This adaptation was found to be similar to those identified in some forms of motor learning. The research adds to understanding of neural responses to strength training by showing that the nervous system can learn to produce high force more effectively, through the strengthening of neural networks in favour of the training direction in extrinsic space. The rapid and transient changes revealed are thought to be a necessary step that precedes more long term neural adaptation that ultimately enhances strength.
Keyword Strength Training
Motor Learning
Use-dependent Learning
Transcranial Magnetic Stimulation
Additional Notes Colour printing: Pg 70, 72, 76

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Created: Thu, 14 Jun 2012, 15:03:29 EST by Victor Selvanayagam on behalf of Library - Information Access Service