J Timothy Inglis
Relevant Degree Programs
Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - Mar 2019)
Humans are unable to stand still, but rather experience continuous oscillations of the body known as postural sway. While the origins of postural sway are largely unknown, theories suggest that postural sway originates from the interaction between movements of the body (centre of mass, COM) and forces beneath the feet (centre of pressure, COP). The COP is commonly assumed to control or correct for deviations of the body from equilibrium, and delays or errors in control result in postural sway. In a sequence of 5 studies, this thesis used a novel experimental paradigm to investigate how postural sway is controlled or used by the central nervous system.The first of five experiments tested whether COP displacements would be reduced when the body was externally stabilized, as traditional theories would predict. Contrary to our hypothesis, COP displacements actually increased, suggesting an exploratory role for postural sway. Using the same experimental protocol, Study 2 provided participants with visual feedback of the COM or COP to determine if increases in COP displacements could be the result of sensory illusions or motor drift. Study 3 provided participants with an explicit verbal cue indicating how and when COM stabilization would occur to determine whether increases in COP displacements reflect an attempt to adapt the internal model of the body during stance. Study 4 examined whether increases in COP displacements could be the result of increases in oscillatory cortical drive. Using an upper limb postural task, the fifth and final study extended the findings from Studies 1-4 to determine whether exploratory behaviour may be a more global phenomenon and observed in other postural tasks that do not involve whole body stability.Individually, the results of Studies 1-4 provide evidence which challenges traditional theories of postural control. In addition, they provide evidence against alternative explanations for increases in COP displacements and suggest that this behaviour may be a more global phenomenon and observed in any postural task (Study 5). Collectively, they provide evidence supporting a potential exploratory role of postural sway and question the basis of current clinical practices designed to deal with balance control deficits due to age or disease.
Master's Student Supervision (2010-2017)
The senses of position and movement are known collectively as kinaesthesia. Muscle spindles are length sensitive receptors, considered pivotal to these senses. Mechanical muscle vibration artificially stimulates muscle spindles and this can lead to illusory limb postures and movements, thereby disrupting kinaesthesia. Exercise induced fatigue also impairs kinaesthesia and this impairment persists if the exercise involves lengthening contractions that create eccentric muscle damage. It is not entirely clear why these lasting impairments occur. Using a targeted movement sequence with the unseen arm (a task that relies heavily on muscle spindles), this study utilized a novel paradigm to investigate task performance both before and after an eccentric based exercise protocol. By investigating the influence of vibration applied to the involved musculature, the results provide insight into how exercise acutely disrupts kinaesthesia. It was found that mechanical muscle vibration created a robust effect on task accuracy at all points during the study, causing participants to undershoot the targets (as previously described in the literature). The effects of exercise also caused a consistent error in task performance, but did not appear to influence the effect of vibration. This suggests that the nervous system continues to rely heavily on muscle spindles, even when they reside in a muscle exposed to damaging eccentric contractions.
The vestibular system is a complex network that plays an important role in balance control. When postural perturbations are exerted on the individual, it appears that the vestibular system plays a role in modulating the amplitude of the responses. The vestibular system is also susceptible to changes in psychosocial and autonomic states. Despite these findings, the inability to precisely record from and directly manipulate the system has hindered the field in completely understanding how the vestibular system is involved in balance. Therefore, the purposes of this thesis were 1) to investigate if there was phase-dependent modulation of the vestibular reflex during the postural responses and 2) to determine if the vestibular reflex was altered with postural threat.Stochastic vestibular stimulation (SVS) was used to electrically probe the vestibular system while participants stood on a rotating platform. The vestibular reflex was analyzed by estimating the vestibulo-muscular (SVS-EMG) relationship using time-dependent SVS-EMG coherence throughout the postural response for the first purpose while, for the second purpose, SVS-EMG coherence, cumulant density, and gain were calculated between non-threatening and threatening conditions. Results from this thesis were unable to determine if there were phase-dependent modulations of the SVS-induced vestibular reflex. However, further testing and pilot data provides a promising method for further investigation. Furthermore, an increase gain in and coupling of the vestibular reflex was observed in the most muscles while a decrease in coupling was observed for the paraspinal muscles in the threatening situation. These results suggest that the central nervous system has the ability to prepare the body for responding to an upcoming postural perturbation by optimizing the vestibular output to the muscles.
Loud acoustic stimuli (>115 dB) are known to evoke electromyographical (EMG) responses in human musculature that differ with body position, presentation rate, and stimulus duration. Long duration acoustic tones (40 ms) with an inter-stimulus interval of 3 – 5 s evoke small amplitude reflex responses in tonically contracted limb musculature, whereas short duration acoustic tones (0.1 – 20 ms) with an inter-stimulus interval of 0.2 – 1 s can evoke EMG responses in limb muscles that are posturally engaged. Therefore the purpose of this study was to investigate the similarities and differences of the EMG responses evoked with repeated short and long duration acoustic tones in tonically contracted axial and limb musculature of supine participants. Methods: Twenty subjects (aged 19 – 30) were exposed to 256 presentations of air conducted (AC) acoustic stimuli that were 7 and 40 ms in duration (500 Hz; 118 dB SPL). Two blocks of 128 AC stimuli at each stimulus duration, and one block of no stimuli were presented randomly and binaurally through calibrated headphones. Surface EMG was sampled from the right sternocleidomastoid (SCM), biceps brachii (BB), and soleus (SOL) while participants maintained low level contractions in each muscle. Results: Repeated 7 and 40 ms AC stimuli evoked a myogenic potential in the tonically contracted SCM, BB, and SOL in at least 80%, 75%, and 75% of participants respectively. Significant effects of stimulus duration were observed in the SCM and SOL, where significant peaks occurred 5.4 and 6.7 ms earlier in the SCM, and 9.3 ms earlier in the SOL with a shorter stimulus. No significant effects were observed in the BB. Conclusion: We have shown that repeated short duration acoustic stimuli presented at a short inter-stimulus interval can evoke reflex responses in tonically contracted limb muscles which has not been shown before. These observations suggest that the EMG responses observed here may differ from those that are influenced by postural engagement.