J Timothy Inglis
Relevant Degree Programs
Affiliations to Research Centres, Institutes & Clusters
Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - April 2022)
There are many structures within the nervous system that, as a whole, are responsible for the control of movement and balance. The spinal cord plays an important role in sensorimotor processing, it integrates sensory signals from the periphery as well as signals from the brain to control muscle activation. The purpose of this thesis was to characterize the short latency (spinally mediated) lower limb muscle responses to proprioceptive perturbations during standing, and examine how they are influenced by ageing and chronic stroke. Chapter 2 develops an innovative methodology to characterize muscle responses to proprioceptive perturbations during standing. Here, we examined the association between noisy (10-115 Hz) suprathreshold Achilles tendon vibration and ongoing triceps surae muscle activity. We observed responses in soleus across a broad frequency bandwidth (~10-80 Hz). Consistent responses were obtained with short trial durations (
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 - 2021)
Proprioceptive feedback from muscle spindles around the ankle joint is critical for balance control. The muscle spindles lie in parallel with muscle fibres and encode length changes in the muscle, including static (i.e. amplitude) and dynamic (i.e. velocity) components of muscle stretch. Further, the spindles’ sensitivity to dynamic and static components of stretch can be functionally modified based on contextual demands. Following rapid stretch, spindle feedback evokes a short latency response (SLR), which scales to stretch velocity, and a medium latency response (MLR), which scales to stretch amplitude. Assessing the slope of scaling for SLR-velocity and MLR-amplitude could be indicative of underlying changes in spindle dynamic and static sensitivity. While the slope of SLR-velocity scaling in ankle muscles during standing has been demonstrated to modulate with height-related arousal, the slope of MLR-amplitude scaling has not yet been characterized during standing. Further, Noisy Tendon Vibration (NTV) is another method of evoking spindle-mediated responses during standing, but it is not yet known to what extent these responses relate to spindle dynamic and static sensitivity. The purpose of this thesis was to assess scaling of SLR-velocity and MLR-amplitude, using unilateral tilts to stretch ankle muscles while standing, and to assess scaling of the NTV-evoked response to NTV amplitude. Additionally, to compare changes in the slope of scaling between all three responses when participants stood in a condition where they could randomly be pushed to perturb balance. Key findings of this thesis include demonstrating MLR-amplitude scaling during standing and characterizing this response by showing it was less linear than SLR-velocity scaling. No changes we observed in slope of scaling between perturbation conditions for any of SLR-velocity, MLR-amplitude, or NTV-evoked response to NTV. Thus, we could not compare the change in slope between the three methods, and the reliance of NTV on spindle dynamic or static sensitivity could not be inferred.
When a muscle is mechanically stretched, a stereotypical EMG response known as the stretch response occurs. The stretch response can be subdivided into a short and long latency response. The short latency response is analogous to the tendon tap reflex, while the long latency response involves both spinal cord circuits and transcortical pathways. The short latency response is typically only modulated by peripheral factors such as the size of the perturbation. The long latency response, in addition to being modulated by peripheral factors, can also be modulated based on the participants motor task. For example, if a participant is asked to respond to the stretch as fast as possible, the amplitude of the long latency response increases which would help the participant complete the task. If you apply mechanical vibration to a muscle tendon, it can also result in activation of the sensory receptors involved in the stretch response. This activation can cause a multitude of effects within the central nervous system, however one main effect that is of particular interest in this thesis is it causes a suppression of the stretch response. However, a few key features remain unknown as followed; how the vibration frequency characteristics and how a participant’s intent to respond to the perturbation effect this suppression. Therefore, the purpose of the current thesis was to investigate if the same effects are present when a periodic vs an aperiodic vibration was used, and whether asking a person to respond the perturbation as fast as possible could modulate this suppression. The key findings from this thesis were that the suppression did not depend on the vibration characteristics, as both periodic and aperiodic vibration resulted in similar amounts of suppression. Additionally, it was found that a participants intent to respond did not modulate the amount of suppression seen. The findings from this thesis provide a more detailed understanding of the stretch response circuitry and the response to tendon vibration.
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.
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.