Biomedical subjects

J Timothy Inglis

Publications and source records attributed to J Timothy Inglis.

At least 19 recordsLinked to original sources

PubMed2007

Electromyography of superficial and deep neck muscles during isometric, voluntary, and reflex contractions.

Increasingly complex models of the neck neuromusculature need detailed muscle and kinematic data for proper validation. The goal of this study was to measure the electromyographic activity of superficial and deep neck muscles during tasks involving isometric, voluntary, and reflexively evoked contractions of the neck muscles. Three male subjects (28-41 years) had electromyographic (EMG) fine wires inserted into the left sternocleidomastoid, levator scapulae, trapezius, splenius capitis, semispinalis capitis, semispinalis cervicis, and multifidus muscles. Surface electrodes were placed over the left sternohyoid muscle. Subjects then performed: (i) maximal voluntary contractions (MVCs) in the eight directions (45 deg intervals) from the neutral posture; (ii) 50 N isometric contractions with a slow sweep of the force direction through 720 deg; (iii) voluntary oscillatory head movements in flexion and extension; and (iv) initially relaxed reflex muscle activations to a forward acceleration while seated on a sled. Isometric contractions were performed against an overhead load cell and movement dynamics were measured using six-axis accelerometry on the head and torso. In all three subjects, the two anterior neck muscles had similar preferred activation directions and acted synergistically in both dynamic tasks. With the exception of splenius capitis, the posterior and posterolateral neck muscles also showed consistent activation directions and acted synergistically during the voluntary motions, but not during the sled perturbations. These findings suggest that the common numerical-modeling assumption that all anterior muscles act synergistically as flexors is reasonable, but that the related assumption that all posterior muscles act synergistically as extensors is not. Despite the small number of subjects, the data presented here can be used to inform and validate a neck model at three levels of increasing neuromuscular-kinematic complexity: muscles generating forces with no movement, muscles generating forces and causing movement, and muscles generating forces in response to induced movement. These increasingly complex data sets will allow researchers to incrementally tune their neck models' muscle geometry, physiology, and feedforward/feedback neuromechanics.

Adult↗

PubMed2006

Interaction between acoustic startle and habituated neck postural responses in seated subjects.

Postural and startle responses rapidly habituate with repeated exposures to the same stimulus, and the first exposure to a seated forward acceleration elicits a startle response in the neck muscles. Our goal was to examine how the acoustic startle response is integrated with the habituated neck postural response elicited by forward accelerations of seated subjects. In experiment 1, 14 subjects underwent 11 sequential forward accelerations followed by 5 additional sled accelerations combined with a startling tone (124-dB sound pressure level) initiated 18 ms after sled acceleration onset. During the acceleration-only trials, changes consistent with habituation occurred in the root-mean-square amplitude of the neck muscles and in the peak amplitude of five head and torso kinematic variables. The subsequent addition of the startling tone restored the amplitude of the neck muscles and four of the five kinematic variables but shortened onset of muscle activity by 9-12 ms. These shortened onset times were further explored in experiment 2, wherein 16 subjects underwent 11 acceleration-only trials followed by 15 combined acceleration-tone trials with interstimulus delays of 0, 13, 18, 23, and 28 ms. Onset times shortened further for the 0- and 13-ms delays but did not lengthen for the 23- and 28-ms delays. These temporal and spatial changes in EMG can be explained by a summation of the excitatory drive converging at or before the neck muscle motoneurons. The present observations suggest that habituation to repeated sled accelerations involves extinguishing the startle response and tuning the postural response to the whole body disturbance.

Acceleration↗

PubMed2006

Visual-vestibular influences on locomotor adjustments for stepping over an obstacle.

Combined visual and vestibular influences on locomotor control, particularly in changing environments, are little understood. We studied such influences on body orientation and foot trajectory control during level walking and obstacle avoidance. Six young adults walked on the level and over an obstacle while vision was present or occluded as well as while vestibular information was intact or perturbed using galvanic vestibular stimulation (GVS). The occlusion of vision caused a slowing of gait during obstacle avoidance as well as increased clearance of the leading limb over the obstruction. GVS caused lateral deviations in head and trunk roll angles as well as in foot and trunk displacements, but these lateral deviations were the same during both level walking and obstacle avoidance. In addition, GVS had no effect at all on sagittal plane factors such as speed, foot proximity to the obstacle and vertical clearance over the obstacle. Overall, there is a complex visual control of bilateral obstacle avoidance, but the lack of differences in GVS effects between level and obstructed walking shows that vestibular information is not upregulated for obstacle avoidance. In addition, the robust indifference of anterior foot placement and body displacement to significant lateral deviations from GVS suggests an orthogonally based sensori-locomotor control.

Adult↗

PubMed2006

Sensorimotor adaptation in response to proprioceptive bias.

Studies investigating visuo-motor adaptation typically introduce sensory conflicts by manipulating visual information (prisms, cursor gains). The purpose of the present study was to determine whether similar adaptation would be observed when a conflict is created through distortion of the proprioceptive sense, rather than through visual distortion. We used a coordinated movement task that required participants to release thumb and index finger at a specific elbow angle during passive elbow extension. Participants could not see their arm, but were shown a cursor representing the forearm on a video screen. In the proprioceptive group, a sensory conflict was introduced by vibrating the biceps brachii muscle, introducing a discrepancy of approximately 7.5 degrees between the proprioceptively perceived and visually perceived elbow angle. In the visual group, a conflict of similar magnitude was obtained by introducing a gain of 7.5 degrees to the cursor with respect to forearm position. Adaptation was assessed by the presence of plastic changes in release elbow angles following a period of exposure to the sensory conflict (i.e., aftereffects). Both groups showed high accuracy during exposure despite the sensory conflicts. More importantly, the visual group presented large and persistent aftereffects, while the proprioceptive group presented none. We suggest that the proprioceptive group's lack of adaptation was due to the artificial muscle spindle activity resulting from vibration, which prevented visual and proprioceptive signals to be merged into a common frame of reference.

Adaptation, Physiological↗

PubMed2006

Long-term intermittent hypoxia increases sympathetic activity and chemosensitivity during acute hypoxia in humans.

We determined the effects of 10 daily exposures of intermittent hypoxia (IH; 1 h day(-1); oxyhaemoglobin saturation = 80%) on muscle sympathetic nerve activity (MSNA, peroneal nerve) and the hypoxic ventilatory response (HVR) before, during and after an acute 20 min isocapnic hypoxic exposure. We also assessed the potential parallel modulation of the ventilatory and sympathetic systems following IH. Healthy young men (n = 11; 25 +/- 1 years) served as subjects and pre- and post-IH measures of MSNA were obtained on six subjects. The IH intervention caused HVR to significantly increase (pre-IH = 0.30 +/- 0.03; post-IH = 0.61 +/- 0.12 l min(-1) %S(aO(2)) (-1)). During the 20 min hypoxic exposure sympathetic activity was significantly greater than baseline and remained above baseline after withdrawal of the hypoxic stimulus, even though oxyhaemoglobin saturation had normalized and ventilation and blood pressure had returned to baseline levels. When compared to the pre-IH trial, burst frequency increased (P < 0.01), total MSNA trended towards higher values (P = 0.06), and there was no effect on burst amplitude (P = 0.82) during the post-IH trial. Following IH the rise in MSNA burst frequency was strongly related to the change in HVR (r = 0.79, P < 0.05) suggesting that these sympathetic and ventilatory responses may have common central control.

Adult↗

PubMed2006

Startle responses elicited by whiplash perturbations.

The human startle response produces muscle contractions throughout the body but the most brisk and synchronized contractions appear in the neck muscles. This response, which is greatest with the first exposure to a startling stimulus, could produce excessive and inappropriately directed muscle contractions that could explain the higher incidence of whiplash injuries in people who are unprepared for the collision. This study seeks neurophysiological evidence of startle responses in the neck muscles of 120 healthy subjects exposed to between 1 and 16 rear-end impacts or forward perturbations of different speeds. Startle responses were quantified by the synchronous electromyographic (EMG) activity between 10 and 20 Hz in bilaterally homologous sternocleidomastoid, scalene and cervical paraspinal neck muscles. Coherence analyses of EMGs from the left and right muscles were used to estimate synchrony for: (i) the first unexpected trial, (ii) subsequent habituated trials, and (iii) the superposition of habituated trials and a loud acoustic stimulus (40 ms, 124 dB sound). The peak in coherent EMG activity between contralateral muscle pairs in the 10-20 Hz bandwidth was related to startle. Synchrony in this bandwidth was observed between the left and right muscles during the first impact or whiplash-like perturbation. This synchrony decreased significantly in the habituated trials, but reappeared when the loud acoustic stimulus was introduced. Its presence in the first trial indicates that startle is part of the neuromuscular response to an unexpected rear-end impact. This startle component of the neuromuscular response could play a role in the aetiology of whiplash injuries.

Acceleration↗

PubMed2006

Auditory startle alters the response of human subjects exposed to a single whiplash-like perturbation.

STUDY DESIGN: Human volunteers were exposed to a single whiplash-like perturbation. OBJECTIVE: To determine how muscle and kinematic responses are affected by the superposition of a rear-end collision and loud startling noise. SUMMARY OF BACKGROUND DATA: Many whiplash studies use forward perturbations without reproducing the sound of a car crash. Loud sounds are known to evoke startle responses in the neck muscles and therefore could affect whiplash injuries. METHODS: Sixty-five subjects (30 female, 35 male) were exposed to a single forward horizontal perturbation. Head and torso kinematics, and electromyographic activity in the sternohyoid, sternocleidomastoid, scalenus, and cervical paraspinal muscles were measured. Two awareness conditions (deceived and unaware subjects) nested in two startle conditions (with or without a 40 milliseconds, 124 dB sound) were tested. RESULTS: Startle and gender affected the amplitude and timing of numerous kinematic and muscle variables. Awareness affected only one muscle variable. Startled individuals exhibited greater peak head and trunk accelerations, increased activity of the cervical paraspinal muscles, and a reduced head retraction and trunk angle. CONCLUSIONS: An acoustic startle alters the neck muscle and kinematic responses and may be as important as gender in the genesis of whiplash injury.

Acoustic Stimulation↗

PubMed2005

Adaptations to normal human gait on potentially slippery surfaces: the effects of awareness and prior slip experience.

Prior knowledge of potentially slippery conditions has been shown to alter normal human gait in slip and fall experiments. Here we quantify the effects of two aspects of prior knowledge - awareness of a possible slip and prior slip experience - on normal gait. Sixty-eight subjects (40F, 28M) each walked over 48 high-friction surfaces (control trials) and 12 low-friction surfaces. Within- and between-subject changes in lower limb muscle activation, gait kinematics and ground reaction forces were analyzed in three non-slip control trials: one before and one after the first unexpected slip exposure, and a third after repeated slip exposures. Subjects knew they might slip in the latter two trials but not the first trial. Twenty subjects slipped during their first low-friction exposure (early slip group), 32 in later low-friction exposures (late slip group), and 16 subjects did not slip at all. Simultaneous changes in awareness and experience between the first two analyzed trials of the early slip group altered the muscle activity in both limbs, reduced the foot and knee angles at heel strike in the slip limb and reduced the ground reaction forces, impulses and utilized friction after heel strike in the slip limb. A change in only awareness between the first two analyzed trials of the late slip group produced the same kinematic changes seen in the early slip group, but only small muscle activity change and no kinetic changes. Subsequent slip experience in the late slip group produced the muscle activation and kinetic changes observed in the early slip group, but no further kinematic changes. These results showed that awareness of a potential slip primarily alters how the slip-limb approaches the floor, whereas prior slip experience primarily alters the anticipatory muscle activation and how the foot interacts with the floor. These muscle, kinematic and kinetic changes were consistent with a more cautious "normal" gait, and can reduce the external validity of slip and fall experiments.

Adolescent↗

PubMed2005

The effect of subject awareness and prior slip experience on tribometer-based predictions of slip probability.

Prior knowledge of potentially slippery conditions has been shown to alter normal human gait in slip and fall experiments. We sought to quantify how the empirical relationship between slip probability and available floor friction was affected by subject awareness and prior slip experience. Sixty-eight subjects (40 females, 28 males) walked over three different low-friction surfaces inserted periodically between non-slip control trials. Three increasing levels of prior knowledge were used: deceived (unaware of the slippery surface), aware (20% chance of a slippery surface, but no prior slip experience) and experienced (aware with prior slip experience). Available friction was measured using a drag sled and a variable incidence tribometer. Of 620 low-friction trials, 124 generated slips greater than 27mm. The proportion of slips, the slip distance and the required friction (taken from the control trial immediately before a low-friction trial) generally decreased with increasing levels of prior knowledge. These adaptations were accommodated by logistically regressing slip outcome (yes/no) against the normalized friction (available friction minus required friction) rather than against available friction alone. The regressions showed that subject awareness biased the slip probability curve toward a lower slip risk for a given normalized friction, but that the subsequent addition of slip experience generated a slip risk curve that was not significantly different from that of deceived (and presumably unprepared) subjects. These findings suggest that data to validate a tribometer's ability to predict the risk of slipping (but not falling) can be acquired from subjects with prior slip experience.

Accidental Falls↗

PubMed2005

Autonomic dysreflexia during sperm retrieval in spinal cord injury: influence of lesion level and sildenafil citrate.

Autonomic dysreflexia (AD) can occur during penile vibratory stimulation in men with spinal cord injury, but this is variable, and the association with lesion level is unclear. The purpose of this study was to characterize the cardiovascular responses to penile vibratory stimulation in men with spinal cord injury. We hypothesized that those with cervical injuries would demonstrate a greater degree of AD compared with men with thoracic injuries. We also questioned whether the rise in blood pressure could be attenuated by sildenafil citrate. Participants were classified as having cervical (n = 8) or thoracic (n = 5) injuries. While in a supine position, subjects were instrumented with an ECG, and arterial blood pressure was determined beat by beat. Subjects reported to the laboratory twice and received an oral dose of sildenafil citrate (25-100 mg) or no medication. Penile vibratory stimulation was performed using a handheld vibrator to the point of ejaculation. At ejaculation during the nonmedicated trials, the cervical group had a significant decrease in heart rate (-5-10 beats/min) and increase in mean arterial blood pressure (+70-90 mmHg) relative to resting conditions, whereas the thoracic group had significant increases in both heart rate (+8-15 beats/min) and mean arterial pressure (+25-30 mmHg). Sildenafil citrate had no effect on the change in heart rate or mean arterial pressure in either group. In summary, men with cervical injuries had more pronounced AD during penile vibratory stimulation than men with thoracic injuries. Administration of sildenafil citrate had no effect on heart rate or blood pressure during penile vibratory stimulation in men with spinal cord injury.

Adult↗

PubMed2005

Vestibular contributions during human locomotor tasks.

This review explores vestibular contributions during dynamic tasks with the goal of identifying the underlying roles of vestibular information in task progression and balance control. Vestibular contributions to upper and lower body control during locomotor tasks were found.

Gait↗

PubMed2005

Touch noise increases vibrotactile sensitivity in old and young.

Stochastic resonance (SR) occurs when the detection of a subthreshold signal is aided by the presence of random energy fluctuations in the signal modality, commonly called noise. SR is counterintuitive because such noise usually worsens performance. Nonetheless, SR has been demonstrated both theoretically and experimentally in human sensory systems. Using a psychophysically sophisticated paradigm, we show that SR aids the detection of vibrating touch stimuli presented to the foot soles of both healthy elderly people with elevated vibrotactile thresholds and healthy young people with normal vibrotactile thresholds. The results also suggest that it is possible to know a priori the amount of noise needed for optimal SR effects given the degree to which the signal is subthreshold. Thus, SR may be practical as a rehabilitative aid for individuals with elevated sensory thresholds.

Adult↗

PubMed2005

Exercise leads to faster postural reflexes, improved balance and mobility, and fewer falls in older persons with chronic stroke.

OBJECTIVES: To determine the effect of two different community-based group exercise programs on functional balance, mobility, postural reflexes, and falls in older adults with chronic stroke. DESIGN: A randomized, clinical trial. SETTING: Community center. PARTICIPANTS: Sixty-one community-dwelling older adults with chronic stroke. INTERVENTION: Participants were randomly assigned to an agility (n=30) or stretching/weight-shifting (n=31) exercise group. Both groups exercised three times a week for 10 weeks. MEASUREMENTS: Participants were assessed before, immediately after, and 1 month after the intervention for Berg Balance, Timed Up and Go, step reaction time, Activities-specific Balance Confidence, and Nottingham Health Profile. Testing of standing postural reflexes and induced falls evoked by a translating platform was also performed. In addition, falls in the community were tracked for 1 year from the start of the interventions. RESULTS: Although exercise led to improvements in all clinical outcome measures for both groups, the agility group demonstrated greater improvement in step reaction time and paretic rectus femoris postural reflex onset latency than the stretching/weight-shifting group. In addition, the agility group experienced fewer induced falls on the platform. CONCLUSION: Group exercise programs that include agility or stretching/weight shifting exercises improve postural reflexes, functional balance, and mobility and may lead to a reduction of falls in older adults with stroke.

Accidental Falls↗

PubMed2004

Can prepared responses be stored subcortically?

Quick voluntary responses to environmental stimuli are required of people on a daily basis. These movements have long been thought to be controlled via cortical loops involving processing of the stimulus and generation of a suitable response. Recent experiments have shown that in simple reaction time (RT) tasks, the appropriate response can be elicited much earlier (facilitated) when the "go" signal is replaced by a startling (124 dB) auditory stimulus. In the present experiment we combined a startling acoustic stimulus with an established RT paradigm that involved simple and choice RT. In a simple RT condition the prepared voluntary response was elicited at very short latencies following the startle. However, when cortical processing was required prior to responding (choice RT task), the startle did not facilitate the voluntary response, and gave rise to more movement production errors. Since movements requiring ongoing cortical processing following the stimulus are not facilitated by startle, it is unlikely that the startle facilitation is due to increased neural activation. In contrast, it appears more likely that the startle acts as an early trigger for subcortically stored prepared movements since movements that are prepared in advance can be initiated at such short latencies (<60 ms).

Acoustic Stimulation↗

PubMed2004

When is vestibular information important during walking?

Locomotion relies on vision, somatosensory input, and vestibular information. Both vision and somatosensory signals have been shown to be phase dependently modulated during locomotion; however, the regulation of vestibular information has not been investigated in humans. By delivering galvanic vestibular stimulation (GVS) to subjects at either heel contact, mid-stance, or toe-off, it was possible to investigate when vestibular information was important during the gait cycle. The results indicated a difference in the vestibular regulation of upper versus lower body control. Upper body responses to GVS applied at different times did not differ in magnitude for the head (P = 0.2383), trunk (P = 0.1473), or pelvis (P = 0.1732) showing a similar dependence on vestibular information for upper body alignment across the gait cycle. In contrast, foot placement was dependent on the time when stimulation was delivered. Changes in foot placement were significantly larger at heel contact (during the double support phase) than when stimulation was delivered at mid-stance (in the single support phase of the gait cycle; P = 0.0193). These latter results demonstrate, for the first time, evidence of phase-dependent modulation of vestibular information during human walking.

Adolescent↗

PubMed2004

Is the use of vestibular information weighted differently across the initiation of walking?

The purpose of this experiment was to examine vestibular contributions at specific times during the initiation of walking in human subjects. Subjects began walking forward at the sound of an auditory tone, with vision present or occluded. Galvanic vestibular stimulation (GVS) was delivered with the anode electrode on the right or left side at either: (1) onset of the anticipatory postural adjustment (APA), (2) toe-off of the first swing limb (TO) or (3) heel contact of the first swing limb (HC). Ground reaction forces and kinematic data were collected. Upper body (roll angles from head, trunk and pelvis) and lower body (foot placement) data were analysed to determine whether the timing and magnitude of the response to GVS, and therefore the level of vestibular contribution, was modulated at different points during the initiation of gait. With vision present and occluded, the magnitude of the lower body response varied depending on the event in the gait cycle at which the stimulation was delivered. These novel results demonstrate evidence that vestibular weighting during gait initiation is dependent upon the specific gait initiation events. Upper body roll also exhibited magnitude differences between events. However, these changes are proposed to occur due to the transition from a stationary position into a dynamic state, prompting the increased weighting of vestibular information. With vision present no significant changes were seen in the segment roll response. The observations suggest a distinction in vestibular regulation of upper body roll versus foot placement for successful completion of the gait initiation task. Changes in upper body roll are influenced by the dynamic nature of the task, whereas foot placement changes are modulated based on the event during gait initiation at which GVS is delivered.

Adult↗

PubMed2004

Galvanic vestibular stimulation alters the onset of motor unit discharge.

We have previously shown that galvanic vestibular stimulation (GVS) can modulate the amplitude of the passive soleus H-reflex. This study examined whether the response resulted from a general change in excitability of the motoneuron pool or a specific modulation of individual motor units (MUs). Subjects performed slow isometric plantarflexor actions in a prone lying and kneeling position until the discharge of a single gastrocnemius MU was detected. During randomly selected trials, a 1-mA bipolar, binaural galvanic stimulus was triggered just prior to the start of plantarflexor activity. With the knee extended and the medial gastrocnemius (MG) at a long muscle length, GVS did not have an effect on MU activity. However, when the knee was flexed and the MG muscle was shortened, GVS significantly modified the onset of activation and the initial firing frequency of MUs. This may reflect a change in the gain of the presynaptic inhibitory mechanisms that act on the motoneuron pool once a muscle reaches a shortened, nonoptimal force-producing length. Thus, GVS may provide an important research tool for activating descending vestibulospinal pathways that act on lower-limb motoneurons and may be applied to test the integrity of the spinal cord.

Adult↗

PubMed2004

Interlimb asymmetry in persons with and without an anterior cruciate ligament deficiency during stationary cycling.

OBJECTIVE: To investigate the power output generation from anterior cruciate ligament (ACL)-injured and noninjured limbs during stationary cycling. DESIGN: Repeated measures. SETTING: Research laboratory. PARTICIPANTS: Ten people with unilateral ACL deficiency and 10 uninjured controls matched for age and sex. INTERVENTIONS: Participants performed 6 randomized bouts of stationary cycling at intensities of 2 cadences (60, 90 rpm) and 3 power outputs (75, 125, 175 W) for approximately 2 minutes for each bout during a single laboratory visit. MAIN OUTCOME MEASURES: The effective component of force (perpendicular to the crank) was measured and used to calculate the power output contribution from each limb to the total power output. RESULTS: Subjects with ACL injury generated significantly more power from uninjured limbs compared with that from injured limbs and the limbs of control subjects. CONCLUSIONS: Results suggest that people with ACL injury have a reduced total output from the injured limb and rely on the uninjured limb for most of the power output. This may compromise the ability to restore lower-limb muscle strength after injury.

Analysis of Variance↗