Biomedical subjects

D J Hewson

Publications and source records attributed to D J Hewson.

17 recordsLinked to original sources

PubMed2006

Surface electromyography as a tool to assess the responses of car passengers to lateral accelerations: Part I. Extraction of relevant muscular activities from noisy recordings.

The aim of this paper is to develop a method to extract relevant activities from surface electromyography (SEMG) recordings under difficult experimental conditions with a poor signal to noise ratio. High amplitude artifacts, the QRS complex, low frequency noise and white noise significantly alter EMG characteristics. The CEM algorithm proved to be useful for segmentation of SEMG signals into high amplitude artifacts (HAA), phasic activity (PA) and background postural activity (BA) classes. This segmentation was performed on signal energy, with classes belonging to a chi(2) distribution. Ninety-five percent of HAA events and 96.25% of BA events were detected, and the remaining noise was then identified using AR modeling, a classification based upon the position of the coordinates of the pole of highest module. This method eliminated 91.5% of noise and misclassified only 3.3% of EMG events when applied to SEMG recorded on passengers subjected to lateral accelerations.

Acceleration↗

PubMed2006

Surface electromyography as a tool to assess the responses of car passengers to lateral accelerations. Part II: Objective comparison of vehicles.

The purpose of this study was to objectively assess the response of car passengers to lateral accelerations. Surface EMG signals were collected bilaterally from the cervical erector spinae (CES), latissimus dorsi (LD), erector spinae (ES), external oblique (EO), and vastus lateralis (VL) muscles of 10 subjects. Lateral acceleration was also recorded. Three chassis-seat configurations AA, BA and BB were tested, with the first letter denoting the chassis and the second the seat. SEMG signals were often contaminated by noise, and were, therefore, denoised using the methods explained in part I. Reciprocal phasic activity was observed for all muscles except for the EO, and the reaction of passengers to lateral accelerations was interpreted as a bust torsion. The RMS of EMG segments was used as an indication of muscle activity. Muscle activation of VL and ES were significantly affected by the configuration tested (p<0.05), with greater activation levels observed for the chassis A than for the chassis B. Such a finding implies that greater roll requires greater muscle activity, thus resulting in less comfortable vehicles. Therefore, SEMG can be used to provide an objective measure of discomfort in passengers subjected to lateral accelerations in a car seat.

Acceleration↗

PubMed2003

Evolution in impedance at the electrode-skin interface of two types of surface EMG electrodes during long-term recordings.

The evolution in impedance at the electrode-skin interface of Beckman and Red Dot electrodes was assessed during long-term recordings. Impedance was measured between each pair of electrodes, arranged in a bipolar configuration on tibialis anterior (n=13). A waveform constructed of sinusoids of known frequencies, evenly distributed on a log scale from 1-16,384 Hz, was applied through the electrodes, and the result recorded by a DAQ system. SEMG signals were recorded at 1000 Hz during isometric dorsiflexion contractions of 30 s, performed every 15 min for 2 h. Impedance data were acquired at 65,536 Hz immediately before and after SEMG recordings. Large individual differences in impedance levels were observed at low frequencies. At high frequencies, impedance values depended only on the electrode type. Impedance decreased steadily with time for Beckman electrodes (p < 0.05), but did not decrease significantly for Red Dot electrodes. The magnitude of the reduction over time varied widely between individuals, and was related to the initial impedance values. The impedance-bandwidth product remained constant for each electrode type (95% confidence intervals 146.2-148.2 and 126.1-127.8 for Beckman and Red Dot electrodes respectively). When skin impedance is electrically modelled with a simple network containing a resistor and a capacitor, the capacitance varies with the properties of the electrode used, whereas resistance is dependent on the subject. Furthermore, the EMG spectrum is unaffected by impedance provided skin preparation is sufficient to reduce the impedance below 55 komega.

Adult↗

PubMed2001

Stretching at the ankle joint: viscoelastic responses to holds and continuous passive motion.

PURPOSE: To compare the effect of static holds and continuous passive motion on stiffness and force relaxation of the soft tissue structures resisting ankle joint dorsiflexion. METHODS: This study used a randomized repeated measures trial design. Twenty-four asymptomatic subjects (15 males and 8 females) with a mean age of 26 yr participated. A Kin-Com dynamometer was used to measure the stiffness and force relaxation response of tissues about the ankle joint in response to a plantar flexor stretch. A comparison was made of the response for a 1 x 60-s hold, 2 x 30-s holds, 4 x 15-s holds, and continuous passive motion for 60 s. All subjects undertook all conditions. The main outcome measures were 1) stiffness at the ankle joint as it moved to 80% of the maximum range of dorsiflexion, and 2) the decrease in force at 80% of the maximum range of motion of the ankle joint. RESULTS: Stiffness was decreased significantly (P < 0.05) for the continuous passive motion condition only. The mean magnitude of the decrease in stiffness was 16%. Across hold times, force decreased significantly (P < 0.05). Bonferonni contrasts indicated that there was a significant difference (P < 0.05) between the continuous passive motion condition and all other hold conditions. There were no significant differences (P > 0.05) between the 4 x 15-s, 2 x 30-s, and 1 x 60-s holds. The magnitude of the decline in force was 10.5%, 21.5%, 21.7%, and 19% for the 0-, 15-, 30-, and 60-s holds, respectively. The greatest decreases in tension were achieved in the first 20 s of a hold. CONCLUSION: If decreasing stiffness is a key aim of a stretching program, the findings indicate that continuous motion is more effective than holds. In contrast, if relaxation of peak tension is the main aim, then holds are most effective.

Adult↗

PubMed2001

Variability of competitive performance of distance runners.

PURPOSE: The typical variation in an athlete's performance from race to race sets a benchmark for assessing the utility of performance tests and the magnitude of factors affecting medal prospects. We report here the typical variation in competitive performance of endurance runners. METHODS: Repeated-measures analysis of log-transformed official race times provided the typical within-athlete variation in performance as coefficients of variation (CV). The types of race were cross-country runs (4 races over 9 wk), summer road runs (5 races over 4 wk), winter road runs (4 races over 9 wk), half marathons (3 races over 13 wk and 2 races over 22 wk), and marathons (2 races over 22 wk). RESULTS: Typical variation of times for the fastest quartile of male runners was 1.2-1.9% in the cross-country and road runs, 2.7% and 4.2% in half marathons, and 2.6% in marathons. Times for the slower half of runners in most events were more variable than those of the faster half (ratio of slower/faster CV, 1.0-2.3). Times of younger adult runners were more variable than times of older runners (ratio of younger/older CV, 1.1-1.8). Times of male runners were generally more variable than those of female runners (ratio of male/female CV, 0.9-1.7). CONCLUSION: Tests of endurance power suitable for assessing the smallest worthwhile changes in running performance for top runners need CV < or = 2.5% and < or = 1.5% for tests simulating half or full marathons and shorter running races, respectively. Most of the differences in variability of race times between types of race, ability groups, age groups, and sexes probably arise from differences in competitive experience and attitude toward competing.

Adolescent↗

PubMed2001

Aircraft control forces and EMG activity in a C-130 Hercules during strength-critical maneuvers.

BACKGROUND: The force levels required to operate aircraft controls should be readily generated by pilots, without undue fatigue or exertion. However, maximum pilot applied forces, as specified in aircraft design standards, were empirically derived from the subjective comments of test pilots, and may not be applicable for the majority of pilots. Further, experienced RNZAF Hercules flying instructors have indicated that endurance and fatigue are problems for Hercules pilots. The aim of this study was to quantify aircraft control forces during emergency maneuvers in a Hercules aircraft and compare these forces with design standards. In addition, EMG data were recorded as an indicator of muscle fatigue during flight. METHODS: Six subjects were tested in a C-130 Hercules aircraft. The maneuvers performed were low-level dynamic flight, one engine-off straight-and-level flight, and a two-engines-off simulated approach. The variables recorded were pilot-applied forces and EMG activity. RESULTS: Left rudder pedal force and vastus lateralis activity were both significantly greater during engine-off maneuvers than during low-level dynamic flight (p < 0.05). Maximum aircraft control forces for all controls were within 10% of the design standards. The mean EMG activity across all muscles and maneuvers was 26% MVC, with a peak of 61% MVC in vastus lateralis during the two-engine-off approach. The median frequency of the vastus lateralis EMG signal decreased 13.0% and 16.0% for the one engine-off and two-engine-off maneuvers, respectively. CONCLUSION: The forces required to fly a Hercules aircraft during emergency maneuvers are similar to the aircraft design standards. However, the levels of vastus lateralis muscle activation observed during the engine-off maneuvers can be sustained for approximately 1 min only. Thus, if two engines fail more than 1 min before landing, pilots may have to alternate control of the aircraft to share the workload and enable the aircraft to land safely.

Adult↗

PubMed2001

Aircraft control forces and EMG activity: comparison of pilots before and after flying training.

BACKGROUND: Skilled performers in most motor tasks tend to require less force, muscle activity, and co-contraction of agonist and antagonist muscles to complete tasks. There have been two previous studies on muscle activation patterns and applied forces of skilled and novice pilots, but no longitudinal measurements have been made. The aim of this study was to compare the muscle activation patterns of pilots who had recently completed pilot training with those of experienced pilots. A secondary aim of the study was to examine co-contraction of novice and experienced pilots. METHODS: Novice (n = 12) and experienced (n = 9) pilots were tested on an Aermacchi flight simulator. The novice pilots were tested before and after completing pilot training. Pilots performed a set of landings, rolls, and turns, which were administered in a random order. Variables recorded included aircraft attitude, pilot applied forces, and electromyographic (EMG) activity. Discriminant function analysis was used to classify pilots as novice or experienced. RESULTS: Muscle activation patterns of the novice pilots after completion of pilot training were similar to those of experienced pilots. Of the recently graduated pilots, 77% were classified as experienced using discriminant function analysis. The maneuver in which the recently graduated pilots most closely resembled experienced pilots was the left aileron roll. During this maneuver, experienced and recently graduated pilots showed lower levels of co-contraction than novice pilots (p < 0.05). CONCLUSION: The similarities in some muscle activity patterns across qualified pilots, regardless of experience level, may be useful in initiating protective systems such as G-suit inflation.

Adult↗

PubMed2001

The effect of aircraft control forces on pilot performance during instrument landings in a flight simulator.

BACKGROUND: Pilots may have difficulty controlling aircraft at both high and low force levels due to larger variability in force production at these force levels. The aim of this study was to measure the force variability and landing performance of pilots during an instrument landing in a flight simulator. METHODS: There were 12 pilots who were tested while performing 5 instrument landings in a flight simulator, each of which required different control force inputs. Pilots can produce the least force when pushing the control column to the right, therefore the force levels for the landings were set relative to each pilot's maximum aileron-right force. The force levels for the landings were 90%, 60%, and 30% of maximal aileron-right force, normal force, and 25% of normal force. Variables recorded included electromyographic activity (EMG), aircraft control forces, aircraft attitude, perceived exertion and deviation from glide slope and heading. Multivariate analysis of variance was used to test for differences between landings. RESULTS: Pilots were least accurate in landing performance during the landing at 90% of maximal force (p < 0.05). There was also a trend toward decreased landing performance during the landing at 25% of normal force. Pilots were more variable in force production during the landings at 60% and 90% of maximal force (p < 0.05). CONCLUSION: Pilots are less accurate at performing instrument landings when control forces are high due to the increased variability of force production. The increase in variability at high force levels is most likely associated with motor unit recruitment, rather than rate coding. Aircraft designers need to consider the reduction in pilot performance at high force levels, as well as pilot strength limits when specifying new standards.

Adult↗

PubMed2000

Aircraft control forces and EMG activity in a UH-1H Iroquois helicopter during routine maneuvers.

BACKGROUND: Flying a helicopter requires greater coordination than flying a fixed-wing aircraft, because the pilot is required to apply force simultaneously to three controls: the cyclic, collective, and pedals. There has been one study of pilot applied forces during helicopter flight, but this investigation did not examine muscle activity patterns. The aim of this study was to examine the muscle activation patterns and control forces of helicopter pilots during routine maneuvers. METHODS: Six pilots were tested in a UH-1 h Iroquois helicopter. The maneuvers involved hovering, winching, under-slung loads, a constant rate turn, and a high-speed valley turn. Variables recorded were pilot applied forces and electromyographic activity (EMG). Multivariate analysis of variance was used to test for differences between maneuvers. RESULTS: Significant differences between the maximum forces recorded from each control across all maneuvers were recorded (p < 0.05). The greatest pilot applied forces were recorded from the pedals. No muscles were activated more than 25% of a maximum voluntary contraction for any maneuver. The greatest magnitude of EMG activity was recorded from vastus lateralis during high-speed valley turns. There were significant differences between the EMG activity of left triceps, right triceps, and right deltoid for some maneuvers (p < 0.05). CONCLUSION: The control forces required to fly a helicopter during routine maneuvers are small. The levels of muscle activation associated with pilot applied forces are also low, but are similar to those reported during routine maneuvers in a fast-jet flight simulator.

Adult↗

PubMed2000

Aircraft control forces and EMG activity: comparison of novice and experienced pilots during simulated rolls, loops and turns.

BACKGROUND: Flying an aircraft requires a considerable degree of coordination, particularly during aerobatic activities such as rolls, loops and turns. Only one previous study has examined the magnitude of muscle activity required to fly an aircraft, and that was restricted to takeoff and landing maneuvers. The aim of this study was to examine the phasing of muscle activation and control forces of novice and experienced pilots during more complex simulated flight maneuvers. METHODS: There were 12 experienced and 9 novice pilots who were tested on an Aermacchi flight simulator while performing a randomized set of rolling, looping, and turning maneuvers. Four different runaway trim settings were used to increase the difficulty of the turns (elevator-up, elevator-down, aileron-left, and aileron-right). Variables recorded included aircraft attitude, pilot applied forces, and electromyographic (EMG) activity. Discriminant function analysis was used to distinguish between novice and experienced pilots. RESULTS: Over all maneuvers, 70% of pilots were correctly classified as novice or experienced. Better levels of classification were achieved when maneuvers were analyzed individually (67-91%), although the maneuvers that required the greatest force application, elevator-up turns, were unable to discriminate between novice and experienced pilots. There were no differences in the phasing of muscle activity between experienced and novice pilots. The only consistent difference in EMG activity between novice and experienced pilots was the reduced EMG activity in the wrist extensors of experienced pilots (p < 0.05). The increased wrist extensor activity of the novice pilots is indicative of a distal control strategy, whereby distal muscles with smaller motor units are used to perform a task that requires precise control. Muscle activity sensors could be used to detect the onset of high G maneuvers prior to any change in aircraft attitude and control G-suit inflation accordingly.

Adolescent↗

PubMed2000

Aircraft control forces and EMG activity in a UH-1H Iroquois helicopter during emergency maneuvers.

BACKGROUND: Some air forces are concerned with the adequacy of existing pilot selection standards in respect to pilot strength. Some studies have provided evidence that a large number of pilots may not be able to match the control force levels specified in both military and civilian aircraft design standards. However, both sets of design standards have been based on the subjective comments of test pilots and may not, therefore, be applicable for the majority of pilots. The aim of this study was to quantify aircraft control forces during emergency maneuvers in an Iroquois helicopter and compare these forces with design standards. The examination of muscle activation patterns of pilots during maneuvers, when normalized, can provide additional information on the relative activation levels that pilots are using to produce the aircraft control forces. METHODS: Six pilots were tested in a UH-1 H Iroquois helicopter. The maneuvers were three engine-out landings and a hydraulics-off landing. The variables recorded were pilot applied forces and EMG activity. Multivariate analysis of variance was used to test for differences between maneuvers. RESULTS: The greatest cyclic and upward collective control forces were observed during constant attitude and variable flare engine-out landings. The greatest downward collective forces were observed during hydraulics-off landing. Greater levels of muscle activation were consistently observed during hydraulics-off landing than during the engine-out landings. Control forces consistently exceeded military design standards for cyclic and collective controls, however muscle activity levels were sub-maximal for all maneuvers. CONCLUSION: Comparisons between existing aircraft control-force design standards and pilot strength may overestimate the number of pilots who are able to produce sufficient force to fly the aircraft. Despite the high control forces observed, all pilots tested were able to successfully maneuver the helicopter without requiring a maximal muscular effort.

Adult↗

PubMed1999

Aircraft control forces and EMG activity: comparison of novice and experienced pilots during simulated take-off and landing.

BACKGROUND: Flying an aircraft requires a considerable degree of coordination, particularly during activities such as takeoff and landing. No studies have examined the magnitude and phasing of muscle activity required to fly an aircraft. The aim of this study was to examine the muscle activation patterns and control forces of novice and experienced pilots during simulated flight. METHODS: Twelve experienced and nine novice pilots were tested on an Aermacchi flight simulator while performing a randomized set of take-off and landing maneuvers. Four different runaway trim settings were used to increase the difficulty of the landings (elevator-up, elevator-down, aileron-left, and aileron-right). Variables recorded included aircraft attitude, pilot applied forces, and electromyographic (EMG) activity. Discriminant function analysis was used to distinguish between novice and experienced pilots. RESULTS: Across all landings, wrist flexors and wrist extensors were the predominant muscles used, with EMG activity consistently around 20-30% maximum voluntary contraction (MVC). In respect to differences in EMG activity between novice and experienced pilots, novices had significantly more activity in wrist extensors during all landings. In contrast, experienced pilots had consistently more vastus lateralis activity for all landings than did novice pilots. Over all landings and take-off, 89.5% of pilots were correctly classified as novice or experienced. When the maneuvers were analyzed individually, normal, elevator-down, and aileron-left landings were the most accurate maneuvers for pilot prediction. EMG and force variables were more important than aircraft attitude in discriminating between novice and experienced pilots (83%, 79%, and 65%, respectively). CONCLUSION: The consistency of the finding that EMG activity and control forces are accurate discriminators of pilot experience is indicative of underlying differences in neuromuscular control strategies between novice and experienced pilots.

Adult↗

PubMed1996

Specificity of training and its relation to the performance of distance runners.

Validated six-month retrospective questionnaires were completed by 119 female and 234 male coached distance runners (59% compliance) for a descriptive study of relationships between specificity of training and best performance in a summer season. Runners were aged 26 +/- 10 y (mean +/- SD), specialising in distances from 800 m to the marathon, with seasonal best paces of 82 +/- 7% of sex- and age-group world records. They reported typical weekly durations of interval and strength training, and typical weekly durations and paces of moderate and hard continuous running, for build-up, precompetition, competition, and postcompetition phases of the season. The training programs showed some evidence of specificity, especially for runners preparing for longer events. A potentially beneficial effect of specificity was evident in a significant (p < 0.01) correlation between performance and seasonal mean weekly duration of moderate continuous running for runners specialising in longer distances (r = 0.29). The only other significant correlates of performance were seasonal mean relative training paces of moderate (r = -0.18) and hard (r = -0.42) continuous running, which exemplified detrimental effects of specificity for most runners. Thus, the training of better runners is not characterised strongly by greater specificity.

Adult↗

PubMed1995

Prescribed and self-reported seasonal training of distance runners.

A survey of 123 distance-running coaches and their best runners was undertaken to describe prescribed seasonal training and its relationship to the performance and self-reported training of the runners. The runners were 43 females and 80 males, aged 24 +/- 8 years (mean +/- S.D.), training for events from 800 m to the marathon, with seasonal best paces of 86 +/- 6% of sex- and age-group world records. The coaches and runners completed a questionnaire on typical weekly volumes of interval and strength training, and typical weekly volumes and paces of moderate and hard continuous running, for build-up, pre-competition, competition and post-competition phases of a season. Prescribed training decreased in volume and increased in intensity from the build-up through to the competition phase, and had similarities with 'long slow distance' training. Coaches of the faster runners prescribed longer build-ups, greater volumes of moderate continuous running and slower relative paces of continuous running (r = 0.19-0.36, P < 0.05), suggesting beneficial effects of not training close to competition pace. The mean training volumes and paces prescribed by the coaches were similar to those reported by the runners, but the correlations between prescribed and reported training were poor (r = 0.2-0.6). Coaches may therefore need to monitor their runners' training more closely.

Adult↗

PubMed1991

Principles behind practice. 4. Natural history.

The busy clinician is confronted daily with a wide range of decisions about patient diagnosis and management. The sharpness and scepticism to constantly ask fundamental questions about the natural history of a patient's disease is a talent that a few lucky individuals have. For the rest of us, it is an acquired skill which must be developed and maintained. We suggest the following questions to guide the clinician in taking account of the natural history of disease in everyday clinical practice. How strong is the evidence that the putative consequence of this illness is or is not attributable to some external factor? Can I be sure that this exposure (drug, surgery, social or behavioural therapy, toxic or environmental agent) does not have a long latency and that subsequent development of illness will not ensue? What do I know from long-term prospective studies about the life history of this disease? Does lead time explain the apparent benefit of this diagnostic test or other screening procedure? Does the evidence of the natural history of this disease specifically relate to the characteristics of my patient--age, sex, set of prognostic factors, selected or unselected referral? Is there good evidence from at least one good randomised controlled trial with a "no treatment" control group that this intervention is efficacious (and not hazardous)?

Epidemiology↗

PubMed1991

Colour duplex flow imaging in carotid arterial disease: correlation with intra-arterial digital angiography.

Colour duplex flow imaging is a highly accurate form of non-invasive assessment of the extra-cranial carotid circulation. This method is precise in characterising the degree of carotid stenosis, when compared with intra-arterial digital subtraction angiography. The accuracy of colour duplex in defining mild (1-30%), moderate (31-70%) and severe (71-99%) stenosis was 96%, 95% and 98%, respectively. Sensitivities and specificities were greater than 90% in each category, while positive and negative predictive values ranged from 96% to 100%.

Angiography, Digital Subtraction↗

PubMed1986

Rapid Doppler screening test for internal carotid disease.

There is increasing evidence that only hemodynamically significant internal carotid lesions (of more than 50% diameter stenosis or occlusion) rather than minor lesions are the precursors of stroke. A Doppler test combining periorbital and direct audible interpretation of internal carotid Doppler signals was performed on 67 patients (134 internal carotids) and the results were compared with those from conventional biplane angiography. The test was 100% sensitive and had a positive predictive value of 91% for hemodynamically significant lesions. Negative predictive value for normal or less than 50% stenosis was 100%. Carotid bifurcation bruits were heard over only 25% of internal carotids with significant stenosis and nearly 60% of bruits were heard over internal carotids with minor disease. The equipment was portable and relatively inexpensive. The test could be performed by vascular technicians in less than 12 minutes with minimal patient discomfort, thus meeting most of the criteria for an ideal non-invasive screening test.

Adult↗