Conflict of interest among researchers.
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Canada↗
Biomedical subjects
R L Kirby
Publications and source records attributed to R L Kirby.
Leg-length discrepancy and postural sway.
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Gait↗
Failure of accentuated vertical body movements to induce cardiac-locomotor coupling.
Cardiac-locomotor coupling (CLC) has been reported during a variety of rhythmic human activities. One reason postulated for such coupling is that axial movements of the viscera during some activities (the "visceral piston") may enhance expulsion of blood from the heart; if so, accentuated vertical movements of the body should provide a powerful stimulus to coupling. To test this hypothesis, we studied 20 subjects hopping and 20 others skipping rope for greater than or equal to 10 min while electrocardiographic and force-platform signals were recorded, from which we derived the subjects' exercise and heart rates. The incidence and intensity of apparent coupling in the test subjects were compared with those of cross-over controls, where the heart rate of each subject was related to the hopping or skipping rate of a matched subject. Ratios consistent with coupling were seen in 10 (50%) hopping subjects under test conditions and in 13 (65%) under control conditions; among skipping subjects, the incidences were 11 (55%) and 10 (50%). In neither group of subjects was the difference in the incidences or the intensities of apparent CLC statistically significant. Our failure to detect CLC while our subjects were hopping or skipping suggests that the visceral piston is unimportant to the CLC phenomenon.
Adult↗
Locomotor-respiratory coupling during wheelchair propulsion.
Visceral movement due to impact loading is believed to play a role in the locomotor-respiratory coupling (LRC) that has been detected in a number of mammalian species. In the bird and bat species in which LRC has been described, the effect of the wing muscles on the timing of respiration appears to be a dominant influence. To test the hypothesis that LRC occurs in humans propelling wheelchairs (where there is no impact loading and the arms are used for locomotion), we studied 10 wheelchair athletes on a motorized treadmill at three speeds. Each subject's data were analyzed by spectral analysis (based on the fast Fourier transform), which detected apparent LRC (rates within 1% of a single-digit integer ratio) in 12 (40%) of the 30 test settings. However, a control analysis, in which each subject's arm-thrust rates were compared with another subject's breathing rates, revealed apparent (but false) coupling in 8 (27%), not significantly less often (using the chi 2 test). These findings appear to refute the hypothesis that LRC occurs during wheelchair propulsion. These data are consistent with the theory that the visceral piston is important to LRC and suggest that rhythmic arm movements are insufficient to induce the phenomenon in this setting.
Adult↗
The cardiac-locomotor coupling phenomenon: the contribution of Coleman.
Cardiac-locomotor coupling (entrainment of heart and exercise rates) has been the subject of a number of recent studies. This paper reviews the early work of W. M. Coleman who, in 1921, published the results of his observations on humans and a variety of other species. The animal studies were carried out at the London Zoological Gardens. Coleman's findings remain relevant to those studying entrainment phenomena. They also illustrate the importance and utility of careful observation.
Animals↗
Influence of caster diameter on the static and dynamic forward stability of occupied wheelchairs.
The hypothesis that the static and dynamic forward stability of an occupied wheelchair would increase as a function of the caster diameter was tested in 20 able-bodied subjects. A device was attached to the wheelchair frame so that casters with different diameters could be used. With platform testing of static stability, the occupied wheelchair equipped with casters with diameters of 10.2, 20.3, 25.4, and 33.0cm, tipped at mean (+/- 1SD) angles of 23.8 degrees (+/- 1.3 degrees), 25.2 degrees (+/- 1.4 degrees), 26.1 degrees (+/- 1.4 degrees), and 28.2 degrees (+/- 1.9 degrees), respectively. The relationship between static stability (y, in degrees) and caster diameter (x, in cm) can be expressed by the equation y = 23.5-.0196 chi + .00484 x2 (p = .0001). Dynamic stability was tested by having subjects descend a 5 degree ramp, by gravity alone, from progressively farther up the ramp until a full forward tip occurred (footrests contacted the floor) when the wheelchair struck a 5-cm-high obstruction with sufficient speed. The mean tipping speeds for the dynamic tests were .85 (+/- .08), .85 (+/- .08), .89 (+/-.07), and 1.04 (+/- .14) m/sec for the wheelchair fitted with the caster diameters ranging from smallest to largest, respectively. The relationship between dynamic stability (y, tipping speed in m/sec) and caster diameter (chi, in cm) can be expressed by the equation y = .788 + .0139 chi-.00110 chi 2 + .0000276 chi 3 (p = .0001). The relationship between caster diameter and forward wheelchair stability should be considered in wheelchair design and prescription.
Equipment Design↗
Cardiac-locomotor coupling while finger tapping: Part II. A cross-over control study.
Cardiac-locomotor coupling (CLC) has been reported by us while people finger tap at cadences natural to them. Since then, we have developed a simple cross-over control strategy in which the heart rate of one subject is related to the finger-tapping rate of another. Of the 20 normal subjects previously studied while tapping a telegraph key at a comfortable rate for 10 min., reevaluation of their data showed that 9 (45%) and 4 (20%) of them, under test and control conditions, respectively, appeared to couple at a single-digit integer ratio. Neither the incidence nor the intensity of apparent CLC under the two conditions was significantly different. Raster plots of the most tightly related rates gave no evidence of phase locking. These results have two implications. First, previously published reports on CLC (and other entrainment phenomena) should be interpreted with caution, and cross-over controls should be considered in future research. Second, the absence of CLC during finger tapping suggests that CLC may only be functionally significant during exercise of large muscle groups (e.g., by minimization of cardiac afterload) or when impact-loading occurs (e.g., by enhancing cardiac ventricular emptying.
Arousal↗
Relationship between electromyographic activity of the vastus lateralis while standing and the extent of bilateral simulated knee-flexion contractures.
The effect of simulated bilateral knee-flexion contractures (KFC) on the electromyographic (EMG) activity of the vastus lateralis was studied by testing 10 normal subjects using surface EMG to test the hypothesis that the activity of the knee extensors would increase as a function of the severity of the contracture. The root mean square of the EMG activity was determined from four 4-s samples taken at 30-s intervals, during 2 min of standing in each of five positions of simulated KFC (0 degree, 10 degrees, 20 degrees, 30 degrees and 40 degrees). A randomly balanced order of conditions was used. KFC were simulated in each subject by means of an adjustable line from the subject's waist to the sole of each foot. An analysis of variance was used to contrast EMG activity, and a significant difference was found between each of the positions (P less than 0.05). The mean (+/- 1 SD) EMG activity, expressed as a percentage of the maximum voluntary contraction, was 0.3% (+/- 0.2) at 0 degree, 7.6% (+/- 5.6) at 10 degrees, 10.9% (+/- 7.6) at 20 degrees, 16.6% (+/- 12.4) at 30 degrees and 24.0% (+/- 14.0) at 40 degrees. A linear relationship was found (r2 = 0.986), expressed by the equation y = 0.62 + 0.56 x, where y represents EMG activity and x represents the extent of simulated KFC (P = 0.0007). The results provide insight into the increased knee extensor activity necessary to stand with KFC and underline the importance of treating this common disorder.
Adult↗
Canes, crutches and walkers.
Canes, crutches and walkers are safe and effective but generally underutilized therapeutic tools. These aids are most helpful to patients who have an unstable gait, whose muscles are weak or who require a reduction in the load on weight-bearing structures. An understanding of the biomechanics of ambulation aids provides insights into how and when these devices should be prescribed. The patient must have sufficient strength, balance and coordination to master the aid and should be trained to use it correctly.
Biomechanical Phenomena↗
The effects of simulated knee-flexion contractures on standing balance.
The effects of simulated unilateral and bilateral knee-flexion contractures on standing balance were studied by testing 15 normal subjects on a Kistler force platform. Postural sway (mediolateral and anteroposterior travel) and the mean position of the center of pressure (as a percentage of the distance between the midlines of the feet and from heels to toes) were determined from 20 s of data. Unilateral and bilateral knee-flexion contractures of 15 degrees and 30 degrees were simulated for each subject by means of an adjustable line from the subject's waist to the sole of each foot. Paired t tests were used to compare balance parameters while standing with the simulated contractures with those during relaxed standing. Mediolateral travel increased by a mean difference of 3.6 cm with a 30 degree unilateral contracture (P less than 0.01) and by 5.0 cm with 30 degrees bilateral contractures (P less than 0.01). Anteroposterior travel increased by 4.7 cm (P less than 0.05) and 8.8 cm (P = 0.08) with 15 degrees and 30 degrees bilateral contractures, respectively. With a unilateral contracture of 30 degrees, the center of pressure shifted 15.6% (P less than 0.0005) toward the unflexed side, changes that were not eliminated by correction of the induced leg-length discrepancy. The center of pressure moved anteriorly by 8.3% with 30 degrees bilateral contractures (P less than 0.001). The results provide insight into how knee-flexion contractures alter standing balance, and underline the importance of preventing and treating this common disorder.
Adult↗
Fatal wheelchair-related accidents in the United States.
Of the 3.3 of every 1000 persons in the United States who use a wheelchair, an estimated 3.3% per year have a serious wheelchair-related accident. Yet, only isolated case reports of fatal accidents have appeared. To obtain a better estimate of the incidence and nature of fatal accidents, a search was carried out of the death certificate database (1973-1987) of the National Information Clearinghouse of the Consumer Product Safety Commission; 770 wheelchair-related deaths were identified. The majority, 596 persons (77.4%), experienced a fall from their chairs or tipped over. Of 85 deaths (11%) caused by environmental factors, stairs were implicated in 51 (60.0%). Of 48 fatal burns (6.2%), 27 (57.3%) were related to smoking. Asphyxia owing to restraints occurred in 44 persons (5.7%) of all ages and caused 10 of the 17 deaths (58.8%) among persons 1-20 yr old. Wheelchair-related accidental death is uncommon (about 0.2% of serious accidents per year), but some types of accidents appear to be preventable.
Accidents↗
Preventing occupied wheelchairs from falling down stairs.
The hypothesis was tested that wheelchairs could be prevented from accidentally falling down stairs. A rigid post was attached to the wheelchair frame immediately behind the caster (clearance of 13 mm) such that it would strike the floor just after the casters dropped off an edge. The device was tested by means of a 3-degree ramp at the lower end of which was a level surface that ended with a 11-cm vertical drop. Twenty able-bodied subjects descended the ramp, by gravity alone, from progressively greater distances up the ramp, to determine the threshold at which the speed of the occupied wheelchair (with and without the device in place) was sufficient to induce a forward tip down the step. Forward tips occurred at a mean (+/- 1 SD) threshold speed of 0.77 (+/- 0.06) m/s with the device and at 0.38 (+/- 0.04) m/s without it, a mean difference of 0.39 (+/- 0.07) m/s (p less than 0.0005). This preliminary study suggests that such a wheelchair feature might improve the safety of wheelchairs in conditions involving inadvertent loss of caster support, as when they drop off a stair or ledge.
Accidental Falls↗
Cardiac-locomotor coupling while finger tapping.
Coupling between cardiac and locomotor rhythms has been identified while people walk, run, hop and cycle at cadences natural to them. To test the hypothesis that cardiac-locomotor coupling occurs during finger tapping, we studied 20 normal subjects tapping a telegraph key at a comfortable rate for 10 min. 15 subjects (75%) coupled significantly at one or more single-digit integer ratio (heart/tapping rate), the most common of which was 1:2. Such coupling should be considered a potentially confounding variable when studying finger tapping in subjects with disease or medication affecting heart rate. Also, the identification of coupling during the repetitive activity of small upper-extremity muscles suggests that neither increases in cardiac load nor impact-loading, two suggested explanations for why coupling occurs, are necessary for the phenomenon.
Adult↗
Prostheses and the forward reach of sitting lower-limb amputees.
The hypothesis that prostheses improve the forward reach of sitting lower-limb amputees was tested. While sitting with only ischial support, ten unilateral below-knee (BK) amputees could reach farther when they wore their prostheses than when they did not, when reaching at 45 degrees toward or away from the side of the amputation, with mean differences (+/- 1 SD) of 10.7cm (+/- 6.9, p less than 0.0001) and 4.4cm (+/- 4.1, p less than 0.01), respectively. With ten unilateral above-knee (AK) amputees, the differences were not statistically significant. Seven bilateral amputees (mixed levels) were able to reach farther with their prostheses on than off, with mean differences of 41.2cm (+/- 9.5) and 31.5cm (+/- 7.9) for reaches at 0 degrees and 45 degrees, respectively (p less than 0.0001). With the prostheses off, their reach was significantly greater when their thighs were supported than when they were sitting with only ischial support. These findings suggest that (1) prostheses improve the anterolateral reach of unilateral BK amputees, and the straight forward and anterolateral reach of bilateral amputees; (2) when prostheses are not being worn, a sitting surface which provides support through the residual limbs improves forward reach; and (3) the design of AK prostheses should reflect the patient's needs, both standing and sitting.
Adult↗
Excellence in rehabilitation through research. Commentary.
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Humans↗
Coupling of cardiac and locomotor rhythms.
The pressure within exercising skeletal muscle rises and falls rhythmically during normal human locomotion, the peak pressure reaching levels that intermittently impede blood flow to the exercising muscle. Speculating that a reciprocal relationship between the timing of peak intramuscular and pulsatile arterial pressures should optimize blood flow through muscle and minimize cardiac load, we tested the hypothesis that heart rate becomes entrained with walking and running cadence at some locomotion speeds, by means of electrocardiography and an accelerometer to provide signals reflecting heart rate and cadence, respectively. In 18 of 25 subjects, 1:1 coupling of heart and step rates was present at one or more speeds on a motorized treadmill, generally at moderate to high exercise intensities. To determine how exercise specific this phenomenon is, and to refute the competing hypothesis that coupling is due to vertical accelerations of the heart during locomotion, we had 12 other subjects cycle on an electronically braked bicycle ergometer. Coupling was found between heart rate and pedaling frequency in 10 of them. Cardiac-locomotor coupling appears to be a normal physiological phenomenon, and its identification provides a fresh perspective from which to study endurance.
Adult↗
Coupling between cardiac and locomotor rhythms: the phase lag between heart beats and pedal thrusts.
During some rhythmic exercises, the heart and exercise rates may become coupled (be within 1% of each other). If the intraarterial and skeletal intramuscular pressure cycles were reciprocal, blood flow to exercising muscle should be maximized and cardiac load minimized. In this study the authors tested the hypothesis that, while coupling is present, the phase lag between the pedaling and cardiac contraction cycles is consistent and appropriate. Twenty-seven subjects pedaled, at a frequency natural to them, on an electronically braked bicycle ergometer that held the power output constant regardless of pedaling rate. To assess the phase lag between pedal thrust (two per revolution) and heart beat, pedal-gated plots of the electrocardiography signal were generated throughout the most coupled five-minute work load for each of the 9 subjects in whom the rates were within 1% of each other for at least two consecutive four-second samples taken every fifteen seconds. During this interval of thirty-seconds in which the rates were within 1% of each other, the phase lag of most subjects gradually lengthened and shortened and there was considerable variation among subjects, refuting the authors' hypothesis. The results of this study illustrate the importance of beat-by-beat analysis when studying coupling phenomena. The preliminary assumption, that the coupling between cardiac and locomotor rhythms during cycling was on the basis of a single ischemic muscle group, has apparently been disproven.
Adult↗
Static and dynamic forward stability of occupied wheelchairs: influence of elevated footrests and forward stabilizers.
With 20 able-bodied subjects, three hypotheses were tested: (1) that elevating the footrests reduces the forward stability of occupied wheelchairs, (2) that placing simulated plaster casts on the occupant's legs further reduces stability, and (3) that forward stabilizers limit the extent of dynamic forward instability. Static stability was studied on a tilting platform; dynamic stability was tested by having subjects descend a ramp. Elevating one or both footrests and adding one or two simulated casts each significantly reduced static forward stability. During dynamic testing with lowered footrests, only seven subjects tipped forward, and the extent of tip was limited by the footrests. With both footrests elevated, eight subjects tipped transiently, and 12 continued to tip until the footrests hit the floor. With the addition of simulated casts, all 20 subjects tipped fully. Elevating one footrest (with or without a cast) caused only transient tips. On a high-friction floor surface, 19 and 18 subjects (with and without casts) experienced full tips or yawing falls to the side of the elevated footrest. The addition of forward stabilizers prevented wheelchairs from full tips or falls, when the users had both legs elevated, or when one of the users' legs (p less than 0.005), was elevated while located on a high friction floor (p less than 0.005). These findings have implications for wheelchair design and prescription.
Adult↗