Dynamics of muscle function during locomotion: accommodating variable conditions.
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Much of what we know about animal locomotion is derived from studies examining animals moving within a single, homogeneous environment, at a steady speed and along a flat grade. As a result, the issue of how musculoskeletal function might shift to accommodate variability within the external environment has remained relatively unexplored. One possibility is that locomotor muscles are differentially recruited depending upon the environment in which the animal is moving. A second possibility is that the same muscles are recruited, but that they are activated in a different manner so that their contractile function differs according to environment. Finally, it is also possible that, in some cases, animals may not need to alter their musculoskeletal function to move under different external conditions. In this case, however, the mechanical behavior appropriate for one environmental condition may constrain locomotor performance in another. To begin to explore the means by which animals accommodate variable conditions in their environment, we present three case studies examining how musculoskeletal systems function to allow locomotion under variable conditions: (1) eels undulating through water and across land, (2) turkeys running on level and inclined surfaces, and (3) ducks using their limbs to walk and to paddle. In all three of these examples, the mechanical behavior of some muscle(s) involved in locomotion are altered, although to different degrees and in different ways. In the running turkeys, the mechanical function of a major ankle extensor muscle shifts from contracting isometrically on a flat surface (producing little work and power), to shortening actively during contraction on an uphill gradient (increasing the amount of work and power generated). In the ducks, the major ankle extensor undergoes the same general pattern of activation and shortening in water and on land, except that the absolute levels of muscle stress and strain and work output are greater during terrestrial locomotion. In eels, a transition to land elicits changes in electromyographic duty cycles and the relative timing of muscle activation, suggesting some alteration in the functional mechanics of the underlying musculature. Only by studying muscle function in animals moving under more variable conditions can we begin to characterize the functional breadth of the vertebrate musculoskeletal system and understand more fully its evolutionary design. (+info)
Acceleration and balance in trotting dogs.
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During quadrupedal trotting, diagonal pairs of limbs are set down in unison and exert forces on the ground simultaneously. Ground-reaction forces on individual limbs of trotting dogs were measured separately using a series of four force platforms. Vertical and fore-aft impulses were determined for each limb from the force/time recordings. When mean fore-aft acceleration of the body was zero in a given trotting step (steady state), the fraction of vertical impulse on the forelimb was equal to the fraction of body weight supported by the forelimbs during standing (approximately 60 %). When dogs accelerated or decelerated during a trotting step, the vertical impulse was redistributed to the hindlimb or forelimb, respectively. This redistribution of the vertical impulse is due to a moment exerted about the pitch axis of the body by fore-aft accelerating and decelerating forces. Vertical forces exerted by the forelimb and hindlimb resist this pitching moment, providing stability during fore-aft acceleration and deceleration. (+info)
Mechanical and thermal hyperalgesia and ectopic neuronal discharge after chronic compression of dorsal root ganglia.
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Chronic compression of the dorsal root ganglion (CCD) was produced in adult rats by implanting a stainless steel rod unilaterally into the intervertebral foramen, one rod at L(4) and another at L(5). Two additional groups of rats received either a sham surgery or an acute injury consisting of a transient compression of the ganglion. Withdrawal of the hindpaw was used as evidence of a nocifensive response to mechanical and thermal stimulation of the plantar surface. In addition, extracellular electrophysiological recordings of spontaneous discharges were obtained from dorsal root fibers of formerly compressed ganglia using an in vitro nerve-DRG-dorsal root preparation. The mean threshold force of punctate indentation and the mean threshold temperature of heating required to elicit a 50% incidence of foot withdrawal ipsilateral to the CCD were significantly lower than preoperative values throughout the 35 days of postoperative testing. The number of foot withdrawals ipsilateral to the CCD during a 20-min contact with a temperature-controlled floor was significantly increased over preoperative values throughout postoperative testing when the floor was 4 degrees C (hyperalgesia) and, to a lesser extent, when it was 30 degrees C (spontaneous pain). Stroking the foot with a cotton wisp never elicited a reflex withdrawal before surgery but did so in most rats tested ipsilateral to the CCD during the first 2 postoperative weeks. In contrast, the CCD produced no changes in responses to mechanical or thermal stimuli on the contralateral foot. The sham operation and acute injury produced no change in behavior other than slight, mechanical hyperalgesia for approximately 1 day, ipsilateral to the acute injury. Ectopic spontaneous discharges generated within the chronically compressed ganglion and, occurring in the absence of blood-borne chemicals and without an intact sympathetic nervous system, were recorded from neurons with intact, conducting, myelinated or unmyelinated peripheral nerve fibers. The incidence of spontaneously active myelinated fibers was 8.61% for CCD rats versus 0.96% for previously nonsurgical rats. We hypothesize that a chronic compression of the dorsal root ganglion after certain injuries or diseases of the spine may produce, in neurons with intact axons, abnormal ectopic discharges that originate from the ganglion and potentially contribute to low back pain, sciatica, hyperalgesia, and tactile allodynia. (+info)
The limping child: epidemiology, assessment and outcome.
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We investigated the epidemiology, assessment and outcome of acute atraumatic limp in 243 children under the age of 14 years presenting to a paediatric accident and emergency department (AED) over a period of six months. Data were collected at presentation and medical notes were re-examined after 18 to 21 months. The incidence of limp was 1.8 per thousand. The male:female ratio was 1.7:1 and the median age 4.35 years. Limp was mainly right-sided (54%) and painful (80%); 33.7% of the children had localised pain in the hip. A preceding illness was found in 40%. The main diagnosis was 'irritable hip'/transient synovitis (39.5%); Perthes' disease accounted for 2%. Most patients (77%) were managed entirely in the AED. Acute atraumatic limp is a common problem in children presenting to the AED. Most can be safely managed there if guidelines are followed and will have a benign outcome. Further studies are needed to identify the role of preceding illness in the aetiology of acute atraumatic limp. (+info)
Load-regulating mechanisms in gait and posture: comparative aspects.
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How is load sensed by receptors, and how is this sensory information used to guide locomotion? Many insights in this domain have evolved from comparative studies since it has been realized that basic principles concerning load sensing and regulation can be found in a wide variety of animals, both vertebrate and invertebrate. Feedback about load is not only derived from specific load receptors but also from other types of receptors that previously were thought to have other functions. In the central nervous system of many species, a convergence is found between specific and nonspecific load receptors. Furthermore, feedback from load receptors onto central circuits involved in the generation of rhythmic locomotor output is commonly found. During the stance phase, afferent activity from various load detectors can activate the extensor part in such circuits, thereby providing reinforcing force feedback. At the same time, the flexion is suppressed. The functional role of this arrangement is that activity in antigravity muscles is promoted while the onset of the next flexion is delayed as long as the limb is loaded. This type of reinforcing force feedback is present during gait but absent in the immoble resting animal. (+info)
Effects of footwear on measurements of balance and gait in women between the ages of 65 and 93 years.
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BACKGROUND AND PURPOSE: Footwear is not consistently standardized in the administration of the Functional Reach Test (FRT), Timed Up & Go Test (TUG), and 10-Meter Walk Test (TMW). This study was conducted to determine whether footwear affected performance on these tests in older women. SUBJECTS: Thirty-five women, aged 65 to 93 years, were recruited from assisted living facilities and retirement communities. METHODS: Each subject performed the FRT, TUG, and TMW wearing walking shoes, wearing dress shoes, and barefooted. Because of space constraints at the facilities where the testing was performed, 22 subjects performed the FRT and TUG on a linoleum floor and 13 subjects performed the tests on a firm, low-pile, carpeted floor. All 35 subjects completed the TMW on a firm, low-pile, carpeted floor. One-way repeated-measures analyses of variance (ANOVAs) and a Tukey Honestly Significant Difference test were used to compare the outcomes for the 3 footwear conditions, with separate ANOVAs conducted for the different floor surfaces for the FRT and TUG. RESULTS: Subjects performed better on the FRT when barefooted or wearing walking shoes compared with when they wore dress shoes, regardless of floor surface. Differences were found among all footwear conditions for the TUG performed on the linoleum floor and for the TMW. For these tests, the women moved fastest in walking shoes, slower barefooted, and slowest wearing dress shoes. CONCLUSION AND DISCUSSION: Footwear should be documented and should remain constant from one test occasion to another when the FRT, TUG, and TMW are used in the clinic and in research. Footwear intervention may improve performance of balance and gait tasks in older women. (+info)
Establishing the reliability and validity of measurements of walking time using the Emory Functional Ambulation Profile.
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BACKGROUND AND PURPOSE: The Emory Functional Ambulation Profile (E-FAP) measures time to walk in different environments and accounts for use of assistive devices. This study assessed the reliability and validity of walking time measurements using these components. SUBJECTS: Twenty-eight subjects who had strokes and 28 subjects without impairment were recruited. METHODS: The E-FAP, Berg Balance Test, Functional Reach Test, and Timed 10-Meter Walk Test were administered in random order during a single data collection session. RESULTS: Interrater reliability for the total E-FAP was > or = .997. Subjects without impairment performed better on all 4 tests than did subjects who had strokes. Increased times on the E-FAP correlated with poor performance on the Berg Balance Test and slow gait speeds on the Timed 10-Meter Walk Test in the subjects who had strokes. The E-FAP scores and the Functional Reach Test scores were not correlated. CONCLUSION AND DISCUSSION: The E-FAP can be administered easily and inexpensively. Because the E-FAP scores differentiated subject groups and correlated with known measures of function, the E-FAP may be a clinically useful measure of ambulation. (+info)
Restricted expression of G86R Cu/Zn superoxide dismutase in astrocytes results in astrocytosis but does not cause motoneuron degeneration.
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Evidence garnered from both human autopsy studies and genetic animal models has suggested a potential role for astrocytes in the pathogenesis of amyotrophic lateral sclerosis (ALS). Currently, mutations in the gene encoding Cu/Zn superoxide dismutase (SOD1) represent the only known cause of motoneuron loss in the disease, producing 21q linked familial ALS (FALS). To determine whether astrocytic dysfunction has a primary role in familial ALS, we have generated multiple lines of transgenic mice expressing G86R mutant SOD1 restricted to astrocytes. In GFAP-m SOD1 mice, astrocytes exhibit significant hypertrophy and increased GFAP reactivity as the animals mature. However, GFAP-mutant SOD1 transgenic mice develop normally and do not experience spontaneous motor deficits with increasing age. Histological examination of spinal cord in aged GFAP-mSOD1 mice reveals normal motoneuron and microglial morphology. These results indicate that 21q linked FALS is not a primary disorder of astrocytes, and that expression of mutant SOD1 restricted to astrocytes is not sufficient to cause motoneuron degeneration in vivo. Expression of mutant SOD1 in other cell types, most likely neurons, is critical for the initiation of disease. (+info)