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Biomedical subjects

J M Macpherson

Publications and source records attributed to J M Macpherson.

At least 19 recordsLinked to original sources
PubMed1999

Attributes of quiet stance in the chronic spinal cat.

Standing is a dynamic task that requires antigravity support of the body mass and active regulation of the position of the body center of mass. This study examined the extent to which the chronic spinal cat can maintain postural orientation during stance and adapt to changes in stance distance (fore-hindpaw separation). Intact cats adapt to changes in stance distance by maintaining a constant horizontal orientation of the trunk and changing orientation of the limbs, while keeping intralimb geometry constant and aligning the ground reaction forces closely with the limb axes. Postural adaptation was compared in four cats before and after spinalization at the T(6) level, in terms of the forces exerted by each paw against the support, body geometry (kinematics) and electromyographic (EMG) activity recorded from chronic, indwelling electrodes, as well as the computed net torques in the fore and hindlimbs. Five fore-hindpaw distances spanning the preferred distance were tested before spinalization, with a total range of 20 cm from the shortest to the longest stance. After spinalization, the cats were trained on a daily basis to stand on the force platform, and all four cats were able to support their full body weight. Three of the four cats could adapt to changes in stance distance, but the range was smaller and biased toward the shorter distances. The fourth cat could stand only at one stance distance, which was 8 cm shorter than the preferred distance before spinalization. All cats shifted their center of pressure closer to the forelimbs after spinalization, but the amount of shift could largely be accounted for by the weight loss in the hindquarters. The three cats that could adapt to changes in stance distance used a similar strategy as the intact cat by constraining the trunk and changing orientation of the limb axes in close relation with the forces exerted by each limb. However, different postures in the fore- and hindlimbs were adopted, particularly at the scapula (more extended) and pelvis (tipped more anteriorly). Other changes from control included a redistribution of net extensor torque across the joints of the forelimb and of the hindlimb. We concluded that the general form of body axis orientation is relatively conserved in the spinal cat, suggesting that the lumbosacral spinal circuitry includes rudimentary set points for hindlimb geometry. Both mechanical and neural elements can contribute toward maintaining body geometry through stiffness regulation and spinal reflexes.

Adaptation, Physiological↗
PubMed1999

Weight support and balance during perturbed stance in the chronic spinal cat.

The intact cat maintains balance during unexpected disturbances of stance through automatic postural responses that are stereotyped and rapid. The extent to which the chronic spinal cat can maintain balance during stance is unclear, and there have been no quantitative studies that examined this question directly. This study examined whether the isolated lumbosacral cord of the chronic spinal cat can generate automatic postural responses in the hindlimbs during translation of the support surface. Responses to 16 directions of linear translation in the horizontal plane were quantified before and after spinalization at the T(6) level in terms of forces exerted by each paw against the support, motion of the body segments (kinematics), and electromyographic (EMG) activity. After spinalization, the cats were trained on a daily basis to stand on the force platform, and all four cats were able to support their full body weight. The cats usually required assistance for balance or stability in the horizontal plane, which was provided by an experimenter exerting gentle lateral force at the level of the hips. Three of the four animals could maintain independent stance for a brief period (10 s) after the experimenter stabilized them. The fourth cat maintained weight support but always required assistance with balance. Perturbations were delivered during the periods of independent stance in three cats and during assisted stance in the fourth. A response to translation in the spinal cats was observed only in those muscles that were tonically active to maintain stance and never in the flexors. Moreover, latencies were increased and amplitudes of activation were diminished compared with control. Nevertheless, flexors and extensors were recruited easily during behaviors such as paw shake and stepping. It is concluded that centers above the lumbosacral cord are required for the full elaboration of automatic postural responses. Although the spinal cat can achieve good weight support, it cannot maintain balance during stance except for brief periods and within narrow limits. This limited stability is probably achieved through spinal reflex mechanisms and the stiffness characteristics of the tonically active extensors.

Adaptation, Physiological↗
PubMed1998

Activity of thoracic and lumbar epaxial extensors during postural responses in the cat.

This study examined the role of trunk extensor muscles in the thoracic and lumbar regions during postural adjustments in the freely standing cat. The epaxial extensor muscles participate in the rapid postural responses evoked by horizontal translation of the support surface. The muscles segregate into two regional groups separated by a short transition zone, according to the spatial pattern of the electromyographic (EMG) responses. The upper thoracic muscles (T5-9) respond best to posteriorly directed translations, whereas the lumbar muscles (T13 to L7) respond best to anterior translations. The transition group muscles (T10-12) respond to almost all translations. Muscles group according to vertebral level rather than muscle species. The upper thoracic muscles change little in their response with changes in stance distance (fore-hindpaw separation) and may act to stabilize the intervertebral angles of the thoracic curvature. Activity in the lumbar muscles increases along with upward rotation of the pelvis (iliac crest) as stance distance decreases. Lumbar muscles appear to stabilize the pelvis with respect to the lumbar vertebrae (L7-sacral joint). The transition zone muscles display a change in spatial tuning with stance distance, responding to many directions of translation at short distances and focusing to respond best to contralateral translations at the long stance distance.

Animals↗
PubMed1998

The cat vertebral column: stance configuration and range of motion.

This study examined the configuration of the vertebral column of the cat during independent stance and in various flexed positions. The range of motion in the sagittal plane is similar across most thoracic and lumbar joints, with the exception of a lesser range at the transition region from thoracic-type to lumbar-type vertebrae. The upper thoracic column exhibits most of its range in dorsiflexion and the lower thoracic and lumbar in ventroflexion. Lateral flexion is limited to less than 5 degrees at all segments. The range in torsion is almost 180 degrees and occurs primarily in the midthoracic region, T4-T11. Contrary to the depiction in most atlases, the standing cat exhibits several curvatures, including a mild dorsiflexion in the lower lumbar segments, a marked ventroflexion in the lower thoracic and upper lumbar segments, and a profound dorsiflexion in the upper thoracic (above T9) and cervical segments. The curvatures are not significantly changed by altering stance distance but are affected by head posture. During stance, the top of the scapula lies well above the spines of the thoracic vertebrae, and the glenohumeral joint is just below the bodies of vertebrae T3-T5. Using a simple static model of the vertebral column in the sagittal plane, it was estimated that the bending moment due to gravity is bimodal with a dorsiflexion moment in the lower thoracic and lumbar region and a ventroflexion moment in the upper thoracic and cervical region. Given the bending moments and the position of the scapula during stance, it is proposed that two groups of scapular muscles provide the major antigravity support for the head and anterior trunk. Levator scapulae and serratus ventralis form the lateral group, inserting on the lateral processes of cervical vertebrae and on the ribs. The major and minor rhomboids form the medial group, inserting on the spinous tips of vertebrae from C4 to T4. It is also proposed that the hypaxial muscles, psoas major, minor, and quadratus lumborum could support the lumbar trunk during stance.

Animals↗
PubMed1998

Effect of head position on postural orientation and equilibrium.

This study examined (1) how changes in head position affect postural orientation variables during stance and (2) whether changes in head position affect the rapid postural response to linear translation of the support surface in the horizontal plane. Cats were trained to stand quietly on a moveable platform and to maintain five different head positions: center, left, right, up, and down. For each head position, stance was perturbed by translating the support surface linearly in 16 different directions in the horizontal plane. Postural equilibrium responses were quantified in terms of the ground reaction forces, kinematics, dynamics (net joint torques), body center of mass, and electromyographic (EMG) responses of selected limb and trunk muscles. A change in head position involved rotation of not only the neck but also the scapulae and anterior trunk. Tonic EMG levels were modulated in several forelimb and scapular muscles but not hindlimb muscles. Finally, large changes in head orientation in both horizontal and vertical planes did not hamper the ability of cats to maintain postural equilibrium during linear translation of the support surface. The trajectory of the body's center of mass was the same, regardless of head position. The main change was observed in joint torques at the forelimbs evoked by the perturbation. Evoked EMG responses of forelimb and scapular muscles were modulated in terms of magnitude but not spatial tuning. Hindlimb responses were unchanged. Thus, the spatial and temporal pattern of the automatic postural response was unchanged and only amplitudes of evoked activity were modulated by head position.

Animals↗
PubMed1997

Postural orientation, equilibrium, and the spinal cord.

In summary, adequate control of postural orientation and equilibrium is a prerequisite for virtually all skilled motor acts, even those apparently simple tasks of standing and walking. In the cat, certain aspects of postural orientation appear to be organized at the spinal level. In contrast, postural equilibrium is not achieved by spinal circuits in isolation, but seems to require input from higher centers. The challenge is to identify those higher centers and the means by which they produce appropriate postural reactions for maintaining balance. If, indeed, postural equilibrium is a function of descending systems, then in the future, the amount of recovery of postural equilibrium after injury and/or treatment could provide a quantitative measure of the degree of sparing and/or re-establishment of functional connections between higher centers and the spinal cord.

Animals↗
PubMed1996

Two functional muscle groupings during postural equilibrium tasks in standing cats.

1. This study examined the relation between electromyographic (EMG) activation and the contact force and joint torques of the left hindlimb during postural equilibrium tasks in the standing cat. It is the appropriate application of force by the limbs against the support surface that allows the animal to control its center of mass and maintain equilibrium. 2. Cats were trained to stand quietly on a moveable force platform. During quiet stance, the cat was perturbed by a platform translation in each of 12 directions evenly spaced in the horizontal plane. EMG activity of mono- and biarticular thigh muscles, three-dimensional ground reaction force under the paw (contact force), and kinematics of the hindlimb segments were recorded Net joint torques were computed using inverse dynamics. The analysis focused on the functional organization of the rapid, automatic postural response in relation to the sagittal plane contact force and joint torques. 3. The muscles of the thigh were subdivided into two functional groups, based on the relationship of the evoked response to the various components of the sagittal plane contact force or joint torques. The first group, consisting of the monoarticular and some biarticular muscles, was correlated with the vertical force component, Fz. The second group, consisting of a separate group of biarticular muscles, was correlated with the difference between knee and hip torque. This torque difference is a function of both sagittal plane force components, Fz and Fy, and is related to contact force direction. 4. It is suggested that this subdivision of muscle activations reflects a neural strategy of parallel control of the two muscle groups in relation to their influence on Fz and Fy. Such a control mechanism could be a strategy for simplifying the control of the multisegmented limb in contact force tasks such as maintaining postural equilibrium.

Animals↗
PubMed1995

Bilateral labyrinthectomy in the cat: effects on the postural response to translation.

1. This study examined the role of vestibular afferent information on the postural responses of four cats, evoked by movements of the support surface during stance. Animals were exposed to linear translations of the supporting surface in eight evenly spaced directions in the horizontal plane, before and after bilateral labyrinthectomy. Postural responses were quantified in terms of the ground reaction forces under each paw and the evoked activity in selected muscles. 2. The cats were able to stand on the platform within 1-3 days after labyrinthectomy and were able to maintain balance during all perturbations of stance, even when they stood in total darkness, completely deprived of visual information. After lesion, postural responses were characterized by normal latency and normal spatial and temporal patterning of electromyographic (EMG) response. The pattern of force response showed the force constraint strategy that characterizes postural responses in the intact animal. 3. The only deficit in the postural response after lesion was a hypermetria, or active over-response that caused the animals to overbalance somewhat but did not impair their ability to remain upright. Analysis of the trajectory of the animal's center of mass during the trials indicated that the hypermetria was due to an abnormally large, active response on the part of the animal and could not be attributed to changes in the passive stiffness of the musculoskeletal system. The hypermetria was transient, and response amplitude returned to control levels after the rapid compensation phase of 10-15 days. 4. It is concluded that vestibular information is not essential for triggering the rapid, automatic postural response to translations of the support surface, nor is it necessary for the selection or shaping of the evoked response. Instead, somatosensory information appears to predominate in these postural adjustments. However, vestibular afferent input does influence the scaling of the postural response.

Afferent Pathways↗
PubMed1995

Determinants of postural orientation in quadrupedal stance.

The purpose of this study was to investigate the determinants of postural orientation by examining stance kinematics and kinetics at various interpaw distances. Four adult cats were trained to stand, unrestrained, on a force platform. Three-dimensional ground reaction forces and kinematics, as well as EMG activities, were recorded during stance at five different anteroposterior (AP) distances and two widths. Stance distance was varied by changing the distance between the four force plates mounted on the platform. All cats used a strategy of maintaining a constant horizontal orientation of the trunk and levering the limbs at the girdles while maintaining constant intralimb geometry to accommodate the changes in stance distance. The direction of the ground reaction forces covaried with the limb axes. As a result, the joint torques were conserved in the forelimb, and varied within small ranges in the hind. Increased tonic activity in the extensors of the back, the hip, and the ankle was observed at shorter distances while increased knee extensor activity was observed at longer distances. A cost function, defined as the sum of squared 3-D joint torques, was minimal for the hindlimb at a stance distance which corresponded to the preferred distance naturally assumed by each cat on the floor. Thus, in the maintenance of stance posture, trunk orientation and intralimb geometry is constrained, the goal of which is to minimize muscular effort or energy expenditure.

Animals↗
PubMed1994

The force constraint strategy for stance is independent of prior experience.

The purpose of this study was to examine the effect of prior experience concerning direction of a postural perturbation on the balance response of cats to translations of their support surface. Previous work has shown that, when cats are translated in many directions in the horizontal plane, they respond by exerting active forces with each paw in only two directions, termed the force constraint strategy. This study examined whether the force constraint strategy could be modified based on predictability of the direction of translation and whether this strategy is used by the naive animal with no prior experience of platform translation. Four cats were trained to stand quietly on the force platform using positive reinforcement, and then were implanted with chronically indwelling electrodes for recording electromyographic (EMG) activity. The first experiment concerned the response of the naive cats to their first exposure to platform translation and consisted of translations presented randomly in four different directions in the horizontal plane. The second experiment consisted of two complete sets of 16 directions of translation (15 trials per direction), with the direction of translation randomized in one set and serially ordered in the other, to make the direction of translation unpredictable or predictable, respectively. Forces exerted by the cat, EMG activity, and platform position were recorded during the 1-s trials. The use of the force constraint strategy was independent of prior experience with direction of translation, as was the amplitude of the response.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Changes in a postural strategy with inter-paw distance.

1. The purpose of this study was to examine the effect of changing initial stance conditions on the postural response of the cat to horizontal plane translations of the support surface. Cats were trained to stand, unrestrained, on a moveable force platform. The platform was translated linearly in each of 16 directions in the horizontal plane, with a ramp-and-hold displacement. The animal's response was quantified in terms of the forces exerted at the ground. The trajectory of the center of mass (CoM) was computed from the forces. 2. Stance length was varied along the longitudinal (sagittal) axis by adjusting the distance between the forepaw and hindpaw force plates. Translation perturbations of the platform were recorded at stance distances varying from 66 to 110% of the preferred stance distance. 3. Changing stance distance had a significant effect on the amplitude and direction of the active forces exerted by the cat both during quiet stance and during the response to platform translation. At long stance distances, each limb exerted a force outward, along the diagonals during quiet stance. The response to translation was characterized by an invariance in the direction of force exerted against the ground, a strategy that was described previously. At short stance distances, quiet stance forces were more laterally directed. The force constraint strategy was usually not observed for the response to translation. Nevertheless, the cats were equally effective at all stance distances in restoring the position of the center of mass after translation of the support surface. 4. There was no discrete boundary between the presence and absence of the force constraint, suggesting that the strategy for exerting forces against the support surface is characterized by a continuum of response, from a bimodal, or anisotropic distribution of force vectors on the one extreme, to a uniform, or isotropic distribution on the other. Arguments are developed to suggest that the force constraint strategy may be useful in stabilizing the vertebral column during the response to platform translation, to allow linear translation of the CoM rather than bending of the trunk.

Animals↗
PubMed1994

Stance control in the chronic spinal cat.

1. A longitudinal study of the control of quiet and perturbed stance was conducted before and for 1 yr after complete spinal transection (T12) in a cat trained to stand on a moveable force platform. 2. With daily training, the spinal cat recovered full weight support and some intermittent control of lateral stability within 1 mo. Within the second month postspinalization, the spinal cat achieved the ability to maintain independent, unassisted stance (no external support or stimulation) for up to 45 s during quiet stance, as well as for 62-97% of the trials of horizontal translations of the support surface. 3. Control of lateral stability in the spinal cat was severely compromised, however, as eventually the spinal cat always lost its balance. Head movements and the tendency for the hindlimbs to initiate stepping movements were more destabilizing than platform translations. 4. Our preliminary results indicate that the recovery of partial lateral stability of the hindquarters in the spinal cat is the product of passive muscle properties and segmental reflexes, which, in isolation can provide only limited balance control in the chronic spinal cat.

Animals↗
PubMed1993

Stance and balance following bilateral labyrinthectomy.

Although vestibular input codes head acceleration, it is not clear whether or not this signal is critical for triggering the initial postural response to a perturbation of stance, and for determining the appropriate direction of response. These experiments were designed to examine the contribution of vestibular inputs to the control of balance in the freely standing cat. Four cats were trained to stand quietly on a moveable force platform. The animal's stance was unexpected perturbed by applying a linear ramp-and-hold translation to the support surface in each of eight different directions in the horizontal plane. The characteristics of quiet stance and the response to the perturbations were quantified in terms of the 3-D ground reaction forces under each paw and the EMG activity in selected muscles. The animals were bilaterally labyrinthectomized, and their responses compared before and after lesion. The cats were able to stand stably on the platform within 2-3 days of the lesion. During quiet stance, there was no change in the distribution of vertical forces under the paws and no increase in sway area. Horizontal plane forces, which were normally outwardly directed on the diagonals, became more laterally directed and transiently larger in amplitude. The level of tonic EMG activity increased in some extensors and flexors, and decreased in others, compared to control. The responses to platform translation were characterized by normal spatial and temporal patterns and latencies of EMG activity. Furthermore, all cats continued to use the force constraint strategy that is characteristic of the intact animal (Macpherson, 1988a). The only clear deficit in performance was a transient hypermetria, characterized by an over-response to the translation. Although the cats over-responded, they were still able to maintain their balance successfully. The moderate changes in quiet stance and in response to perturbation gradually returned to control values over 8-10 days following the lesion. These results suggest that vestibular information is not necessary for triggering appropriate postural responses evoked by support surface translations, nor for selecting the direction of response.

Animals↗
PubMed1993

Activity of neuromuscular compartments in lateral gastrocnemius evoked by postural corrections during stance.

1. The electromyographic (EMG) activity of the four neuromuscular compartments in lateral gastrocnemius (LG) of cats was investigated to determine whether these intramuscular subdivisions could be activated differentially during automatic postural corrections. EMG electrodes were surgically implanted into each of the four compartments of left LG-LG1, LG2, LG3, and LGm--in two cats. Electrodes were also implanted into soleus and gluteus medius for comparative purposes. 2. Quiet quadrupedal stance was disturbed first by linearly translating the cats on a movable platform in each of 16 different horizontal directions. Mechanical events during corrections were characterized in terms of the three-dimensional forces exerted by each paw on the platform. EMG and force traces were quantified (area under the curve) and normalized, and tuning curves were constructed that relate muscle response and force change to direction of platform movement. 3. In a second series of trials, translations were presented along one direction only over a series of six velocities ranging from 5 to 16 cm/s. The third series of perturbations, termed the pop-up, consisted of a rapid upward displacement of the support under the left hindlimb only over a series of six amplitudes ranging from 1 to 10 mm. Evoked EMG activity and average change in force were normalized and regressions were computed onto velocity and amplitude, respectively. The slopes of the regressions were compared. 4. EMG tuning curves associated with the multidirectional horizontal translations revealed no differential activity across LG compartments. Similarly, there was no statistical difference among the slopes of the regressions within LG. In contrast, soleus exhibited significantly different slopes from LG for the regressions. Thus it is concluded that LG compartments are not differentially activated during automatic postural responses to perturbations of the support surface.

Animals↗
PubMed1991

Head acceleration following linear translations in the freely-standing cat.

The aim of this study was to determine whether vestibular information related to head acceleration is available for triggering postural responses to perturbations of stance in the freely-standing cat. Linear accelerations of the head were recorded during postural responses evoked by linear translations of the support surface. A consistent initial peak of acceleration was observed at an average latency of 22 ms and magnitude of 0.03 g (g is acceleration due to gravity, 9.8 m/s/s). The acceleration peak preceded the first evoked EMG activity by an average of 24 ms. It was concluded that stimulation of the vestibular apparatus was both adequate and early enough for the vestibular system to have triggered the automatic postural response.

Animals↗
PubMed1991

Functionally complex muscles of the cat hindlimb. III. Differential activation within biceps femoris during postural perturbations.

The biceps femoris (BF) muscle is divided into three neuromuscular compartments defined by the innervation patterns of the main nerve branches (English and Weeks 1987). The goals of this study were i) to determine how different regions of the biceps femoris muscle are activated in the intact cat during a broad range of limb movements evoked by perturbations of stance posture, and ii) to determine the relationship between the anatomical compartments of biceps femoris and the functional units as defined in this task. Cats were trained to stand on a moveable platform with each paw on a triaxial force plate. The animal's stance was perturbed by linear translation of the platform in each of sixteen different directions in the horizontal plane. EMG activity was recorded from eight sites across the width of the left biceps femoris muscle. During quiet stance only the anterior compartment was tonically active, presumably contributing to hip extensor torque in the maintenance of stance. During platform translation, evoked EMG activity was recorded from each electrode pair for a wide range of directions of perturbation; as direction changed progressively, the amplitude of evoked activity from any electrode pair increased to a maximum and then decreased. When the EMG amplitude was plotted in polar coordinates as a function of translation direction, the region of response formed a petal shaped area in the horizontal plane, termed the EMG tuning curve. The compartments of the BF muscle were not activated homogeneously. The tuning curve of the anterior BF compartment was similar to that of other hip extensors, and coincided with the region of postero-lateral force production by the hindlimb against the support. The tuning curve of the middle BF compartment was shifted in a counterclockwise direction from that of the anterior compartment, but overlapped extensively with it; the middle BF tuning curve was similar to that of anterior gracilis. The tuning curve of the posterior biceps compartment was rotated further counterclockwise and overlapped very little with that of the middle BF compartment. The posterior BF was activated in a pattern similar to that of other knee flexors. The functional units of BF activation were not identical with the neuromuscular compartments defined by the main nerve branches. As direction of the perturbation changed, the region of BF that was activated moved progressively across the muscle. This progression of the active region was continuous across BFa and BFm, whereas there was a jump, or discontinuity at the border between BFm and BFp.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Bilateral labyrinthectomy in the cat: motor behaviour and quiet stance parameters.

The purpose of this study was to examine the effect of bilateral labyrinthectomy on quiet stance in the freely-standing cat. Since loss of the vestibular end organs produces marked deficits in motor behaviour, including ataxia and problems with balance, we hypothesized that labyrinthectomized animals would show impairment in quantitative measures of stance. Stance was quantified in terms of the ground reaction forces under each limb and the tonic electromyographic (EMG) activity of selected muscles. Animals were labyrinthectomized by drilling into the vestibule and removing the vestibular epithelium. Following lesion, animals were able to stand unsupported on the force platform within 2 days. To our surprise, the lesioned animals showed little change in stance parameters from the control, pre-lesion state. Thus, our hypothesis of changes in stance parameters was not supported. There was no change in the distribution of vertical forces under the limbs and no increase in sway, as measured by the area of excursion of the centre of pressure over time. The horizontal plane forces, which were diagonally directed prior to lesion, became more laterally directed and larger in amplitude. The change in direction persisted even after the animals had fully compensated for the lesion, but the force amplitudes returned to control values within 10-12 days. The change in horizontal force direction was similar to that observed in normal animals that were required to stand with their paws closer than preferred in the sagittal plane (unpublished observations). EMG activity changed in some muscles but not others, and usually transiently.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗