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PubMed1991

Activity feedback to the mammalian circadian pacemaker: influence on observed measures of rhythm period length.

In the mouse, activity is precisely timed by the circadian clock and is normally most intense in the early subjective night. Since vigorous activity (e.g., wheel running) is thought to induce phase shifts in rodents, the temporal placement of daily exercise/activity could be a determinant of observed circadian rhythm period. The relationship between spontaneous running-wheel activity and the circadian period of free-running rhythms was studied to assess this possibility. With ad libitum access to a running wheel, mice exhibited a free-running period (tau) of 23.43 +/- 0.08 hr (mean +/- SEM). When running wheels were locked, tau increased (23.88 +/- 0.04 hr, p less than 0.03), and restoration of ad libitum wheel running again produced a shorter period (tau = 23.56 +/- 0.06 hr, p less than 0.05). A survey of free-running activity patterns in a population of 100 mice revealed a significant correlation between the observed circadian period and the time of day in which spontaneous wheel running occurred (r = 0.7314, p less than 0.0001). Significantly shorter periods were observed when running was concentrated at the beginning of the subjective night (tau = 23.23 +/- 0.04), and longer periods were observed if mice ran late in the subjective night (tau = 23.89 +/- 0.04), F (1, 99) = 34.96, p less than 0.0001. It was previously believed that the period of the circadian clock was primarily responsive to externally imposed tonic or phasic events. Systematic influences of spontaneous exercise on tau demonstrate that physiological and/or behavioral determinants of circadian timekeeping exist as well.

Animals↗

PubMed2004

Proprioceptive control of human wrist extensor motor units during an attention-demanding task.

The responsiveness of the tonically firing single motor units (SMU) to Ia afferent volleys elicited by either mechanical (T-reflex) or electrical nerve stimulation (H-reflex) was tested in the extensor carpi radialis muscle (ECR) while the subjects were maintaining a steady wrist extension force using visual feedback set either at low or high gain. The aim was to determine whether the proprioceptive control of tonic motoneuronal activity depends on the level of attentiveness required by the behavioural context. The response probability of the SMUs to tendon taps was significantly higher (p<0.0001) and that to electrical nerve stimulation was lower (p<0.001) during the more demanding task. Since these changes in SMU responsiveness were not accompanied by any differences in either the motor unit firing patterns or the mean levels of EMG muscle activity, it can be concluded that there were no attention-related changes in the net excitatory drive to the ECR motoneurons. These results are consistent with the idea that fusimotor sensitization of the muscle spindle may have occurred in the more demanding task: an increase in the mechanical sensitivity of the muscle spindles would certainly account for both the T-reflex facilitation and the H-reflex depression observed. The attention-demanding task therefore seemed to involve an independent fusimotor drive activation process. The results of this study suggest that an adaptation of the fusimotor system occurs in humans, depending on the levels of attention and accuracy required to perform the ongoing motor task, as previously reported to occur in animals.

Action Potentials↗

PubMed1987

Neural control of limb coordination. I. Comparison of hatching and walking motor output patterns in normal and deafferented chicks.

Previous work has shown that the neural circuits underlying the leg movements of walking and hatching coexist in post-hatching chicks (Bekoff and Kauer, 1984). In the present study, quantitative analysis of leg EMGs shows that there are some similarities, but also significant differences, in the motor output patterns of walking and hatching. This study examines the effect of removing sensory feedback from the legs on the production of the distinctive leg motor patterns. The temporal characteristics and interlimb coordination of hatching and walking are little affected. However, major changes in intralimb motor output patterns are seen when compared to records from normal chicks. These changes fall into one of 2 categories. Some parameters show similar changes in both behaviors after deafferentation (e.g., increases in flexor burst durations and cycle period). This suggests that certain features of sensory input from the legs normally modulate the hatching and walking pattern-generating circuitry in similar ways. Other parameters show convergence. That is, these aspects of the 2 intralimb motor patterns become more similar to each other after removal of sensory input. This is consistent with the hypothesis that some feature of sensory input from the legs normally modulates one set of multiuse intralimb circuitry to produce different output patterns. In general, the walking pattern becomes more like hatching after deafferentation, rather than the reverse, which suggests that the hatching pattern is a more basic one. The maintenance of some residual differences in intralimb motor patterns after leg deafferentation suggests that other sources of modulation must also be involved, or that there are some additional elements of circuitry that are called into play during the normal production of walking and hatching.

Animals↗

PubMed2005

Changes in neuromuscular function after tasks involving control of EMG versus torque feedback of the same duration.

This study was designed to compare alterations in neuromuscular function after two tasks of similar duration involving the control of (1) torque level fixed at 40% maximal voluntary contraction (MVC) torque (torque task) and (2) EMG level when exerting 40% MVC torque on the knee extensor muscles. Ten healthy subjects volunteered to participate in two testing sessions separated by approximately 2 h. Contraction duration for the EMG task was fixed for each subject to the time to task failure of the torque task (104+/-20s). MVC, maximal voluntary activation level, muscle compound action potential (M-wave), peak twitch and potentiated peak doublet were assessed before and immediately after each task using electrical stimulation of the femoral nerve. Average EMG activity of quadriceps muscle increased (p<0.01) during the torque task from 27.7+/-5.4% to 46.2+/-19.3% maximal EMG, whereas torque decreased during the EMG task from 41.5+/-2.9% to 28.9+/-3.8% MVC torque. Alterations in MVC torque (p<0.01) and maximal voluntary activation level (p<0.05) were comparable at termination of the two tasks. Rate of perceived exertion was greater (p<0.05) at the end of the torque task compared to the EMG task. Despite the absence of change in the M-wave for either task, potentiated peak doublet was altered after the torque task (-18+/-14%, p<0.01), whereas there was no change after the EMG task (p>0.05). The absence of peripheral failure at the end of the EMG task could be attributed to (1) a lower intramuscular pressure allowing a lesser accumulation of metabolites and (2) a slower rate of PCr hydrolysis compared to the torque task.

Adult↗

PubMed1994

Twitch potentiation following voluntary diaphragmatic contraction.

The purpose of this study was to determine whether twitch potentiation (a transient augmentation of twitch tension following vigorous muscular contraction) occurs in the human diaphragm in vivo. Six healthy subjects were studied. To determine whether twitch potentiation occurs, the subjects attempted to maximally activate their diaphragm by performing the combined Mueller-expulsive maneuver with visual feedback (Pdi max maneuver). Twitches were obtained before, at 10 s after the transdiaphragmatic pressure (Pdi) maneuver, and at intervals over the ensuing 10 min. We also determined whether twitch potentiation would occur following submaximal voluntary diaphragmatic contractions (33 and 66% of Pdi max). In addition, we examined whether repeated voluntary contractions would result in greater twitch potentiation compared with that observed after a single voluntary contraction. Twitch potentiation was observed in every subject. The number of maximal voluntary contractions (MVC) (one, two, or four) had no significant effect on the degree of twitch potentiation. The increase in twitch amplitude (expressed as a percentage of the control value) averaged 63 +/- 35% (SD)(pooled data from one, two, and four contraction trials). Twitch potentiation decayed in a monoexponential fashion (r = 0.99) with a time constant of 125 s (95% Cl = 100 to 160 s). Twitch potentiation was also observed after submaximal voluntary diaphragmatic contractions. Again, the number of voluntary contractions (one or four) had no significant effect on the degree of twitch potentiation. After submaximal diaphragmatic contractions of 66% of Pdi max, the degree of twitch potentiation was not significantly different, 61 +/- 36% (pooled data from one and four contraction trials) from that observed following the maximal voluntary contraction maneuvers.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗

PubMed2001

Effects of increased hardness on jaw movement and muscle activity during chewing of visco-elastic model foods.

When food is chewed, sensory feedback adapts the motor program to the characteristics of the food. However, the relationship between the physical properties of different foods and the motor response is poorly understood. In this study, we developed edible and well-controlled model foods in order to describe some of the stimulus-response functions of the food-mastication loop. Four gelatine-based visco-elastic model foods identical in shape and size but differing in hardness were prepared. They displayed reproducible sensory and physical characteristics and were distributed on a wide hardness scale. Electromyographic activity of masseter and temporalis muscles and jaw movements in the frontal plane were simultaneously recorded during mastication in 15 young men with intact dentition and good oral status. Almost all EMG and jaw movement parameters were clearly affected by increasing hardness of model foods. However, it is possible to summarise the results by reducing the number of parameters to three: the number of chewing cycles, EMG activity of any one of the two temporal or the two masseter muscles and the amplitude of the opening mandibular movements. Indeed, these were the best transcriptors of the hardness range of the model foods used in this study. As inferred from these parameter recordings, the food hardness modifications were strongest during the first five strokes, began as early as the first stroke and lasted for the whole sequence.

Adult↗

PubMed2006

Threshold control of motor actions prevents destabilizing effects of proprioceptive delays.

It is usually assumed that proprioceptive feedback comes to motoneurons too late to contribute to the initial activity of agonist muscles during fast arm movements, leading to the suggestion that this feedback is only efficient in slow movements and postural control. The argument does not take into account that the changes in the motoneuronal membrane potentials and the associated changes in the state of spinal neurons preceding the initial activity of muscles deeply affect, in a forward way, the state of reflex systems by shifting their thresholds, as suggested in the lambda model for motor control. As a result, the initial muscle activity emerges with full contribution of these systems so that the effects of reflex delays become negligible. We tested the hypothesis that threshold control of muscle activation may be instrumental in preventing destabilizing effects of proprioceptive delays in spinal and trans-cortical pathways to motoneurons. The analysis was made by recording fast elbow movements (peak velocity approximately 300-500 degrees/s) and simulating them in a dynamic model that incorporates the notion of threshold control of intrinsic and reflex muscle properties. The model was robust in reproducing experimental movement patterns (R (2)>0.95). It generated stable output despite substantial proprioceptive (up to 100 ms) and electromechanical (40 ms) delays. Stability was thus ensured for delays not only in segmental (about 25-50 ms) but also in trans-cortical loops (50-70 ms). Our study illustrates that a natural physiological process--threshold control--may manifest feed-forward properties hitherto attributed to hypothetical internal neural models.

Adult↗

PubMed1993

The effect of an occlusal stabilization splint and the mode of visual feedback on the activity balance between jaw-elevator muscles during isometric contraction.

The aim of the present study was to gain an insight into the influence of a vertical bite-rise (clenching in intercuspal occlusion vs. clenching on an occlusal stabilization splint), the mode of visual feedback (VF; obtained from the compound masseter signal, from the compound anterior temporalis signal, or from the compound signal of both masseter and anterior temporalis muscles) and the EMG clenching level (10% MVC and 50% MVC) on the muscle balance between the masseter and the anterior temporalis muscles. The muscle balance was quantified as the logarithmic value of the ratio between the summated mean rectified EMG activity of the masseter muscles and this activity of the anterior temporalis muscles. The muscle balance was influenced significantly by the mode of VF (p < 0.01), the muscle balance shifting toward the group of muscles from which VF was obtained. When VF was obtained from the masseter muscles, a decrease in the anterior temporalis EMG activity was observed when the vertical dimension was increased (p < 0.05-0.01). When VF was obtained from the anterior temporalis muscles, the activity of the masseter muscles was raised with respect to that of the anterior temporalis muscles during clenching with a vertical bite-rise (p < 0.05-0.01). When VF was obtained from both groups of muscles, the masseteric EMG activity increased, whereas the anterior temporalis EMG activity decreased. Hence, regardless of the mode of VF, a relatively lower activity level of the anterior temporalis muscles was achieved after insertion of an occlusal stabilization splint.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1992

Respiratory and abdominal muscle responses to expiratory threshold loading in cystic fibrosis.

We hypothesized that the hyperinflation and pulmonary dysfunction of cystic fibrosis (CF) would distort feedback and therefore alter the abdominal muscle response to graded expiratory threshold loads (ETLs). We compared the respiratory and abdominal muscle responses with graded ETLs of seven CF patients with severe lung dysfunction with those of matched healthy control subjects in the supine and 60 degrees head-up positions. Breathing frequency, tidal volume, and ventilatory timing were determined from inspiratory flow recordings. Abdominal electromyograms (EMGs) were detected with surface electrodes placed unilaterally over the external and internal oblique and the rectus abdominis muscles. Thresholds, times of onset, and durations of phasic abdominal activity were determined from raw EMGs; peak amplitudes were determined from integrated EMGs. Graded ETLs were imposed by submerging a tube from the expiratory port of the breathing valve into a column of water at depths of 0-25 cmH2O. We found that breathing frequency, tidal volume, and expired minute ventilation were higher in CF patients than in control subjects during low ETLs; a change in body position did not alter these ventilatory responses in the CF patients but did in the control subjects. All CF patients, but none of the control subjects, had tonic abdominal activity while supine. CF patients recruited abdominal muscles at lower loads, earlier in the respiratory cycle, and to a higher recruitment level in both positions than the control subjects, but burst duration of phasic activity was not different between groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Abdominal Muscles↗

PubMed1988

The trajectory of human wrist movements.

1. To determine the form of human movement trajectories and the factors that determine this form, normal subjects performed wrist flexion movements against various elastic, viscous, and inertial loads. The subjects were instructed with visual and auditory feedback to make a movement of prescribed amplitude in a present period of time, but were free to choose any trajectory that fulfilled these constraints. 2. The trajectories were examined critically to determine if they corresponded to those which would minimize the root mean square (RMS) value of some kinematic variable or of energy consumption. The data agreed better with the trajectory that minimized the RMS value of jerk (the third derivative of length) than that of acceleration. However, systematic deviations from the minimum jerk predictions were consistently observed whenever movements were made against elastic and viscous loads. 3. Improved agreement could generally be obtained by assuming that the velocity profile varied according to a normal (Gaussian) curve. We conclude that minimization of jerk is not a general principle used by the nervous system in organizing voluntary movements, although movements may approach the predicted form, particularly under inertial loading conditions. 4. The EMG of the agonist muscles consisted of relatively simple waveforms containing ramplike increases and approximately exponential decays. The form of the movements could often be predicted quite well by using the EMG as an input to a linear second-order model of the muscle plus load. Rather than rigorously minimizing a kinematic variable or energy consumption, the nervous system may generate simple waveforms and adjust the parameters of these waveforms by trial and error until a trajectory is achieved that meets the requirements for a given load.

Adult↗

PubMed1997

Electromyographic heterogeneity in the human temporalis and masseter muscles during dynamic tasks guided by visual feedback.

The complex architecture of the human jaw muscles suggests regional differences in function within these muscles. This study examines the way the temporalis and masseter muscle regions are activated when free mandibular movements with various speeds and against various external leads are carried out guided by visual feedback. Electromyographic (EMG) activity was registered in six temporalis and three masseter muscle regions with bipolar fine-wire electrodes. Recordings were made during open/close excursions, protrusion/retrusion movements, and laterodeviations. During open/close excursions and protrusion/retrusion movements, an anterior and posterior temporalis part could be distinguished, whereas during laterodeviations a more complex partitioning of this muscle was observed. During the protrusion/retrusion movements and the laterodeviations, the temporalis muscle demonstrated higher EMG peak activities than the masseter muscle, and within the masseter muscle the deep masseter showed higher EMG peaks than the superficial one. In contrast to this, during the open/close excursions the masseter showed higher peak activities than the temporalis muscle, while the superficial masseter showed higher EMG peak activities than the deep masseter. Within the deep masseter, differences were also found. During open/close excursions, the anterior deep region demonstrated higher EMG peak activities than the posterior region, whereas during protrusion/retrusion and laterodeviations the posterior deep region showed higher peaks. In general, speed had a greater effect on the EMG peak activity than external load. Only during laterodeviations did speed and load equally influence peak activity in both the deep and superficial masseter. During protrusion/retrusion movements, load showed no significant effect on EMG peak activity in the masseter muscle. A general finding was that, according to task, different regions were activated preferentially. This points to a partitioning of the excitatory command of the motoneuron pool.

Adult↗

PubMed2003

Contribution of cutaneous inputs from the hindpaw to the control of locomotion. I. Intact cats.

The goal of this study was to evaluate the role of hindpaw cutaneous feedback in the control of locomotion, by cutting some (in one cat) or all (in 2 cats) cutaneous nerves bilaterally at ankle level. Kinematic and electromyographic (EMG) recordings were obtained before and for several weeks after denervation during level and incline (15 degrees up and down) treadmill walking. Ladder walking and ground reaction forces were also documented sporadically. Early after the denervation (1-3 days), cats could not walk across a ladder, although deficits were small during level treadmill walking. Increased knee flexion velocity caused a 14% reduction in swing phase duration. EMG activity was consistently increased in knee, ankle, and toe flexors, and in at least one knee or ankle extensor. The adaptive changes during walking on the incline were much reduced after denervation. Ladder walking gradually recovered within 3-7 wk. By this time, level treadmill walking kinematics had completely returned to normal, but EMG activity in flexors remained above control. Incline walking improved but did not return to normal. Mediolateral ground reaction forces during overground walking were increased by 200%. It is concluded that in intact cats, cutaneous inputs contribute more to demanding situations such as walking on a ladder or on inclines than to level walking. Active adaptive mechanisms are likely involved given that the EMG locomotor pattern never returned to control level. The companion paper shows on the other hand that when the same cats are spinalized, these cutaneous inputs become critical for foot placement during locomotion.

Animals↗

PubMed2001

Neural compensation for compliant loads during rhythmic movement.

An experiment was performed to characterise the movement kinematics and the electromyogram (EMG) during rhythmic voluntary flexion and extension of the wrist against different compliant (elastic-viscous-inertial) loads. Three levels of each type of load, and an unloaded condition, were employed. The movements were paced at a frequency of 1 Hz by an auditory metronome, and visual feedback of wrist displacement in relation to a target amplitude of 100 degree was provided. Electromyographic recordings were obtained from flexor carpi radialis (FCR) and extensor carpi radialis brevis (ECR). The movement profiles generated in the ten experimental conditions were indistinguishable, indicating that the CNS was able to compensate completely for the imposed changes in the task dynamics. When the level of viscous load was elevated, this compensation took the form of an increase in the rate of initial rise of the flexor and the extensor EMG burst. In response to increases in inertial load, the flexor and extensor EMG bursts commenced and terminated earlier in the movement cycle, and tended to be of greater duration. When the movements were performed in opposition to an elastic load, both the onset and offset of EMG activity occurred later than in the unloaded condition. There was also a net reduction in extensor burst duration with increases in elastic load, and an increase in the rate of initial rise of the extensor burst. Less pronounced alterations in the rate of initial rise of the flexor EMG burst were also observed. In all instances, increases in the magnitude of the external load led to elevations in the overall level of muscle activation. These data reveal that the elements of the central command that are modified in response to the imposition of a compliant load are contingent, not only upon the magnitude, but also upon the character of the load.

Acoustic Stimulation↗

PubMed1998

Afferent feedback in the triphasic EMG pattern of leg muscles associated with rapid body sway.

Electromyographic (EMG) patterns of leg muscles associated with rapid body sway were studied in relation to displacement of the center of foot pressure (CFP). Standing subjects were instructed to shift the CFP by swaying their bodies, pivoting at the ankle as rapidly and accurately as possible after an auditory signal. CFP position was designated as N when the subject maintained a relaxed bending posture and as F when a maximally forward-leaning posture was maintained. A serial, stereotyped triphasic EMG pattern was observed in the rapid shift of CFP from N to F: cessation of EMG activity in the gastrocnemius (GC) muscle was followed by a burst in the tibialis anterior (TA) muscle (acceleration phase), and then resumed discharge occurred in the GC muscle with cessation of activity in the TA muscle (deceleration and stop). When the subject shifted the CFP from N to F to different degrees, the duration and amount of EMG activity in the TA muscle during acceleration and the GC muscle in deceleration were proportionate to the amount of CFP displacement associated with forward body sway. To determine the functional roles of sensory inputs from the foot on the triphasic EMG pattern, body sway was studied under the condition of sensory block in the feet induced by ischemia from tourniquets placed bilaterally just above the ankle joints. The triphasic EMG pattern persisted during ischemia. The time of GC cessation and the onset of TA burst at acceleration remained unchanged, but the times of TA cessation and resumption of GC discharge at deceleration were altered during ischemia. Moreover, subjects were unable to stop at F and eventually fell. These results indicate that both amount and duration of EMG activity associated with rapid body sway are functions of the amount of CFP displacement. Somatic sensation from the feet is important for control of burst and cessation timing and duration in leg muscle activity.

Adult↗

PubMed2003

Matching different levels of isometric torque in elbow flexor muscles after eccentric exercise.

Human subjects generated a specified level of isometric torque with elbow flexor muscles of one arm, the reference arm, under visual feedback. They were then asked to generate what they perceived to be the same level, with the other arm, the indicator, but with no visual feedback. A number of torque levels, between 2% and 30% of maximum were used in the matching trials. Elbow flexors of one arm were then exercised eccentrically on a dynamometer. Immediately after the exercise, there was a large (40%) drop in maximum voluntary torque, as well as some soreness and swelling 24 h later, indicative of muscle damage. When the torque-matching experiment was repeated after the indicator arm had been exercised, the indicator signalled torque levels significantly below the reference level (P<0.05). When the reference arm was exercised, errors were in the opposite direction. Over the 4 days of testing post-exercise, errors became less as torque levels returned to normal. When errors were expressed in terms of maximum torque post-exercise, they were significantly reduced. This suggested that subjects were using as a matching cue the perceived effort required to generate a given level of torque rather than the level of torque itself. Persisting matching errors, from 24 h onwards after the eccentric contractions, were proposed to include a component attributable to the muscle soreness. Changes in electromyogram recorded after eccentric exercise were consistent with the effort-matching hypothesis. The muscle's torque-angle relationship was used to estimate matching ability in the absence of fatigue. One forearm was placed at various angles and its reference torque was matched by the other, the indicator, always at 90 degrees. Again, matching errors were consistent with an interpretation based on a match of effort rather than torque.

Adolescent↗

PubMed1999

Context-dependent force coding in motor and premotor cortical areas.

In three monkeys trained to finely grade grip force in a visuomotor step-tracking task, the effect of the context on neuronal force correlates was quantitatively assessed. Three trial types, which differed in force range, number, and direction of the force steps, were presented pseudo-randomly and cued with the color of the cursor serving as feedback of the exerted force. Quantitative analyses were made on 85 neurons with similar discharge patterns in the three trial types and significant linear positive (54 cells) or negative (31 cells) correlation coefficients between firing rate and force. An analysis of covariance (ANCOVA) showed that the population slopes for 2-step were steeper than for 3-step trials. Another ANCOVA at the population level, computed on the differences in firing rate and force between force steps, persistently disclosed a significant effect of trial type. For the first two force steps, the differences in firing rate were significantly larger in the 2-step than in the 3-step increase trials. Further analyses revealed that neither the force range nor the number of steps was a unique factor. A small group of neurons was tested in an additional trial series with a uniform cue for all three trials, leading to either a loss of context-dependency or to unexpected changes in firing rate. This demonstrates that the cue color was an important instruction for task performance and neuronal activity. The most important findings are that the context-dependent changes were occurring "on-line", and that neurons displaying context-dependency were found in all three lateral premotor cortex hand regions and in the primary motor cortex. Finger muscle activity did not show any context dependency. The context-dependent effect leads to a normalization of the cortical activity. The advantage of normalization is discussed and mechanisms for the gain regulation are proposed.

Analysis of Variance↗

PubMed2001

Vestibulospinal reflexes: quantitative effects of sensory feedback and postural task.

Vestibulospinal and vestibulocollic reflexes evoked by galvanic stimulation were studied in 20 normal volunteers. In an initial "baseline" study, subjects stood unsupported on a flat surface and a narrow base with their eyes shut and with their heads rotated to the left. The effects of vision, external support and increasing stance width were examined both individually and in combination. In a second series, the effects of the same factors were examined while subjects stood on a compliant surface. Short latency (SL) and medium latency (ML) vestibulospinal reflex responses were evoked using 4 mA/20 ms galvanic vestibular stimulation (GVS) and measured from the right soleus muscle. Vestibulocollic reflexes to short duration galvanic stimulation (4 mA/2 ms) were also measured under similar conditions. Both SL and ML vestibulospinal reflexes decreased significantly and to a similar degree with vision, external support and increasing stance width on a flat surface. On the compliant surface, stance width did not result in a decrease in the ML reflex. Reflex amplitudes further decreased in a non-linear fashion with each additional sensory modality ("factor") that was made available; the degree of attenuation due to the addition of a second and third factor closely approximated the product of the effect of each factor in isolation. Standing on a compliant surface resulted in enhancement of average SL and ML reflexes under all conditions. Vestibulocollic reflexes in contrast showed no significant modulation with vision, external support and stance width. The dissociation between vestibulocollic and vestibulospinal reflexes indicates that the modulation of vestibulospinal reflexes with task occurs proximal to the primary vestibular afferents. Vestibulospinal reflexes were largest when subjects stood on a narrow base, on a compliant surface, deprived of vision and external support, consistent with the importance of vestibular function under these conditions. Although attenuated, vestibulospinal reflexes were preserved in most subjects even when vision and external support were available and a wider stance width was adopted. The combination of different factors caused a multiplicative attenuation of the initial response.

Adult↗

PubMed2007

Physiologically based controller for generating overground locomotion using functional electrical stimulation.

The physiological control of stepping is governed both by signals descending from supraspinal systems and by circuitry residing within the lumbosacral spinal cord. The goal of this study was to evaluate the capacity of physiologically based controllers to restore functional overground locomotion after neurological damage, such as spinal cord injury when used in conjunction with functional electrical stimulation. For this purpose we implemented and tested two controllers: 1) an intrinsically timed system that generated a predetermined rhythmic output and 2) a sensory-based system that used feedback signals to make appropriate transitions between the unloaded (flexion) and loaded (extension) phases of the gait cycle. A third controller, a combination of the intrinsically timed and sensory-driven controllers, was implemented and two sessions were conducted to demonstrate the functional advantages of this approach. The controllers were tested in anesthetized cats, implanted with intramuscular electrodes in six major extensor and flexor muscles of the hindlimbs. The cats were partially supported on a sliding trolley that was propelled by the hindlimbs along a 2.5-m instrumented walkway. Ground reaction forces and limb positions were measured by force plates in the walkway and by accelerometers secured to the legs of the cat, respectively. The controllers were used to generate patterns of stimulation that would elicit alternating flexor (swing) and extensor (stance) movements in the hindlimbs. Using either the intrinsically timed or sensory-driven controllers, the cats were able to travel a distance of 2.5 m, taking five to 12 steps. Functional stepping sequences were more easily achieved using the intrinsically timed controller as the result of a lower sensitivity to the selection of initial stimulation parameters. However, unlike the sensory-driven controller, the intrinsically timed controller was unable to adjust to overcome walkway resistance and muscle fatigue. Neither system was consistently able to ensure load-bearing stepping. Therefore we propose the use of a "combined controller" that relies heavily on intrinsic timing but that can be reset based on sensory signals. A combined controller such as this one may provide the best solution for restoring robust overground locomotion after spinal cord injury.

Animals↗