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PubMed2002

Dynamic and static subjective visual vertical with aging.

OBJECTIVE: Our vestibular function is gradually deteriorating during aging, although, its behavioral consequences are not easily recognized due to a substitution process by other sensory modalities as visual or proprioceptive inputs. METHODS: To reveal such a hidden substitution process by visual signals, the measurement of the static as well as the dynamic subjective visual vertical (SVV) was performed among 63 healthy subjects of different age. RESULTS: The static SVV was found to be stable among all subjects, whereas the shift of the dynamic SVV during rotation of a background scene gradually increased with age. CONCLUSION: This result indicates that the substitution process identified as a function of age in a perceptual test may have its counterpart in postural stabilizing reflex.

Adult↗
PubMed2003

Postural effects on cardiac output and mixed venous oxygen saturation in humans.

The activation of cardiovascular reflexes for postural adaptation questions whether, in healthy humans, the central blood volume is optimised to support the upright position. A functional definition of an 'optimal circulating volume' that provides the heart with enough central blood volume to establish a maximal cardiac output (CO) and mixed venous oxygen saturation (S(v,O2)) at rest was evaluated in nine healthy subjects. Preload to the heart was varied by passively changing the body position from 70 deg head-up to 20 deg head-down tilt. The S(v,O2) was compared with simultaneously measured estimates of CO by computer-controlled thermodilution. The CO was in the range 8.7-3.8 l min(-1) and S(v,O2) was in the range 79-58 %. Neither CO (median 6.0 (range 5.3-8.7) l min(-1)) nor S(v,O2) (mean +/- S.D. 73.6 +/- 2.6 %) changed from the supine to the 20 deg head-down position. During sustained 70 deg head-up tilt, S(v,O2) decreased to 64 +/- 4 % together with a decline in CO to 4.7 (3.9-5.6) l min(-1) (P < 0.05). Under conditions of varying tilt angles, a change in CO is paralleled by concordant changes in S(v,O2). Maximal values for CO and S(v,O2) during supine rest suggest that the horizontal position provides for an 'optimal' central blood volume.

Adaptation, Physiological↗
PubMed2002

Identification of intrinsic and reflexive components of human arm dynamics during postural control.

In this study a new methodology to quantify reflexive feedback gains from the mechanical behavior of the human arm during posture maintenance is proposed. Disturbance experiments were carried out on human subjects using continuous random force inputs. The task instruction was 'minimize displacements', prescribing a maximum performance task. For the separation of intrinsic and reflexive components, system identification in the frequency domain is applied. From the time records of position and force, frequency response functions (FRFs) are estimated. Given a model structure and an appropriate estimate of the intrinsic component, an estimate of the reflex gains for length and velocity are obtained from the FRFs. The feedback gains vary considerably with the frequency content of the disturbance signal. The results show that reflexive dynamics are substantial for narrow-band and especially low-frequency input signals. It is likely that high reflex gains are most effective for low-frequency inputs (< 3 Hz) that do not excite the closed-loop system's eigenfrequency. Also significant negative reflex gains are estimated for near-sinusoidal inputs (> 1.5 Hz). It is concluded that this new methodology can offer interesting insights into the ability of the central nervous system to modulate reflexive feedback gains.

Adult↗
PubMed1989

Modulation of flexor carpi radialis H reflex by lateral tilts in man.

Static vestibular influences on upper limb flexor tone were studied in man by analyzing the changes in flexor carpi radialis H reflex area with lateral tilting of the longitudinal body axis. Ten healthy volunteers and 2 labyrinthine defective patients were tested in an experimental situation designed to minimize all afferent inputs except vestibular ones. Each subject was seated on a chair which could be tilted laterally to the left or the right from the vertical. Head and trunk were fixed upright, upper and lower limbs in half-flexed position and forearm in an intermediate position between supination and pronation. Lateral tilting was applied at random from the vertical (0 degrees control position) to left and right (4 degrees, 8 degrees, 12 degrees, 16 degrees, 20 degrees test positions). Each test position was followed by a return to 0 degrees and in each control and test position 20 consecutive H reflexes were recorded. The data observed in the normal subjects showed flexor tone inhibition in the arm which was tilted downwards and facilitation in the contralateral arm. These findings suggest that in man, like in animals, labyrinth reflexes act asymmetrically and in the opposite direction to neck reflexes.

Adult↗
PubMed1993

Fusimotor control of proprioceptive feedback during locomotion and balancing: can simple lessons be learned for artificial control of gait?

The possibilities for central control of primary spindle afferents through fusimotor efferents for gain control in motor control mechanisms are briefly reviewed. While the existence of separate pathways for independent control of static and dynamic gamma-motoneurones is well established, it proved more difficult to demonstrate that gain control of spindle feedback, attributable to alterations in static and dynamic fusimotor drive, indeed took place in voluntary movements. However, earlier qualitative indications, that Ia sensitivity (and hence the balance of static over dynamic drive) was adjusted differently in different motor tasks, have recently been confirmed in experimental simulation studies, in which the fusimotor activation profiles, that were required to reproduce chronically recorded spindle Ia discharge patterns, were reconstructed. These studies indicated that Ia sensitivity and dynamic gamma-drive were low in routine movements (walking), but that they could be dramatically increased in motor tasks which were either difficult or unfamiliar (landing from falls, balancing on narrow walk beams, adjustment to imposed disturbances). This suggested that sensitization of spindle feedback could play a significant role in motor adaptation. In line with this, studies in patients with large fibre sensory (including proprioceptive) neuropathies indicated that long-term motor deficits (affecting motor adaptation and learning) could be at least as serious as short-term motor dysfunction (due to loss of reflex control). It is suggested that spindle Ia feedback may play a dual role: in addition to its contribution to short-term reflex control of posture and movement, it may also be used for optimization or maintenance of motor programs, especially if its gain is increased by significant dynamic fusimotor drive.(ABSTRACT TRUNCATED AT 250 WORDS)

Afferent Pathways↗
PubMed1994

Primitive reflexes. Their contribution to the early detection of cerebral palsy.

In evaluating the infant for significant motor impairment, a framework to organize and analyze data is presented. Five general areas of information are essential: (1) motor milestone attainment, (2) the classic neurological examination, (3) primitive reflex and postural reaction patterns, (4) progressive vs static nature of the dysfunction, and (5) associated evidence for neurological dysfunction and/or structural nervous system damage. A sixth criterion, age of onset, is relevant--but controversial--to making the specific diagnosis of cerebral palsy. The treatment of and prognosis for children with cerebral palsy have been linked to the historical and examination features outlined, particularly primitive reflexes. For the practicing pediatrician, understanding the fundamentals of the primitive reflex/postural reaction transition and achieving clinical facility with the elicitation of these signs represent skills that are readily acquired, easy to incorporate in routine clinical practice, and diagnostically useful.

Cerebral Palsy↗
PubMed2005

Evidence for reflex and perceptual vestibular contributions to postural control.

Vestibular signals are known to have an important role in stance under specific conditions. Potentially these effects could be modulated by vestibular reflexes or by voluntary responses to perceived vestibular signals. Our preliminary aim was to confirm that vestibulospinal reflexes change in parallel with sway under different postural conditions, and then to determine whether any relationship was present between these reflexes and body sway within fixed postural conditions. Sixteen subjects (eight male, eight female) were tested in conditions assessing the effects of vision (eyes open or closed), support surface (firm or compliant), external support (with or without) and stance width (feet apart or together). Sway (centre of pressure) in the anteroposterior (AP) and mediolateral planes was measured using a force platform. A subgroup of 11 subjects (five male, six female) underwent testing to measure short (SL) and medium latency (ML) reflexes from soleus. Bipolar, transmastoid galvanic stimulation (1 mA, 200 ms) was administered while subjects stood in the most unstable of our conditions (eyes closed, compliant surface and feet together). In the final part, to assess possible perceptual contributions to body sway, short duration AP sway levels were measured and expressed in angular terms (sway in mrad, velocity in mrad s(-1)) in the 11 subjects for both our baseline (eyes open, firm surface and feet apart) and most unstable conditions. Average sway levels increased more than seven-fold between conditions and had significant, positive correlations with reported changes in mean vestibulospinal reflexes under similar conditions (overall r = 0.75, P < 0.001). However, the SL reflex for the subgroup of 11 subjects had a significant negative correlation (r = -0.71; P = 0.014) with the degree of AP sway in the condition with maximum reliance on vestibular inputs (eyes closed, compliant surface, and feet together). Under baseline conditions, 5/125 (4%) of the short-term AP sway displacements were above the threshold previously reported for the detection of imposed sway. In the unstable condition, when sway was increased, 43/138 (31%) of the short-term AP sway movements were above the threshold for perception of imposed body sway based on vestibular signals. Our results confirm that vestibulospinal reflexes appear to be acutely facilitated as body sway increases. For the most unstable condition, when non-vestibular information was absent or attenuated, subjects with larger SL reflexes had less AP sway, suggesting that the SL reflex acted to attenuate sway. Under the same condition, short duration sway levels increased such that 31% were above the previously published threshold for detection using vestibular afferents. We conclude that both vestibular reflexes and perceptual signals appear to have a specific role in the maintenance of upright stance, under conditions in which other sources of postural information are attenuated or absent.

Adult↗
PubMed1995

Cutaneomuscular reflexes recorded from the lower limb in man during different tasks.

1. Cutaneomuscular reflexes have been recorded in ten adult subjects from extensor digitorum brevis (EDB), tibialis anterior (TA), soleus (Sol), quadriceps femoris (Quad) and erector spinae (ES) following electrical stimulation of the digital nerves of the second toe. 2. Recordings were made while subjects were instructed to activate voluntarily the relevant muscles and also when these muscles were active posturally. 3. Reflex responses could comprise three components: an initial increase in EMG (E1), followed by a decrease (I1), followed by a second increase (E2). E1 and I1 were confined to muscles acting at the ankle and in the foot. E2 was most pronounced in EDB but also found in TA, Sol, Quad (1 subject) and ES. No responses were recorded contralateral to the stimulus. 4. E2 was significantly larger when the reflex was recorded during voluntary contraction of the muscle, rather than when the muscle was active posturally. 5. E1 and I1 components are mediated via spinal pathways. E2 requires the integrity of the dorsal columns, sensorimotor cortex and corticospinal tract (Jenner & Stephens, 1982; Rowlandson & Stephens, 1985b). The present study suggests that one or more of these supraspinal pathways is more active during voluntary contraction of lower limb muscles than when these muscles are active posturally.

Adult↗
PubMed1989

Age related decline in postural control mechanisms.

In order to study voluntary and reflexive mechanisms of postural control, young and elderly persons were given large-fast and small-slow ankle-rotation postural disturbances while standing on a movable platform capable of measuring ground reaction forces. Large-fast rotations were employed to activate long-loop reflexes, and small-slow rotations were employed to tap the higher level sensory integration aspects of postural control. Overall, the elderly persons exhibited more perturbation induced sway and showed a slowing in voluntary, as opposed to reflexive mechanisms of correcting postural disturbance. For both age groups, reflexive mechanisms were found to be relatively intact. When small perturbations were given, the elderly persons swayed more than young participants and produced sporadic reflexive activity. Moreover, elderly persons did not adapt to the small perturbations and exhibited increased postural sway to repetitive presentation of the perturbation, whereas young participants substantially decreased their postural sway. These data demonstrate that elderly persons are at some disadvantage when posture is under the control of slower, higher level sensory integrative mechanisms.

Adult↗
PubMed2001

Medio-lateral balance adjustments preceding reflexive limb withdrawal are modified by postural demands.

We have recently observed medio-lateral balance adjustments (BA) preceding reflexive stepping elicited by noxious stimulation. While task specific modulation is evident for BA prior to voluntary leg movement, it is unclear whether rapid BA reactions (prior to 'reflexive' stepping) represent a generic response to evoked limb withdrawal or can be modified to suit task-conditions. This study was designed to establish whether the CNS is able to modify rapid onset latency BAs to match task conditions. Reflexive stepping was evoked by applying a noxious stimulus (50 ms stimulus train, 1 ms pulses, 300 Hz, 4 x perceptual threshold) to the plantar surface of the either the left or right foot. Task conditions were varied prior to stimulation by having subjects maintain one of three different static positions: (1) lean left (70% body weight (BW) on left), (2) neutral (50% BW both sides), (3) lean right (70% BW on right). BAs were denoted by centre-of-pressure (CoP) excursions towards the swing foot after the onset of noxious stimulation (average onset latency of 128 ms). There was a significant increase in frequency of occurrence and a significant increase in magnitude of CoP shift when the stimulation was applied to a loaded limb (leaning with 70% BW on the stimulated foot) as compared to an unloaded limb (30% BW). In addition, 78% of loaded trials featured steps taken with the unstimulated foot, which delayed removal of the stimulated foot. Collectively, the results indicate modifiability of the very rapid onset balance adjustments that precede the onset of limb withdrawal revealing complex control of balance exists even over very brief latencies.

Adult↗