Postural adjustments and reaching in 4- and 6-month-old infants: an EMG and kinematical study

Adequate postural control is a prerequisite for daily activities such as reaching for an object. However, knowledge on the relationship between postural adjustments and the quality of reaching movements during human ontogeny is scarce. Therefore we evaluated the development of the relationship between the kinematic features of reaching movements and the accompanying postural adjustments in young infants. Twelve typically developing (TD) infants were assessed twice, i.e. at 4 and 6 months of age, in supine and supported sitting position. Reaching was elicited by presenting toys in the midline at an arm-length distance while simultaneously surface EMG-activity was recorded from multiple arm-, neck-, trunk- and leg muscles. Concurrently kinematics of reaching were recorded with an ELITE system; kinematic analysis was restricted to the behaviour of so-called movement units, which are sub movements of reaching determined with the help of peaks in the velocity profile of the hand, maximum movement velocity and movement duration. A computer-algorithm determined significant phasic muscle activity. Activity in neck and trunk muscles (postural activity) was related to the onset of the prime mover, which was the arm muscle being activated first. The results indicated that about 50% of reaching movements in lying and sitting infants aged 4 and 6 months were accompanied by direction-specific postural adjustments. At 4 months variation dominated, but at 6 months a preference to recruit muscles in a top-down order (during sitting) and in the configuration of the complete pattern, i.e. the pattern in which all dorsal neck- and trunk muscles are activated in concert, (both conditions) emerged. Interestingly, the postural characteristics such as the presence of direction-specificity, recruitment of the complete pattern and top-down recruitment, were related to how successful the reaching was and the kinematics of reaching. It was concluded that the presence of direction-specific activity is not a prerequisite for the emergence of reaching movements. Nevertheless, already from 4 months onwards a better postural control is associated with a larger success and a better quality of reaching.     

Postural adjustments in sitting humans following external perturbations: muscle activity and kinematics

There are several controversies concerning the organization and induction of postural adjustments in standing humans. Some investigators suggest the responses are triggered by somatosensory inputs (especially from the ankle in standing subjects), while others emphasize the vestibular input induced by head acceleration. We examined postural responses in sitting subjects in order to describe the muscle activation pattern during various perturbations and to test whether somatosensory or vestibular stimulation elicited the responses. The kinematics and EMG patterns in respons to perturbations caused by movements of the support surface were studied in adults. The postural muscle activation following a backward sway was mainly the same, whether it was elicited by a forward translation or a legs-up rotation. This is remarkable, since, except for pelvis rotation, the movements of all body segments including the head differed in the two conditions. Furthermore, a second experiment showed that the direction of the initial head movement could be reversed with retainment of the same postural muscle activation pattern. The results suggest that somatosensory signals derived from the backward rotation of the pelvis, and not vestibular information from the head, trigger postural responses during sitting. There was a slight but consistent difference in the muscle activation pattern, whether the backward sway was elicited by a forward translation or legs-up rotation. The difference seemed to reflect the sensory information from head and other body parts (except the pelvis). This finding allowed us to speculate in a central pattern generator for postural adjustments containing two levels. At the first level, a simple format of the muscle activation would be generated; at the second level, the centrally generated pattern could be shaped and timed by interaction from the entire somatosensory, vestibular, and visual input.     

Development of postural adjustments during reaching in sitting children

We evaluated the development of postural adjustments accompanying reaching movements in sitting children. Twenty-nine typically developing children aged, 2–11 years, and ten adults were studied with multiple surface electromyograms (EMGs) and kinematics during reaching in four conditions: sitting with the seat-surface oriented horizontally with and without an additional task load, and sitting with the seat-surface tilted 15° forward and 15° backward. The development of postural adjustments during reaching in a sitting position turned out to have a non-linear and protracted course, which is not finished by the age of 11 years. The development of these adjustments is characterised by variation, yet specific developmental sequences could be distinguished. Firstly, the development of postural adjustments during reaching from the age of 2 years onwards lacked a preference for an en bloc strategy, which consists of an in concert activation of the direction-specific neck and trunk muscles. Secondly, anticipatory postural muscle activity, which was consistently present in adults, was virtually absent between 2 and 11 years of age. Thirdly, the data demonstrated that with increasing age the head gradually becomes the dominant frame of reference. In addition, the study suggested that, in terms of postural control, the forward-tilted position is the most efficient one.     

The development of postural adjustments during reaching in 6- to 18-month-old infants Evidence for two transitions

 The present study focused on the developmental changes of postural adjustments accompanying reaching movements in healthy infants. We made a longitudinal study of ten infants between 6 and 18 months of age. During each session multiple surface electromyograms of arm, neck, trunk and leg muscles at the right side of the body were recorded during right-handed reaching movements in two positions (”upright sitting” in an infant chair and ”long-leg” sitting without support). Simultaneously the whole session was recorded on video. Comparable data were present from the same infants at 3–5 months. Additionally, 18 infants (8–15 months) were assessed once during similar reaching tasks, but in these infants electromyographic activity of the trunk and neck muscles at both sides of the body were recorded. Our data revealed two transitions in the development of postural adjustments. The first transition was present around 6 months of age. At this age the postural muscles were infrequently activated during reaching movements. At 8 months ample postural activity reappeared and the infants developed the ability to adapt the postural adjustments to task-specific constraints such as arm movement velocity or the sitting position at the onset of the reaching movement. The second transition occurred between 12 and 15 months. Before 15 months the infants did not show consistent anticipatory postural activity, but from 15 months onwards they did, particularly in the neck muscles. Received: 3 March 1998 / Accepted: 5 February 1999     

Epigenetic development of postural responses for sitting during infancy

This study examined whether postural responses emerge in children in a predetermined way before independent sitting is achieved, and in what respect postural responses in infants differ from those in adults. Children just able to sit independently and children not yet able to sit were exposed to surface perturbations (translation and rotation) while body movement and electromyographic (EMG) responses were recorded. Perturbations causing a backward sway of the body (i.e., forward translation and legs-up rotation), elicited consistent patterns of muscle activity in ventral hip, trunk, and neck muscles in the independently sitting children. A high tonic EMG background activity in trunk and neck extensor muscles was inhibited at the onset of the ventral muscle activity. Kinematic analysis revealed that backward rotation of the pelvis was the first detectable body movement, while head movements (linear and angular displacement) were irregular and occurred later than the pelvis movement. Perturbations in the opposite direction, causing a forward sway, evoked variable responses in dorsal trunk and neck muscles, suggesting that the excitability level for postural responses was set according to the stability limits of the body. Children not yet able to sit without support were tested when the support around the waist, given by the experimenter's hands, was released prior to the onset of the platform perturbation. Postural responses were elicited in ventral muscles following a backward sway in all children and in about 60% of all trials. Often, only some of the ventral muscles were activated. No distinct responses were evoked during perturbations imposing a forward sway. These results suggest that (1) backward rotation of the pelvis triggers the postural adjustments in the independently sitting children; (2) a basic form of the postural adjustment develops in a predetermined manner before children practice independent sitting; and (3) the basic structure of ventral muscle activation pattern resembles that of adults, while the activation of the dorsal muscles (inhibition) differs in several aspects. These findings are in agreement with a recent model of central pattern generators for postural responses consisting of two operative levels. At the first level, which is triggered by backward rotation of the pelvis, the basic activation pattern is generated. At the second level, the pattern is shaped and fine-tuned by multisensory interactions from all activated sensory systems. The basic pattern present in the youngest infants may be produced mainly by neural networks at the first level, while the shaping function develops during practice, the shaping function being subjected to a learning process in which appropriate responses are formed in conjunction with the establishment of an internal neural representation for sitting.     

Preparatory process for anticipatory postural adjustments: Modulation of leg muscles reflex pathways during preparation for arm movements in standing man

Summary We have investigated, in 6 standing subjects, the time course of amplitude changes in the short latency (40–60 ms) and long latency (60–80 ms) reflex response components of gastrocnemius medialis (GM) and soleus (S) muscles during the preparatory period (time between a warning signal and a response signal) as a function of the precued direction (pull or push) of arm movement. Subjects maintained their standing posture by visual feedback during the 1.5 s preparatory period of a reaction time task. A warning signal gave advance information concerning the voluntary response to be performed which consisted of either a pull or push movement of the right arm. The excitability of the reflex pathways was evaluated by triggering a rapid rotation of the right ankle joint (dorsiflexion) applied randomly at 100, 300 or 500 ms before the response signal for arm movement. Statistical analysis of EMG amplitude (ANOVA) showed that preparatory effects were different for the two synergistic muscles with both short (spinal) and longer latency components of GM showing generalized facilitation and of S showing generalized inhibition. In addition, the longer latency, presumably supraspinal, component of the two muscles was differentially modulated according to directional advance information, showing relative facilitation for pull as compared to push trials. These reflex response modulations were emphasized in faster reaction time (RT) performers at the end of the preparatory period. It is concluded that postural preparatory processes are reflected at the spinal level in global effects and at the supraspinal level in directionally specific effects.     

Coexistence of stability and mobility in postural control: evidence from postural compensation for respiration

This study evaluated the extent to which movement of the lower limbs and pelvis may compensate for the disturbance to posture that results from respiratory movement of the thorax and abdomen. Motion of the neck, pelvis, leg and centre of pressure (COP) were recorded with high resolution in conjunction with electromyographic activity (EMG) of flexor and extensor muscles of the trunk and hip. Respiration was measured from ribcage motion. Subjects breathed quietly, and with increased volume due to hypercapnoea (as a result of breathing with increased dead-space) and a voluntary increase in respiration. Additional recordings were made during apnoea. The relationship between respiration and other parameters was measured from the correlation between data in the frequency domain (i.e. coherence) and from time-locked averages triggered from respiration. In quiet standing, small angular displacements ( approximately 0.5 degrees ) of the trunk and leg were identified in raw data. Correspondingly, there were peaks in the power spectra of the angular movements and EMG. While body movement and EMG were coherent with respiration (>0.5), the coherence between respiration and COP displacement was low (<0.2). The amplitude of movement and coherence was increased when respiration was increased. The present data suggest that the postural disturbance that results from respiratory movement is matched, at least partly, and counteracted by small angular displacements of the lower trunk and lower limbs. Thus, stability in quiet stance is dependent on movement of multiple body segments and control of equilibrium cannot be reduced to control of a single joint.     

Postural control of the trunk in response to lateral support surface translations during trunk movement and loading

The postural response to translation of the support surface may be influenced by the performance of an ongoing voluntary task. This study was designed to test this proposal by applying lateral perturbations while subjects handled a load in the frontal plane. Measurements were made of medio-lateral displacement of the centre of pressure, angular displacement of the trunk and thigh in the frontal plane and intra-abdominal pressure. Subjects were translated randomly to the left and right in a variety of conditions that involved standing either quietly or with a 5 kg load in their left hand, which they were required either to hold statically or to lift or lower. The results indicate that when the perturbation occurred towards the loaded left side the subjects were able to return their centre of pressure, trunk and thigh rapidly and accurately to the initial position. However, when the perturbation occurred towards the right (away from the load) this correction was delayed and associated with multiple changes in direction of movement, suggesting decreased efficiency of the postural response. This reduced efficiency can be explained by a conflict between the motor commands for the ongoing voluntary task and the postural response, and/or by the mechanical effect of the asymmetrical addition of load to the trunk.     

A kinematic and kinetic analysis of locomotion during voluntary gait modification in the cat

As part of a study to characterize the postural reactions that occur during voluntary gait modification, we examined the kinematic, electromyographic (EMG), and kinetic responses that occurred when cats stepped over an obstacle placed in their path. Analyses of the kinematics as each of the forelimbs stepped over the obstacle showed that changes in joint angles were most pronounced at the elbow of the first (lead) limb, and at the shoulder of the second (trailing) limb. In the hindlimbs, there was a pronounced change in the knee joint angle in both the leading and trailing limbs. Examination of the horizontal and vertical velocities of the tip of the forepaw suggests that the movements can be divided into two phases: one in which the limb is rapidly lifted above and over the obstacle, and a slower one during which the limb is carefully repositioned on the floor. On the basis of the velocity profiles, we suggest that the repositioning of the paw on the support surface is more critically controlled for the forelimb than for the hindlimb. Analysis of the changes in the ground reaction forces in the supporting limbs during these gait modifications showed that there were two major increases in vertical reaction force. One of these occurred as the two forelimbs were straddling the obstacle, the other when the two hindlimbs were straddling it. There was also a net increase in the anteroposterior force that resulted in a small increase in propulsion as the cat stepped over the obstacle. Each change in the vertical ground reaction force was paralleled by a similar change in the amplitude of the EMG recorded from the respective extensor muscles. An analysis of the vertical displacement of the scapula and of the pelvis showed that there was a slight increase in the height of the scapula in the support limb just prior to and during the swing phase of the trailing forelimb, and a more pronounced and progressive change in the height of the pelvis prior to and during the passage of both hindlimbs over the obstacle. We suggest that the increases in vertical ground reaction force raise the height of the body sufficiently to allow, respectively, passage of the trail forelimb and each of the hindlimbs over the obstacle. The results are discussed with respect to both the biomechanical changes underlying these gait modifications and the neuronal mechanisms implicated in their control.     

Postural control in isometric ramp pushes: the role of Consecutive Postural Adjustments (CPAs)

Postural adjustments, which occur after the end of a voluntary movement (termed Consecutive Postural Adjustments: CPAs), were studied and compared to the corresponding Anticipatory Postural Adjustments (APAs). Seven right-handed male adults were asked to perform horizontal two-handed maximal ramp pushes as quickly as possible, while sitting. A dynamometric bar measured the reaction to push force (F_x) and a custom-designed device measured the resultant reaction forces along the antero-posterior axis (R_x). Two ischio-femoral contacts (100 BP: full ischio-femoral contact of the ischio-femoral length; and 30 BP: one-third contact) were considered. Each session consisted of ten pushes. The reaction forces, as well as push force, increased continuously, displaying similar time course profiles. However, R_x continued to increase after the end of push rise, which ascertained CPAs. CPAs were showed to be consistent kinetic phenomena, using a biomechanical analysis, based on time courses of reaction forces and CoG kinematics. Their coherence was checked precisely, by comparing theoretical and experimental occurrences of remarkable points (extrema and zero crossings). CPA durations and peak amplitudes (dCPA and pCPA) were significantly greater than the corresponding APA values (dAPA and pAPA). Moreover, dAPAs and dCPAs increased (p < 0.001), as did pCPAs (p < 0.001) and pAPAs (p < 0.05) when the peak push force was greater (30 BP), showing that the probability of finding a statistically significant difference is greater for APA duration than amplitude, unlike CPAs. Finally, the present results were discussed in relation to the hypothesis according to which the focal and the postural components are parts of the same motor program.     

Postural responses in the cat to unexpected rotations of the supporting surface: evidence for a centrally generated synergic organization

Summary Postural reactions to disruptions of stance are rapid and automatic in both quadrupeds and bipeds. Current evidence suggests that these postural responses are generated by the central nervous system as patterns involving muscle synergies. This study attempted to test this hypothesis of a centrally generated postural mechanism by determining whether the same postural response could be evoked in the freely-standing cat under two different biomechanical conditions. The present work is an extension of previous experiments in which the stance of cats was perturbed by a horizontal translation of the supporting surface in the anterior and posterior directions (Rushmer et al. 1983). We now tested whether simple rotation of the metacarpo- and metatarsophalangeal (M-P) joints that mimics the digit rotation occurring during platform translation, was sufficient to evoke the translation postural response. The rotational perturbations were biomechanically different from translations in that the rotation did not cause displacement of the centre of mass of the animal, nor did it result in any significant movement about any but the M-P joints. Even so, rotational perturbations did evoke the appropriate translational muscle synergies in all four animals. Both plantar flexion rotation and headward translation activated the posterior hindlimb synergy (which included gluteus medius, semitendinosus and lateral gastrocnemius). Similarly, dorsiflexion rotation and tailward translation both activated the same anterior hindlimb synergy (iliopsoas, vastus lateralis and tibialis anterior) together with the forelimb synergy. The postural responses elicited by rotational perturbations were biomechanically inappropriate, and caused the animal to displace its own centre of mass away from the stable, control position. The most striking finding was that the group of muscles in which the medium latency postural response was evoked was different than the group from which short latency reflex responses were elicited. These data support the hypothesis that postural reactions are not merely reflex responses to local sensory inputs associated with the perturbation but, instead, represent a centrally generated response, with the muscle synergy being the controlled unit.Supported by NIH grants NS19484 and RR05593 as well as Good Samaritan Hospital     

Time-based prediction in motor control: evidence from grip force response to external load perturbations

An object held in precision grip creates predictable load forces on the hand during voluntary hand movement and these are associated with anticipatory modulation of grip force. Conflicting results have been obtained over whether predictable external load perturbations result in anticipatory grip force responses (e.g. Blakemore et al. in J Neurosci 18(18):7511–7518, 1998; Weeks et al. in Exp Brain Res 132:404–410, 2000). This paper investigated whether the discrepancies reflect differences in the methods used in estimating the time delay. Subjects held a manipulandum that delivered load force perturbations in the form of pulses of variable duration and interval or periodic 0.5 and 1 Hz square waves or sinusoids. The grip forces exerted by the subjects were measured. Two methods were used to assess the time delay of the grip force in relation to the load force: (1) cross-spectral analysis, (2) a single threshold method applied on time-locked averaged data. Despite a phase lag shown by the cross-spectral analysis, the threshold method revealed grip force increased 264.8±40.2 ms before the onset of the load force when 0.5 Hz square waves were used as the load force perturbation and 70.2±17.0 ms before the load force when 1 Hz square waves were used. Computer simulations indicated that the single threshold method gives a more sensitive estimate of the onset time than the cross-spectral analysis. We conclude that discrepancies in previous studies reflect differences in the methods used to assess the time-delay and that there is an anticipatory component in the grip force response to predictable external load perturbation.     

Developmental changes in the dynamical structure of postural sway during a precision fitting task

Recent research using measures to assess the time-dependent structure of postural fluctuations has provided new insights into the stability and adaptability of human postural control in adults. To date, little research has examined how postural dynamics reflecting the stability and adaptability of postural control may change as a function of development, especially during supra-postural tasks. The goal of this study was to examine the dynamics of postural fluctuations during a manual-fitting task in which precision, visual and postural task constraints were altered in children and adults. Three age groups were tested: 7-, 10-year olds and college aged adults. Recurrence quantification analysis (RQA) was used to assess the regularity (percent determinism) and complexity (entropy) of the center of pressure (CoP) in the anterior–posterior (AP) and medial-lateral (ML) directions. The CoP patterns exhibited by adults were more deterministic and more complex (higher entropy) than those of the 7-year-old children under the different experimental manipulations. No differences between the adults and the 10-year-old children were observed. The increase in determinism with a corresponding increase in entropy exhibited by the adults and older-children during a manual fitting task may be a prospective mechanism over which postural movements follow a more predictable path allowing for stable and flexible task performance. Our results also support the notion that complex postural fluctuations (as measured by RQA entropy) are functional and typically increase as the precision requirements of a manual task increase.     

Reprogramming of muscle activation patterns at the wrist in compensation for elbow reaction torques during planar two-joint arm movements

The relationship between wrist kinematics, dynamics and the pattern of muscle activation were examined during a two-joint planar movement in which the two joints moved in opposite directions, i.e. elbow flexion/wrist extension and elbow extension/wrist flexion. Elbow movements (ranging from 10 to 70 deg) and wrist movements (ranging from 10 to 50 deg) were performed during a visual, step-tracking task in which subjects were required to attend to the initial and final angles at each joint. As the elbow amplitude increased, wrist movement duration increased and the wrist movement trajectories became quite variable. Analysis of the torques acting at the wrist joint showed that elbow movements produced reaction torques acting in the same direction as the intended wrist movement. Distinct patterns of muscle activation were observed at the wrist joint that were dependent on the relative magnitude of the elbow reaction torque in relation to the net wrist torque. When the magnitude of the elbow reaction torque was quite small, the wrist agonist was activated first. As the magnitude of the elbow reaction torque increased, activity in the wrist agonist decreased significantly. In conditions where the elbow reaction torque was much larger than the net wrist torque, the wrist muscle torque reversed direction to oppose the intended movement. This reversal of wrist muscle torque was directly associated with a change in the pattern of muscle activation where the wrist antagonist was activated prior to the wrist agonist. Our findings indicate that motion of the elbow joint is an important consideration in planning wrist movement. Specifically, the selection of muscle activation patterns at the wrist is dependent on the relative magnitude and direction of the elbow reaction torque in relation to the direction of wrist motion.     

Automatic postural responses in the cat: responses of distal hindlimb muscles to paired vertical perturbations of stance

Summary The active components of the quadrupedal diagonal stance response to rapid removal of the support from beneath a single limb were studied in cats to further define the mechanisms that trigger and generate the response. We recorded EMG activity from lateral gastrocnemius and tibialis anterior muscles in awake, behaving cats while they stood on an hydraulic posture platform. By dropping the support from beneath a single limb, we evoked the diagonal stance response, with its characteristic changes in vertical force and EMG patterns. As the animal responded to this drop, a second perturbation of posture was then presented at intervals of 10 to 100 ms following the first. This second perturbation, which consisted of dropping the support from beneath the two limbs that were loaded as a result of the initial limb drop, made the first response biomechanically inappropriate. The EMG responses observed in both muscles during paired perturbations were triggered by the somatosensory events related to the perturbations. Muscle responses that were appropriate for the first perturbation always occurred with amplitudes and latencies similar to control trials. This was true even when the second perturbation occurred 10–20 ms after the first, that is, when this perturbation either preceded or was coincident with the response to the initial limb drop. The EMG responses that were normally associated with the second perturbation were delayed and/or reduced in amplitude when the time interval between perturbations was short. As the inter-perturbation interval was lengthened beyond 60–100 ms, however, EMG responses to the second perturbation were unaffected by the occurrence of the first perturbation. When the hindlimb containing the recording electrodes was dropped as part of the second perturbation, a myotatic latency response was observed in tibialis anterior. The amplitude of this response to the second perturbation was greater than controls when this displacement was presented during the period between initiation of the first perturbation and execution of the response to it. When the second displacement was presented after execution of the first response began, the amplitude of the myotatic response was reduced below control levels. While the results do not preclude the possibility that these automatic postural responses are segmental or suprasegmental reflexes, they support the hypothesis that the active component of the response to drop of the support beneath a single limb is centrally programmed and that the appropriate response can be riggered very rapidly by the somatosensory information signalling the perturbation.Supported by NIH grants NS19484 and RR05593 as well as the Medical Research Foundation of Oregon, and the Neurological Sciences Center of Good Samaritan Hospital and Medical Center     

Developmental changes in compensatory responses to unexpected resistance of leg lift during gait initiation

The development of the ability to integrate postural adjustments into the gait initiation process was investigated in children, using both kinematic and electromyographic (EMG) analysis. Subjects included children of 1 year of age (1-4 months' walking experience), 2-3 years of age (9-17 months' walking experience), 4-5 years of age (3-4 years' walking experience), and adults. We perturbed the balance of the children during gait initiation to determine the point at which infants begin to develop and finally master the ability to respond to external threats to balance during the gait initiation process. A magnet attached to the force platform on which the child stood was activated and served to resist the child's gait initiation (metal plaques on the soles of the shoes were attracted by the magnet) and thus served as an external perturbation during the gait initiation process. Kinematic and EMG analysis indicated that, while the ability to use preparatory postural adjustments in the gait initiation process emerges early in development, the ability to react efficiently to perturbations during gait initiation does not develop until after about 4-5 years of age. Though even the youngest age groups showed some response to the perturbation, it was highly variable, indicating its primitive form. The main response to the perturbation was a slight decrease in latency and increase in amplitude in the muscles used for push-off for gait initiation. Interestingly there was a shift in response pattern at 4-5 years of age, in both kinematic and EMG patterns. The amplitudes of the lateral and anteroposterior trunk and stance leg oscillations were significantly increased. In addition, the muscle response amplitudes (hamstrings and second quadriceps burst) of the swing leg were significantly increased and delayed (hamstring and gastrocnemius), with coactivation of agonist and antagonist muscles at the knee and ankle joint, concomitant with an exaggerated foot height of the first step.     

Prone sleeping impairs circulatory control during sleep in healthy term infants: implications for SIDS

STUDY OBJECTIVES: To determine the effects of sleeping position on development of circulatory control in infants over the first 6 months of postnatal age (PNA). DESIGN: Effects of sleeping position, sleep state and PNA on beat-beat heart rate (HR) and mean arterial pressure (MAP) responses to a head-up tilt (HUT) were assessed during sleep in infants at 2-4 wks, 2-3 mo and 5-6 mo PNA. MEASUREMENTS: Daytime polysomnography was performed on 20 full-term infants (12 F/8 M) and MAP was recorded continuously and noninvasively (Finometer). HUTs of 15 degrees were performed during active sleep (AS) and quiet sleep (QS) in both the prone and supine sleeping positions. MAP and HR data were expressed as the percentage change from baseline, and responses were divided into initial, middle and late phases. RESULTS: In the supine position HUT usually resulted in an initial increase (P < 0.05) in HR and MAP, followed by decreases (P < 0.05) in HR and MAP in the middle phase; subsequently HR and MAP returned to baseline in the late phase. By contrast, in the prone position the initial HUT-induced rises in HR and MAP were usually absent, and at 2-3 mo MAP actually decreased (P < 0.05); subsequently HR but not MAP returned to baseline. At 2-3 mo, MAP was lower (P < 0.05) in prone than supine sleeping throughout the HUT. CONCLUSIONS: Prone sleeping alters MAP responses to a HUT during QS at 2-3 mo PNA. Decreased autonomic responsiveness may contribute to the increased risk for SIDS of infants sleeping in the prone position.     

Shifting the balance: evidence of an exploratory role for postural sway

Humans and other species are unable to stand perfectly still; their bodies continuously sway during stance even during concentrated efforts to avoid such movement. Traditionally, this phenomenon has been viewed as an inability of the central nervous system (CNS) to maintain perfect equilibrium because of its reliance on feedback from sensory signals to control corrective ground-reaction forces. Using a novel method to minimize movements of the body during stance without subject awareness, we have made the unique discovery that ground-reaction forces are generated independent of body sway, as evidenced by observations of increased centre of pressure variability when postural sway is minimized experimentally. Contrary to traditional views, our results suggest that postural sway may be used by the CNS as an exploratory mechanism to ensure that continuous dynamic inputs are provided by multiple sensory systems. This novel paradigm has the potential to significantly shift long-standing views on balance, and questions the theoretical basis behind conventional treatment strategies for balance deficits associated with age and disease.     

Automatic postural responses in the cat: responses of hindlimb muscles to horizontal perturbations of stance in multiple directions

Summary The effect of the direction of unexpected horizontal perturbations of stance on the organization of automatic postural responses was studied in cats. We recorded EMG activity in eight proximal and distal muscles of the hindlimb along with vertical forces imposed by the limbs in awake behaving cats while they stood on an hydraulic platform. Postural responses to motion of the platform in 16 different horizontal directions were recorded. Vertical force changes were consistent with passive shifts of the center of mass and active correction of stance by the animals. When the perturbation was in the sagittal plane, vertical force changes began about 65 ms following initial platform movement. When the perturbation contained a component in the lateral direction, latency for vertical force changes was about 25 ms and an inflection in the vertical force trace was observed at 65 ms. No EMG responses were observed with latencies that were short enough to account for the early force component and it was concluded that this force change was due to passive shifts of the center of mass. The amplitude of the EMG responses of each muscle recorded varied systematically as perturbation direction changed. The directions for which an individual muscle showed measurable EMG activity were termed the muscle's angular range of activation. No angular range of activation was oriented strictly in the A-P or lateral directions. Most muscles displayed angular ranges of activation that encompassed a range of less than 180°. Onset latencies of EMG responses also varied systematically with perturbation direction. The amplitude and latency relationships between muscles, which made up the organization of postural responses, also varied systematically as perturbation direction was changed. This result suggests that direction of perturbation determines organizational makeup of postural responses, and for displacements in the horizontal plane, is considered a continuous variable by the nervous system.     

Assessment of torque-steadiness reliability at the ankle level in healthy young subjects: implications for cerebral palsy

It was the primary objective of this study to investigate whether quantifying fluctuations in dorsi and plantarflexor torque during submaximal isometric contractions is a reliable measurement in young healthy subjects. A secondary objective was to investigate the reliability of the associated muscle activity (EMG) data. Eighteen young subjects (12.8 ± 3.1 years, mean ± 1 SD) were examined twice. At each visit, fluctuations in exerted torque (torque steadiness) and muscle activity from the tibialis anterior, gastrocnemius and soleus muscles were determined during submaximal isometric dorsi and plantarflexions. The relative reliability of the torque steadiness variables was substantial (0.80 < ICC_3.1 < 0.92), with an absolute reliability (average coefficient of variation) of 13–17%. The relative reliability of the muscle activity data was generally moderate (0.51 < ICC_3.1 < 0.90), with an absolute reliability of 6–26%. The reliability of dorsi and plantarflexion torque-steadiness measurements proved to be good in young healthy subjects.