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.     

Human automatic postural responses: responses 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 human subjects. We recorded EMG activity from eight proximal and distal muscles acting on joints of the legs and hip known to be involved in postural corrections, while subjects stood on an hydraulic platform. Postural responses to horizontal motion of the platform in 16 different directions were recorded. The amplitude of the EMG responses of each muscle studied varied continuously as perturbation direction was changed. The directions for which an individual muscle showed measurable EMG activity were termed the muscle's angular range of activation. There were several differences in the response characteristics of the proximo-axial muscles as opposed to the distal ones. Angular ranges of activity of the distal muscles were unipolar and encompassed a range of less than 180°. These muscles responded with relatively constant onset latencies when they were active. Proximo-axial muscles, acting on the upper leg and hip showed larger angular ranges of activation with bimodal amplitude distributions and/ or onset latency shifts as perturbation direction changed. While there were indications of constant temporal relationships between muscles involved in responses to perturbations around the sagittal plane, the onset latency relationships for other directions and the response amplitude relationships for all directions varied continuously as perturbation direction was changed. Responses were discrete in that for any particular perturbation direction there appeared to be a single unique response. Thus, while the present results do not refute the hypothesis that automatic postural responses may be composed of mixtures of a few elemental synergies, they suggest that composition of postural responses is a complex process that includes perturbation direction as a continuous variable.     

The influence of vision on the automatic postural muscle responses of newly standing and newly walking infants

In adults, visual inputs do not appear to contribute significantly to automatic postural muscle responses (90–100 ms latency) activated by transient support surface displacements causing threats to standing balance, but are activated through slow pathways with latencies of more than 200 ms. However, it has been shown that the postural sway behavior of early walking infants is strongly influenced by visual flow cues that falsely signal self-movement. To determine whether there also are significant contributions of vision to automatic postural muscle responses in this age group, two groups of infants were tested on a moveable platform; pre-walkers (n=6) and early walkers (n=6). Pre-walkers did not show any measurable effect of visual condition (vision vs no vision) on muscle response characteristics. However, the integrated gastrocnemius activity of early walkers increased significantly in vision versus no vision conditions (P<0.05). These results show that visual cues contribute to, or modulate, the automatic postural responses in children who are in the developmental transition to independent walking. Received: 10 May 1996 / Accepted: 23 January 1998     

Influence of auditory precuing on automatic postural responses

An experiment was conducted to determine the influence of auditory precuing on posture control. Specifically, the influence of a warning signal on the onset latencies of the gastrocnemius (G) and tibialis anterior (TA) muscles was determined. An audible 50-ms tone was presented to subjects standing on a moveable platform and preceded a perturbation to standing balance by 500 ms. The perturbations were produced by an anterior or posterior translation (3 cm at 30 cm/s) of the support surface. Unilateral electromyographic activity was recorded from G and TA muscles. In the first series of trials (series A), the muscle onset latencies following perturbations with a nondirectionally specific precue, an invalid precue, and no precue were compared. In the second series of trials (series B), muscle onset latencies following perturbations with a directionally specific precue, invalid precue, and no precue perturbations were compared. In series A, mean muscle onset latencies decreased following nondirectionally specific precues during forward and backward platform perturbations; respectively, TA 6% (91±9 ms to 86±9 ms) and G 7% (93±6 ms to 87±5 ms). During series B, the TA and G muscle onset latencies decreased following directionally specific precues by 10.4% (92±12 ms to 82±6ms) and 9.8% (92±9ms to 83±6ms), respectively. There were no significant differences between the types of precues. Thus, prior knowledge of a forthcoming balance perturbation reduces postural muscle onset latency times. In addition, specific prior knowledge reduces muscle onset latency time in the same manner as does nonspecific prior knowledge.     

Automated postural responses are modified in a functional manner by instruction

The restoration of upright balance after a perturbation relies on highly automated and, to a large extent, stereotyped postural responses. Although these responses occur before voluntary control comes into play, previous research has shown that they can be functionally modulated on the basis of cognitive set (experience, advanced warning, instruction, etc.). It is still unknown, however, how the central nervous system deals with situations in which the postural response is not necessarily helpful in the execution of a task. In the present study, the effects of instruction on automated postural responses in neck, trunk, shoulder, and leg muscles were investigated when people were either instructed to recover balance after being released from an inclined standing posture [balance recovery (BR) trials], or not to recover at all and fall onto a safety mattress in the most comfortable way [fall (F) trials], in both backward and leftward directions. Participants were highly successful in following the instructions, consistently exhibiting stepping responses for balance recovery in BR trials, and suppressing stepping in the F trials. Yet EMG recordings revealed similar postural responses with onset latencies between 70 and 130 ms in both BR and F trials, with slightly delayed responses in F trials. In contrast, very pronounced and early differences were observed between BR and F trials in response amplitudes, which were generally much higher in BR than in F trials, but with clear differentiation between muscles and perturbation directions. These results indicate that a balance perturbation always elicits a postural response, irrespective of the task demands. However, when a specific balance recovery response is not desired after a perturbation, postural responses can be selectively downregulated and integrated into the motor output in a functional and goal-oriented way.     

Reversals of anticipatory postural adjustments during voluntary sway in humans

We describe reversals of anticipatory postural adjustments (APAs) with the phase of a voluntary cyclic whole-body sway movement. Subjects ( n = 9) held a standard load in extended arms and released it by a bilateral shoulder abduction motion in a self-paced manner at different phases of the sway. The load release task was also performed during quiet stance in three positions: in the middle of the sway range and close to its extreme forward and backward positions. Larger APAs were seen during the sway task as compared to quiet stance. Although the direction of postural perturbation associated with the load release was always the same, the direction of the APAs in the leg muscles reversed when the subjects were close to the extreme forward position as compared to the APAs in other phases and during quiet stance. The trunk muscles showed smaller APA modulation at the extreme positions but larger modulation when passing through the middle position, depending on the direction of sway, forward or backward. The phenomenon of APA reversals emphasizes the important role of safety in the generation of postural adjustments associated with voluntary movements. Based on these findings, APAs could be defined as changes in the activity of postural muscles associated with a predictable perturbation that act to provide maximal safety of the postural task component.     

Postural adjustments associated with voluntary contraction of leg muscles in standing man

Summary The postural adjustments associated with a voluntary contraction of the postural muscles themselves have been studied in the legs of normal standing men. We focussed on the following questions. Do postural adjustments precede the focal movement as in the case of movements of the upper limb? Which muscle(s) are involved in the task of stabilizing posture? Can the same postural muscle be activated in postural stabilization and in voluntary movement at the same time, in spite of the opposite changes in activity possibly required by these conditions? Six subjects standing on a dynamometric platform were asked to rise onto the tips their toes by contracting their soleus muscles, or to rock on their heels by contracting their tibialis anterior muscles. The tasks were made in a reaction time (RT) situation or in a self-paced mode, standing either freely or holding onto a stable structure. Surface EMGs of leg and thigh muscles, and the foot-floor reaction forces were recorded. The following results were obtained in the RT mode, standing freely. 1. Rising onto toe tips: a striking silent period in soleus preceded its voluntary activation; during this silent period, a tibialis anterior burst could be observed in three subjects; these anticipatory activities induced a forward sway, as monitored by a change in the force exerted along the x axis of the platform. 2. Rocking on heels: an enhancement in tonic EMG of soleus was observed before tibialis anterior voluntary burst, at a mean latency from the go-signal similar to that of the silent period; this anticipatory activity induced a backward body sway. 3. Choice RT conditions showed that the above anticipatory patterns in muscle activity were pre-programmed, specific for the intended tasks, and closely associated with the focal movement. When both tasks were performed in a self-paced mode, all the above EMG and mechanical features were more pronounced and unfolded in time. If the subjects held onto the frame, the early features in the soleus or tibialis anterior EMG were absent, and the corresponding changes in the foot-floor reaction forces were lacking. The anticipatory phenomena observed are considered postural adjustments because they appear only in the free-standing situation, and induce a body sway in the appropriate direction to counteract the destabilizing thrust due to the voluntary contraction of soleus or tibialis anterior. The central organization and descending control of posture and movements are briefly discussed in the light of the short latency of the anticipatory phenomena and of their close association with the focal movement.     

Influence of local sensory afference in the calibration of human balance responses

Summary Peripheral sensory modulation of balance behavior may require a calibrated mechanism which would maintain upright standing by a feedback control of torque at the ankle joint. The calibration of human balance was studied using a systematic presentation of perturbation excursions and velocities in normal freely standing subjects. All perturbations (posterior movements of a force platform) induced a forward body sway and were presented by first increasing and then decreasing the magnitude of perturbation. In preselected conditions the stability of the ankle and hence the accuracy of surface orientation inputs was altered using a foam base placed under the subjects feet. Each subject pressed a hand held response key at the moment a postural disturbance was detected. The automatic neuromuscular response (ANR) was recorded from the gastrocnemius muscles bilaterally and the perturbation detection time (DT) was obtained from the onset of thenar muscle discharge. The major findings in this study were: (1) Conscious DT changed as a function of step variations in perturbation excursion and was disassociated from the ANR latency. The ANR latency remained essentially constant in all conditions and did not have any influence on the kinematics of body sway. (2) Normalized peak body sway decreased during unstable ankle conditions and the reduction of body sway could be attributed to an increase in the gain of the ANR across a 200 ms integration period. The ANR 200 ms amplitude also showed higher correlations with perturbation magnitude during unstable (versus stable) ankle conditions. (3) The 200 ms gastrocnemius amplitude was modulated by excursion and velocity of platform displacement but the amplitude integrated over 100 ms was dependent on only the velocity of perturbation. Our results indicate that balance is controlled by a centrally initiated postural response but regulated in amplitude by local sensory information. These results establish that the gain of the ANR is functional, peripherally driven, and occurs subconsciously to alter the kinematics of body sway.     

Effects of 20-day bed rest with and without strength training on postural sway during quiet standing

Aim: To examine the effect of unweighting as a possible contributory factor to a reduced calf muscle volume on postural sway during quiet standing, changes in postural sway following bed rest with or without strength training were investigated. Methods: Twelve young men participated in a 20‐day bed‐rest study. Subjects were divided into a non‐training group (BR‐Con) and a strength training group (BR‐Tr). For the BR‐Tr group, training was comprised of dynamic calf‐raise and leg‐press exercises to maintain the muscle volume of the plantar flexors. Before and after bed rest, subjects maintained quiet standing in a barefoot position on a force platform with their eyes open or closed. During the quiet stance, foot centre‐of‐pressure (CoP) and the mean velocity of CoP was calculated. Muscle volume of the plantar flexors was computed using axial magnetic resonance images of the leg. Results: After the bed‐rest period, the muscle volume decreased in the BR‐Con group but not in the BR‐Tr group. The mean velocity of CoP as an assessment of postural sway, however, increased in both groups. These results indicate that the strength training during bed rest cannot counteract the increase in postural sway. Conclusion: We concluded that postural sway increases following 20 days of bed rest despite maintenance of the muscle volume of plantar flexors as the main working muscles for the human postural standing.     

Lateral balance organisation in human stance in response to a random or predictable perturbation

The effect of the predictability of perturbation to standing balance was evaluated in terms of the muscle activity and response dynamics of five subjects exposed to horizontal forces at the pelvis producing sideways or forward sway. Rapid (EMG onset latencies of 70–80 ms recorded from the left gluteus medius and gastrocnemius) and qualitatively different patterns of response were produced by forward pushes and pushes to either side. However, the EMG response to left push was constant in pattern and timing, whether the push direction was constant and, therefore, predictable over a block of trials or whether the left push trials were interleaved randomly with right push or forward push trials. Moreover, there were no systematic effects of perturbation direction uncertainty on the latency and rate of increase of ground reaction forces. We conclude that prior information does not speed postural responses that differ quantitatively according to the direction of perturbation to balance. Received: 13 October 1997 / Accepted: 17 July 1998     

Relation between muscle response onset and body segmental movements during postural perturbations in humans

Summary This study has examined the individual movements of the body segments of a group of 10 standing adults during anterior and posterior platform displacements (3 and 6 cm amplitudes), and compared body movements to neck and ankle muscle response onset times. Differences in the kinematics of movement were observed for anterior vs. posterior platform displacements: hip, shoulder, and head began to move much earlier for posterior compared to anterior platform movements. This could explain differences in postural muscle temporal response organization for the two directions of body movement. Though anterior/posterior neck and head displacements were late in comparison to neck flexor muscle response onset, small vertical movements of the shoulder and head occurred early (40 and 67 ms after platform movement onset). These movements were consistently directed upward for anterior platform displacements and downward for posterior platform displacements. In order to determine whether neck proprioceptors were responsible for response activation in the neck we repeated the experiment using a neck stabilization device, on one of our subjects. In this condition, we found normal neck muscle response latencies. This suggests that neck proprioceptors are not the primary contributors to the early neck muscle responses seen during horizontal support surface displacements. In studying the effect of repeated exposure to horizontal platform displacements we found a diminution in the amplitude and an increase in onset latencies in neck and antagonist ankle muscle responses over the sequence of 16 trials, in many of the subjects tested. This corresponded to smaller head accelerations, and smaller displacements of the head and shoulder in later trials in the experimental sequence. The result implies that these subjects changed their postural set during the course of the experiment, possibly by relaxing the muscles of the body to allow the viscoelastic properties of the lower body segments to absorb more of the impact of platform displacement.     

The influence of natural body sway on neuromuscular responses to an unpredictable surface translation

Previous research has shown that the postural configuration adopted by a subject, such as active leaning, influences the postural response to an unpredictable support surface translation. While those studies have examined large differences in postural conditions, it is of additional interest to examine the effects of naturally occurring changes in standing posture. Thus, it was hypothesized that the normal postural sway observed during quiet standing would affect the responses to an unpredictable support surface translation. Seventeen young adults stood quietly on a moveable platform and were perturbed in either the forward or backward direction when the location of the center of pressure (COP) was either 1.5 standard deviations anterior or posterior to the mean baseline COP signal. Postural responses, in the form of electromyographic (EMG) latencies and amplitudes, were recorded from lower limb and trunk muscles. When the location of the COP at the time of the translation was in the opposite, as compared to the same, direction as the upcoming translation, there was a significantly earlier onset of the antagonists (10-23%, i.e. 15-45 ms) and a greater EMG amplitude (14-39%) in four of the six recorded muscles. Stepping responses were most frequently observed during trials where the position of the COP was opposite to the direction of the translation. The results support the hypothesis that postural responses to unpredictable support surface translations are influenced by the normal movements of postural sway. The results may help to explain the large variability of postural responses found between past studies.     

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.     

Fixed patterns of rapid postural responses among leg muscles during stance

Summary The aim of this study has been to present firmer evidence that during stance functionally related postural muscles in the legs are activated according to fixed patterns. The importance of fixed patterns of activation for stabilization, balance, and movement control has received considerable theoretical and experimental attention. With regard to postural adjustment in humans, however, evidence for fixed activation patterns has been circumstantial only. Previous studies could not rule out the possibility that fixed patterns were caused by the mechanical coupling of rotatory movements among the joints of the body.This study has shown that in subjects employing FSR adjustments during stance activation patterns among leg muscles at FSR latency (functional stretch response, 100–120 msec) are preprogrammed prior to a response and are, on the average, fixed, independent of the associated motions among the ankle, knee, and hip joints. The identical fixed patterns were produced by sway rotation about the ankle joints and by direct rotation of the ankles. A pattern was characterized as fixed when, during a 1 hr session, the ratios of estimated force between pairs of functionally related leg muscles remained constant. In addition, the sequence of activation among muscles was fixed and followed a course beginning at the ankle muscle and proceeding proximally.The discussion of these results considered the functional implications of fixed contractile patterns during stance posture control.     

Control of bipedal posture following localised muscle fatigue of the plantar-flexors and finger-flexors

The present experiment investigated the control of bipedal posture following localised muscle fatigue of the plantar-flexors and finger-flexors. Twelve young healthy adults voluntarily participated in this study. They were asked to stand upright as still as possible with their eyes closed in two randomly ordered experimental sessions. Each session consisted of pre- and post-fatigue bipedal static postural control measurements immediately before and after a designated fatiguing protocol for plantar-flexor and finger-flexor muscles. Centre of foot pressure (CoP) displacements were recorded using a force platform. The results showed that the postural effects of localised muscle fatigue differed between the muscles targeted by the fatiguing procedures. Indeed, localised muscle fatigue of the plantar-flexors yielded increased CoP displacements, whereas localised muscle fatigue of the finger-flexors had no significant effect on the CoP displacements. In other words, fatigue localised to muscles which are involved in the performance of the postural task (plantar-flexors) degraded postural control, whereas fatigue localised to muscles which are not involved in the performance of the postural task did not. Taken together, the present findings support the recent conclusions that the effects of localised muscle fatigue on upright postural control is joint- and/or muscle-specific, and suggest that localised muscles fatigue of the plantar-flexors could mainly affect bipedal postural control via sensorimotor rather than cognitive processes.     

A kinematic and electromyographic analysis of the development of sitting posture in infants

This study was designed to describe the development of posture control in sitting in response to a natural perturbation. Seven normal infants 2 to 5 months of age were tested at two stages of independent sitting development. Trunk support was removed from infants while sitting erect and the postural responses were videotaped and EMG recorded from the upper trunk extensors, lumbar paraspinals, gluteus maximus, rectus femoris, hamstrings, and abdominals. Kinematic variables (trunk displacement, trunk velocity, trunk curvature) and an EMG variable (pattern of muscle activation) were analyzed with computer programs. Between Stages 1 and 2 of sitting development, anterior trunk displacement and velocity decreased significantly, although the trunk extension curve did not change significantly. Infants had variable muscle responses during Stage 1; however, during Stage 2 EMG analysis revealed less variability and the emergence of postural synergies. Overall, lumbar paraspinals, hamstrings, and quadriceps were the muscles most frequently active during the postural response. Each subject had a preferred synergy, with the most common synergies being a lumbar paraspinalhamstring synergy and a lumbar paraspinal-quadriceps synergy. These data provide evidence that trunk displacement and trunk velocity decrease in infants develop independent sitting posture, and these variables may be used to measure improvement in sitting control. We suggest that the control of sitting posture is related to the emergence and preferred use of the paraspinal-hamstring and paraspinal-quadriceps synergies. © 1993 Johan Wiley & Sons, Inc.     

A history of low back pain associates with altered electromyographic activation patterns in response to perturbations of standing balance

People with a history of low back pain (LBP) exhibit altered responses to postural perturbations, and the central neural control underlying these changes in postural responses remains unclear. To characterize more thoroughly the change in muscle activation patterns of people with LBP in response to a perturbation of standing balance, and to gain insight into the influence of early- vs. late-phase postural responses (differentiated by estimates of voluntary reaction times), this study evaluated the intermuscular patterns of electromyographic (EMG) activations from 24 people with and 21 people without a history of chronic, recurrent LBP in response to 12 directions of support surface translations. Two-factor general linear models examined differences between the 2 subject groups and 12 recorded muscles of the trunk and lower leg in the percentage of trials with bursts of EMG activation as well as the amplitudes of integrated EMG activation for each perturbation direction. The subjects with LBP exhibited 1) higher baseline EMG amplitudes of the erector spinae muscles before perturbation onset, 2) fewer early-phase activations at the internal oblique and gastrocnemius muscles, 3) fewer late-phase activations at the erector spinae, internal and external oblique, rectus abdominae, and tibialis anterior muscles, and 4) higher EMG amplitudes of the gastrocnemius muscle following the perturbation. The results indicate that a history of LBP associates with higher baseline muscle activation and that EMG responses are modulated from this activated state, rather than exhibiting acute burst activity from a quiescent state, perhaps to circumvent trunk displacements.     

Development of somatosensory-motor integration: an event-related analysis of infant posture in the first year of independent walking

The ability to integrate sensation with action is considered an important factor underlying the development of upright stance and locomotion. While many have studied sensory influences on posture, the nature of these influences and how they change with development have yet to be thoroughly characterized in infancy. Six infants were examined from 1 month prior to walk onset until 9 months of independent walking experience while standing quietly and touching either a static or a dynamic surface. Five adults were examined performing an analogous task. An event-related, time-frequency analysis was used to assess the relationship between postural sway and the motion of the somatosensory stimulus. Phase consistency between sway and stimulus was observed for both adults and infants, and with walking experience the infants increased their phase consistency rather than changing aspects of response amplitude. It is concluded that walking experience provides opportunities for an active tuning of sensorimotor relations for adequate estimation of body position in space and thus facilitates refined control over temporal aspects of postural sway.     

Task-specific adaptations of postural sway in sitting infants

When engaging in manual or visual tasks while sitting, infants modify their postural sway based on concurrent task demands. It remains unclear whether these modulations are sensitive to differences in concurrent task demands (holding a toy vs. looking at a toy being held by someone else), and whether the properties of the support surface impact these adaptations. We investigated infants’ ability to modify postural sway when holding a toy or visually attending to a toy someone else was holding while sitting on different support surfaces. Twenty-six independently sitting infants sat on solid and compliant surfaces placed on a force plate while looking at or holding a toy. Measures of postural sway were calculated from the center of pressure data. Visually attending to a toy was associated with less sway and lower sway velocity than when holding a toy. Surprisingly, surface compliance did not affect sway and there were no interaction effects. Whereas sway modulations may facilitate infants’ performance on both manual and visual concurrent tasks, the visual task placed more constraints on the postural system leading to greater adaptations in postural sway. These findings provide insights into how infants are allocating attention and coordinating perceptual-motor information in developing sitting skills.     

Postural adjustments due to external perturbations during sitting in 1-month-old infants: evidence for the innate origin of direction specificity

The aim of the study was to examine whether infants, at an age when they have no or little experience in sitting, can produce direction specific postural adjustments, i.e. synergies of muscle activity on the ventral side of the body during backward sway and on the dorsal side during forward sway. In addition, we addressed the question whether postural adjustments at this young age are restricted to single muscle responses or consist of a variable repertoire of muscle activation patterns including one during which all direction specific muscles participate (complete pattern). Postural adjustments due to external perturbations in a sitting position were studied in eight healthy infants aged 1 month. Multiple surface EMGs of neck, trunk and leg muscles and kinematics were recorded while the infants were exposed to horizontal forward (Fw) and backward (Bw) displacements of the surface of support. Direction specific postural adjustments, defined as adjustments during which agonist activation or antagonist inhibition preceded antagonist activation, were present in 85% of Bw and 72% of Fw translations. The direction specific adjustments showed a large variability with the repertoire of adjustments including the activation of one, two or all of the recorded direction specific muscles. The finding of direction specific adjustments at 1 month of age support the opinion that the basic level of organisation of postural adjustments has an innate origin. The finding of a variable repertoire of muscle response patterns, including the complete pattern, refutes the idea that the development of postural adjustments results from gradual addition of appropriate muscles to the synergies.Electronic Supplementary Material Electronic supplementary material is available in the online version of this article at The study was supported financially by Folke Bernadotte Stiftelsen, Linnéa och Josef Carlssons Stiftelse, Norrbacka-Eugeniastiftelsen, Riksförbundet för Rörelsehindrade Barn och Ungdomar and Sunnerdahls Handikappfond