***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: The purpose of this study was to identify how the postural system weights coincident yet discordant disturbances of the visual and proprioceptive/vestibular systems . Eleven healthy subjects ( 25 - 38 yrs ) received either fore-aft translations of an immersive , wide field-of-view visual environment ( 0.1 Hz , + /- 3.7 m/sec ) , or anterior-posterior translations of the support surface ( 0.25 Hz , + /- 15 cm/sec ) , or both concurrently .  Kinematics of the head , trunk , and shank were collected with an Optotrak system and angular motion of each segment plotted across time . With only support surface translation , segmental responses were small ( 1 degrees -2 degrees ) and mostly opposed the direction of sled translation . When only the visual scene was moving , segmental responses increased as the trial progressed . When the inputs were presented coincidentally , response amplitudes were large even at the onset of the trial . Mean RMS values across subjects were significantly greater with combined stimuli than for either stimulus presented alone and areas under the power curve across subjects were significantly increased at the frequency of the visual input when both inputs were presented . Thus , intra-modality dependencies were observed , such that responses to the visual inputs significantly increased and responses to the somatosensory signals reflected the stimulus amplitude only when the two inputs were combined . We believe it unlikely that the role of any single pathway contributing to postural control can be accurately characterized in a static environment if the function of that pathway is context dependent To characterize muscle synergy organization underlying multidirectional control of stance posture , electromyographic activity was recorded from 11 lower limb and trunk muscles of 7 healthy subjects while they were subjected to horizontal surface translations in 12 different , r and omly presented directions .  The latency and amplitude of muscle responses were quantified for each perturbation direction . Tuning curves for each muscle were examined to relate the amplitude of the muscle response to the direction of surface translation . The latencies of responses for the shank and thigh muscles were constant , regardless of perturbation direction . In contrast , the latencies for another thigh [ tensor fascia latae ( TFL ) ] and two trunk muscles [ rectus abdominis ( RAB ) and erector spinae ( ESP ) ] were either early or late , depending on the perturbation direction . These three muscles with direction-specific latencies may play different roles in postural control as prime movers or as stabilizers for different translation directions , depending on the timing of recruitment . Most muscle tuning curves were within one quadrant , having one direction of maximal activity , generally in response to diagonal surface translations . Two trunk muscles ( RAB and ESP ) and two lower limb muscles ( semimembranosus and peroneus longus ) had bipolar tuning curves , with two different directions of maximal activity , suggesting that these muscle can play different roles as part of different synergies , depending on translation direction . Muscle tuning curves tended to group into one of three regions in response to 12 different directions of perturbations . Two muscles [ rectus femoris ( RFM ) and TFL ] were maximally active in response to lateral surface translations . The remaining muscles clustered into one of two diagonal regions . The diagonal regions corresponded to the two primary directions of active horizontal force vector responses . Two muscles ( RFM and adductor longus ) were maximally active orthogonal to their predicted direction of maximal activity based on anatomic orientation . Some of the muscles in each of the synergic regions were not anatomic synergists , suggesting a complex central organization for recruitment of muscles . The results suggest that neither a simple reflex mechanism nor a fixed muscle synergy organization is adequate to explain the muscle activation patterns observed in this postural control task . Our results are consistent with a central ly mediated pattern of muscle latencies combined with peripheral influence on muscle magnitude . We suggest that a flexible continuum of muscle synergies that are modifiable in a task-dependent manner be used for equilibrium control in stance Sudden loading of the  spine  is not only considered a risk factor for the development of low-back pain but also enables an evaluation of the stability of the spine when conducted under laboratory conditions . In the present study the  upper spine  was pulled in the anterior direction and the stiffness as well as activity in the erector spinae muscle was measured with different pre-tension in the erector spinae . The results showed that increased activity in the erector spinae prior to loading led to increased stiffness ( stiffness coefficients from 297 Nm rad−1 to 438 Nm rad−1 ) and a decrease in the extra neural signal input to the muscles to maintain the stability . It is therefore clear that increased tension in the erector spinae muscle will create a larger stability of the spine to anterior perturbations . However , contracting the muscles around the spine increases the load on the spinal structures . In 34 % of the experiments a silent period in the electromyographic signal was present after loading in the period when the torso was moving in the anterior direction . This phenomenon is discussed Abstract A large body of evidence has been collected which describes the response parameters associated with automatic balance corrections in man to perturbations in the pitch plane . However , perturbations to human stance can be expected from multiple directions . The purpose of the present study was to describe the directional sensitivities of muscle responses re-establishing disturbed stance equilibrium in normal subjects . The contributions of stretch reflex and automatic balance-correcting responses to balance control , and concomitant biomechanical reactions , were examined for combinations of pitch and roll perturbations of the support surface . More specifically , muscle responses , initial head accelerations and trunk velocities were analyzed with the intention of identifying possible origins of directionally specific triggering signals and to examine how sensory information is used to modulate triggered balance corrections with respect to direction . Fourteen healthy adults were required to st and on a dual-axis rotating platform capable of delivering rotational perturbations with constant amplitude ( 7.5 ° ) and velocity ( 50 ° /s ) through multiple directions in the pitch and roll planes . Each subject was r and omly presented with 44 support surface rotations through 16 different directions separated by 22.5 ° first under eyes-open , and then , for a second identical set of rotations , under eyes-closed conditions . Bilateral muscle activities from tibialis anterior , soleus , lateral quadriceps and paraspinals were recorded , averaged across direction , and areas calculated over intervals with significant bursts of activity . Trunk angular velocity and ankle torque data were averaged over intervals corresponding to significant biomechanical events . Stretch reflex ( intervals of 40–100 , 80–120 ms ) and automatic balance-correcting responses ( 120–220 , 240–340 ms ) in the same muscle were sensitive to distinctly different directions . The directions of the maximum amplitude of balance-correcting activity in leg muscles were oriented along the pitch plane , approximately 180 ° from the maximum amplitude of stretch responses . Ankle torques for almost all perturbation directions were also aligned along the pitch plane . Stretch reflexes in paraspinal muscles were tuned along the 45 ° plane but at 90 ° to automatic balance corrections and 180 ° to unloading responses in the same muscle . Stretch reflex onsets in paraspinal muscles were observed at 60 ms , as early as those of soleus muscles . In contrast , unloading reflexes in released paraspinal muscles were observed at 40 ms for perturbations which caused roll of the trunk towards the recorded muscle . Onsets of trunk roll velocities were earlier and more rapid than those observed for pitch velocities . Trunk pitch occurred for pure roll directions but not vice versa . When considered together , early stretch and unloading of paraspinals , and concomitant roll and pitch velocities of the trunk requiring a roll- and -pitch-based hip torque strategy , bring into question previous hypotheses of an ankle-based trigger signal or ankle-based movement strategies for postural balance reactions . These findings are compatible with the hypothesis that stretch- , force- and joint-related proprioceptive receptors at the level of the trunk provide a directionally sensitive triggering mechanism underlying a , minimally , two-stage ( pitch-based leg and pitch- and -roll-based trunk ) balance-correcting strategy . Accelerometer recordings from the head identified large vertical linear accelerations only for pitch movements and angular roll accelerations only during roll perturbations with latencies as early as 15 ms . Thus , it appears that balance corrections in leg and trunk muscles may receive strong , receptor-dependent ( otolith or vertical canal ) and directionally sensitive amplitude-modulating input from vestibulospinal signals Abstract The effect of the predictability of perturbation to st and ing 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 r and omly 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 Background The effects of lumbosacral orthoses ( LSOs ) on neuromuscular control of the trunk are not known . There is a concern that wearing LSOs for a long period may adversely alter muscle control , making individuals more susceptible to injury if they discontinue wearing the LSOs . The purpose of this study was to document neuromuscular changes in healthy subjects during a 3-week period while they regularly wore a LSO .  Methods Fourteen subjects  wore LSOs 3 hrs a day for 3 weeks . Trunk muscle activity prior to and following a quick force release ( trunk perturbation ) was measured with EMG in 3 sessions on days 0 , 7 , and 21 . A longitudinal , repeated- measures , factorial design was used . Muscle reflex response to trunk perturbations , spine compression force , as well as effective trunk stiffness and damping were dependent variables . The LSO , direction of perturbation , and testing session were the independent variables . Results The LSO significantly ( P < 0.001 ) increased the effective trunk stiffness by 160 Nm/rad ( 27 % ) across all directions and testing sessions . The number of antagonist muscles that responded with an onset activity was significantly reduced after 7 days of wearing the LSO , but this difference disappeared on day 21 and is likely not clinical ly relevant . The average number of agonist muscles switching off following the quick force release was significantly greater with the LSO , compared to without the LSO ( P = 0.003 ) . Conclusions The LSO increased trunk stiffness and result ed in a greater number of agonist muscles shutting-off in response to a quick force release . However , these effects did not result in detrimental changes to the neuromuscular function of trunk muscles after 3 weeks of wearing a LSO 3 hours a day by healthy subjects BACKGROUND Studies examining rapid spine loading have documented the influence of steady-state trunk preloads , and the result ing levels of trunk muscle preactivation , on the control of spine stability . However , the effects of different levels of muscle coactivation , and result ing spine loads , on the response to a perturbation of the externally unloaded trunk are unclear . METHODS Fourteen male subjects coactivated the abdominal muscles  at four different levels ( approximately 0 % , 10 % , 20 % and 30 % of the maximal voluntary contraction ) monitored by an electromyography biofeedback system while  semi-seated in a  neutral lumbar spine position .  They were loaded posteriorly in two directions ( 0 degrees and 30 degrees from the sagittal plane ) and with two different loads ( 6.80 and 9.07 kg ) . Force perturbation , spine displacement and electromyography activity were measured , and torso compression and stability were modeled . FINDINGS Abdominal coactivation significantly increased spine stability and reduced the movement of the lumbar spine after perturbation , but at the cost of increasing spinal compression . Preactivation also reduced the frequency and magnitude , and delayed the onset of muscle reactions , mainly for the back muscles and the internal oblique . The higher magnitude load and the load applied in an oblique direction both showed more potentially hazardous effects on the trunk . INTERPRETATION Torso coactivation increases spinal stiffness and stability and reduces the necessity for sophisticated muscle responses to perturbation . Although further investigation is needed , it appears there is an asymptotic function between coactivation and both stiffness and stability . There also appears to be more hazard when buttressing twisting components of a sudden load compared to sagittal components . Patients with trunk instability and intolerance to spine compression may benefit from low to moderate levels of coactivation Trunk instability during sitting is a major problem following neuromuscular injuries such as stroke and spinal cord injury . In order to develop new strategies for alleviating this problem , a better underst and ing of the intrinsic contributions of the healthy trunk to sitting control is needed . As such , this study set out to propose and vali date a novel methodology for determining multidirectional trunk stiffness during sitting using r and omized transient perturbations .  Fifteen healthy individuals sitting naturally on a custom-made seat  were r and omly perturbed in eight horizontal directions .  Trunk stiffness and damping were quantified using force and trunk kinematics in combination with translational and torsional models of a mass-spring-damper system . The results indicate that stiffness and damping of the healthy trunk are roughly symmetrical between the two body sides . Moreover , both quantities are smallest in the anterior and largest in the lateral directions . In conclusion , a novel protocol for identifying intrinsic trunk stiffness and damping has been developed , eliminating anticipation effects with respect to perturbation timing and direction . Subsequent studies will use these findings as a reference not only for quantifying trunk stiffness and damping in individuals with various neuromuscular disorders , but also for assessing whether neuroprostheses could increase upper body stiffness and , hence , stability Tripping during gait occurs frequently . A successful balance recovery implies that the forward body rotation is sufficiently reduced . In view of this , adequate 
    Output:
    
When aim ing to assess trunk stabilization with system identification , we propose a perturbation method where the trunk is studied in isolation , the perturbation is unpredictable , force controlled , directly applied to the upper body , completely known and results in small fluctuations around the working point