Stance Perturbations Research Articles

Responses to brief perturbations of stance: EMG, midline cortical, and subcortical changes.

We studied simultaneous EMG and midline EEG responses, including over the cerebellum, in 10 standing subjects (35 ± 15 yr; 5 females, 5 males). Recordings were made following repeated taps to the sternum, stimuli known to evoke short-latency EMG responses in leg muscles, consistent with postural reflexes. EEG power had relatively more high-frequency components (>30 Hz) when recorded from electrodes over the cerebellum (Iz and SIz) compared with other midline electrodes. We confirmed a previous report using a similar stimulus that evoked short-latency potentials over the cerebellum. We showed clear midline-evoked EEG potentials occurring at short latency over the cerebellum (P23, N31, N42, and P54) and frontally (N28 and N57) before the previously described perturbation-evoked potential (P1/N1/P2). The P23 response correlated with the subsequent EMG response in the tibialis anterior muscles (r = 0.72, P = 0.018), confirming and extending previous observations. We did not find a correlation with the N1 amplitude. We conclude that early activity occurs from electrodes over the inion in response to a brief tap to the sternum. This is likely to represent cerebellar activity and it appears to modulate short-latency postural EMG responses.NEW & NOTEWORTHY We studied the effects of a brief tap to the sternum in human subjects, known to evoke short-latency postural responses. Using an extended EEG recording system, we showed early evoked responses over the midline cerebellum, including the P23 potential, which correlated with the EMG responses in tibialis anterior, consistent with a cerebellar role in postural reflexes. The stimulus also evoked later EEG responses, including the perturbation potential.

Neuromuscular mechanisms of motor adaptation to repeated treadmill-slip perturbations during stance in healthy young adults.

Treadmill-based repeated perturbation training (PBT) induces motor adaptation in reactive balance responses, thus lowering the risk of slip-induced falls. However, little evidence exists regarding intervention-induced changes in neuromuscular control underlying motor adaptation. Examining neuromuscular changes could be an important step in identifying key elements of adaptation and evaluating treadmill training protocols for fall prevention. Moreover, identifying the muscle synergies contributing to motor adaptation in young adults could lay the groundwork for comparison with high fall-risk populations. Thus, we aimed to investigate neuromuscular changes in reactive balance responses during stance slip-PBT. Lower limb electromyography (EMG) signals (4/leg) were recorded during ten repeated forward stance (slip-like) perturbations in twenty-six young adults. Muscle synergies were compared between early-training (slips 1-2) and late-training (slips 9-10) stages. Results showed that 5 different modes of synergies (named on dominant muscles: WTA, WS_VLAT, WR_GAS, WR_VLAT, and WS_GAS) were recruited in both stages. 3 out of 5 synergies (WTA, WR_VLAT, and WS_GAS) showed a high similarity (r>0.97) in structure and activation between stages, whereas WR_GAS and WS_VLAT showed a lower similarity (r<0.83) between the two stages, and the area of activation in WTA, the peak value of activation in WR_VLAT and the activation onset in WR_GAS showed a reduction from early-to late-training stage (p<0.05). These results suggest that a block of stance slip-PBT resulted in modest changes in muscle synergies in young adults, which might explain the smaller changes seen in biomechanical variables. Future studies should examine neuromuscular changes in people at high risk of falls.

Does footedness affect bilateral plantar flexor responses to sudden stance perturbations under unilateral lower leg muscular fatigue?

BACKGROUND: Analysis of performance asymmetries has indicated that the right and left cerebral hemispheres are specialized for specific functions of motor control. AIM: In the current investigation, we aimed to evaluate the effect of leg dominance on electrical activation of the plantar flexor muscles in responses to unanticipated stance perturbations in a state muscular fatigue of the dominant leg. METHOD: Fatigue was induced through ankle isometric contraction targeting 40% of maximum voluntary contraction. Muscle activation of the triceps surae of the dominant and non-dominant legs were compared in reactive responses to unanticipated load released from the trunk, leading to forward body sway. Muscular responses were analyzed in two states: pre-fatigue and fatigue of the triceps surae muscles of the dominant leg only. RESULTS: Analysis of magnitude of muscular activation for balance recovery following perturbations revealed fatigue-related compensatory activation in the lateral gastrocnemius muscle, as indicated by over-activation of the non-fatigued/non-dominant leg to compensate for the low muscular activation of the dominant/fatigued leg in the unilateral fatigue state. CONCLUSION: Compensatory behavior between the legs was not evident in the medial gastrocnemius and soleus muscles. Lack of effects related to leg dominance indicates that footedness did not affect automatic muscular responses either in the pre-fatigue or fatigue states.

Cognitive Task Domain Influences Cognitive-Motor Interference during Large-Magnitude Treadmill Stance Perturbations.

Reactive balance is postulated to be attentionally demanding, although it has been underexamined in dual-tasking (DT) conditions. Further, DT studies have mainly included only one cognitive task, leaving it unknown how different cognitive domains contribute to reactive balance. This study examined how DT affected reactive responses to large-magnitude perturbations and compared cognitive-motor interference (CMI) between cognitive tasks. A total of 20 young adults aged 18-35 (40% female; 25.6 ± 3.8 y) were exposed to treadmill support surface perturbations alone (single-task (ST)) and while completing four cognitive tasks: Target, Track, Auditory Clock Test (ACT), Letter Number Sequencing (LNS). Three perturbations were delivered over 30 s in each trial. Cognitive tasks were also performed while seated and standing (ST). Compared to ST, post-perturbation MOS was lower when performing Track, and cognitive performance was reduced on the Target task during DT (p < 0.05). There was a larger decline in overall (cognitive + motor) performance from ST for both of the visuomotor tasks compared to the ACT and LNS (p < 0.05). The highest CMI was observed for visuomotor tasks; real-life visuomotor tasks could increase fall risk during daily living, especially for individuals with difficulty attending to more than one task.

Older Adults with Pre-sarcopenia/Sarcopenia have Increased Gastrocnemius Muscle Activity to Maintain Upright Postural Control at Stance Perturbation

Older Adults with Pre-sarcopenia/Sarcopenia have Increased Gastrocnemius Muscle Activity to Maintain Upright Postural Control at Stance Perturbation

Between-leg asymmetry in automatic postural responses to stance perturbations in people with Parkinson's disease

BackgroundPeople with Parkinson's disease (PwPD) showed impairments of balance control which can be aggravated by the presence of higher interlateral postural asymmetry caused by a distinct dopaminergic loss in the substantia nigra between cerebral hemispheres. Research questionWe evaluate asymmetries between the more and the less affected leg in PwPD in responses to unanticipated stance perturbations. MethodsSixteen 16 PwPD participated in the experiment that consisted of recovering a stable upright stance, keeping the feet in place, in response to a perturbation caused by a sudden release of a load equivalent to 7 % of the participant's body mass. Anterior displacement and velocity of the center of pressure (CoP), the latency of gastrocnemius medialis muscle (GM) activation onset, rate of GM activation, and normalized magnitude of muscular activation were analyzed. ResultsAnalysis revealed significantly rate (p = 0.04) and magnitude (p = 0.02) higher activation of GM in the less affected limb. No significant effects of the leg were found for GM activation latency or CoP-related variables. SignificanceThere is a higher contribution of the less affected leg in automatic postural responses in PwPD.

Which lower limb joints compensate for destabilizing energy during walking in humans?

Current approaches to investigating stabilizing responses during locomotion lack measures that both directly relate to perturbation demands and are shared across different levels of description (i.e. joints and legs). Here, we investigated whether mechanical energy could serve as a 'common currency' during treadmill walking with transient unilateral belt accelerations. We hypothesized that by delivering perturbations in either early or late stance, we could elicit net negative or positive work, respectively, from the perturbed leg at the leg/treadmill interface, which would dictate the net demand at the overall leg level. We further hypothesized that of the lower limb joints, the ankle would best reflect changes in overall leg work. On average across all seven participants and 222 perturbations, we found early stance perturbations elicited no change in net work performed by the perturbed leg on the treadmill, but net positive work by the overall leg, which did not support our hypotheses. Conversely, late stance perturbations partially supported our hypotheses by eliciting positive work at the leg/treadmill interface, but no change in net work by the overall leg. In support of our final hypothesis, changes in perturbed ankle work, in addition to contralateral knee work, best reflected changes in overall leg work.

Association of balance control mechanisms with brain structural integrity in older adults with mild cognitive impairment

BackgroundOlder adults with mild cognitive impairment (OAwMCI) exhibit subtle balance control and gait deficits which are predominantly associated with structural brain pathologies such as impaired white matter integrity and reduced gray matter volume. However, the relationship between balance recovery mechanisms and neural substrates in OAwMCI remains unknown. This study thus aimed to explore the associations of volitional (self-initiated) and reactive balance (in response to an external perturbation) control with structural brain integrity. MethodsTen OAwMCI (MoCA 18-25/30; greater than 55 years) were examined on the limits of stability test (volitional balance via Equitest), stance perturbation test (reactive balance via ActiveStep treadmill) and underwent magnetic resonance imaging. Forward movement (frequently performed functional activity of daily living) was quantified by maximum excursion (maximum ability to shift one’s center of gravity toward the theoretical limit [MXE-%])and directional control (amount of movement exhibited towards the target proportional to the movement away from the target [DCL-%]) on the limits of stability test. Slip-like (prevalent type of accidental falls) perturbations were quantified by postural stability (shortest distance of the COM motion state, i.e., its position and velocity, to the theoretical boundary) on the ActiveStep treadmill. White matter integrity was quantified by fractional anisotropy (FA, movement of water molecules directionality) and gray matter volume measured in mm3. ResultsFor volitional balance control, reduced forward MXE was significantly (p < 0.05) associated with lower FA in left (R2 = 0.56) and right (R2 = 0.60) corticospinal tract, left (R2 = 0.49) and right (R2 = 0.51) corticothalamic tract, left (R2 = 0.70) and right (R2 = 0.57) frontopontine tract, right (R2 = 0.67) cingulum, anterior commissure (R2 = 0.82), and corpus callosum (R2 = 0.62). Reduced forward DCL was significantly (p < 0.05) associated with reduced gray matter volume in the left (R2 = 0.75) and right (R2 = 0.81) cerebellum, brainstem (R2 = 0.64), right (R2 = 0.49) thalamus. For reactive balance control, reduced postural stability (p < 0.05) was significantly associated with reduced FA in the left (R2 = 0.75) and right (R2 = 0.64) corticospinal tract, left (R2 = 0.67) and right (R2 = 0.65) frontopontine tract. Reduced postural stability was significantly (p < 0.05) associated with reduced gray matter volume in the brainstem (R2 = 0.72) and right cerebellum (R2 = 0.70). ConclusionOur results indicate that structural brain integrity influences stability control in OAwMCI for both volitional and reactive balance tasks, which may share some common cortico-subcortical motor pathways and relay centers. Results also show that the integrity of descending pathways from cortical attentional centers could influence stability control for both tasks.

Judokas Show Increased Resilience to Unpredictable Stance Perturbations

Combat sports are characterized by frequent large-scale stance perturbations that may lead to falls. In the present investigation, we compared compensatory arm and leg movements in response to unpredictable stance perturbations between judokas and other athletes whose sports present reduced balance demand, relative to combat sports. Specifically, we tested judokas (n = 9), and a group of swimmers and runners (n = 11, controls) in sudden support base displacements in the mediolateral direction, generated by a movable electronic platform, in the following modes: (a) rotation, (b) translation, and (c) combined rotation-translation. The platform was displaced to either side, in three peak velocities (cm/second or o/second) of 20 (low), 30 (moderate), or 40 (high), resulting in 18 distinct perturbations. We evaluated postural responses with a scale for analyzing the stability of compensatory arm and leg movements (CALM). Results showed that, in the most challenging perturbations, judokas had higher stability scores (arm, leg, and global) than did the comparison group. Higher scores for judokas reflected their increased rate of motionless arm and leg responses and absence of near-falls, compared to 30% falls in the most challenging perturbations for the swimmers and runners. As a practical application, judo training may help achieve stable compensatory limb movements in a way that parallels the benefits obtained from perturbation-based balance training in laboratory settings.

Postural control in response to unilateral and bilateral external perturbations in older adults

BackgroundBalance is an important determinant of physical function and falls risk. The ability to withstand external perturbations is important when walking on icy or uneven surfaces, whether the perturbations are bilateral or unilateral. Research questionThis study sought to determine the effect of unilateral and bilateral standing perturbations on leg muscle activity in healthy older adults. MethodsParticipants experienced unilateral and bilateral standing perturbations of the treadmill. Surface electromyography (EMG) from lower limb muscles was recorded unilaterally. EMG onset latency and root mean square (RMS) amplitude of the muscle bursts were calculated. ResultsOlder adults demonstrated a combined ankle/hip strategy, along with pre-activation and co-contraction of muscles in response to unilateral and bilateral stance perturbations. As well, older adults demonstrated higher levels of EMG, but no difference in the latency of burst onset, in bilateral than unilateral perturbation types. SignificanceWhen the stance limb was perturbed in the bilateral condition, the older adults responded with a Gastrocs EMG burst nearly 100% of the maximum EMG. The high level of EMG used, especially in the Gastrocs, during the bilateral perturbations may reduce the safety factor for falls in older adults. Older adults responded to the different perturbation demands by modulating EMG amplitude as opposed to the onset timing of EMG.

Application of neuromuscular electrical stimulation on the support limb during reactive balance control in persons with stroke: a pilot study.

The aim of the present study was to investigate the effect of the application of neuromuscular electrical stimulation to the quadriceps muscle of the paretic limb during externally induced stance perturbations on reactive balance control and on fall outcomes in people with chronic stroke. Ten participants experienced 12 stance treadmill perturbation trails, 6 forward balance perturbation trials and 6 backward balance perturbation trials. For each perturbation condition, three perturbation trials were delivered synchronized with neuromuscular electrical stimulation applied to the quadriceps of the paretic limb and three perturbation trials were delivered without stimulation. Behavioral outcome measures, such as incidence of laboratory falls and number of compensatory steps, kinematic outcome measures, such as margin of stability and minimum hip high values after the perturbation, step initiation time, step execution time and step length of the stepping leg were analyzed. The application of neuromuscular electrical stimulation on the paretic quadriceps between the range of 50 and 500ms after stance forward and backward perturbations reduced the laboratory falls incidence (p < 0.05), improved stability values (p < 0.05) and reduced the hip height descent (p < 0.05) compared to the experimental condition in which participants were exposed to stance perturbations without neuromuscular electrical stimulation. Additionally, step initiation time of the recovery step was lower in neuromuscular electrical stimulation condition during the forward balance perturbation protocol. Our results showed that the application of neuromuscular electrical stimulation on the knee extensor muscles of the paretic limb reduces the incidence of laboratory falls, enhances reactive stability control and reduces vertical limb collapse after stance forward and backward perturbations in people with chronic stroke.

Low back pain leads to a protective action of pain on dynamic postural stability

ABSTRACT Low back pain (LBP) is a painful manifestation in the lower part of the spine which causes disabilities changing the sensitivity of sensory neurons resulting in weakness of postural muscles interfering with the balance. It is not already clear if LBP people’s muscle changes affect the centre of pressure (CoP) recovery in challenging stance perturbations. The aim of this study was to identify differences in the muscle reactions of people with and without LBP to control CoP in challenging stance perturbations. We applied low (Vel1) and high (Vel2) magnitude stance perturbation by a movable platform and evaluated (a) the magnitude and latency time of trunk and lower limb muscle activation, (b) and the displacement and the latency time of the first CoP peak. The latency of trunk and hip muscle activation on Vel2 was lower for LBP group. The CoP latency and displacement did not change between groups and velocities indicating that the muscles took the same time to overcome the external forces. In conclusion, the maintenance of CoP latency at both velocities was not affected on Vel2, suggesting that such alterations have protective action and preservation of the pain on the LPB group in challenging stance perturbations.

Compensatory control between the legs in automatic postural responses to stance perturbations under single-leg fatigue.

In response to sudden perturbations of stance stability, muscles of both legs are activated for balance recovery. In conditions that one of the legs has a reduced capacity to respond, the opposite leg is predicted to compensate by responding more powerfully to restore stable upright stance. In this investigation, we aimed to evaluate between-leg compensatory control in automatic postural responses to sudden perturbations in a situation in which plantar flexor muscles of a single leg were fatigued. Young participants were evaluated in response to a series of perturbations inducing forward body sway, with a focus on activation of plantar flexor muscles: lateral and medial gastrocnemii and soleus. Muscular responses were analyzed through activation magnitude and latency of muscular activation onset. For evaluation of balance and postural stability, we also analyzed the center of pressure and upper trunk displacement and weight-bearing asymmetry between the legs. Responses were assessed in three conditions: pre-fatigue, under single-leg fatigue, and following the recovery of muscular function. Results showed (a) compensation of the non-fatigued leg through the increased magnitude of muscular activation in the first perturbation under fatigue; (b) adaptation in the non-fatigued leg over repetitive perturbations, with a progressive decrement of muscular activation over trials; and (c) maintenance of increased muscular activation of the non-fatigued leg following fatigue dissipation. These findings suggest that the central nervous system is able to modulate the descending motor drive individually for each leg's muscles apparently based on their potential contribution for the achievement of the behavioral aim of recovering stable body balance following stance perturbations.

Does Exercise-Based Conventional Training Improve Reactive Balance Control among People with Chronic Stroke?

Background: Exercise-based conventional training has predominantly benefited fall-associated volitional balance control domain; however, the effect on reactive balance control is under-examined. Therefore, the purpose of this study was to examine the effect of exercise-based conventional training on reactive balance control. Methods: Eleven people with chronic stroke (PwCS) underwent multi-component training for six weeks (20 sessions) in a tapering manner. Training focused on four constructs-stretching, functional strengthening, balance, and endurance. Volitional balance was measured via movement velocity on the Limits of Stability (LOS) test and reactive balance via center of mass (COM) state stability on the Stance Perturbation Test (SPT). Additionally, behavioral outcomes (fall incidence and/or number of steps taken) were recorded. Results: Movement velocity significantly increased on the LOS test (p < 0.05) post-intervention with a significant decrease in fall incidence (p < 0.05). However, no significant changes were observed in the COM state stability, fall incidence and number of recovery steps on the SPT post-intervention. Conclusion: Although volitional and reactive balance control may share some neurophysiological and biomechanical components, training based on volitional movements might not significantly improve reactive balance control for recovery from large-magnitude perturbations due to its task-specificity.

Adaptation of balancing behaviour during continuous perturbations of stance. Supra-postural visual tasks and platform translation frequency modulate adaptation rate.

When humans are administered continuous and predictable perturbations of stance, an adaptation period precedes the steady state of balancing behaviour. Little information is available on the modulation of adaptation by vision and perturbation frequency. Moreover, performance of supra-postural tasks may modulate adaptation in as yet unidentified ways. Our purpose was to identify differences in adaptation associated to distinct visual tasks and perturbation frequencies. Twenty non-disabled adult volunteers stood on a platform translating 10 cm in antero-posterior (AP) direction at low (LF, 0.18 Hz) and high frequency (HF, 0.56 Hz) with eyes open (EO) and closed (EC). Additional conditions were reading a text fixed to platform (EO-TP) and reading a text stationary on ground (EO-TG). Peak-to-peak (PP) displacement amplitude and AP position of head and pelvis markers were computed for each of 27 continuous perturbation cycles. The time constant and extent of head and pelvis adaptation and the cross-correlation coefficients between head and pelvis were compared across visual conditions and frequencies. Head and pelvis mean positions in space varied little across conditions and perturbation cycles but the mean head PP displacements changed over time. On average, at LF, the PP displacement of the head and pelvis increased progressively. Adaptation was rapid or ineffective with EO, but slower with EO-TG, EO-TP, EC. At HF, the head PP displacement amplitude decreased progressively with fast adaptation rates, while the pelvis adaptation was not apparent. The results show that visual tasks can modulate the adaptation rate, highlight the effect of the perturbation frequency on adaptation and provide evidence of priority assigned to pelvis stabilization over visual tasks at HF. The effects of perturbation frequency and optic flow and their interaction with other sensory inputs and cognitive tasks on the adaptation strategies should be investigated in impaired individuals and considered in the design of rehabilitation protocols.

Mixed slip-trip perturbation training for improving reactive responses in people with chronic stroke.

This study determined the effect of mixed (slip- and trip-like stance perturbation) training on reactive responses in people with chronic stroke (PwCS) and examined modulation of their reactive responses on higher intensity perturbations posttraining (scaling). Twelve PwCS were exposed to consecutive blocks of treadmill-based slip-like and trip-like perturbations and mixed-stance perturbations. A higher intensity trial was provided postblock and postmixed training. Postural stability [center-of-mass position (CoMP) and velocity (CoMV)], compensatory step length, step count, and trunk angle were examined. PwCS demonstrated an anterior positioning of the CoM, increased step length, and reduced compensatory step count with slip-like block training (P < 0.05). Trip-like block training resulted in reductions in step count, step length, and trunk angle (P < 0.05); however, CoMP remained unchanged (P > 0.05). With mixed training, there was a decrease rather than an increase in step length for slip-like perturbations but a continued decrease in step length and trunk angle was seen on trip-like perturbations (P < 0.05); however, CoMP and step count remained unchanged for both. For both perturbations, the higher intensity trials demonstrated no change from the last block trial. Postmixed block, the higher intensity trial demonstrated an increase only in step count on trip-like perturbation. Between postblock and postmixed higher intensity trials, an increase in step count and decrease in step length was noted only for slip-like perturbations. Block training with slip- and trip-like stance perturbations can enhance reactive responses among PwCS. Although mixed perturbation training continued to improve trip-induced adaptation, prior slip-induced adaptive changes were not maintained and further slip-adaptation was not seen. PwCS demonstrated partial scaling of reactive responses postblock and postmixed training.NEW & NOTEWORTHY Block perturbation training led to development of favorable reactive responses to counteract treadmill-based, slip-like and trip-like stance perturbations among people with chronic stroke. During mixed block, previously acquired adaptive changes in reactive responses from slip-block training were not maintained, probably due to interference offered by trip block. Instead, on trip-like perturbations, trip block-induced adaptation was maintained and continued to show further improvement. Our findings might provide future direction for designing effective mixed perturbation training paradigms to counteract both opposing perturbation types.

Acoustic cues and postural control

The effect of auditory input on postural control was evaluated in separate experiments performed in three groups of healthy volunteers. Auditory input took the form either of feedback signals generated by a force platform in response to the subject's postural control movements, or of field orientation (frame of reference) input provided by repeated clicks emitted by loudspeakers in a normally reverberative environment. The effect of these acoustic cues was measured in terms of body sway recorded on a force platform during stance perturbations induced by vibratory stimuli applied to the calf muscles either at low (120mW) or high (850 mW) intensity, the subject standing with eyes closed or open, as instructed. In the presence of feedback auditory input, body sway in response to low intensity vibratory stimulation was significantly reduced, but not that in response to high intensity stimulation. This may be due to the fact that the head and body movements induced by high intensity vibratory stimulation are so rapid and powerful that they override the information available or to the subject using other strategies for postural control in which auditory feedback, at least in the form used here, does not contribute useful information. The availability of field orientation input did not reduce body sway in response to vibratory stimulation at low intensity. This was probably due to the cognitive lag which precluded use being made of the input before the fast proprioceptive responses to vibratory stimulation had already occurred.

Evaluation of balance recovery stability from unpredictable perturbations through the compensatory arm and leg movements (CALM) scale

Following unpredictable large-magnitude stance perturbations diverse patterns of arm and leg movements are performed to recover balance stability. Stability of these compensatory movements could be properly estimated through qualitative evaluation. In the present study, we present a scale for evaluation of compensatory arm and leg movements (CALM) in response to unpredictable displacements of the support base in the mediolateral direction. We tested the CALM scale for intra- and inter-rater reliability, correlation with kinematics of arm and leg movement amplitudes, and sensitivity to mode (rotation, translation and combined) and magnitude (velocity) of support base displacements, and also to perturbation-based balance training. Results showed significant intra- and inter-rater coefficients of agreement, ranging from moderate (0.46–0.53) for inter-rater reliability in the arm and global scores, to very high (0.87–0.99) for inter-rater leg scores and all intra-rater scores. Analysis showed significant correlation values between scale scores and the respective movement amplitudes both for arm and leg movements. Assessment of sensitivity revealed that the scale discriminated the responses between perturbation modes, platform velocities, in addition to higher balance recovery stability as a result of perturbation-based balance training. As a conclusion, the CALM scale was shown to provide adequate integrative evaluation of compensatory arm and leg movements for balance recovery stability after challenging stance perturbations, with potential application in fall risk prediction.

Stumbling reactions in hypo and hyper gravity \u2013 muscle synergies are robust across different perturbations of human stance during parabolic flights

The control of bipedal stance and the capacity to regain postural equilibrium after its deterioration in variable gravities are crucial prerequisites for manned space missions. With an emphasize on natural orthograde posture, computational techniques synthesize muscle activation patterns of high complexity to a simple synergy organization. We used nonnegative matrix factorization to identify muscle synergies during postural recovery responses in human and to examine the functional significance of such synergies for hyper-gravity (1.75 g) and hypo-gravity (0.25 g). Electromyographic data were recorded from leg, trunk and arm muscles of five human exposed to five modes of anterior and posterior support surface translations during parabolic flights including transitional g-levels of 0.25, 1 and 1.75 g. Results showed that in 1 g four synergies accounted for 99% of the automatic postural response across all muscles and perturbation directions. Each synergy in 1 g was correlated to the corresponding one in 0.25 and 1.75 g. This study therefore emphasizes the similarity of the synergy organization of postural recovery responses in Earth, hypo- and hyper-gravity conditions, indicating that the muscle synergies and segmental strategies acquired under terrestrial habits are robust and persistent across variable and acute changes in gravity levels.

Stumbling Reactions in Partial Gravity - Neuromechanics of Compensatory Postural Responses and Inter-Limb Coordination During Perturbation of Human Stance.

Spontaneous changes in gravity play a significant role in interplanetary space missions. To preserve the astronauts’ capability to execute mission-critical tasks and reduce the risk of injury in transit and on planetary surfaces, a comprehensive understanding of the neuromuscular control of postural responses after balance deterioration in hypo- or hyper-gravity conditions is essential. Therefore, this study aimed to evaluate the effect of acute gravitational variation on postural adjustments in response to perturbations. Gravitational changes were induced using parabolic flight. Postural set was manipulated by randomly providing unilateral left, bilateral or split perturbations which require balance corrections to restore postural stability. In six subjects, postural reactions were recorded after anterior and posterior surface perturbations for progressively increased gravitational conditions spanning from 0.25 to 1.75 g. Ankle and knee joint kinematics and electromyograms (EMG) of eight leg muscles were recorded prior (PRE) and after perturbation onset. Muscle activation onset latencies and amplitudes in the short-, medium-, and long-latency responses (SLR, MLR, LLR) were assessed. Results demonstrate an increased muscle activity (p < 0.05) and co-contraction in the lower extremities (p < 0.05) prior to perturbation in hypo- and hyper-gravity. After perturbation, reduced muscle onset latencies (p < 0.05) and increased muscle activations in the MLR and LLR (p < 0.05), concomitant with an increased co-contraction in the SLR, were manifested with a progressive rise in gravity. Ankle and knee joint deflections remained unaffected, whereas angular velocities increased (p < 0.05) with increasing gravitation. Effects were more pronounced in bi- compared to unilateral or split perturbations (p < 0.05). Neuro-mechanical adaptations to gravity were more distinct and muscle onset latencies were shorter in the displaced compared to the non-displaced leg. In conclusion, the timing and magnitude of postural reflexes involved in stabilization of bipedal stance are gravity-dependent. The approximately linear relationship between gravity and impulse-directed EMG amplitudes or muscle onset latencies after perturbation indicates that the central nervous system correctly predicts the level of gravity. Moreover, it accurately governs contractions in the antigravity musculature to counterbalance the gravitational pull and to regain upright posture after its disturbance. Importantly, unilateral perturbations evoked fast reflex responses in the synergistic muscles of the non-displaced contralateral leg suggesting a synchronized inter-limb coordination mediated by spinal circuitries.