State-dependent corrective reactions for backward balance losses during human walking

We investigated corrective reactions for backward balance losses during walking. Several biomechanical studies have suggested that backward falling can be predicted from the horizontal position and velocity of the body center of mass (COM) related to the stance foot. Our hypothesis was that corrective reactions for backward balance losses depend on whether the body moves forward or backward after a perturbation. Using a split-belt treadmill, backward balance losses during walking were induced by rapid decreases of belt speed from 3.5 km/h to 2.5, 2.0, 1.5 and 1.0 km/h. We measured kinematic data and surface electromyography (EMG) during corrective reactions while walking on the treadmill. Phase portrait analysis of COM trajectories revealed that backward balance stability was decreased by the perturbations. When the perturbed belt speed was 1.0 km/h, the COM states at toe-off were significantly lower than the stability limit; a rapid touch-down of the swing foot posterior to the stance foot then occurred, and the gait rhythm was modulated so that the phase advanced. EMG recordings during perturbed steps revealed a bilateral response, including modulation of the swing leg during the recovery. For weaker perturbations, the swing foot placements were anterior to the stance foot and there was a phase delay. In contrast to the bilateral responses for stronger perturbations, unilateral EMG responses were observed for weaker perturbations. The differences in joint kinematics and EMG patterns in the unperturbed swing leg depended on the COM states at toe-off, suggesting the existence of different responses consisting of ongoing swing movements and rapid touch-down. Thus, we conclude that corrective reactions for backward balance losses are not only phase-dependent but also state-dependent. In addition, the control system for backward balance losses predicts the feasibility of forward progression and modulates swing movement and walking rhythm according to backward balance stability.     

Stepping Responses in Young and Older Adults Following a Perturbation to the Support Surface During Gait

The objective of this work was to investigate the influence perturbation direction has on postural responses during overground gait, and whether these responses are age related. Differences in stepping patterns following perturbations of the support surface were examined in the frontal and sagittal planes during forward walking. Eleven young and 10 older adults completed Mini BESTest, hip strength tests, and 45 perturbed walking trials, triggered on heel contact. Lateral perturbations were more challenging to postural stability for both groups. Step length measures showed young adults recovered in the step proceeding the perturbation, while older adults needed additional steps to regain balance. Young adults arrested center of mass movement by producing larger step widths than older adults following the support surface perturbation.     

Kinematic adaptations to perturbations as a function of practice in rhythmic drawing movements

Kinematic adaptations in multijoint rhythmic drawing movements were investigated under unexpected perturbations in friction levels between stylus and writing surface. Changes in coupling and stability properties were assessed as a function of practice level by applying perturbations to subjects' dominant and nondominant limbs. Under nonperturbation and perturbation conditions, joint motions of right-handed subjects were highly coupled in the nondominant limb and uncoupled in the dominant limb. Stability analyses of the kinematic responses in the phase plane showed a relatively higher intrajoint resistance to perturbations in the nondominant limb as compared to the dominant limb for the elbow joint. indicating a decrease in global joint stiffness with practice. These changes in joint coupling and stiffness with practice were not observed for left-handed subjects. In addition, the stability to perturbations in the end-effector (stylus) kinematics was related to the amount of joint coupling in the nondominant limb, whereas in the dominant limb there existed no such coupling. It was concluded that (a) practice changes the responses to perturbations from anatomically specific early in practice to task-specific late in practice, and (b) this shift is related to the stability in the joint phase-plane dynamics, degree of coupling between joint angles, and the decoupling of the dynamics in the intrinsic and extrinsic control spaces.     

Effects of visual focus and gait speed on walking balance in the frontal plane

We investigated how head position and gait speed influenced frontal plane balance responses to external perturbations during gait. Thirteen healthy participants walked on a treadmill at three different gait speeds. Visual conditions included either focus downward on lower extremities and walking surface only or focus forward on a stationary scene with horizontal and vertical lines. The treadmill was positioned on a platform that was stationary (non-perturbed) or moving in a pattern that appeared random to the subjects (perturbed). In non-perturbed walking, medial–lateral upper body motion was very similar between visual conditions. However, in perturbed walking, there was significantly less body motion when focus was on the stationary visual scene, suggesting visual feedback of stationary vertical and horizontal cues are particularly important when balance is challenged. Sensitivity of body motion to perturbations was significantly decreased by increasing gait speed, suggesting that faster walking was less sensitive to frontal plane perturbations. Finally, our use of external perturbations supported the idea that certain differences in balance control mechanisms can only be detected in more challenging situations, which is an important consideration for approaches to investigating sensory contribution to balance during gait.     

Older adults exhibit variable responses in stepping behaviour following unexpected forward perturbations during gait initiation

With the socioeconomic burden associated with falls expected to rise as the average age of the Canadian population increases, research is needed to elucidate the nature of postural responses generated by older adults (OA) following a posture-destabilizing event. This knowledge is even more imperative for novel and difficult tasks, such as gait initiation (GI), a task known to pose a postural threat to stability for OA. A common technique to regain stability following an unexpected perturbation is reactive stepping. A deficiency in the execution of a reactive control strategy following a destabilizing event may be the cause of many unexpected falls in OA. The purpose of this study is to explore age related changes in the nature of these responses during a challenging GI task combined with an unexpected forward perturbation of the support surface. A total of 18 young adults (YA) and 16 OA performed 36 trials containing 20 unexpected perturbations. We calculated step width, length, time and COM velocity in the first unperturbed step and the second perturbed step. Results revealed that, during unperturbed GI, OA had a reduced forward velocity and took shorter, faster steps. Following forward perturbations, OA altered stepping patterns, perhaps to reduce single support duration, via reduced base of support and shorter step length compared to YA. Additionally, OA executed both forward and backwards directed steps however YA only generated forward steps. Regression analyses revealed that reduced forward velocity was predictive of step direction; which is possibly an unfavorable motor control strategy as OA who walk slower generated a posterior directed step immediately following the perturbation. This strategy is of concern as rapid responses by the trail limb are required to recover successfully, and these alterations may be associated with an elevated risk of falls.     

A concurrent attention-demanding task did not interfere with balance recovery function in standing and walking among young adults – An explorative laboratory study

This study aimed to explore the ability to overcome unannounced surface perturbations of different magnitudes during standing and walking under single-task and dual-task conditions. Balance recovery abilities during perturbed walking and concurrently performing cognitive tasks has rarely been investigated although it provides more ecological information in regard to real-life situations than perturbations during single-task conditions (i.e., just walking). Thirteen young adults were asked to perform: 1) a cognitive task while sitting; 2) perturbed standing; 3) a concurrent cognitive task during perturbed standing; 4) perturbed walking; and 5) a concurrent cognitive task during perturbed walking. The cognitive task was to perform number subtractions by seven. The participants were instructed to “try to avoid a fall” during the perturbation trials. Step threshold, cognitive task performance, and 3D kinematic analysis of the first recovery step, i.e., the spatiotemporal characteristics, were compared between all conditions. Step threshold and the spatiotemporal parameters of the first recovery stepping responses were similar between all task conditions. Cognitive performance was also unaffected by the postural challenges in all task conditions. These results suggest that the first balance recovery stepping response among young adults is automatic. Furthermore, young adults seem to have sufficient motor-cognitive resources to perform concurrently both balance recovery and cognitive tasks with no interference effects.     

“Posture first”: Interaction between posture and locomotion in people with low back pain during unexpectedly cued modification of gait initiation motor command

The ability to adapt anticipatory postural adjustments (APAs) in response to perturbations during single-joint movements is altered in people with chronic low back pain (LBP), but a comprehensive analysis during functional motor tasks is still missing. This study aimed to compare APAs and stepping characteristics during gait initiation between people with LBP and healthy controls, both in normal (without cue occurrence) condition and when an unexpected visual cue required to switch the stepping limb. Fourteen individuals with LPB and 10 healthy controls performed gait initiation in normal and switch conditions. The postural responses were evaluated through the analysis of center of pressure, propulsive ground reaction forces, trunk and whole-body kinematics, and activation onsets of leg and back muscles. During normal gait initiation, participants with LBP exhibited similar APAs and stepping characteristics to healthy controls. In the switch condition, individuals with LBP were characterized by greater mediolateral postural stability but decreased forward body motion and propulsion before stepping. The thorax motion was associated with forward propulsion parameters in both task conditions in people with LBP but not healthy controls. No between-group differences were found in muscle activation onsets. The results suggest that postural stability is prioritized over forward locomotion in individuals with LBP. Furthermore, the condition-invariant coupling between thorax and whole-body forward propulsion in LBP suggests an adaptation in the functional use of the thorax within the postural strategy, even in poor balance conditions.     

Some Characteristics of EMG Patterns During Locomotion

Myoelectric signals from several muscles of the lower limb were studied under various speed and stride length conditions. The main purpose was to determine invariant and variant features among these myoelectric patterns. A pattern recognition algorithm was used to analyze these activity patterns. Within-condition analysis revealed some common features among the EMG patterns. This suggests that the nervous system does not have to generate all the muscle activity patterns, only the common features that can, in appropriate combination, produce the necessary activity patterns. From the across condition analysis, the following rules emerged. First, both phasic component and magnitude (d.c. level) of the muscle activity patterns have to be modulated to meet the demands imposed by the various conditions. Second, the variability in the proximal muscle activity patterns across conditions are higher than the distal muscle activity patterns. Within each group, the extensor muscles and double-jointed muscles show greater variability than the flexor muscles and single-jointed muscles. And finally, the changes in the average value (d.c. level) of the muscle activity patterns across conditions are not uniform but show muscle and task specificity. For example, within the speed condition, the increase in d.c. level of the extensors with speed of locomotion show a proximal to distal trend. Based on these results, a conceptual model for the human locomotor control process is proposed.     

Some characteristics of EMG patterns during locomotion: implications for the locomotor control process

Myoelectric signals from several muscles of the lower limb were studied under various speed and stride length conditions. The main purpose was to determine invariant and variant features among these myoelectric patterns. A pattern recognition algorithm was used to analyze these activity patterns. Within-condition analysis revealed some common features among the EMG patterns. This suggests that the nervous system does not have to generate all the muscle activity patterns, only the common features that can, in appropriate combination, produce the necessary activity patterns. From the across condition analysis, the following rules emerged. First, both phasic component and magnitude (d.c. level) of the muscle activity patterns have to be modulated to meet the demands imposed by the various conditions. Second, the variability in the proximal muscle activity patterns across conditions are higher than the distal muscle activity patterns. Within each group, the extensor muscles and double-jointed muscles show greater variability than the flexor muscles and single-jointed muscles. And finally, the changes in the average value (d.c. level) of the muscle activity patterns across conditions are not uniform but show muscle and task specificity. For example, within the speed condition, the increase in d.c. level of the extensors with speed of locomotion show a proximal to distal trend. Based on these results, a conceptual model for the human locomotor control process is proposed.     

Harnessing the Power of a Novel Program for Dynamic Balance Perturbation with Supported Body Weight

Abstract

Self-initiated postural adjustments commonly occur in daily life. To accessibly measure this type of dynamic balance, we developed a simple computer program to induce virtual perturbations and combined it with a commercially available balance board and portable EMG system to measure resulting self-initiated postural adjustments. When performing perturbed balance tests, safety harness with body weight support (BWS) is often used. However, influences of these harnesses on postural reactions are not well known. This study investigated the sensitivity of our assessment tool under different BWS conditions and muscle responses during postural adjustments following perturbation at different directions. Fifteen neurologically intact participants performed self-initiated postural adjustments under conditions with: (1) no harness; (2) harness with no BWS; and (3) harness with 10% BWS. Postural adjustment time and muscle activities of the lower leg were measured. We observed significant increases in postural adjustment time in the harness with no BWS condition and differneces in lower leg muscles response to virtual perturbation. Our findings suggest that the combination of our customized program with EMG is a sensitive and convenient tool to measure postural adjustments that approximate real-world scenarios. This method can be used with light body weight support to ensure safety without influencing muscle synergies.     

Effects of plantar cutaneo-muscular and tendon vibration on posture and balance during quiet and perturbed stance

Modulation of lower limb somatosensory information by tendon or plantar vibration produces directionally specific, vibration-induced falling reactions that depend on the tendon or the region of the sole that is vibrated. This study characterized the effects of different patterns of plantar cutaneo-muscular vibration and bilateral Achilles tendon vibration (ATV) on the postural strategies observed during quiet and perturbed stance. Twelve healthy young participants stood barefooted, with their vision blocked, on two sets of plantar vibrators placed on two AMTI force plates embedded in a moveable support surface. Two other vibrators were positioned over the Achilles tendons. Participants were randomly exposed to different patterns of plantar cutaneo-muscular and ATV. Tilts of the support surface in the toes-up (TU) and toes-down (TD) directions were given 5-8 s after the beginning of vibration. Body kinematics in 3D and ground reaction forces were recorded. Bilateral ATV applied with or without rearfoot vibration (RFV) during quiet stance resulted in a whole-body backward leaning accompanied by an increase in trunk extension and hip and knee flexion. RFV alone produced a forward whole-body tilt with increased flexion in trunk, hip, and ankle. When stance was perturbed by TU tilts, the center of mass (CoM) and center of pressure (CoP) displacements were larger in the presence of RFV or ATV and associated with increased peak trunk flexion. TD tilts with or without ATV resulted in no significant difference in CoM and CoP displacements, while larger trunk extension and smaller distal angular displacements were observed during ATV. RFV altered the magnitude of the balance reactions, as observed by an increase in CoP displacements and variable response in trunk displacement. Significant interactions between ATV and RFV were obtained for the peak angular excursions for both directions of perturbations, where ATV either enhanced (for TU tilts) or attenuated (for TD tilts) the influence of RFV. Manipulating somatosensory information from the plantar cutaneo-muscular and muscle spindle Ia afferents thus results in altered and widespread postural responses, as shown by profound changes in body kinematics and CoM and CoP displacements. This suggests that the CNS uses plantar cutaneo-muscular and ankle spindle afferent inputs to build an appropriate reference of verticality that influences the control of equilibrium during quiet and perturbed stance.     

Responses of human ankle muscles to mediolateral balance perturbations during walking

During walking our balance is maintained by muscle action. In part these muscle actions automatically respond to the imbalance. This paper considers responses to balance perturbations in muscles around the ankle, peroneus longus (PL), tibialis anterior (TA) and soleus (SO). It is investigated if their action is related to previously observed balance mechanisms: the ‘braking reaction’ and the mediolateral ankle strategy.Subjects walked on a treadmill and received pushes to the left and pulls to the right in various phases of the gait cycle. Muscle actions were divided into medium latency R1 (100–150 ms), long latency R2 (170–250 ms), and late action R3 (270–350 ms). Short latency responses, before 100 ms, were not observed but later responses were prominent. With inward perturbations (e.g. pushes to the left shortly before or during stance of the right foot) responses in RPL were seen. The forward roll-over of the CoP was briefly stalled in mid stance, so that the heel was not lifted. Stance was shortened. With outward perturbations, pushes to the left shortly before or during stance of the left foot, responses in all three muscles, LTA, LSO, and LPL were seen. Our interpretation is that these muscle activations induce a ‘braking reaction’ but could also contribute to the ‘mediolateral ankle strategy’. The resultant balance correction is small but fast, and so diminishes the need for later corrections by the stepping strategy.     

Aging-related cocontraction effects during ankle strategy balance recovery following tether release in women

Increased antagonistic muscle activation during balance recovery has been documented during proprioceptive reliant responses in older adults. The authors examined ankle muscle cocontraction levels in young and older adults during balance recovery from a tether-release-induced, vestibular-input-reliant perturbation. Nine healthy young adult and 9 older women without history of falls performed maximum isometric plantar flexion and dorsiflexion trials followed by balance recovery trials using the ankle strategy. Surface electromyography data normalized to isometric conditions were analyzed during the 100 ms prior to release, the 50 ms immediately after release, and the 100-ms epochs from 100-400 ms following release. No differences existed in gastrocnemius and soleus activity levels (p > .05), though antagonistic tibialis anterior activity was greater in the older adults during the 300-400 ms epoch (young: 23.5 ± 5.8%, older: 38.7 ± 9.4%; p = .001). Vestibular-dominated perturbations may increase antagonistic activity during the recovery phase in older adults, inhibiting efficient balance recovery during proprioceptive and vestibular reliant perturbations.     

Functions and recruitment patterns of one- and two-joint muscles under isometric and walking conditions

Hypotheses advanced concerning the functions and advantages of the two-joint (and multi-joint) muscles in the lower limb include transferring energy, ease of control, muscle bulk reduction and decreased velocity of contraction. The aim of this investigation was to assess quantitatively the generality of one such suggestion seen in the literature. It was hypothesized that two-joint muscles would be recruited preferentially when they produced appropriate moments at the joints they crossed. This organizing strategy was used to partition the sagittal plane joint moment at the hip, knee and ankle between the one- and two-joint muscles crossing those joints. If the conditions of the strategy were not met, the moment was considered to be producted by one-joint muscles only. Ten representative muscles were modelled: tibialis anterior, soleus, gastrocnemius, short head of biceps femoris, vasti, rectus femoris, long head of biceps femoris, sartorius, gluteus maximus and iliopsoas. A number of static loading and walking conditions were recorded and then compared to simultaneously measured linear envelope EMG records of each activity. The joint moments were determined from a sagittal plane kinetic analysis using cinematography and measurements of the ground reaction force. Overall, the strategy partitioned the moment between the one- and two-joint muscles in accordance with the EMG records. The strategy tended to underestimate the contributions of the one-joint musculature, implying the existence of other important control strategies, such as cocontraction of antagonists for joint stability, or of synergistic activation to share the joint moment. It was, however, observed that predicted activity of two-joint musculature did agree well with recorded EMG activity.     

Neuromuscular effects of shifting the focus of attention in a simple force production task

Research on the focus of attention has begun exploring the physiological changes that underlie the difference between internal and external foci of attention. However, previous electromyography studies have used dynamic tasks, making it difficult to interpret electrophysiological data. The authors analyzed how the focus of attention affects a subject's ability to perform an isometric force production task (focus was directed either at the force platform or the muscles responsible for force production). Subjects received practice without attentional focus instructions and then completed blocks of trials with an external and internal attentional focus separately. An external focus led to significantly less error overall and reduced surface electromyography activity with lower median power frequencies in the antagonist muscle, but attentional focus had no effects on the agonist muscle. Thus, an external focus of attention led to more efficient motor unit recruitment patterns (reduced cocontraction) and improved performance. Posttest surveys revealed subjects were aware of their improved performance with an external focus.     

Task-dependent compensatory responses to perturbations applied during rhythmic movements in humans

Modulation of the responses to perturbation applied during different phases of three rhythmic movements in humans-running, cycling, and hopping-was studied. The perturbation was an electrical stimulus. The results showed gating and modulation of the responses in both ipsi- and contralateral limb muscles. The responses during running and cycling were only excitatory in nature, while during hopping an inhibitory response was observed. These responses were not correlated with the normal activity during the movement. The latency of the response in general was not altered for different stimulation phases. The alterations in the step cycle demonstrated overt behavioral changes due to the responses. There were differences between the responses observed during these movements and walking. In running, the major adaptation to perturbations appears to be in the contralateral side as seen in the changes in the step cycle. During cycling (except for one phase) and hopping, the same set of muscles was activated in response to perturbation. This represents a simplifying strategy in response organization. The dependency of the response on the task characteristics, postural stability requirement, and external constraints imposed on the subject is discussed. These studies provide insights into task-dependent strategies adopted by the nervous system to meet unexpected perturbation during rhythmic movements in humans.     

Task-Dependent Compensatory Responses to Perturbations Applied During Rhythmic Movements in Humans

Modulation of the responses to perturbation applied during different phases of three rhythmic movements in humans—running, cycling, and hopping—was studied. The perturbation was an electrical stimulus. The results showed gating and modulation of the responses in both ipsi-and contralateral limb muscles. The responses during running and cycling were only excitatory in nature, while during hopping an inhibitory response was observed. These responses were not correlated with the normal activity during the movement. The latency of the response in general was not altered for different stimulation phases. The alterations in the step cycle demonstrated overt behavioral changes due to the responses. There were differences between the responses observed during these movements and walking. In running, the major adaptation to perturbations appears to be in the contralateral side as seen in the changes in the step cycle. During cycling (except for one phase) and hopping, the same set of muscles was activated in response to perturbation. This represents a simplifying strategy in response organization. The dependency of the response on the task characteristics, postural stability requirement, and external constraints imposed on the subject is discussed. These studies provide insights into task-dependent strategies adopted by the nervous system to meet unexpected perturbation during rhythmic movement in humans.     

An Ecological Approach to Modeling Vision: Quantifying Form Perception Using the Circle Map Equation

Abstract

Object perception occurs within a dynamic world, where the environment and the observer (both body and eyes) are continually moving, shifting and changing. We seek to characterize and quantify this process from a perspective accounting for the interconnected system of motion in the environment, the perceiver and the eye, unfolding through time. Specifically, we build a mathematical representation for object perception based off the circle map equation. We describe an interaction between the eyes’ movement and the movement in the world, in order to better understand how those work together to result in perception. Across three experiments, we show that the stability of the relationship between object perception and complex eye movements can be perturbed and will have a predictable response to said perturbations. In so doing, we provide a different context – a dynamical systems framework – under which we can begin to consider the ecological validity of visual perception models, while recognizing the degree to which the visuo-spatial world is continuously being perturbed and disrupted. In fact, we postulate that such perturbations are capitalized on by the perceptual system, contributing to accurate object and motion identification.     

A Note on Constant Error Shifts in Post-Perturbation Responses

Response biasing was examined in the production of well-learned discrete timing responses. Interpolated movements consisted of trials which were briefly perturbed by an accelerating or decelerating force with subjects requested to amend the response in order to complete the trial successfully. Movement time analysis indicated that the response immediately following the perturbation trial demonstrated large biasing effects with the direction of the constant error shift a function of the direction of the perturbation. Responses following deceleration perturbations were produced too rapidly and those following acceleration perturbations were produced too slowly. Analysis of kinematic variables associated with these responses showed that post perturbation trials were characterized by systematic changes in peak acceleration and peak deceleration as well as the timing of these parameters. The biasing effects were temporary and showed other similarities to findings from short-term motor memory investigations. A number of differences were also noted along with methodological considerations for perturbation paradigms.     

A note on constant error shifts in post-perturbation responses

Response biasing was examined in the production of well-learned discrete timing responses. Interpolated movements consisted of trials which were briefly perturbed by an accelerating or decelerating force with subjects requested to amend the response in order to complete the trial successfully. Movement time analysis indicated that the response immediately following the perturbation trial demonstrated large biasing effects with the direction of the constant error shift a function of the direction of the perturbation. Responses following deceleration perturbations were produced too rapidly and those following acceleration perturbations were produced too slowly. Analysis of kinematic variables associated with these responses showed that post perturbation trials were characterized by systematic changes in peak acceleration and peak deceleration as well as the timing of these parameters. The biasing effects were temporary and showed other similarities to findings from short-term motor memory investigations. A number of differences were also noted along with methodological considerations for perturbation paradigms.