Biomechanical analysis of movement strategies in human forward trunk bending. II. Experimental study

 The large mass of the human upper trunk, its elevated position during erect stance, and the small area limited by the size of the feet, stress the importance of equilibrium control during trunk movements. The objective of the present study was to perform a biomechanical analysis of fast forward trunk movements in order to understand the coordination between movement and posture. The analysis is based on a comparison between experimentally observed bending and hypothetical “optimal bending” performed on an infinitely narrow support, as presented in a companion paper. The experimental data were obtained from 16 subjects who performed fast forward bending while standing on a wide platform or on a narrow beam. The analysis is performed by decomposition of the movement into three dynamically independent components, each representing a movement along one of the three eigenvectors of the motion equation. The eigenmovements are termed “hip”, “ankle”, and “knee” eigenmovements, according to the dominant joint. The experimentally observed movement is characterized mainly by the hip and ankle eigenmovements, whereas the knee eigenmovement is negligible. Similarly to the “optimal bending” the ankle eigenmovement starts earlier and lasts longer than the hip eigenmovement. An early forward acceleration of the center of gravity in the ankle eigenmovement is caused by anticipatory changes in the ankle joint torque. This clarifies the role of the early tibialis anterior burst and/or soleus inhibition usually observed in electromyographic recordings during forward bending. The results suggest that the hip and the ankle eigenmovements can be treated as independently controlled motion units aimed at functionally different behavioral goals: the bending per se and postural adjustment. It is proposed that the central nervous system has to control these motion units sequentially in order to perform the movement and maintain equilibrium. It is also suggested that the hip and ankle eigenmovements can be regarded as a biomechanical background for the hip and ankle strategies introduced by Horak and Nashner (1986) on the basis of electromyographic recordings and kinematic patterns in response to postural perturbations. Received: 1 July 1999 / Accepted in revised form: 23 October 2000     

Influence of an additional movement task for the upper extremities on equilibrium maintenance in frontal and sagittal level in standing man

The postural oscillations of standing man were studied during additional manual motor task that consisted of maintaining of the moving ball in the center of flat box. The movement of a center of pressure (CP) in frontal and sagittal plane were analyzed during standing on stable rigid support and on moving unstable support. The influence of the additional motor task on CP movement depend on level of support stability. Sagittal CP movement increased while the additional task was executed during standing on moving support but it did not when the support was stable. Frontal CP movement decreased when the additional task was executed during standing on stable support but it did not while the support was unstable. Thus execution of the additional motor task execution led to the reduction of efficacy of the postural control on the moving unstable support. This result suggests that the cortical influence on the postural mechanism was stronger during standing on moving support in comparison to the standing on the stable support.     

Specific and Nonspecific Visual Influences on the Stability of the Vertical Posture in Humans

We studied physiological mechanisms of vision-related stabilization of the vertical posture in humans using a stabilographic technique; spontaneous deviations of the projection of the center of gravity during quiet stance and magnitudes of the postural response to vibratory stimulation of proprioceptors of the lower leg muscles under varied conditions of visual control were measured. The stability of quiet stance, as estimated according to the root mean square value of the sagittal component of the stabilogram, was the best with eyes open. Vibration-induced postural responses were the smallest also under these conditions. Spontaneous postural sway and the amplitude of response to vibratory stimulation increased when only a central sector of visual field (20 ang. deg) was preserved and, especially, under conditions of closed eyes and horizontal inversion of visual perception using prismatic spectacles. Parallel changes in the quantitative stabilographic indices and amplitude of vibration-induced postural responses show that the intensity of the latter is probably determined by the background stiffness of the musculoskeletal system. We tried to estimate separately the contributions of the stiffness factor, on the one hand, and specific visual influences, on the other hand, by testing the parameters of quiet stance and postural responses under conditions of standing while lightly touching a support with the index finger. We found that the influence of the conditions of visual control on the stability of quiet stance while touching the support was eliminated. At the same time, the magnitude of postural responses to vibratory stimulation decreased but, nonetheless, changed with visual conditions in the same manner as when standing without additional support. We conclude that vision performs a dual function in the control of the vertical posture; it forms the basis for the spatial reference system and serves the source of information on the movements of one's body.     

The role of anticipatory postural adjustments in compensatory control of posture: 1. Electromyographic analysis

Anticipatory (APAs) and compensatory (CPAs) postural adjustments are the two principal mechanisms that the central nervous system uses to maintain equilibrium while standing. We studied the role of APAs in compensatory postural adjustments. Eight subjects were exposed to external predictable and unpredictable perturbations induced at the shoulder level, while standing with eyes open and closed. Electrical activity of leg and trunk muscles was recorded and analyzed during four epochs representing the time duration typical for anticipatory and compensatory postural control. No anticipatory activity of the trunk and leg muscles was seen in the case of unpredictable perturbations; instead, significant compensatory activation of muscles was observed. When the perturbations were predictable, strong anticipatory activation was seen in all the muscles: such APAs were associated with significantly smaller compensatory activity of muscles and COP displacements after the perturbations.The outcome of the study highlights the importance of APAs in control of posture and points out the existence of a relationship between the anticipatory and the compensatory components of postural control. It also suggests a possibility to enhance balance control by improving the APAs responses during external perturbations.     

Individuals with non-specific low back pain use a trunk stiffening strategy to maintain upright posture

There is increasing evidence that individuals with non-specific low back pain (LBP) have altered movement coordination. However, the relationship of this neuromotor impairment to recurrent pain episodes is unknown. To assess coordination while minimizing the confounding influences of pain we characterized automatic postural responses to multi-directional support surface translations in individuals with a history of LBP who were not in an active episode of their pain. Twenty subjects with and 21 subjects without non-specific LBP stood on a platform that was translated unexpectedly in 12 directions. Net joint torques of the ankles, knees, hips, and trunk in the frontal and sagittal planes as well as surface electromyographs of 12 lower leg and trunk muscles were compared across perturbation directions to determine if individuals with LBP responded using a trunk stiffening strategy. Individuals with LBP demonstrated reduced peak trunk torques, and enhanced activation of the trunk and ankle muscle responses following perturbations. These results suggest that individuals with LBP use a strategy of trunk stiffening achieved through co-activation of trunk musculature, aided by enhanced distal responses, to respond to unexpected support surface perturbations. Notably, these neuromotor alterations persisted between active pain periods and could represent either movement patterns that have developed in response to pain or could reflect underlying impairments that may contribute to recurrent episodes of LBP.     

Support surface related changes in feedforward and feedback control of standing posture

The aim of the study was to investigate the effect of different support surfaces on feedforward and feedback components of postural control. Nine healthy subjects were exposed to external perturbations applied to their shoulders while standing on a rigid platform, foam, and wobble board with eyes open or closed.Electrical activity of nine trunk and leg muscles and displacements of the center of pressure were recorded and analyzed during the time frames typical of feedforward and feedback postural adjustments. Feedforward control of posture was characterized by earlier activation of anterior muscles when the subjects stood on foam compared to a wobble board or a firm surface. In addition, the magnitude of feedforward muscle activity was the largest when the foam was used. During the feedback control, anterior muscles were activated prior to posterior muscles irrespective of the nature of surface. Moreover, the largest muscle activity was seen when the supporting surface was foam. Maximum CoP displacement occurred when subjects were standing on a rigid surface.Altering support surface affects both feedforward and feedback components of postural control. This information should be taken into consideration in planning rehabilitation interventions geared towards improvement of balance.     

The relationship between dynamic balancing ability and posture-related modulation of the soleus H-reflex

Soleus H-reflex reveals down modulation with increased postural difficulty. Role of this posture-related reflex modulation is thought to shift movement control toward higher motor centers in order to facilitate more precise postural control. Present study hypothesized that the ability to modulate H-reflex is related to one’s ability to dynamically balance while in an unstable posture. This study examined the relationship between dynamic balancing ability and soleus H-reflex posture-related modulation. Thirty healthy adults participated. The soleus maximal H-reflex (Hmax), motor response (Mmax), and background EMG activity (bEMG) were obtained during three postural conditions: prone, open-legged standing, and closed-legged standing. Hmax/Mmax ratios were normalized via the corresponding bEMG in order to remove the effects of background muscle activity from the obtained H-reflex. Reflex modulation was calculated as the ratio of the normalized Hmax/Mmax ratios in one postural condition to another posture in a more difficult condition. Dynamic balancing ability was assessed by testing stability while standing on a wobble board. A significant negative correlation was observed between balancing scores and reflex modulation from open-legged standing to closed-legged standing. This suggests that the ability to modulate monosynaptic stretch reflex excitability in response to a changing posture is a significant factor for dynamic balancing.     

Interaction between the vertical posture and movement control systems during a quick voluntary arm raise

The study was aimed at a deeper understanding of the interaction between the system of vertical posture control and the system of voluntary movement control based on the analysis of postural muscle activity components resulting from the action of the former or the latter system. For this purpose, a quick arm raise was performed in the standing and sitting positions with body fixation at different levels, when the task of maintaining a vertical posture was simplified or completely eliminated. Under these conditions, the muscle activity associated with posture control was supposed to change, while the activity of muscles raising the arm was supposed to remain invariable. The results showed that the simplification of the posture control resulted in a decrease or elimination of anticipatory changes in the activity of some muscles. However, most of the muscle activity variations were retained even in the sitting position, and these variations appeared simultaneously with the activity of muscles raising the arm. The so-called “anticipatory postural activity” during an arm raise in a normal standing position is supposed to consist of two components: an initial component reflecting the work of the posture control system and a later component reflecting the work of the movement control system. It is suggested that the planning of muscle activity and exchange of information between these two systems take place only before the beginning of the movement; after that, they act independently and in parallel.     

Neural control of quadrupedal and bipedal stance: implications for the evolution of erect posture

The transition among hominids from quadrupedalism to bipedalism resulted in modifications in their musculoskeletal morphology. It is unclear, however, whether changes in the circuitry of the CNS were also necessary in order to accommodate the unique balance requirements of two-limb support. This study addresses the issue of modifications in control strategies by investigating the rapid, automatic postural responses of feline and human subjects to sudden disturbances of balance in the anteroposterior (AP) direction while they stand quadrupedally and bipedally on movable platforms. Postural responses are characterized in terms of segmental adjustments, generated AP shear forces, and electromyographic activity. Feline and human subjects correct posture similarly when standing quadrupedally. Furthermore, both species correct stance primarily with their hindlimbs and use their forelimbs as supportive struts. In contrast, both species use completely different correctional strategies when standing bipedally. Morphological restrictions, however, prevent cats from adopting the pillar-like plantigrade posture of human beings. Thus, the correctional strategies of bipedal cats are distinct from those of bipedal human subjects. It is concluded that 1) automatic postural response patterns of quadrupedal Felis and bipedal Homo reflect the different biomechanical characteristics of the initial postures rather than species differences in CNS circuitry controlling stance; 2) hindlimb-dominated posture control is probably a common and relatively ancient pattern; and 3) reorganization of hominid CNS circuitry was probably unnecessary because hindlimb control was already a feature of the system.     

Postural changes associated with public speech tests lead to mild and selective activation of stress hormone release.

We tested whether simulation of postural changes, which occur during public speech test procedures, activates cardiovascular system and stress hormone release that could interfere with the effect of psychosocial stress load. Young healthy male volunteers (n=8) underwent procedure imitating exactly all postural changes present in the psychosocial stress model based on public speech used in this laboratory (namely changes from sitting to standing and repeated sitting). Postural changes were associated with increases in heart rate, blood pressure, plasma concentrations of noradrenaline and aldosterone and elevation in plasma renin activity. In contrast to cardiovascular parameters, adrenocorticotropic hormone, cortisol and adrenaline, the main characteristics of hormonal response during mental stress, were not significantly influenced. The overall magnitude of all observed alterations was much smaller than that seen following mental stress procedures in our previous studies. This study provides evidence that changes in body posture during public speech test procedure influence hemodynamics and endocrine responses in a mild manner. Though this influence may represent a source of unspecific variance, substantial confounding effects on responses to the psychosocial component of the procedure are unlikely. In any case, models combining mental stressors and changes in body posture must be interpreted as complex stress stimuli.     

Anticipation of direction and time of perturbation modulates the onset latency of trunk muscle responses during sitting perturbations

Trunk muscles are responsible for maintaining trunk stability during sitting. However, the effects of anticipation of perturbation on trunk muscle responses are not well understood. The objectives of this study were to identify the responses of trunk muscles to sudden support surface translations and quantify the effects of anticipation of direction and time of perturbation on the trunk neuromuscular responses. Twelve able-bodied individuals participated in the study. Participants were seated on a kneeling chair and support surface translations were applied in the forward and backward directions with and without direction and time of perturbation cues. The trunk started moving on average approximately 40 ms after the perturbation. During unanticipated perturbations, average latencies of the trunk muscle contractions were in the range between 103.4 and 117.4 ms. When participants anticipated the perturbations, trunk muscle latencies were reduced by 16.8 ± 10.0 ms and the time it took the trunk to reach maximum velocity was also reduced, suggesting a biomechanical advantage caused by faster muscle responses. These results suggested that trunk muscles have medium latency responses and use reflexive mechanisms. Moreover, anticipation of perturbation decreased trunk muscles latencies, suggesting that the central nervous system modulated readiness of the trunk based on anticipatory information.     

Biomechanical capabilities influence postural control strategies in the cat hindlimb

During postural responses to perturbations, horizontal plane forces generated by the cat hindlimb are stereotypically directed either towards or away from the animal's center of mass, independent of perturbation direction. We used a static, three-dimensional musculoskeletal model of the hindlimb to investigate possible biomechanical determinants of this "force constraint strategy." We hypothesized that directions in which the hindlimb can produce large forces are preferentially used in postural control. We computed feasible force sets (FFSs) based on hindlimb configurations of three cats during postural equilibrium tasks and compared them to horizontal plane postural force directions. The grand mean FFS was bimodal, with maxima near the posterior-anterior axis (-86+/-8 degrees and 71+/-4 degrees ), and minima near the medial-lateral axis (177+/-8 degrees and 8+/-8 degrees ). Experimental postural force directions clustered near both maxima; there were no medial postural forces near the absolute minimum. However, the medians of the anterior and posterior postural force direction histograms in the right hindlimb were rotated counter-clockwise from the FFS maxima (p<0.05; Wilcoxon signed-rank test). Because the posterior-anterior alignment of the FFS is consistent with a hindlimb structure optimized for locomotion, we conclude that the biomechanical capabilities of the hindlimb strongly influence, but do not uniquely determine the force directions observed in the force constraint strategy. Forces used in postural control may reflect a balance between a neural preference for using forces in the directions of large feasible forces and other criteria, such as the stabilization of the center of mass, and muscular coordination strategies.     

Blood pressure and blood flow variation during postural change from sitting to standing: model development and validation.

Short-term cardiovascular responses to postural change from sitting to standing involve complex interactions between the autonomic nervous system, which regulates blood pressure, and cerebral autoregulation, which maintains cerebral perfusion. We present a mathematical model that can predict dynamic changes in beat-to-beat arterial blood pressure and middle cerebral artery blood flow velocity during postural change from sitting to standing. Our cardiovascular model utilizes 11 compartments to describe blood pressure, blood flow, compliance, and resistance in the heart and systemic circulation. To include dynamics due to the pulsatile nature of blood pressure and blood flow, resistances in the large systemic arteries are modeled using nonlinear functions of pressure. A physiologically based submodel is used to describe effects of gravity on venous blood pooling during postural change. Two types of control mechanisms are included: 1) autonomic regulation mediated by sympathetic and parasympathetic responses, which affect heart rate, cardiac contractility, resistance, and compliance, and 2) autoregulation mediated by responses to local changes in myogenic tone, metabolic demand, and CO(2) concentration, which affect cerebrovascular resistance. Finally, we formulate an inverse least-squares problem to estimate parameters and demonstrate that our mathematical model is in agreement with physiological data from a young subject during postural change from sitting to standing.     

Postural adjustments to head movements in the cat

Head movements induced by motor cortex stimulation in the cat are accompanied by variations in the vertical force exerted by each limb. These postural responses were found to show stereotyped patterns: with head dorsiflexions an increase was observed in the force exerted by the anterior limbs and a decrease at the posterior limb level. From comparison between the latencies of the force variations, the beginning of head acceleration, and EMG activity in the limb extensor muscles, it was concluded that triggering of these postural responses is not reflex, but depends on the same command as the movement itself. This early response might be a means of avoiding the downward movement of the trunk which would otherwise result from the reaction force corresponding to the upward head movement.     

Somatosensory influence on postural response to galvanic vestibular stimulation

We investigated how postural responses to galvanic vestibular stimulation were affected by standing on a translating support surface and by somatosensory loss due to diabetic neuropathy. We tested the hypothesis that an unstable surface and somatosensory loss can result in an increase of vestibulospinal sensitivity. Bipolar galvanic vestibular stimulation was applied to subjects who were standing on a force platform, either on a hard, stationary surface or during a backward platform translation (9 cm, 4.2 cm/s). The intensity of the galvanic stimulus was varied from 0.25 to 1 mA. The amplitude of the peak body CoP displacement in response to the galvanic stimulus was plotted as a function of stimulus intensity for each individual. A larger increase in CoP displacement to a given increase in galvanic current was interpreted as an increase of vestibulospinal sensitivity. Subjects with somatosensory loss in the feet due to diabetes showed higher vestibulospinal sensitivity than healthy subjects when tested on a stationary support surface. Control subjects and patients with somatosensory loss standing on translating surface also showed increased galvanic response gains compared to stance on a stationary surface. The severity of the somatosensory loss in the feet correlated with the increased postural sensitivity to galvanic vestibular stimulation. These results showed that postural responses to galvanic vestibular stimulus were modified by somatosensory information from the surface. Somatosensory loss due to diabetic neuropathy and alteration of somatosensory input during stance on translating support surface resulted in increased vestibulospinal sensitivity.     

Evidence for an exposure-response relationship between trunk flexion and impairments in trunk postural control

Prolonged trunk flexion alters passive and active trunk tissue behaviors, and exposure-response relationships between the magnitude of trunk flexion exposure and changes in these behaviors have been reported. This study assessed whether similar exposure-response relationships exist between such exposures and impairments in trunk postural control. Twelve participants (6 M, 6 F) were exposed to three distinct trunk flexion conditions (and a no-flexion control condition), involving different flexion durations with/without an external load, and which induced differing levels of passive tissue creep. Trunk postural control was assessed prior to and immediately following trunk flexion exposures, and during 10 min of standing recovery, by tracking center of pressure (COP) movements during a seated balance task. All COP-based sway measures increased following each flexion exposure. In the anteroposterior direction, these increases were larger with increasing exposure magnitude, whereas such a relationship was not evident for mediolateral sway measures. All measures were fully recovered following 10 min of standing. The present results provide evidence for an exposure-response relationship between trunk flexion exposures and impairments in trunk postural control; specifically, larger impairments following increased exposures (i.e., longer flexion duration and presence of external load). Such impairments in trunk postural control may result from some combination of reduced passive trunk stiffness and altered/delayed trunk reflex responses, and are generally consistent with prior evidence of exposure-dependent alterations in trunk mechanical and neuromuscular behaviors assessed using positional trunk perturbations. Such evidence suggests potential mechanistic pathways through which trunk flexion exposures may contribute to low-back injury risk.     

Differential joint coordination in the tasks of standing up and sitting down

We studied similarities and differences in the use of goal-equivalent patterns of joint coordination to stand up and sit down from different support surfaces, performed without vision. Sagittal plane motion of major body segments was measured and joint angles for the left upper and lower extremities and the trunk were calculated. We used a modeling strategy relating motion in the redundant space of the joints to motion of individual performance variables, such as the center of mass (CM) or head, and determined how the variability of joint combinations across trials was structured; i.e. variations in joint combinations leading to a consistent value of a performance variable (goal-equivalent variability) and variations resulting in variability of the performance variable (non goal-equivalent variability). We found the variability of joint combinations to be selectively channeled into goal-equivalent directions, leading to stable horizontal motion of the CM and of the head, during both standing up and sitting down. In contrast, when evaluating the effect of joint combination variability on the control of vertical CM motion, we found differences in the variability components between standing up and sitting down. In general, more variable vertical CM motion occurred. An important finding was an enhanced use of goal-equivalent joint combinations under challenging task conditions, whether standing up or sitting down.     

Simulating mechanical consequences of voluntary movement upon whole-body equilibrium: the arm-raising paradigm revisited

Voluntary arm-raising movement performed during the upright human stance position imposes a perturbation to an already unstable bipedal posture characterised by a high body centre of mass (CoM). Inertial forces due to arm acceleration and displacement of the CoM of the arm which alters the CoM position of the whole body represent the two sources of disequilibrium. A current model of postural control explains equilibrium maintenance through the action of anticipatory postural adjustments (APAs) that would offset any destabilising effect of the voluntary movement. The purpose of this paper was to quantify, using computer simulation, the postural perturbation due to arm raising movement. The model incorporated four links, with shoulder, hip, knee and ankle joints constrained by linear viscoelastic elements. The input of the model was a torque applied at the shoulder joint. The simulation described mechanical consequences of the arm-raising movement for different initial conditions. The variables tested were arm inertia, the presence or not of gravity field, the initial standing position and arm movement direction. Simulations showed that the mechanical effect of arm-raising movement was mainly local, that is to say at the level of trunk and lower limbs and produced a slight forward displacement of the CoM (1.5 mm). Backward arm-raising movement had the same effect on the CoM displacement as the forward arm-raising movement. When the mass of the arm was increased, trunk rotation increased producing a CoM displacement in the opposite direction when compared to arm movement performed without load. Postural disturbance was minimised for an initial standing posture with the CoM vertical projection corresponding to the ankle joint axis of rotation. When the model was reduced to two degrees of freedom (ankle and shoulder joints only) the postural perturbation due to arm-raising movement increased compared to the four-joints model. On the basis of these results the classical assumption that APAs stabilise the CoM is challenged.     

Renal sympathetic nerve activity and arterial pressure responses to changes in postural position of conscious dogs

Experiments were conducted in conscious dogs to determine the relationships between postural position, arterial pressure, and renal sympathetic nerve activity. Observations of the changes in arterial pressure and renal nerve activity were made when animals spontaneously changed postural position from lying to sitting, sitting to standing, standing to sitting, and sitting to lying. Rising to sit from lying down increased arterial pressure from 109 +/- 5 to 125 +/- 3 mm Hg and increased renal nerve activity by 96 +/- 58 microV/sec (61% of control). Movement from the sitting to standing position decreased renal nerve activity by 90 +/- 39 microV/sec (48% of control) without changing mean arterial pressure. Sitting down from standing also did not change arterial pressure, whereas renal nerve activity increased by 56 +/- 17 microV/sec (33% of control). Returning to the lying position (from sitting) decreased arterial pressure, and this hypotension was associated with significant reductions in renal nerve activity. These results indicate that nonuniform changes in sympathetic outflow from the central nervous system must occur to various vascular beds during changes in postural position of conscious dogs. Thus, renal sympathetic outflow may or may not reflect changes in nerve traffic which contribute to alterations in arterial pressure.     

Responsiveness of the H reflex to loading and posture in patients following stroke

The objective of the research was to examine the effects of loading and posture on motoneuronal excitability of the triceps surae (TS) for patients with hemiplegia. Twelve healthy subjects and 12 patient subjects with post-stroke hemiparesis (onset period: 3–60 months) were enrolled in this study. The subjects were instructed to remain in quiet sitting with the test knee straight and three standing conditions of different superincumbent loads by shifting body weight to the test leg (10%, 50%, and 90% of body weight), while the H reflexes and M waves of the TS were measured. The results clearly indicated that H reflex amplitudes were not affected by different loading conditions in standing for both healthy subjects and patients who had a previous stroke. In addition, the H reflex amplitude in quiet standing for healthy subjects was significantly downward modulated relative to that in relaxed sitting with the test knee straight, but this posturally driven modulation was impaired in patients following stroke. Current electrophysiological findings imply that body weight as a means for rehabilitation facilitation had little immediate effect on paretic TS, and absence in postural gating of reflex excitability appeared to be an incentive for postural instability resulting from post-stroke hemiparesis.