The role of anticipatory postural adjustments and gravity in gait initiation

This study analyzes the anticipatory postural adjustments which precede heel-off by considering the participation of the gravitational and muscular actions about the ankle joints during the gait initiation process. The resultant moment about the ankle joints and the gravitational moment were calculated using a biomechanical model in five normal subjects for three different speed conditions. The results show that the variations of these two moments are correlated to the velocity at the end of the first step. Nevertheless, a significant variation of the ankle joints moment occurs at the beginning of the anticipatory phase, whereas the gravity effect is still insignificant. These findings show how the successive controls of the muscular actions acting during the anticipatory movement and of the gravity action acting principally during the step execution allow the subject to reach the velocity which has been initially and centrally decided, by the end of the first step.     

Voluntary toe-walking gait initiation: electromyographical and biomechanical aspects

This study was carried out to investigate the biomechanical constraints of toe-walking gait initiation and the associated changes in motor program compared to the well documented heel-toe walking gait initiation. Seven healthy subjects volunteered for this study. Gait was initiated on a force platform, at three self-selected speed conditions, from an erect spontaneous posture in the control situation (CS) and from a posture with heels raised in the test situation (TS). Surface electromyographical (EMG) activities of muscles soleus (Sol) and tibialis anterior (TA) were recorded on both limbs. We analysed the consequences of the heel-off posture on EMG patterns and biomechanical activities. Whatever the speed condition, the centre of foot pressure was initially located more ahead of the ankle axis in TS than in CS, with an increased Sol and TA EMG activity. The EMG pattern which expresses the motor program governing gait initiation was modified in TS in comparison to CS and induced adaptations of the anticipatory postural adjustments (APA) that precede toe-off of the stepping limb. A lengthening of the APA duration allowed the subjects in TS to reach a gait velocity similar to the one obtained in CS at the end of the anticipatory movements and also at the end of the first step. In TS, the velocity of the centre of gravity at time of toe-off covaried, as in CS, with CG velocity at the end of the first step, still resulting from TA muscular actions during the APA but also probably from other combined muscular actions. Electronic Publication     

Phase-dependent modulations of anticipatory postural activity during human locomotion

1. The ability of the CNS to coordinate several motor tasks was studied in humans walking on a treadmill while pulling on a handle. Subjects were instructed to respond to an audio signal that was presented in different phases of the step cycle. Electromyograph (EMG) and movements were recorded from the left arm and leg. 2. The activity of the arm muscle was preceded by postural activity in the leg muscles. The pattern of the anticipatory postural activity differed in the various phases of the step cycle. Lateral gastrocnemius and hamstring muscles were activated during responses occurring in the early support phase whereas tibialis anterior and quadriceps muscles were activated when the pull was exerted during the late support phase and during the swing phase. In the middle of the support phase the combination of both muscle activity was present. 3. The temporal sequencing and the spatial distribution of the anticipatory muscle activity changed gradually. Early during the support phase the hamstring muscles were activated before the gastrocnemius muscle, whereas the order was reversed during midstance. The EMG amplitude of the hamstring and gastrocnemius muscles was largest in the beginning of the support phase and then gradually decreased, whereas the amplitude of the tibialis anterior and quadriceps muscles increased during the later parts of the support phase. 4. The anticipatory responses to pulls exerted during the first part of the support phase reduced the ankle flexion during the single support phase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pain reported during prolonged standing is associated with reduced anticipatory postural adjustments of the deep abdominals

Within the context of low back pain, the measurement of deep abdominal anticipatory postural adjustments (APAs) during rapid limb movement has received much interest. There is dispute about the association between APAs and back pain. Moreover, there is limited evidence examining compensatory postural adjustments (CPAs) in back pain. This study examined the relationship between APAs and CPAs with pain reported in the low back during 2 h of prolonged standing. Twenty-six participants with no history of severe back pain performed 2-h prolonged standing. APAs and CPAs of the deep abdominal muscles (transverse abdominis/internal obliques) were measured by surface electromyography during rapid shoulder flexion and extension. APAs and CPAs measured pre-standing revealed symmetrical anticipatory activity, but an asymmetry between the different sides of the abdominal wall for CPAs. APAs and CPAs measured pre-standing were not associated with pain reported during standing. For the whole group, APA amplitudes were reduced post-standing during shoulder flexion (p = 0.005). Pain reported during standing was associated with the changes in APA amplitudes post-standing (rs = 0.43, p = 0.002). These findings support previous research using hypertonic saline injections to induce back pain that showed reduced APA amplitudes, and extends findings to suggest pain does not effect compensatory postural adjustments.     

Effect of lower limb muscle fatigue on anticipatory postural adjustments associated with bilateral-forward reach in the unipedal dominant and non-dominant stance

Voluntary arm movements are preceded by dynamical and electromyographical (EMG) phenomena in “postural segments” (i.e. body segments not directly involved in the voluntary movement) called “anticipatory postural adjustments” (APA). The present study examined how the central nervous system organizes APA under fatigued state of postural musculature elicited by series of high-level isometric contractions (HIC), i.e. corresponding to 60% of maximal voluntary contraction. Subjects (N = 14) purposely performed series of bilateral-forward reach task (BFR) under unipodal stance (dominant and non-dominant) before (“no fatigue” condition, NF) and after (“fatigue” condition, F) a procedure designed to obtain major fatigue in hamstrings. Centre-of-gravity acceleration, centre-of-pressure displacement, and electrical activity of trunk and leg muscles were recorded and quantified within a time-window typical of APA. Results showed that there was no significant effect of fatigue on the level of muscle excitation and APA onset in any of the postural muscles recorded. Similarly, no change in APA onset could be detected from the biomechanical traces. In contrast, results showed that the amplitude of anticipatory centre-of-pressure displacement and centre-of-gravity acceleration reached lower value in F than in NF. Similar results were obtained whether dominant or non-dominant leg was considered. The changes in biomechanical APA features could not be ascribed to a different focal movement performance (maximal BFR velocity and acceleration) between F and NF. These results suggest that, when fatigue is induced by HIC, the capacity of the central nervous system to adapt APA programming to the fatigued state of the postural muscle system might be altered.     

Influence of fear of falling on anticipatory postural control of medio-lateral stability during rapid leg flexion

During leg flexion from erect posture, postural stability is organized in advance during “anticipatory postural adjustments” (APA). During these APA, inertial forces are generated that propel the centre of gravity (CoG) laterally towards stance leg side. This study examined how fear of falling (FoF) may influence this anticipatory postural control of medio-lateral (ML) stability. Ten young healthy participants performed a series of leg flexions at maximal velocity from low and high surface heights (6 and 66 cm above ground, respectively). In this latter condition with increased FoF, stance foot was placed at the lateral edge of the support surface to induce maximal postural threat. Results showed that the amplitude of ML inertial forces generated during APA decreased with FoF; this decrease was compensated by an increase in APA duration so that the CoG position at time of swing foot-off was located further towards stance leg side. With these changes in ML APA, the CoG was propelled in the same final (unipodal) position above stance foot as in condition with low FoF. These results contrast with those obtained in the literature during quiet standing which showed that FoF did not have any influence on the ML component of postural control. It is proposed that ML APA are modified with increased FoF, in such a way that the risk of a sideway fall induced by the large CoG motion is attenuated.     

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

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

Fatigue-induced adaptive changes of anticipatory postural adjustments

To examine the fatigue-induced adaptive changes (e.g., timing) of anticipatory postural adjustments (APAs), APAs of 30 research participants were recorded before (baseline) and after (post-test) conditions of either rest (control group, n = 15) or fatigue (fatigue group, n = 15). Muscle fatigue was generated using a dead-lift exercise performed to exhaustion. Self-initiated postural perturbations were induced using a rapid unilateral arm-raising maneuver (focal movement), and APAs were obtained using electromyography (EMG) recorded bilaterally in the lumbar and thoracic paraspinal muscles as well as the hamstring muscles. Postural stability during the focal movement was assessed using a force plate. Results showed that fatigue had no effect on postural stability during the focal movement, and yet caused earlier APA onsets in three of the six muscles evaluated. In spite of early APA activation, the APA EMG integrals of two of the three postural control muscles which exhibited fatigue-induced early APA onsets (T9 and L4 contralateral paraspinals) did not differ between baseline and post-test measures. The findings suggest that early APA onset may enhance postural stability by permitting a longer duration APA which can counteract fatigue-induced decreases in the force-producing capability of muscles that contribute to postural stability.     

Environmental constraints on foot trajectory reveal the capacity for modulation of anticipatory postural adjustments during rapid triggered stepping reactions

This study used environmental restrictions on foot movement to challenge the capacity of the central nervous system (CNS) to counter the lateral instability that arises after foot-lift during rapid triggered stepping reactions evoked by unpredictable postural perturbation. The objective was to determine the extent to which lateral stability could be regulated via modulation of the mediolateral (m-l) anticipatory postural adjustment (APA) that precedes foot-lift. A high frontal obstacle was used to double the required swing duration, and thereby increase the potential for the center of mass (COM) to fall laterally toward the unsupported side, during forward-step reactions. The capacity to use lateral step placement to recover lateral stability was restricted by means of lateral barriers. Six healthy young adults were tested. In obstacle-only trials, the APA was insufficient to prevent increased lateral COM motion during the prolonged swing phase; hence, lateral step placement was necessitated. However, when lateral stepping was obstructed, the CNS was able to upregulate the APA amplitude so as to prevent this increase in lateral COM motion. The swing foot was placed medially, with no detriment to clearing the frontal obstacle or recovering equilibrium. There was no change in step timing or anteroposterior (a-p) COM motion. While previous studies have suggested that the a-p COM progression may determine the extent to which the m-l APA is expressed or truncated during triggered stepping reactions evoked by unpredictable perturbation, the present findings demonstrate that prior knowledge of environmental demands can lead to predictive efforts to modulate the APA during such reactions. An apparent preference to underscale anticipatory efforts when lateral step placement is permitted suggests that the CNS may be acting to avoid some potential risk or cost associated with the execution of a large APA.     

Anticipatory postural adjustments and the latency of compensatory stepping reactions in humans

A compensatory stepping response is a commonly used strategy in recovering balance control after a postural perturbation. Unlike gait initiation, the compensatory stepping often occurs without an anticipatory postural adjustment (APA), in which body weight is shifted to the swing leg first and then back to the stance leg prior to foot lifting. In postural perturbation studies using a moving platforms stepping responses without an APA were found to have shorter latency to foot lifting than trials with an APA. We studied stepping responses of healthy young adults under postural perturbation of a pulling force impulse on the subject's waist. In contrast to previous studies, the latency of foot lifting was found in the current study to be shorter in the trials with an APA than trials without an APA. Furthermore, greater amplitude of an APA was associated with a shorter latency of foot lifting. Response with an APA of large amplitude may indicate high level of determinant for foot lifting. A pause as to whether or not to initiate/complete a stepping response is suggested to be partially the cause of delayed foot lifting in trials without an APA or with small amplitude of the APA.     

Strategies for the integration of posture and movement during reaching in the cat

We have examined the relationship between the movement and the anticipatory postural adjustments (APAs) that precede that movement during a reaching task in the cat. We recorded ground reaction forces in all 3 planes from all 4 limbs as well as electromyographic (EMG) activity from limb and axial muscles. The reaching movement was always preceded by an APA that was characterized by a loading of the reaching forelimb and an unloading of the support forelimb. This loading of the reaching forelimb was preceded, and accompanied, by increased activity in shoulder and limb extensor muscles of the reaching limb; extensor muscle activity in the supporting limb was simultaneously decreased. An important finding from this study was that the onset of the APA and of the movement was temporally decoupled. Analyses of the onset of EMG activity showed that most of the muscles that we recorded could be classified as either related to the APA or related to the movement. These results support the idea of distributed, and perhaps independent, systems for the execution of the APA and of the prime movement. There was also postural activity in the supporting limb during the movement. Analysis of this activity, which is also anticipatory in nature, suggests that it was tightly linked to the movement. We suggest that this postural response is signaled as part of the command for movement. Some muscles, particularly the extensors of the reaching limb, received convergent input from the command signals for the APA and for the movement.     

Decreased variability in postural control strategies in young people with non-specific low back pain is associated with altered proprioceptive reweighting

Optimal postural control is an essential capacity in daily life and can be highly variable. The purpose of this study was to investigate if young people have the ability to choose the optimal postural control strategy according to the postural condition and to investigate if non-specific low back pain (NSLBP) influences the variability in proprioceptive postural control strategies. Young individuals with NSLBP (n?=?106) and healthy controls (n?=?50) were tested on a force plate in different postural conditions (i.e., sitting, stable support standing and unstable support standing). The role of proprioception in postural control was directly examined by means of muscle vibration on triceps surae and lumbar multifidus muscles. Root mean square and mean displacements of the center of pressure were recorded during the different trials. To appraise the proprioceptive postural control strategy, the relative proprioceptive weighting (RPW, ratio of ankle muscles proprioceptive inputs vs. back muscles proprioceptive inputs) was calculated. Postural robustness was significantly less in individuals with NSLBP during the more complex postural conditions (p?<?0.05). Significantly higher RPW values were observed in the NSLBP group in all postural conditions (p?<?0.05), suggesting less ability to rely on back muscle proprioceptive inputs for postural control. Therefore, healthy controls seem to have the ability to choose a more optimal postural control strategy according to the postural condition. In contrast, young people with NSLBP showed a reduced capacity to switch to a more multi-segmental postural control strategy during complex postural conditions, which leads to decreased postural robustness.     

Systematic review of postural control and lateral ankle instability, part I: can deficits be detected with instrumented testing

Objective:

To answer the following clinical questions: (1) Is poor postural control associated with increased risk of a lateral ankle sprain? (2) Is postural control adversely affected after acute lateral ankle sprain? (3) Is postural control adversely affected in those with chronic ankle instability?

Data Sources:

PubMed and CINAHL entries from 1966 through October 2006 were searched using the terms ankle sprain, ankle instability, balance, chronic ankle instability, functional ankle instability, postural control, and postural sway.

Study Selection:

Only studies assessing postural control measures in participants on a stable force plate performing the modified Romberg test were included. To be included, a study had to address at least 1 of the 3 clinical questions stated above and provide adequate results for calculation of effect sizes or odds ratios where applicable.

Data Extraction:

We calculated odds ratios with 95% confidence intervals for studies assessing postural control as a risk factor for lateral ankle sprains. Effect sizes were estimated with the Cohen d and associated 95% confidence intervals for comparisons of postural control performance between healthy and injured groups, or healthy and injured limbs, respectively.

Data Synthesis:

Poor postural control is most likely associated with an increased risk of sustaining an acute ankle sprain. Postural control is impaired after acute lateral ankle sprain, with deficits identified in both the injured and uninjured sides compared with controls. Although chronic ankle instability has been purported to be associated with altered postural control, these impairments have not been detected consistently with the use of traditional instrumented measures.

Conclusions:

Instrumented postural control testing on stable force plates is better at identifying deficits that are associated with an increased risk of ankle sprain and that occur after acute ankle sprains than at detecting deficits related to chronic ankle instability.     

Perturbations of ground support alter posture and locomotion coupling during step initiation in Parkinson��s disease

During the initiation of stepping, anticipatory postural adjustments (APAs) for lateral weight transfer and propulsion normally precede the onset of locomotion. In Parkinson's disease (PD), impaired step initiation typically involves altered APA ground force production with delayed step onset and deficits in stepping performance. If, as in stance and gait, sensory information about lower limb load is important for the control of stepping, then perturbations influencing loading conditions could affect the step initiation process. This study investigated the influence of changes in lower limb loading during step initiation in patients with PD and healthy control subjects. Participants performed rapid self-triggered step initiation with the impending single stance limb positioned over a pneumatically actuated platform. In perturbation trials, the stance limb ground support surface was either moved vertically downward (DROP) or upward (ELEVATE) by 1.5 cm shortly after the onset of the APA phase. Overall, PD patients demonstrated a longer APA duration, longer time to first step onset, and slower step speed than controls. In both groups, the DROP perturbation reinforced the intended APA kinetic changes for lateral weight transfer and resulted in a significant reduction in APA duration, increase in peak amplitude, and earlier time to first step onset compared with other conditions. During ELEVATE trials that opposed the intended weight transfer forces both groups rapidly adapted their stepping to preserve standing stability by decreasing step length and duration, and increasing step height and foot placement laterally. The findings suggested that sensory information associated with limb load and/or foot pressure modulates the spatial and temporal parameters of posture and locomotion components of step initiation in interaction with a centrally generated feedforward mode of neural control. Moreover, impaired step initiation in PD may at least acutely be enhanced by augmenting the coupling between posture and locomotion.     

Anticipatory adjustments of multi-finger synergies in preparation for self-triggered perturbations

We studied changes in multi-finger synergies associated with predictable and unpredictable force perturbations applied to a finger during a multi-finger constant total force production task. The main hypothesis was that indices of multi-finger synergies can show anticipatory changes in preparation for a predictable perturbation. Subjects sat in a chair and pressed on force sensors with the four fingers of the right hand. The task was to produce a constant level of total force. The fingers acted against loads that produced upward directed forces. The loads (applied either to the index or to the ring finger) could be disengaged either by the subject or by the experimenter. An index of finger co-variation, ΔV was computed across sets of 12 trials at each time sample and for all tasks separately. During steady-state force production, all subjects showed positive ΔV values corresponding to strong negative covariation among finger forces interpreted as a force-stabilizing synergy. Prior to self-triggered unloading, subjects showed an anticipatory drop in ΔV that started 100–125 ms prior to the unloading time. Such early changes were absent in trials with experimenter-triggered unloading. After an unloading, subjects changed forces of both perturbed and unperturbed fingers and reached a new sharing pattern of the total force. In experimenter-triggered conditions, changes in the forces of unperturbed fingers could be seen as early as 120 ms following an unloading. The index ΔV dropped following a perturbation and then recovered; the recovery occurred faster in self-triggered conditions. We conclude that humans can use feed-forward changes in multi-finger synergies (anticipatory synergy adjustments) in anticipation of a predictable perturbation. These changes may help avoid prolonged weakening of a multi-digit force-stabilizing synergy. We discuss a possibility that anticipatory postural adjustments may represent a particular case of the phenomenon of anticipatory synergy adjustments and suggest a hierarchical control scheme that incorporates a possibility of independent control over the output of a multi-element system and covariation patterns among outputs of its elements.     

Why anticipatory postural adjustments in gait initiation need to be modified when stepping up onto a new level?

The study examined why anticipatory postural adjustments (APA) associated to gait initiation in a stepping up to a new level situation (SU) are reduced as compared to a level walking situation (LW), as previously reported. Five young adults performed gait initiation in both situations at normal and fast speed. Data from a force platform provided gait parameters related to the motion of the body's centre of mass (CM) on the anteroposterior (progression) and vertical axes. The electromyographic activity of the soleus of the stance limb (SOst) and the vastus lateralis of the swing limb (VLsw) were analyzed prior to and after the onset of the double stance phase. The results showed that APA and progression CM velocity at the time of foot contact were smaller in SU, whereas the peak of this velocity was similar in both situations. Thus, the change in progression velocity during the double stance phase had to be greater in SU than in LW. In both velocity conditions, the activity of SOst stopped after the time of foot contact in both situations, but clearly later in SU. So, this ankle plantar flexor muscle would be involved not only in the change of body lift but also in forward CM progression. The latter role of this muscle brought supporting evidence for the reduction of APA in SU, enabling the peak of progression velocity to be similar in both situations. Only in SU, the timing of activation of VLsw and deactivation of SOst strongly co-varied, showing the implementation of a motor synergy to fulfil the new requirements of the task, i.e. body lift.     

Systematic Review of Postural Control and Lateral Ankle Instability,Part II: Is Balance Training Clinically Effective

Objective:

To answer the following clinical questions: (1) Can prophylactic balance and coordination training reduce the risk of sustaining a lateral ankle sprain? (2) Can balance and coordination training improve treatment outcomes associated with acute ankle sprains? (3) Can balance and coordination training improve treatment outcomes in patients with chronic ankle instability?

Data Sources:

PubMed and CINAHL entries from 1966 through October 2006 were searched using the terms ankle sprain, ankle instability, balance, chronic ankle instability, functional ankle instability, postural control, and postural sway.

Study Selection:

Only studies assessing the influence of balance training on the primary outcomes of risk of ankle sprain or instrumented postural control measures derived from testing on a stable force plate using the modified Romberg test were included. Studies had to provide results for calculation of relative risk reduction and numbers needed to treat for the injury prevention outcomes or effect sizes for the postural control measures.

Data Extraction:

We calculated the relative risk reduction and numbers needed to treat to assess the effect of balance training on the risk of incurring an ankle sprain. Effect sizes were estimated with the Cohen d for comparisons of postural control performance between trained and untrained groups.

Data Synthesis:

Prophylactic balance training substantially reduced the risk of sustaining ankle sprains, with a greater effect seen in those with a history of a previous sprain. Completing at least 6 weeks of balance training after an acute ankle sprain substantially reduced the risk of recurrent ankle sprains; however, consistent improvements in instrumented measures of postural control were not associated with training. Evidence is lacking to assess the reduction in the risk of recurrent sprains and inconclusive to demonstrate improved instrumented postural control measures in those with chronic ankle instability who complete balance training.

Conclusions:

Balance training can be used prophylactically or after an acute ankle sprain in an effort to reduce future ankle sprains, but current evidence is insufficient to assess this effect in patients with chronic ankle instability.     

Cardiac Cycle Time Effects on Performance, Phasic Cardiac Responses, and Their Intercorrelation in Choice Reaction Time

Temporal relationships between phasic cardiac responses and choice reaction time (RT) performance were studied in 8 human volunteers using a speed-accuracy tradeoff design. “Cardiac cycle time” was varied by presenting stimuli at either the R wave of the electrocardiogram or 350 msec afterward. The speed and accuracy of choice RT performance were systematically manipulated by rewarding subjects for responding as close as possible to five different RT targets (150, 200, 250, 300, and 350 msec). The magnitude of both the cardiac deceleration which preceded stimulus onset and the cardiac acceleration which followed the response varied systematically with RT; both anticipatory deceleration and accelerative recovery were larger for faster RTs. More importantly, the timing of the shift from anticipatory deceleration to accelerative recovery varied systematically as a function of both cardiac cycle time and RT. Correlations between performance and cardiac interbeat intervals (IBIs) before, during, and after the RT were uniformly low, but showed consistent variation as a function of temporal proximity of the IBI to the stimulus. An hypothesis which relates timing of task completion, timing of vagal inhibition, and phasic cardiac responses was proposed to account for the timing of the shift from anticipatory deceleration to accelerative recovery.     

Directional specificity of postural muscles in feed-forward postural reactions during fast voluntary arm movements

Healthy subjects performed bilateral fast shoulder movements in different directions while standing on a force platform. Anticipatory postural adjustments were seen as changes in the electrical activity of postural muscles as well as displacements of the center of pressure and center of gravity. Postural muscle pairs of agonist-antagonist commonly demonstrated triphasic patterns starting prior to the first electromyographic (EMG) burst in the prime-mover muscle. Proximal postural muscles demonstrated the largest anticipatory increase in the background activity during movements in one of the two opposite directions (forward or backwards). These changes progressively decreased when movements deviated from the preferred direction and frequently disappeared during movements in the opposite direction. The patterns in distal muscles varied across subjects and could demonstrate larger anticipatory changes during movements forward and backwards as compared to movements in intermediate directions. Bilateral addition of inertial loads to the wrists did not change the general anticipatory patterns, while making some of their features more pronounced. Anticipatory postural adjustments were followed by later changes in the activity of postural muscles, also reflected in the mechanical variables. Changes in leg joint angles revealed a hip-ankle strategy during shoulder flexions and an ankle strategy during shoulder extensions. The study demonstrates different behaviors of proximal and distal muscles during anticipatory postural adjustments in preparation for fast arm movements. We suggest that the proximal muscles produce a general pattern of postural adjustments, while distal muscles take care of fine adjustments that are more likely to vary across subjects.     

Sex differences, hormone fluctuations, ankle stability, and dynamic postural control

Context:

Hormonal fluctuation as a risk factor in anterior cruciate ligament injury has been investigated with conflicting results. However, the influence of hormone fluctuations on ankle laxity and function has not been thoroughly examined.

Objective:

To examine the potential hormone contributions to ankle laxity and dynamic postural control during the preovulatory and postovulatory phases of the menstrual cycle using an ankle arthrometer and the Star Excursion Balance Test in healthy women. The cohort group consisted of male control participants.

Design:

Cohort study.

Setting:

Research laboratory.

Patients or Other Participants:

Twenty healthy women (age = 23.8 ± 6.50 years, height = 163.88 ± 8.28 cm, mass = 63.08 ± 12.38 kg) and 20 healthy men (age = 23.90 ± 4.15 years, height = 177.07 ± 7.60 cm, mass = 80.57 ± 12.20 kg).

Intervention(s):

Ankle stability was assessed with anterior-posterior and inversion-eversion loading. Dynamic postural control was assessed with the posteromedial reaching distance of the Star Excursion Balance Test.

Main Outcome Measure(s):

Female participants used ovulation kits for 3 months to determine the time of ovulation; during their preovulatory and postovulatory phases, they were tested in the laboratory with an ankle arthrometer and the Star Excursion Balance Test. Male participants were tested on similar dates as controls. For each dependent variable, a time by side by sex repeated-measures analysis of variance was performed. Statistical significance was set a priori at P < .05.

Results:

For anterior-posterior laxity, a side main effect was noted (F_1,38 = 10.93, P = .002). For inversion-eversion laxity, a sex main effect was seen (F_1,38 = 10.75, P = .002). For the posteromedial reaching task, a sex main effect was demonstrated (F_1,38 = 8.72, P = .005). No influences of time on the dependent variables were evident.

Conclusions:

Although women presented with more ankle inversion-eversion laxity and less dynamic postural control, hormonal fluctuations during the menstrual cycle (preovulatory compared with postovulatory) did not affect ankle laxity or dynamic postural control, 2 factors that are associated with ankle instability.