The effect of prior leaning on human postural responses

This study examines how human postural responses are altered by leaning about the ankles to five different initial stance positions prior to anterior or posterior horizontal translations of the support surface. When subjects leaned in the same direction as the translation-induced sway, postural strategies changed to use of less ankle torque and more horizontal shear forces at the surface to return to equilibrium. This change in strategy was associated with reduced and delayed activation of the stretched ankle muscles and an increased activation of proximal muscles producing rapid hip flexions or extensions. The changes in ankle muscle activation strength and latencies cannot be predicted based on simple stretch or load reflexes, but match predictions from computational, biomechanical models of human stance co-ordination^1–4.     

Examination of static and dynamic postural stability in individuals with functionally stable and unstable ankles.

OBJECTIVE: To determine static and dynamic postural stability differences between functional ankle instability and stable ankle groups. DESIGN: Subjects were required to balance on a single leg and remain motionless for 20 seconds. After completing 3 trials, they performed a jump-landing test, which required them to jump 50% to 55% of their maximum vertical jump height. They landed on a single leg, stabilized quickly, and remained motionless for 20 seconds. SETTING: Sports Medicine Research Laboratory. PARTICIPANTS: Subjects with functional ankle instability (n = 14) who reported at least 2 sprains and "giving way" sensations at their ankle joint within the year prior to testing. Fourteen subjects with no history of ankle sprain injury were matched to subjects with functional ankle instability. MAIN OUTCOME MEASURES: Anterior/posterior and medial/lateral mean sway quantified static postural stability during single-leg stance. Dynamic postural stability was quantified with anterior/posterior and medial/lateral time to stabilization during single-leg jump landing. RESULTS: Mean sway was not significantly different between groups in the anterior/posterior (P = 0.28) and medial/lateral (P = 0.65) directions. The functional ankle instability group took significantly longer to stabilize in the anterior/posterior (3.27 +/- 0.72 seconds vs. 2.33 +/- 0.33 seconds; P < 0.001) and medial/lateral (2.48 +/- 0.50 seconds vs. 2.00 +/- 0.65 seconds; P = 0.04) directions. CONCLUSIONS: Individuals with functional ankle instability took significantly longer to stabilize than individuals with stable ankles after a single-leg jump landing. Differences between groups were not detected with mean sway measured during single-leg stance.     

Ankle joint proprioception and postural control in basketball players with bilateral ankle sprains

BACKGROUND: Deficiencies in ankle proprioception and standing balance in basketball players with multiple ankle sprains have been reported in separate studies. However, the question of how ankle proprioceptive inputs and postural control in stance are related is still unclear. HYPOTHESIS: Ankle repositioning errors and the amount of postural sway in stance are increased in basketball players with multiple ankle sprains. STUDY DESIGN: Controlled laboratory study. METHODS: Twenty healthy male basketball players and 19 male basketball players who had suffered bilateral ankle sprains within the past 2 years were examined. Both groups were similar in age. Passive ankle joint repositioning errors at 5 degrees of plantar flexion were used to test for ankle joint proprioception. The Sensory Organization Test was applied with dynamic posturography to assess postural sway angle under 6 sensory conditions. RESULTS: A significant increase in ankle repositioning errors was demonstrated in basketball players with bilateral ankle sprains (P < .05). The mean errors in the right and left ankles were increased from 1.0 degrees (standard deviation, 0.4 degrees ) and 0.8 degrees (standard deviation, 0.2 degrees ), respectively, in the healthy players to 1.4 degrees (standard deviation, 0.7 degrees ) and 1.1 degrees (standard deviation, 0.5 degrees ) in the injured group. A significant increase in the amount of postural sway in the injured subjects was also found in conditions 1, 2, and 5 of the Sensory Organization Test (P < .05). Furthermore, there were positive associations between averaged errors in repositioning both ankles and postural sway angles in conditions 1, 2, and 3 of the Sensory Organization Test (r = 0.39-0.54, P < .05). CONCLUSIONS: Ankle repositioning errors and postural sway in stance increased in basketball players with multiple ankle sprains. A positive relationship was found between these 2 variables. CLINICAL RELEVANCE: Such findings highlight the need for the rehabilitation of patients with multiple ankle sprains to include proprioceptive and balance training.     

Direct measurement of ankle stiffness during quiet standing: implications for control modelling and clinical application

In this study, we describe a device for the direct measurement of intrinsic ankle stiffness in quiet standing. It consists of a motorised footplate mounted on a force platform. By generating random sequences of step-like disturbances (1° amplitude, 150 ms duration) and measuring the corresponding displacements of the center of pressure in the antero-posterior direction, we obtained torque-rotation patterns after aligning, averaging, and scaling the postural responses. Such patterns were used for estimating the value of the ankle stiffness, which was normalized as a fraction of the critical value. In order to be confident that the measurements addressed the intrinsic ankle stiffness and were not affected in a significant way by the reflex activation of the muscles in response to the test disturbances, we performed the estimates in different ways: least squares estimates with time windows of different widths and an instantaneous estimate at the time in which the angular acceleration vanishes. The statistical analysis showed that there is no significant difference among the different methods of estimate and the inspection of the electromyographic activity in response to the perturbations showed that at least two of the estimates were certainly outside the possible influence of reflex patterns. The intrinsic ankle stiffness was evaluated to be 64 ± 8% of the critical stiffness for test disturbances of the order of 1°. We argue that this figure identifies the lower bound of the range of values which characterise normal sway in quiet standing, whereas the upper bound is given by the estimates performed with much smaller test disturbances [1] which yield a higher value: 91 ± 23%. The two estimation paradigms (with very small and very large test disturbances, respectively) are complementary also because they behave in a different way as regards the sensitivity to a bias torque: it is close to zero in the Loram & Lakie’s paradigm, whereas it is significant in our paradigm. Thus, as the bias grows, it appears that the range of stiffness values is narrowed and is pushed towards the upper bound. There is a clear potential for the clinical application of these methods, in the sense that the identification of the range of stiffness values used by a patient is a measurable index of motor organisation/reorganisation.     

Dynamic postural stability deficits in subjects with self-reported ankle instability

PURPOSE: A limited understanding of how functional ankle instability (FAI) affects dynamic postural stability exists because of a lack of reliable and valid measures. Therefore, the purpose of this investigation was to determine whether a new reliable index for dynamic postural stability could differentiate between those with stable ankles and those with FAI. METHODS: Data were collected on 108 subjects (54 subjects with stable ankles (STABLE group); 54 subjects with functionally unstable ankles (FAI group)). Subjects performed a single-leg-hop stabilization maneuver in which they stood 70 cm from the center of a force plate, jumped off both legs, touched a designated marker placed at a height equivalent to 50% of their maximum vertical leap, and landed on a single leg. The dynamic postural stability index and directional stability indices (medial/lateral, anterior/posterior, and vertical) were calculated. The raw and normalized (to energy dissipated) indices were compared between groups. RESULTS: Significant differences were noted for the anterior/posterior stability index (FAI = 0.36 +/- 0.09, STABLE = 0.30 +/- 0.06). Similar results were seen for the vertical stability index (FAI = 0.73 +/- 0.17, STABLE = 0.61 +/- 0.13), the normalized dynamic postural stability index (FAI = 0.85 +/- 0.17, STABLE = 0.73 +/- 0.12), the normalized vertical stability index (FAI = 0.007 +/- 0.004, STABLE = 0.005 +/- 0.001), and the dynamic postural stability index (FAI = 0.008 +/- 0.003, STABLE = 0.006 +/- 0.001). CONCLUSIONS: These results indicate that the dynamic postural stability index is a sensitive measure of dynamic postural stability and is capable of detecting differences between individuals with stable ankles and individuals with functionally unstable ankles.     

Intra- and inter-foot coordination in quiet standing: footwear and posture effects

Intra-foot coordination between center of pressure (COP) of the ball and heel of the foot in single leg standing and inter-foot coordination of the right and left foot during bipedal stance was examined as a function of postural stance (two legs, one leg, and toe postures), footwear (barefoot, different area based high heel shoes) and postural training (ballet group and regular exercising group). Young adult females performed three 20s trials in each postural condition. In general, the traditional variability measures of COPnet motion increased under the less stable postural support conditions and ballet dancers had better balance in single leg standing. Regularity analysis revealed a negative relation between the variability of foot coupling (both intra- and inter-foot) and the standard deviation of COPnet that was mediated by the interaction of shoe support and postural stance. The findings show that shoe support and postural stance modulate collective postural motion (COPnet) through the adaptability of the coupling of foot dynamics.     

Individuals with chronic ankle instability exhibit decreased postural sway while kicking in a single-leg stance

Individuals with chronic ankle instability (CAI) usually experience deficits in balance control, which increase displacement in the body's center of pressure (COP) when they balance on a single leg. Little is known, however, about whether or not these individuals use the strategies of postural adjustment properly, especially during functional tasks that may predispose them to ankle sprain. The aim of this study was to investigate anticipatory (APA) and compensatory (CPA) postural adjustments in individuals with and without CAI as they kick a ball while standing in a single-leg stance with their ankle in neutral and supinated positions. COP displacements were calculated and their magnitudes (range) analyzed during APA and CPA intervals and over the duration of the whole task, represented by the COP area of sway and mean velocity. The CAI group exhibited a significant decrease in CPA and area of sway over the whole task, relative to controls. These results suggest that the decreased balance sway could be caused by the need for further stabilization of the ankle in more unstable postures to prevent recurrent sprain. Our findings could help clinicians to better understand the strategies of postural adjustments in individuals with CAI, and may assist and motivate new investigations into balance control interventions in such individuals, as well as proactively address recurrent ankle sprain conditions.     

Effects of Levodopa on Postural Strategies in Parkinson's disease

Altered postural control and balance are major disabling issues of Parkinson's disease (PD). Static and dynamic posturography have provided insight into PD's postural deficits; however, little is known about impairments in postural coordination. We hypothesized that subjects with PD would show more ankle strategy during quiet stance than healthy control subjects, who would include some hip strategy, and this stiffer postural strategy would increase with disease progression.We quantified postural strategy and sway dispersion with inertial sensors (one placed on the shank and one on the posterior trunk at L5 level) while subjects were standing still with their eyes open. A total of 70 subjects with PD, including a mild group (H&Y≤2, N = 33) and a more severe group (H&Y≥3, N = 37), were assessed while OFF and while ON levodopa medication. We also included a healthy control group (N = 21).Results showed an overall preference of ankle strategy in all groups while maintaining balance. Postural strategy was significantly lower ON compared to OFF medication (indicating more hip strategy), but no effect of disease stage was found. Instead, sway dispersion was significantly larger in ON compared to OFF medication, and significantly larger in the more severe PD group compared to the mild. In addition, increased hip strategy during stance was associated with poorer self-perception of balance.     

Is balance impaired by recurrent sprained ankle?

OBJECTIVE: To evaluate uninjured and recurrent sprained ankles during single leg standing, both with and without visual input, and the contribution of related proprioceptive feedback in this event. METHODS: A force measuring system was used for monitoring reaction forces in the anteroposterior and mediolateral directions during single leg standing. Differences between selected variables obtained in the uninjured and sprained ankles were analysed using two way analysis of variance. RESULTS: Foot-ground reaction forces in both anteroposterior and mediolateral directions were the same in normal and sprained ankles of each subject while standing with either open or closed eyes. However, standing with closed eyes, irrespective of the ankle status, always produced significantly higher reaction forces than those obtained with open eyes (P < 0.05). CONCLUSIONS: The amount of postural sway during single leg standing is similar in the chronically sprained and the uninjured ankle joint.     

Increase in muscle tone promotes the use of ankle strategies during perturbed stance

Background:To maintain an upright standing posture against external disturbances, the human body mainly employs two types of postural control strategies: “ankle strategy” and “hip strategy.” While it has been reported that the magnitude of the disturbance alters the use of postural control strategies, it has not been elucidated how the level of muscle tone, one of the crucial parameters of bodily function, determines the use of each strategy. We have previously confirmed using forward dynamics simulations of human musculoskeletal models that an increased muscle tone promotes the use of ankle strategies. The objective of the present study was to experimentally evaluate a hypothesis: an increased muscle tone promotes the use of ankle strategies.Research question:Do changes in the muscle tone affect the use of ankle strategies?Methods:Participants were asked to maintain their standing posture on a movable platform sliding horizontally at several accelerations. Postural reactions for support surface translations were examined under three instructions with or without handgrips: relax state, squeezing a handgrip, and an increased muscle tone of the whole body. Surface-electromyography and marker locations of joints were measured to calculate the index of muscle tone and postural control strategies. The relationship of the indexes was evaluated based on correlation coefficients.Results:In half of the conditions, weak negative correlations were noted between the muscle tone index and postural control strategy index. In other words, an increased muscle tone rather promoted the use of the ankle strategy than the hip strategy. These findings are consistent with our previous simulation results.Significance:The results recognized a positive response to the research question. This suggests that it is crucial to take muscle tone into account to understand postural control strategies.     

Postural responses to lateral perturbation in healthy subjects and ankle sprain patients.

The purpose of this study was to investigate postural responses of healthy subjects and patients with recent ankle sprains following a perturbation that created sway in the frontal plane. EMG data were taken from the posterior tibialis (PT) (not monitored in patients), peroneal longus (PL), and tibialis anterior muscles (TA). Subjects stood on a platform that provided a rotational perturbation (approximately 70 degrees.s-1) in the frontal plane. This perturbation had the effect of everting and loading one limb while inverting and unloading the contralateral limb. An initial response in the PT of the loaded limb and the PL of the unloaded limb was noted at approximately 50 ms following the perturbation. This was followed by a bilateral response in the TA at 60 ms. The amplitude of the TA muscle was significantly greater in the loaded limb. For ankle sprain patients a bilateral TA response and a PL response in the unloaded limb was noted at approximately 65 ms. TA response amplitude ratios between the loaded and unloaded limbs were similar to that of the healthy subjects. These data suggest that ankle sprain patients use a modified postural response following lateral perturbation as a compensation for the injury.     

Prolonged peroneal reaction time in ankle instability

The peroneal reflex time to sudden ankle inversion and the postural control of 15 athletes with functionally instable ankles were compared with 15 stable controls. A trapdoor produced sudden ankle inversion. Surface electrodes recorded electromyographic activity of the peroneal muscles. Postural sway was expressed by a transverse sway value obtained during single limb stance on a force plate. Increased postural sway was found in subjects with functional instability (p less than 0.01). This is in accordance with previous studies. Functionally instable subjects also displayed an increased peroneal reaction time (p less than 0.01) supporting the theory that functional instability is induced by a proprioceptive reflex defect. Nine of the 15 instable subjects were unilaterally instable and showed lower peroneal reaction time and postural sway values for the stable ankle, but the difference was not significant. There was a high degree of correlation between postural sway and peroneal reaction time (Spearman's rho = .92). In ten functionally instable athletes tested with and without ankle taping, it could not be verified that a reflex enhancing effect of taping occurs through stimulation of cutaneous afferents.     

In vitro measurement of intraarticular pressure in the ankle joint

Ankle joint affections and injuries are common problems in sports traumatology and in the daily routine of arthroscopic surgeons. However, there is little knowledge regarding intraarticular loads. Pressures on the ankle were determined in a dynamic model on 8 cadaver specimens, applying forces to tendons of the foot over the stance phase under vertical loading. A characteristic course of loading in the tibiotalar joint with a rapid increase upon heel contact was observed. It increased gradually to reach a maximum after 70% of the stance phase, during the push-off phase. The major torque in the ankle joint is located anterolaterally. A dynamic loading curve of the ankle joint can be demonstrated. These observations explain phenomena such as the appearance of osteophytes on the anterior tibia in the case of ankle osteoarthritis and the relatively low incidence of posterior tibial edge fragments in the case of trimalleolar ankle fracture. Furthermore, the medial side of the talus is less loaded compared to the lateral side, which appears relevant to the treatment of osteochondrosis dissecans.     

Anticipatory activation of postural muscles associated with bilateral arm flexion in subjects with different quiet standing positions

We investigated changes in activation timing and magnitude of the postural muscles according to initial standing positions. The subjects were divided into three groups depending on the position of the center of foot pressure (CFP) during quiet standing, namely backward, middle, and forward. Subjects maintained standing postures at various CFP positions in the anteroposterior direction, and then started bilateral arm movement at their own pace. The activation magnitude of the biceps femoris (BF) and erector spinae (ES) did not differ among any of the initial CFP positions. In only the BF, the preceding action to the anterior deltoid (AD) was clearly observed at more forward CFP positions in the order of the forward, middle and backward groups. Between initial CFP positions adjacent to quiet standing posture, the smallest change was observed in the preceding activation time of the BF. Significant correlation was observed between the background activity and activation time in both the BF and ES.     

Ankle muscle coactivation during gait is decreased immediately after anterior weight shift practice in adults after stroke

Increased ankle muscle coactivation during gait has frequently been observed as an adaptation strategy to compensate for postural instability in adults after stroke. However, it remains unclear whether the muscle coactivation pattern increases or decreases after balance training. The aim of this study was to investigate the immediate effects of balance practice on ankle muscle coactivation during gait in adults after stroke. Standing balance practice performed to shift as much weight anteriorly as possible in 24 participants after stroke. The forward movement distance of the center of pressure (COP) during anterior weight shifting, gait speed, and ankle muscle activities during 10-m walking tests were measured immediately before and after balance practice. Forward movement of the COP during anterior weight shifting and gait speed significantly increased after balance practice. On the paretic side, tibialis anterior muscle activity significantly decreased during the single support and second double support phases, and the coactivation index at the ankle joint during the first double support and single support phases significantly decreased after balance practice. However, there were no significant relationships between the changes in gait speed, forward movement of the COP during anterior weight shifting, and ankle muscle coactivation during the stance phase. These results suggested that ankle muscle coactivation on the paretic side during the stance phase was decreased immediately after short-term anterior weight shift practice, which was not associated with improved gait speed or forward movement of the COP during anterior weight shifting in adults after stroke.     

Hip external rotation stiffness and midfoot passive mechanical resistance are associated with lower limb movement in the frontal and transverse planes during gait

BackgroundHip external rotation stiffness, midfoot passive mechanical resistance and foot alignment may influence on ankle, knee and hip movement in the frontal and transverse planes during gait.Research questionAre hip stiffness, midfoot mechanical resistance and foot alignment associated with ankle, knee and hip kinematics during gait?MethodsHip stiffness, midfoot mechanical resistance, and foot alignment of thirty healthy participants (18 females and 12 males) with average age of 25.4 years were measured. In addition, lower limb kinematic data during the stance phase of gait were collected with the Qualisys System (Oqus 7+). Stepwise multiple linear regressions were performed to identify if hip stiffness, midfoot torque, midfoot stiffness and foot alignment were associated with hip and knee movement in the transverse plane and ankle movement in the frontal plane with α = 0.05.ResultsReduced midfoot torque was associated with higher hip range of motion (ROM) in the transverse plane (r² = 0.18), reduced hip stiffness was associated with higher peak hip internal rotation (r² = 0.16) and higher ROM in the frontal plane (r² = 0.14), reduced midfoot stiffness was associated with higher peak knee internal rotation (r² = 0.14) and increased midfoot torque and midfoot stiffness were associated with higher peak knee external rotation (r² = 0.36).SignificanceThese findings demonstrated that individuals with reduced hip and midfoot stiffness have higher hip and knee internal rotation and higher ankle eversion during the stance phase of gait. On the other hand, individuals with increased midfoot torque and stiffness have higher knee external rotation. These relationships can be explained by the coupling between ankle movements in the frontal plane and knee and hip movements in the transverse plane. Finally, this study suggests that midfoot passive mechanical resistance and hip stiffness should be assessed in individuals presenting altered ankle, knee and hip movement during gait.     

DOMS-associated changes in ankle and knee joint dynamics during running

PURPOSE: The purpose of this study was to determine whether leg mechanics change due to DOMS by examining ankle and knee joint kinematics and stiffness before and after a down hill run. METHODS: Sagittal plane kinematics were recorded with high-speed (120 Hz) video at a speed representing 75% of VO2peak of nine well-trained male runners before (RE1) and 48 h after (RE2) a 30-min downhill run. From the recorded video, 10-12 consecutive strides were digitized, and the following variables were calculated for each stride: ankle and knee range of motion (ROM), ankle and knee peak angular velocity, ankle and knee stiffness, and leg vertical stiffness. A repeated measures ANOVA was calculated for each variable (alpha = 0.05). RESULTS: Both knee and ankle ROM during stance decreased with DOMS, but otherwise there were few changes in ankle mechanics with DOMS. Knee stiffness tended to increase during the early portion of stance (from initial stance to maximum angular velocity of flexion) with DOMS, immediately followed by a decrease (to maximum knee flexion) in stiffness. Changes in knee stiffness caused vertical leg stiffness to increase for the initial portion of stance with DOMS. CONCLUSION: Knee mechanics changed such that the knee stiffness increased at initial stance, resulting in an increase in vertical leg stiffness. This change in knee stiffness possibly serves as a protective mechanism to prevent further damage or pain in the knee extensor musculature.     

Aging effects on postural responses to self-imposed balance perturbations

The present study investigated how young and older individuals organize their posture in response to self-induced balance perturbations evoked by oscillatory single limb movements. Eleven old (70.1+/-4.3 years) and nine young (20.1+/-2.4 years) participants performed repeatedly for 5s hip flexion/extension movements using full range of motion and maximum velocity. Two-dimensional joint kinematics (sampling rate: 60Hz), center of pressure (CoP) and EMG activity of tibialis anterior (TA), medial gastrocnemius (MGAS) rectus femoris (RF) and, semitendinosus (ST) in the stance limb were recorded and analysed. Cross-correlation function (CCF) analysis was used to identify the degree of coupling between the swinging limb (SL), center of gravity (CoG) and CoP motions. Old adults significantly limited SL, CoG and CoP range of anterior/posterior (A/P) motion in response to the forceful leg swinging. In the stance limb, significantly lower levels of ankle muscle activity resulted in reduced hip and knee joint excursions and increased ankle instability. By contrast, young performers produced sufficient ankle muscle activity to stabilize the foot to the ground while progressively increasing joint range of motion from the ankle to the hip. Center of pressure and SL movements were strongly correlated in an anti-phase relationship in both age groups. In older adults, however, the relationship between CoG-SL and CoG-CoP movements was neither strong nor synchronous, reflecting a weaker coupling and lack of coordination between component movements. It is concluded that insufficient ankle muscle activity, central integration deficits and increased anxiety to postural threat are important factors implicated for the weaker postural synergies and freezing of degrees of freedom seen in the elderly during performance of single limb oscillations.     

Differences in lower extremity muscular coactivation during postural control between healthy and obese adults

IntroductionIt is well established that obesity is associated with deterioration in postural control that may reduce obese adults’ autonomy and increase risks of falls. However, neuromuscular mechanisms through which postural control alterations occur in obese adults remain unclear.ObjectiveTo investigate the effects of obesity on muscle coactivation at the ankle joint during static and dynamic postural control.Materials and methodsA control group (CG; n = 20; age = 32.5 ± 7.6 years; BMI = 22.4 ± 2.2 Kg/m²) and an obese group (OG; n = 20; age = 34.2 ± 5.6 years; BMI = 38.6 ± 4.1 Kg/m²) participated in this study. Static postural control was evaluated by center of pressure (CoP) displacements during quiet standing. Dynamic postural control was assessed by the maximal distance traveled by the CoP during a forward lean test. Electromyography activity data for the gastrocnemius medialis (GM), soleus (SOL) and tibialis anterior (TA) were collected during both quiet standing and forward lean tests. Muscle activities were used to calculate two separate coactivation indexes (CI) between ankle plantar and dorsal flexors (GM/TA and SOL/TA, respectively).ResultsCoP displacements were higher in the OG than in the CG for quiet standing (p < 0.05). When leaning forward, the maximal distance of the CoP was higher in the CG than in the OG (p < 0.05). Only the CI value calculated for SOL/TA was higher in the OG than in the CG for both static and dynamic tasks (p < 0.05). The SOL/TA CI value in the OG was positively correlated with CoP displacements during quiet standing (r = 0.79; p < 0.05).ConclusionObesity increases muscle coactivation of the soleus and tibialis anterior muscles at the ankle joint during both static and dynamic postural control. This adaptive neuromuscular response may represent a joint stiffening strategy for enhancing stability. Consequently, increased ankle muscle coactivation could not be considered as a good adaptation in obese adults.