Ankle Angular Displacement Research Articles

"Going Backward": Effects of age and fatigue on posterior-directed falls in Parkinson disease.

Nearly half of persons with Parkinson disease (PD) report fatigue as a factor in their fall history. However, it is unknown whether these self-reported falls are caused by a sensation of fatigue or performance fatigue. We sought to investigate the influences of performance fatigue and age on postural control in persons with PD. Individuals with PD (n = 14) underwent postural control assessments before (T0) and immediately after (T1) fatiguing exercise. Biomechanical data were gathered on participants completing a treadmill-induced, posterior-directed fall. Performance fatigue was produced using lower extremity resistance exercise on an isokinetic ergometer. Repeated measures ANCOVAs were used with age as a covariate to determine the effects of performance fatigue on biomechanical variables. After adjustment for age, there was a statistically significant difference in peak center of pressure (COP) latency during the support phase of recovery. Pairwise comparisons demonstrated a decrease in peak ankle displacement from T0 to T1. Age was also found to be significantly related to reaction time and peak knee displacement while participants were fatigued. The decreased peak COP latency, along with decreased ankle angular displacement, suggest that persons with PD adopt a stiffening strategy in response to backward directed falls. Postural stiffening is not uncommon in persons with PD and could be a risk factor for falls. Older individuals with PD demonstrate slower mobility scores and decreased reaction times in the setting of fatigue, suggesting a combined effect of the aging and fatigue processes.

BIOMECHANICAL EFFECTS OF HYPERPRONATION ON MULTIDIRECTIONAL ANKLE ANGULAR DISPLACEMENT AND STIFFNESS

Hyperpronation of the foot is believed to contribute to ankle hypermobility and associated stiffness reduction, but the underlying biomechanical mechanisms remain unknown. This study aimsed to investigate multidirectional ankle displacement and associated stiffness when a posterior–anterior impact force was applied to the posterior knee compartment. Forty healthy adults with and without foot hyperpronation were recruited. A three-dimensional motion capture system and force plates were used to acquire angular displacement and ankle joint moment data. The independent [Formula: see text]-test and Mann–Whitney [Formula: see text] test were used to compare the group differences in ankle angular displacement, moment, and stiffness. Spearman’s rho test was performed to determine the relationship between ankle angular displacement and stiffness. The hyperpronation group demonstrated significantly greater sagittal ([Formula: see text]) and frontal plane ([Formula: see text]) angular displacements and reduced sagittal plane ankle stiffness ([Formula: see text]) than the neutral group. The Spearman’s correlation analysis showed a close inverse relationship between the ankle angular displacement and stiffness, ranging from [Formula: see text] to [Formula: see text]. The biomechanical data in our study suggest that individuals with foot hyperpronation present with multidirectional hypermobility and a reduction in ankle stiffness. These factors contribute to an increased risk of ankle-foot injury in individuals with foot hyperpronation.

Prophylactic Ankle Braces and the Kinematics and Kinetics of Half-Squat Parachute Landing.

The objective of the study was to investigate the effects of dropping heights and prophylactic ankle braces on ankle joint biomechanics during half-squat parachute landing from two different heights. There were 30 male elite paratroopers with formal parachute landing training and more than 2 yr of parachute jumping experience who were recruited for this study. The subjects tested three different ankle brace conditions (no-brace, elastic brace, semirigid brace). Each subject was instructed to jump off a platform from two different heights of 0.4 m and 0.8 m, and land on a force plate in a half-squat posture. The Vicon 3D motion capture system and force plate were used to record and calculate kinematic and kinetic data. Dropping height had a significant effect on peak vertical ground reaction force (vGRF), maximum ankle angular displacement, and time to vGRF. As compared with the no-brace group, use of an elastic ankle brace significantly reduced peak vGRF by 18.57% and both braces significantly reduced the maximal angular displacements of dorsiflexion. The semirigid brace provided greater restriction against maximal angular displacement of inversion. The elastic and semirigid ankle braces both effectively restricted motion stability of the ankle joint in the sagittal plane, and the semirigid ankle brace prevented excessive inversion, although the comfort of this device should be improved overall.Wu D, Zheng C, Wu J, Hu T, Huang R, Wang L, Fan Y. Prophylactic ankle braces and the kinematics and kinetics of half-squat parachute landing. Aerosp Med Hum Perform. 2018; 89(2):141-146.

Dynamic joint stiffness of the ankle in healthy and rheumatoid arthritis post-menopausal women

The purpose of this study was to compare rheumatoid arthritis post-menopausal women (RAPW) with pathological involvement of the lower limb joints and age-matched healthy post-menopausal women (AHPW) in regard to the dynamic joint stiffness of the ankle (DJSankle) during the stance phase of gait. Data were collected from 18 RAPW and 18 AHPW. Gait was assessed by a three-dimensional motion analysis system synchronised with a force plate. Subjects walked barefoot at natural and self-selected speed, performing 14 valid trials (comprising 7 left and 7 right foot-steps on a force plate). The stance phase was split into three sub-phases that corresponded to the three angular displacements of the ankle that occurred during this phase, namely, controlled plantar flexion (CPF), controlled dorsiflexion (CDF), and powered plantar flexion (PPF). A linear model represented each sub-phase and computed DJSankle. Model fitting was assessed by the coefficient of determination (R2). The coefficient of variation (CV) was used to assess intra-individual variability. In all sub-phases, R2 values for both groups were higher than 0.85. There were no differences in the R2 values among groups. RAPW showed a higher DJSankle during the CPF (p < 0.05). CDF and PPF yielded no differences among groups. During CPF, RAPW yielded a higher CV for DJSankle (p < 0.01). RAPW also yielded lower ankle angular displacements during CPF and PPF (p < 0.05). Findings suggested that the stance phase of RAPW and AHPW can be studied by a linear ankle ‘moment of force −– angle’ relationship. During CPF, RAPW exhibited excessive ankle stiffness and presented a higher intra-individual DJSankle variability.

A novel protocol to evaluate ankle movements during reaching tasks using pediAnklebot.

The aim of the study is to design a novel protocol to characterize the ankle movements during dorsal and plantar flexion reaching tasks using the pediAnklebot. Five healthy children were instructed to control a pointer and hit targets appearing on the monitor, by moving their ankle alternatively up and down. The protocol consisted of 60 targets, 30 up and 30 down, reachable via dorsiflexion and plantarflexion movements, respectively. Ankle angular displacements and torques were gathered by encoders and load cells embedded in the robot. Ankle motor performance was evaluated by means of kinematic, submovements and dynamic indices. Results suggest that (i) plantarflexion movements are faster and more accurate than the dorsiflexion ones, but children are able to perform with a higher level of smoothness the latter ones; (ii) children are able to stop the ankle movement more easily at the end of dorsiflexion rather than plantarflexion; (iii) the central nervous system plans plantarflexion and dorsiflexion movements with the same efficiency; (iv) children apply different torque levels during the two motor tasks and they cannot balance the inversion and eversion moments during dorsiflexion. These findings provide an important starting point for the assessment of a reference baseline of motor indices for the ankle joint.

Collaborative robotic biomechanical interactions and gait adjustments in young, non-impaired individuals.

BackgroundCollaborative robots are used in rehabilitation and are designed to interact with the client so as to provide the ability to assist walking therapeutically. One such device is the KineAssist which was designed to interact, either in a self-driven mode (SDM) or in an assist mode (AM), with neurologically-impaired individuals while they are walking on a treadmill surface. To understand the level of transparency (i.e., interference with movement due to the mechanical interface) between human and robot, and to estimate and account for changes in the kinetics and kinematics of the gait pattern, we tested the KineAssist under conditions of self-drive and horizontal push assistance. The aims of this study were to compare the joint kinematics, forces and moments during walking at a fixed constant treadmill belt speed and constrained walking cadence, with and without the robotic device (OUT) and to compare the biomechanics of assistive and self-drive modes in the device.MethodTwenty non-neurologically impaired adults participated in this study. We evaluated biomechanical parameters of walking at a fixed constant treadmill belt speed (1.0 m/s), with and without the robotic device in assistive mode. We also tested the self-drive condition, which enables the user to drive the speed and direction of a treadmill belt. Hip, knee and ankle angular displacements, ground reaction forces, hip, knee and ankle moments, and center of mass displacement were compared “in” vs “out” of the device. A repeated measures ANOVA test was applied with the three level factor of condition (OUT, AM, and SDM), and each participant was used as its own comparison.ResultsWhen comparing “in” and “out” of the device, we did not observe any interruptions and/or reversals of direction of the basic gait pattern trajectory, but there was increased ankle and hip angular excursions, vertical ground reaction force and hip moments and reduced center of mass displacement during the “in device” condition. Comparing assistive vs self-drive mode in device, participants had greater flexed posture and accentuated hip moments and propulsive force, but reduced braking force.ConclusionsAlthough the magnitudes and/or range of certain gait pattern components were altered by the device, we did not observe any interruption from the mechanical interface upon the advancement of the trajectories nor reversals in direction of movement which suggests that the KineAssist permits relative transparency (i.e.. lack of interference of movement by the device mechanism) to the individual’s gait pattern. However, there are interactive forces to take into account, which appear to be overcome by kinematic and kinetic adjustments.

Sex differences in lower extremity stiffness and kinematics alterations during double-legged drop landings with changes in drop height

The aim of this study was to determine whether sex differences and effect of drop heights exist in stiffness alteration of the lower extremity during a landing task with a drop height increment. Twelve male participants and twelve female participants performed drop landings at two drop heights (DL40 and DL60; in cm). The leg and joint stiffnesses were calculated using a spring–mass model, and the joint angular kinematics were calculated using motion capture. Ground reaction forces (GRFs) were recorded using a force plate. The peak vertical GRF of the females was significantly increased when the drop height was raised from 40 to 60 cm. Significantly less leg and knee stiffness was observed for DL60 in females. The ankle, knee, and hip angular displacement during landing were significantly increased with drop height increment in both sexes. The knee and hip flexion angular velocities at contact were significantly greater for the 60 cm drop height relative to the 40 cm drop height in males. These sex disparities regarding the lower extremity stiffness and kinematics alterations during drop landing with a drop height increment would predispose females to lower extremity injury.

Recovery of powerful kick biomechanics after intense running fatigue in male and female soccer players.

Background:Fatigue seems to have a significant effect on soccer kick performance. However, the duration of these effects has not been previously investigated.Objectives:The purpose of the present study was to investigate the duration of the acute effects of fatigue on soccer kick performance in males and females.Patients and Methods:Ten male (age: 26.3 ± 4.9 years, height: 178.1 ± 5.1 cm, mass: 81.3 ± 8.1 kg) and ten female (age: 24.4 ± 4.2 years, height: 169.7 ± 5.7 cm, mass: 61.8 ± 5.1 kg) amateur soccer players performed three instep kicks prior to and after running on a treadmill till exhaustion. Three-dimensional kinematics were collected pre- and post-fatigue.Results:Analysis of variance indicated a statistically significant decline in ball speed during the first and the second trial after fatigue (P < 0.05), but recovered to pre-fatigue levels during the third post-fatigue kicking trial (P > 0.05). Similarly, maximum ankle, knee and hip linear velocity and ankle angular displacement were significantly lower during the first two trials (P < 0.05), but not during the third trial after fatigue (P > 0.05).Conclusions:Soccer kick parameters recovered to pre-fatigue levels approximately within a minute after the end of the protocol. These findings have a practical meaning for players who have to perform set piece kicks under fatigue conditions and for coaches who have to guide their players.

Horizontal and Declined Squats in Healthy Individuals: A Study of Kinematic and Muscle Patterns

Squats are frequently incorporated in physiotherapy programmes and performed in different ways. Accordingly, muscle and kinematic patterns also differ. The objective was to compare the kinematics and EMG patterns of the major muscles of knee and ankle joints during different squat exercises on horizontal (HP) and 25° decline (DP) boards. Seventeen healthy individuals performed squats on HP and DP with bilateral support and restriction of trunk movements in the sagittal plane. The knee and ankle angular displacements in the sagittal plane and the EMG activities of the major muscles of the lower limb in four subphases of movement were recorded and analyzed. For the descending phase of the HP squats, the angular excursion, as well as the initial and final positions, was smaller for the knee (P&lt;0.05) but larger for the ankle (P=0.01). For the ascending phase of the HP squats, the initial and final positions were larger for the ankle (P&lt;0.02) and the final position was smaller for the knee (P=0.05). All muscles remained activated and showed similarity between the tasks (P&gt;0.44), except for the tibialis anterior in both squat exercises (P&lt;0.03). The HP and DP squats produced different kinematics (knee and ankle joints) but did not modify the EMG strategy for both movement phases.

Analyzing individual and group differences in multijoint multiwaveform gait data using the Parafac2 model

Locomotion research often involves analyzing multiwaveform data (e.g., velocities, accelerations, etc.) from various body locations (e.g., knees, ankles, etc.) of several subjects. Therefore, some multivariate technique such as principal component analysis is often used to examine interrelationships between the many correlated waveforms. Despite its extensive use in locomotion research, principal component analysis is for two-mode data, whereas locomotion data are typically collected in higher mode form. In this paper, we present the benefits of analyzing four-mode locomotion data (subjects × time × joints × waveforms) using the Parafac2 model, which is a component model designed for analyzing variation in multimode data. Using bilateral hip, knee, and ankle angular displacement, velocity, and acceleration waveforms, we demonstrate Parafac2's ability to produce interpretable components describing (i) the fundamental patterns of variation in lower limb angular kinematics during healthy walking and (ii) the fundamental differences between normal and atypical subjects' multijoint multiwaveform locomotive patterns. Also, we illustrate how Parafac2 makes it possible to determine which waveforms best characterize the individual and/or group differences captured by each component. Our results indicate that different waveforms should be used for different purposes, confirming the need for the holistic analysis of multijoint multiwaveform locomotion data, particularly when investigating atypical motion patterns.

Skin sensory information from the dorsum of the foot and ankle is necessary for kinesthesia at the ankle joint

Previous research has shown that skin is capable of providing kinesthetic cues at particular joints but we are unsure how these cues are used by the central nervous system. The current study attempted to identify the role of skin on the dorsum of the ankle during a joint matching task. A 30 cm patch of skin was anesthetized and matching accuracy in a passive joint matching task was compared before and after skin anesthetization. Goniometers were used to measure ankle angular displacement. Four target angles were used in the matching task, 7° of dorsiflexion, 7°, 14° and 21° of plantarflexion. We hypothesized that, based on the location of skin anesthetized, only the plantarflexion matching tasks would be affected. Absolute error (accuracy) increased significantly for all angles when the skin was anesthetized. Directional error indicated that overall subjects tended to undershoot the target angles, significantly more so for 21° of plantarflexion when the skin was anesthetized. Following anesthetization, variable error (measure of task difficulty) increased significantly at 7° of dorsiflexion and 21° of plantarflexion. These results indicate that the subjects were less accurate and more variable when skin sensation was reduced suggesting that skin information plays an important role in kinesthesia at the ankle.

Injuries to the triceps surae muscle of elite rugby union front rowers: A 3 dimensional kinematic analysis of the front row during live scrimmaging

Introduction: The increasing incidence of injury to the triceps surae muscle in high performance front row rugby union (rugby) players has stimulated a need to investigate the events surrounding this injury. Previous research has linked triceps surae muscle injuries with both scrummaging and activities which mimic those of scrummaging, but possible mechanisms of injury remain uninvestigated. In addition, limited published data exists that has reported the biomechanics of scrum technique in high performance players. Of these studies the majority have been undertaken using scrummaging machines, with research on the biomechanics of live scrummaging even less well documented. Accordingly, the purpose of this study was to investigate the lower limb kinematics during a series of scrummaging drills. It was hypothesized that analysing the biomechanics of this aspect of game play would provide greater understanding of the possible causes and mechanisms for triceps surae injury. Methodology: Eleven high performance front row rugby players were landmarked for 16 anatomical points and then videoed during a series of 2-on-1 (attacking) and 1 -on-2 (defensive) live scrummaging drills using four digital camcorders (Panasonic NV-GS180GN) operating at 50 Hz. On completion of data collection, the 16 land marked anatomical points were digitised for each frame using APAS motion analysis software and a three-dimensional model of the trunk and lower limb developed. A series of one way ANOVA were used to determine if the spatio temporal descriptors of the lower limb kinematics differed between defensive and attacking scrummaging technique. All statistical analyses were performed using SPSS for Windows (version 17). A significance level of P < 0.05 was used for all analysis. Results/conclusions: Results (presented as mean ± SD) showed numerous significant differences in a range of spatio temporal variables between defensive and attacking scrum types. Analysis indicated a clear trend towards more extended positions in the defensive scrum drills. For example, relative ankle angular displacement from foot strike to toe-off varied significantly (P < 0.05) between the attacking (−13° [±7]) and defensive scrum drills (3° [±8]), as did relative ankle angular displacement at peak extension velocity between the attacking (108 deg [±12]) and defensive scrum drills (97° [±9]) during single leg stance. It was concluded that these more extended joint positions contribute to increased triceps surae load during attacking scrummaging activity. This study also showed that the scrummaging techniques of the subjects had the potential to predispose them to triceps surae injury.

A study of semi-rigid support on ankle supination sprain kinematics

Ankle sprain injury is very common in sports and the use of ankle support is crucial. This research investigated the effect of an ankle brace in reducing the ankle angular displacement and angular velocity during sudden supination. In the experiment, 11 healthy males were tested. The bracing condition, semi-rigid ankle braces were investigated. The angular displacement and angular velocity of the ankle were computed. The motion-capture system was adopted to capture the three-dimensional coordinates of the reflective markers. The coordinates of the reflective markers were used to compute the ankle kinematics during simulated ankle supination. A mechanical supination platform was used to simulate the sprain motions. Experimental results showed that the semi-rigid brace tested significantly reduced the ankle angular displacement and angular velocity compared with control conditions during sudden supination. In conclusion, the semi-rigid-type brace can provide significant restriction to reduce the magnitudes of the angular displacement and angular velocity of the ankle during sudden supination sprain. The semi-rigid-type brace is suggested as the prophylactic bracing for the ankle.

THE EFFECT OF HIP AND ANKLE FLEXIBILITY TRAINING ON OLDER ADULT GAIT.

Purpose/Hypothesis: The purpose of this study was to examine effects of a hip and ankle stretching intervention on flexibility and gait biomechanics of older adults not previously active in exercise participation. It was hypothesized that increases in hip extension and ankle plantar flexion range of motion (ROM) would occur from the intervention. Accompanying the flexibility increases were expected increases in freely chosen gait speed (FCGS), stride length (SL), and joint angular displacement. Subjects: 37. Materials/Methods: Healthy older adults 62 to 82 years of age participated. Assessment of each participant was made before and after an 8 week period. The experimental (EXP) group participated in home flexibility training for the 8 weeks while the control (CON) group made no changes to their physical activity routine. Both groups had weekly visits with the primary investigator to encourage compliance and receive exercise training as needed. Hip extensor flexibility was measured with participants in the Thomas Test position and ankle plantar flexor flexibility was measured as dorsiflexion ROM with the knee extended. Gait variables were measured during both FCGS and set gait speed (SGS) (1.5 m/s) trials. Data were captured for gait variables using a video motion analysis system. Results: Hip extensor flexibility increased as measured by combined passive ROM of hip extension and knee flexion in the Thomas Test position from 59.7 deg pre- to 66.5 deg post-assessment in the EXP group (p = 0.002). Ankle plantar flexor flexibility increased from 7.8 deg pre- to 11.3 deg post-assessment in the EXP group (p = 0.001). Freely chosen walking speed increased from 1.23 m/s pre- to 1.30 m/s post-assessment in the EXP group (p < 0.001). During gait, increasing trends were noted in hip angular displacement and SL from pre- to post-assessment in the EXP group at both FCGS and SGS, but increases were not statistically significant compared to the CON group. No trends of change were noted in ankle angular displacement at either gait speed. Conclusions: Healthy, previously non-exercising older adults demonstrate functional benefit from 8 weeks of flexibility training. The implication is that joint ROM is a specific age-associated impairment that can be modified to enhance older adult gait function. The promotion of routine stretching exercise for healthy individuals has been criticized due to insufficient evidence of benefit. Based on results of this study, recommendations for routine hip extensor and ankle plantar flexor stretching appear reasonable for healthy older adults. Clinical Relevance: The findings of this study are clinically significant. For a person to functionally ambulate in the community, a minimum level of gait speed is required. Decline in gait speed is directly related to age-associated disability. This study supports previous suggestions that joint ROM may be a key impairment dictating gait change in older adults. Application of such a flexibility program for older adults with conditions that exacerbate ROM impairment may result in even greater improvements.

Positive force feedback in human walking

The objective of this study was to determine if load receptors contribute to the afferent-mediated enhancement of ankle extensor muscle activity during the late stance phase of the step cycle. Plantar flexion perturbations were presented in late stance while able-bodied human subjects walked on a treadmill that was declined by 4%, inclined by 4% or held level. The plantar flexion perturbation produced a transient, but marked, presumably spinally mediated decrease in soleus EMG that varied directly with the treadmill inclination. Similarly, the magnitude of the control step soleus EMG and Achilles' tendon force also varied directly with the treadmill inclination. In contrast, the ankle angular displacement and velocity were inversely related to the treadmill inclination. These results suggest that Golgi tendon organ feedback, via the group Ib pathway, is reduced when the muscle-tendon complex is unloaded by a rapid plantar flexion perturbation in late stance phase. The changes in the unload response with treadmill inclination suggest that the late stance phase soleus activity may be enhanced by force feedback.

The Acute Effects of Static Stretching on Human Hopping Performance

Static stretching is related to increases in joint range of motion (ROM), however, the relationship between stretching and movement performance is unclear. Some suggest an acute bout of stretching enhances neuromuscular performance, whereas other evidence points to a detrimental effect. PURPOSE: To determine the acute effects of static stretching on performance measurements of a bouncing gait represented by bipedal hopping. METHODS: Participants were randomly assigned to either a control (C) or static stretching (EXP) group (n=12 each group). Plantar-flexor muscle activity, active ankle joint stiffness (kA), and leg stiffness (kL) were measured during preferred-height hopping. These variables were calculated for the lowering phase of ground contact, from initial contact to the lowest vertical position of the center of mass (COM). Muscle activity was measured with surface electromyographic (EMG) electrodes over the gastrocnemius (lateral and medial) and soleus muscles. Active kA was calculated as the ratio of net muscle torque to ankle angular displacement and kL was calculated as the ratio of change in vertical GRF to vertical displacement of the COM. Hopping frequency was maintained at 2.2 Hz with the aid of a metronome. In addition, measurements of active ankle dorsiflexion ROM were recorded before each trial of hopping. Participants performed two trials of 60-s hopping. Between trials the EXP group participated in static stretching of the plantar-flexor muscles and C group rested. Comparison of the change in hopping variables from one trial to the next was made between groups using a one-way MANO VA and comparison of change in active ROM between groups was made with a one-way ANOVA. RESULTS: Univariate examination of changes in active dorsiflexion ROM between the C and EXP groups demonstrated a difference (AROM: 0.0(1.1) vs. 1.6(2.0) degrees, p=0.02). Multivariate examination of the changes in hopping variables from one trial to the next showed no difference between groups (muscle activity ratio: 0.9(0.2) vs. 1.0(0.2) IEMG post/pre, δkA: 8(50) vs. 20(32) Nm rad−1, δkL 0.3(1.9) vs. 0.1(1.2) kN m−1, p=0.46). All data are presented as mean (SD), C and EXP respectively. CONCLUSIONS: The observed increase in ankle dorsiflexion ROM after static stretching is in agreement with previous findings. There is, however, no support for either an enhancing or detrimental effect of static stretching on performance measures during bipedal hopping.

Responses to whole head-and-body tilts with and without passive ankle dorsiflexion in the absence of visual feedback

Objectives. We have investigated lower limb responses in seven blindfolded healthy subjects to well controlled tilts in the standing position. Our aims were (1) to determine the effect of head acceleration magnitude on responses evoked by whole head-and-body tilts, and (2) to establish whether tilt-evoked responses are modifiable by passive ankle dorsiflexion. Whole head-and-body tilts evoked responses in the biceps femoris, medial gastrocnemius and tibialis anterior muscles. Methods. Seven young healthy subjects stood on a spring-activated tilting apparatus and underwent sudden whole head-and-body tilts of about 15° from the vertical position, with or without passive ankle dorsiflexion. Head acceleration was recorded with a linear accelerometer and ankle angular displacement with a potentiometer. Surface EMG signals were recorded in the right biceps femoris, medial gastrocnemius and tibialis anterior muscles. Results. As the peak of head acceleration was increased from 0.5 g to 1.8 g, the frequency of occurrence of tilt-evoked responses increased from 7% to 60% of trials in the biceps femoris muscle during whole head-and-body tilts. In general, the more proximal muscle (biceps femoris) was activated before the more distal muscle (medial gastrocnemius) during whole head-and-body tilts, while the opposite pattern was found during tilt with dorsiflexion. Conclusions. Our results indicate that the occurrence of tilt-evoked responses increases with an increase in the amplitude of tilting acceleration. This suggests that tilt-evoked responses are dependent, at least in part, on vestibular stimulation. In addition, the spatio-temporal pattern of biceps femoris and medial gastrocnemius muscle activation was opposite during whole head-and-body tilts and tilts with dorsiflexion. This finding suggests that foot/ankle somatosensory inputs can modify tilt-evoked responses.

Kinematic and kinetic validity of the inverted pendulum model in quiet standing.

Movements of the whole-body center of mass during quiet standing have been estimated from measurements of body segment movements. These whole-body center of mass movements have been compared with movements of the center of mass as predicted from a simple inverted-pendulum model of standing. However, the total body center of mass is a weighted average of the center of mass of all individual body segments. The question arises as to how well the total body center of mass represents the individual segments and lower limb joint angles. This study focuses on the validity of how well the individual segments and lower limb angles temporally and spatially synchronize with the total body center of mass. Eleven healthy university students volunteered to participate. Kinematic data were collected using a 3D optoelectronic camera system; kinetic data were collected using a 3D force plate. Participants stood quietly, with eyes open, for 120 s. Segment and whole body centers of mass were calculated from a 14 segment, 3D bilateral model. Segment and joint angles were calculated for the lower limbs, bilaterally, and the trunk. Segment center of mass root-mean-square displacements were strongly correlated with center of mass height relative to the ankle joint and were synchronized, or temporally locked, to the movement of the whole body center of mass. Sagittal plane ankle angular displacements were highly correlated to sagittal plane center of mass movement; stronger correlations between body center of mass and lower limb angular displacement were observed, the result of compensatory knee joint angular displacements. These data support and extend the use of an inverted pendulum model to represent quiet standing postural control.

On the Origin of the Soleus H-Reflex Modulation Pattern During Human Walking and Its Task-Dependent Differences

Recently, Brooke and colleagues have suggested "that the strong inhibition arising from passive movement about the knee and hip joints, lays down the base for the soleus H-reflex gain modulation seen during human gait." In particular stretch-evoked afferent activity from the quadriceps muscle was emphasized as the most important source of movement-induced inhibition of the H-reflex. To test this hypothesis we examined the kinematics and electromyographic (EMG) activity of the leg during human walking and correlated these with the modulation pattern of the soleus H-reflex. To further test the possible contribution of stretch-evoked quadriceps afferent activity to the soleus H-reflex modulation pattern during walking different walking gaits were studied. In one condition subjects were asked to walk with their knee locked in full extension by a rigid knee brace. In a second condition subjects were asked to walk backwards. During normal walking, the soleus H-reflex modulation pattern is strongly correlated with the EMG events of the soleus and tibialis anterior (TA), but not with hip, knee, or ankle angular displacement or velocity. When subjects walked with the knee locked in full extension, the amplitude of the H-reflex, its modulation pattern, and the task-dependent changes of its amplitude were the same as during normal walking. During backward walking, the H-reflex increases in late swing before activity of the soleus has begun and while the knee is flexing, an observation that highlights central control of the H-reflex amplitude. The effects of imposed flexion of the knee in passive subjects were also reexamined. The knee flexion imposed by the experimenter followed the same trajectory as that which occurred during the swing phase of the subject's step cycle. It was found that imposed knee flexions elicited a burst of TA EMG activity with an average latency of 81.6 ms (SD = 21 ms) in six out of eight subjects. Inhibition of the H-reflex, when it occurred, was associated with the occurrence of this burst. When subjects voluntarily flexed their right knee from an initial quiet standing posture, the inhibition of the soleus H-reflex began before flexion of the knee or that of any other leg segment. Once again the onset of inhibition was closely associated with the onset of activity in the TA. In the discussion section the present observations are examined in light of the predictions made by the movement-induced inhibition hypothesis of Brooke et al. It will be concluded that none of the predictions of this hypothesis were corroborated by present tests done during human walking. In consequence, we suggest that the modulation pattern of the H-reflex observed during normal human walking is centrally determined, as are the task-dependent differences of its amplitude (e.g., standing versus the stance phase of human walking).

Effects of Varying Acceleration of Platform Translation and Toes-Up Rotations on the Pattern and Magnitude of Balance Reactions in Humans

Different movement synergies used to restore balance in response to sudden support surface displacements have been described, which include the ankle movement synergy and a number of multisegmental movement synergies. The purpose of this study was to extend the analysis of the effects of stimulus magnitude on the pattern and scaling of balance reactions to larger magnitudes of balance disturbances, and to other types of balance disturbances, in particular, forward translations (FT), backward translations (BT), and toes-up rotations (RT). In addition, we examined whether the timing and magnitude of center of body mass (CM) displacement is an invariant feature of corrective responses to varying magnitudes of balance disturbances. Thirteen healthy adults were subjected to FT, BT, and RT of varying acceleration/velocity. The balance disturbance induced by FT and BT was fundamentally different from that induced by RT. The balance requirement during FT and BT was to rapidly translate the CM forward/backward to the new position within the displaced base of support. For RT, the requirement was to minimize the backward displacement of the CM. As evidenced from the initial phase of ankle, knee, and hip angular displacements and anterior-posterior (A-P) center of foot pressure displacement, the magnitude of the balance disturbance increased with increasing platform acceleration/velocity. For FT and BT, the present findings are consistent with the view that trajectory of CM is a control variable, as the timing, peak magnitude, and time to peak CM displacement did not vary as a function of platform acceleration/velocity. However, for RT, the peak magnitude and time to peak CM displacement did increase with increasing platform acceleration/velocity. The results demonstrate that in response to FT, BT, and RT, stability was restored by distinct multisegmental movement synergies. The corrective response to FT consisted of early knee flexion then ankle dorsiflexion and hip extension. The corrective response to BT consisted of hip flexion and ankle plantar flexion. For RT early hip flexion and knee flexion was observed. All muscles recorded (tibialis anterior, soleus, gastrocnemius, hamstrings, and quadriceps) were activated within a range of 60 to 170 ms from onset of platform displacement. For FT, BT, and RT, the pattern and timing of angular displacements and muscle responses did not vary as a function of platform acceleration/velocity, while there was a significant effect of platform acceleration/velocity on the magnitude of the corrective response, that is, peak magnitude of corrective hip, knee, and ankle angular displacements and magnitude of muscle responses. The present findings indicate that multiple sources of spatial information are necessary for the selection and initiation of the appropriate corrective response to meet the requirements of the different balance tasks. The present results strongly endorse the concept of a postural control network for recovery of standing balance, as opposed to positive feedback through local segmental or long loop reflex circuits.