Both anticipatory and compensatory postural adjustments are adapted while catching a ball in unstable standing posture.

The main objective of this study was to analyze the role of balance exercises on anticipatory (APA) and compensatory (CPA) postural adjustments in different conditions of postural stability. Sixteen subjects were required to catch a ball while standing on rigid floor, trampoline and foam cushion surfaces. Electromyographic activities (EMG) of postural muscles were analyzed during time windows typical for APAs and CPAs. Overall there were a reciprocal activation of the muscles around the ankle and co-activations between ventral and dorsal muscles of the thigh and trunk during the catching a ball task. Compared to the rigid floor, the tibialis anterior activation was greater during the trampoline condition (CPA: p = 0.006) and the soleus muscle inhibition was higher during foam cushion condition (APA: p = 0.001; CPA: p = 0.007). Thigh and trunk muscle activities were similar across the conditions. These results advance the knowledge in postural control during body perturbations standing on unstable surfaces.

Age-related changes of grip force control in physically active adults.

The age-related changes for controlling grip force in adults who practice regular and supervised physical activity have not been investigated. Fifty-five physically active adults ages 50 years and older and a group of 20 young adults (18-30 years old) performed a task of grasping and lifting an object. The grip force and the object's acceleration were recorded and used to calculate the parameters of grip force control. The levels of daily physical activities were also examined in all age groups. There were no differences in grip force control between the age groups. Conversely, the group over 76 years old used stronger grip force and increased latency (the time from grip force application to the time the object is lifted off the table) to manipulate the object. The results of this study suggest that middle-aged and older adults who exercise regularly can reduce or delay the possible grip force control deficits associated with aging.

Compensatory but not anticipatory adjustments are altered in older adults during lateral postural perturbations.

OBJECTIVE: This study investigated anticipatory postural adjustments (APAs) and compensatory postural adjustments (CPAs) and their relationship in older adults during lateral postural perturbations. METHODS: Unpredictable and predictable postural disturbances were induced by a swinging pendulum that impacted at the shoulder level of two groups of older adults, non-fallers (20) and fallers (20), and in a group of young control subjects (20). The electromyographic (EMG) activity of the postural muscles and the center of pressure (COP) displacement were recorded and quantified within the time intervals typical for APAs and CPAs. RESULTS: Both groups of older adults (non-fallers and fallers) showed higher magnitude of EMG activity in the lateral muscles and increased COP displacement, particularly, during the CPAs time interval when compared to the young group. Older adults, however, were able to change the electrical activity of the muscles during the predictable task by generating APAs with similar magnitudes of those found in young subjects. CONCLUSIONS: Compensatory but not anticipatory adjustments are altered in older adults during predictable lateral postural perturbations. SIGNIFICANCE: These findings provide new data on the role of APAs and CPAs in their relationship in older adults during external lateral perturbations and may advance current rehabilitative management strategies to improve balance control in older individuals.

Patterns of inter-joint coordination during a single-limb standing.

Past studies have documented contributions of multiple joints in maintaining a single-limb standing, but no reports on patterns of inter-joint coordination. It is also unknown whether such inter-joint coordination, if exists, depends on visual feedback. Eight health young volunteers took part in this study. The inter-joint coordination during a single-limb standing were examined using 3D joint kinematics. There were five testing trials with eyes open (EO) and five trials with eyes close (EC) conditions. During each trial the subject stood on the right leg on an even platform for 20 s while 3D kinematic data was recorded. Recorded data was processed for an "adjusted coefficient of multiple determinations (ACMD)" to evaluate the inter-joint similarities in joint motions. Under both EO and EC conditions moderate to good similarities were found in axial rotation between the ankle and hip joints, and between ankle inversion/eversion and hip axial rotation. This pattern of the inter-joint coordination might be a unique feature of biomechanical configuration of the lower extremity. The significant increases in joint rotations but maintained inter-joint coordination from EO to EC condition may indicate a minimal influence of vision on the inter-joint coordination. Future studies to test patients with pathological conditions in single-limb stance need to examine any alternation/impairment of the inter-joint coordination pattern.

Heightened flexor withdrawal responses following ACL rupture are enhanced by passive tibial translation.

OBJECTIVE: Hyperexcitability of nociceptive pathways has been demonstrated with several musculoskeletal conditions but not anterior cruciate ligament (ACL) injury. The purpose was to investigate flexor withdrawal reflex (FWR) excitability following ACL rupture and determine if painless stretch of knee joint structures enhanced reflexive responses. METHODS: Ten subjects with and 10 subjects without unilateral ACL rupture were compared. FWRs were induced through sural nerve stimulus in symmetrical stance and recumbent positions, with the knee in relaxed and stressed condition. Latencies and amplitudes of hamstring electromyographic activity were analyzed. RESULTS: FWR thresholds were significantly diminished (p=0.05) on the injured limb (11.8±8 mA) compared to non-injured limb (18.6±13 mA) and controls (22.5±3 mA). Anterior tibial translation resulted in increased (p=0.001) amplitude of EMG hamstring response on the injured limb (70±50%) versus control (-1±20%) and decreased latency (p=0.01) of hamstring activation (82.0±13 ms). CONCLUSIONS: Individuals with ACL rupture demonstrated increased excitability of FWR responses indicated by decreased FWR threshold and reduced hamstring muscle latency. Responses were enhanced by passive stretch of the knee joint. SIGNIFICANCE: Subjects with ACL rupture demonstrated hyperexcitability of nociceptive pathways on the injured limb which may trigger the FWR more readily and promote the sensation of instability at the knee.

The role of anticipatory postural adjustments in compensatory control of posture: 2. Biomechanical analysis.

The central nervous system (CNS) utilizes anticipatory (APAs) and compensatory (CPAs) postural adjustments to maintain equilibrium while standing. It is known that these postural adjustments involve displacements of the center of mass (COM) and center of pressure (COP). The purpose of the study was to investigate the relationship between APAs and CPAs from a kinetic and kinematic perspective. Eight subjects were exposed to external predictable and unpredictable perturbations induced at the shoulder level while standing. Kinematic and kinetic data were recorded and analyzed during the time duration typical for anticipatory and compensatory postural adjustments. When the perturbations were unpredictable, the COM and COP displacements were larger compared to predictable conditions with APAs. Thus, the peak of COM displacement, after the pendulum impact, in the posterior direction reached 28+/-9.6mm in the unpredictable conditions with no APAs whereas it was 1.6 times smaller, reaching 17+/-5.5mm during predictable perturbations. Similarly, after the impact, the peak of COP displacement in the posterior direction was 60+/-14 mm for unpredictable conditions and 28+/-3.6mm for predictable conditions. Finally, the times of the peak COM and COP displacements were similar in the predictable and unpredictable conditions. This outcome provides additional knowledge about how body balance is controlled in presence and in absence of information about the forthcoming perturbation. Moreover, it suggests that control of posture could be enhanced by better utilization of APAs and such an approach could be considered as a valuable modality in the rehabilitation of individuals with balance impairment.

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

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

Grip force control in individuals with multiple sclerosis.

BACKGROUND: Appropriate regulation of grip force is essential in performance of various activities of daily living such as drinking, eating, buttoning a shirt, and so on. The extent to which individuals with multiple sclerosis (MS) are able to regulate grip forces while performing elements of the activities of daily living is largely unknown. OBJECTIVE: . To investigate how individuals with MS control grip force during performance of functional tasks. METHODS: . This study evaluated the grip force control in selected individuals with MS (n = 9) and healthy control subjects (n = 9) while they performed the task of lifting and placing an instrumented object on a shelf and the task of lifting the object and bringing it close to the mouth to mimic drinking. The grip forces, object acceleration, force ratio, and time lag were recorded and analyzed. RESULTS: . The individuals with MS used significantly larger peak grip force and force ratio than control subjects while performing both tasks and for both hands. In addition, the time lag between the peaks of grip and load forces was significantly longer in individuals with MS. CONCLUSION: . The application of excessive grip force could predispose individuals with MS to additional fatigue and musculoskeletal overuse trauma. Rehabilitation protocols for the MS population may need to account for increased levels of grip force applied during the performance of functional tasks.

Role of movement velocity on the magnitude of grip force while lifting an object with touch from the contralateral finger.

We investigated whether slower velocity of arm movement affects grip-force generation in conditions with the finger touch provided to the wrist of the target arm. Nine subjects performed the task of lifting and transporting an object at slow, intermediate, and fast velocities with a light finger touch from the contralateral arm and without it. There was an effect of velocity of arm movement on grip-force generation in both conditions. However, when the no touch and touch trials performed with similar velocity were matched, the effect of touch on grip-force reduction was statistically significant (p < .001). The observed decrease in grip force could not be explained by slower movement execution in the touch conditions and underlines the importance of using a contralateral touch in the performance of activities of daily living. It also points to a possibility of the development of therapeutic advances for the enhancement of grip-force control in patients with neurological impairments.

Effect of contralateral finger touch on grip force control in individuals with multiple sclerosis.

OBJECTIVE: It was recently shown that the magnitude of grip force utilized to lift and transport a hand-held object decreased if a light touch from the contralateral finger is provided [Aruin AS. Support-specific modulation of grip force in individuals with hemiparesis. Arch Phys Med Rehabil 2005;86:768-75]. The rationale of the study is to find out whether or not the individuals with multiple sclerosis (MS) have the same behavior as control subjects. METHODS: Eight patients with MS and eight control subjects performed the task of lifting and transporting an instrumented object with provision of light touch of the contralateral index finger to the wrist of the target arm and without it. The tasks of placing the object on a shelf and a simulation of a drinking maneuver were studied. Peak grip force, force ratio, latency, and time lag were recorded and analyzed. RESULTS: The mean peak grip force decreased in conditions with application of light finger touch in patients and control subjects while performing both the experimental tasks. The force ratio also improved with application of light finger touch. CONCLUSIONS: Individuals with MS apply smaller grip force if they utilize a finger touch. SIGNIFICANCE: Findings provide better understanding of how patients with MS control grip force and suggest that a light touch approach could be considered as a valuable modality in the rehabilitation of these patients.

Effects of lateral perturbations and changing stance conditions on anticipatory postural adjustment.

The study investigates the role of lateral muscles and changing stance conditions in anticipatory postural adjustments (APAs). Subjects stood laterally to an aluminum pendulum released by an experimenter and were required to stop it with their right or left hand. Stance conditions were manipulated by having the subjects stand in the following positions: on a single limb (SS), with feet together (narrow base of support, NB), and with feet shoulder width apart (regular base of support, RB). Bilateral EMG activity of dorsal, ventral, and lateral trunk and leg muscles and ground reaction forces were recorded and quantified within the time intervals typical of APAs. Anticipatory postural adjustments were seen in all experimental conditions, and their magnitudes depended on the stance and the side of perturbation. Accordingly, APAs in lateral muscles increased on the side of perturbation in SS condition, while simultaneous activation of dorsal muscles occurred on the contralateral side. Smaller APAs were seen in lateral muscles in conditions with a wider base of support (NB, RB) and APAs in dorsal muscles were smaller in NB - in comparison to RB - stance. The results of the present study provide new data on the role of lateral, ventral, and dorsal muscles in anticipatory postural control when dealing with lateral perturbations in conditions of postural instability.

Anticipatory postural control following fatigue of postural and focal muscles.

OBJECTIVE: To investigate the effect of fatigue of postural and focal muscles on anticipatory postural adjustments (APAs). METHODS: Nine healthy adults performed rapid bilateral arm raising movements before and after isometric hamstring (postural) and deltoid (focal/prime mover) muscle fatigue. Muscle force and peak acceleration of the arm movements were recorded to assess the presence of fatigue. Ground reaction forces, EMG activity of trunk and leg muscles and center of pressure (COP) displacements were recorded and quantified within the time intervals typical of APAs. RESULTS: Early APA onset was seen in erector spinae and semitendinosus muscles post-deltoid fatigue. Anticipatory EMG integrals were reduced in the semitendinosus muscle post-hamstring fatigue, and were increased in the gastrocnemius muscle post-deltoid fatigue. No changes in COP displacement were observed following fatigue of both muscle groups. CONCLUSION: A common pattern of APA adaptations seen following fatigue of either muscle groups along with no changes observed in COP displacements emphasizes the efficiency of the CNS in maintaining dynamic postural stability in the presence of fatigue. SIGNIFICANCE: The outcome of the study is important for better understanding of the effect of muscular fatigue on feedforward mechanisms of postural control with possible implications for the elderly and individuals with neurological disorders.

The effectiveness of active exercise as an intervention for functional ankle instability: a systematic review.

Functional ankle instability (FAI) is a term used to describe an ankle that easily 'gives way' with activity. There have been many proposed causes of FAI including joint proprioceptive deficiency, muscle weakness, balance control impairments, and delayed muscle reaction time, none of which has proven to be the exclusive cause of FAI. Treatment becomes difficult when the causative factor of the injury is obscure. This systematic review evaluates the clinical trials involving conservative exercise interventions in FAI and examines the changes induced by the exercise treatments to the various potential FAI factors. Sixteen articles describing the active exercise treatment of FAI were analysed using Sackett's levels of evidence and were examined for scientific rigor. From this review, it can be concluded that conservative treatment interventions including balance, proprioceptive and muscle strengthening exercises are effective for patients with FAI in decreasing the incidence of giving-way episodes, improving balance stability, and improving function.

Possible factors related to functional ankle instability.

STUDY DESIGN: Case control study. OBJECTIVES: To classify individuals with functional ankle instability (FAI) into deficit and non-deficit categories based on the evaluation of the most common factors that have been proposed to be related to FAI. BACKGROUND: Recent studies have suggested that FAI may be secondary to a combination of factors including ankle proprioceptive deficit, muscular weakness, impaired balance, delayed neuromuscular reaction time, and joint laxity. However, only a few authors have investigated the prevalence and association among these factors in a single group of individuals. METHODS AND MEASURES: The above 5 factors were tested bilaterally in 21 individuals with FAI and in 16 healthy control subjects. Data were analyzed for (1) within- and between-group comparison, (2) classification of subjects with FAI into deficit and no-deficit categories, and (3) magnitude of association between factors in the subjects with FAI using Pearson bivariate correlation. RESULTS: Balance control and evertors' strength were significantly less on the affected side in comparison to the unaffected side in subjects with FAI. The evertors' strength was also significantly different between the side difference of the FAI group and the side difference of the control group. Passive ankle stiffness was significantly correlated to balance control, ankle proprioception, and evertor peak torque. Individuals with FAI demonstrated a large variation in the deficit categories ranging from multiple deficits to no noticeable deficits. CONCLUSION: Mechanical alterations in the ankle joint may influence several aspects of the ankle's functional ability. Alterations in the afferent processes, represented in this study by ankle proprioception, may effect the evertors' strength or vice versa. More importantly, individuals with FAI might exhibit high variability in ankle deficits.

Role of lateral muscles and body orientation in feedforward postural control.

The study investigates the role of lateral muscles and body orientation in anticipatory postural adjustments (APAs). Subjects stood in front of an aluminum pendulum and were required to stop it with their right or left hand. An experimenter released the pendulum inducing similar body perturbations in all experimental series. The perturbation directions were manipulated by having the subjects standing on the force platform with different body orientations in relation to the pendulum movements. Consequently, perturbations were induced in sagittal, oblique, and frontal planes. Ground reaction forces and bilateral EMG activity of dorsal, ventral, and lateral trunk and leg muscles were recorded and quantified within the time intervals typical of APAs. Anticipatory postural adjustments were seen in all experimental conditions; their magnitudes depended on the body orientation in relation to the direction of perturbation. When the perturbation was produced in the lateral and oblique planes, APAs in the gluteus medius muscles were greater on the side opposite to the side of perturbation. Conversely, simultaneous anticipatory activation of the external obliques, rectus abdominis, and erector spinae muscles was observed on the side of perturbation when it was induced in the lateral plane. The results of the present study provide additional information on the directional specificity of anticipatory activation of ventral and dorsal muscles. The findings provide new data on the role of lateral muscles in feedforward postural control and stress the importance of taking into consideration their role in the control of upright posture.

Unloading reactions in functional ankle instability.

Past studies have suggested that unloading reactions may be a strategy to prevent ankle sprains. This study tested unloading reactions in individuals with functional ankle instability (FAI) in order to better understand this phenomenon. We provoked unloading reactions in 20 individuals with FAI and 18 healthy controls by delivering nociceptive electrical stimulation to the lateral aspect of the ankle during standing. Ground reaction forces, lower extremity kinematics, and EMG activity of five muscles of the lower limb were recorded. Individuals with FAI demonstrated increased and faster body weight unloading after stimulation. This hyper-reactivity may partially account for the sensation of the ankle "giving way" in those with a history of severe ankle sprain and subsequent functional instability.

Does the location of the touch from the contralateral finger application affect grip force control while lifting an object?

It was recently shown that the magnitude of grip force used to lift and transport a hand-held object decreased if a light finger touch from the contralateral arm was provided to the wrist of the target arm [A.S. Aruin, Support-specific modulation of grip force in individuals with hemiparesis, Arch. Phys. Med. Rehabil. 86 (2005) 768-775]. In this study, we investigated whether the location of the finger touch along the target arm affects the way grip force is reduced. Subjects performed the same task of lifting and transporting an instrumented object with no involvement of the contralateral arm and when an index finger touch of the contralateral arm was provided to the wrist, thumb, mid-forearm, and the hand-held object. Grip force was reduced by approximately the same amount in all conditions with the finger touch compared to the no touch condition suggesting that its reduction was not associated with a particular point of contact of the finger with the target arm. The results of the study provide additional evidence to support of the use of a second arm in the performance of activities of daily living and stress the importance of future studies investigating contralateral arm sensory input on grip force control.

The effect of joint instability on latency and recruitment order of the shoulder muscles.

Several shoulder dysfunctions are generally characterized as Shoulder Impingement Syndrome (SIS). Specifically, glenohumeral instability (GI) has been viewed as a primary cause of SIS in young individuals, mainly in overhead athletics. Past studies have associated GI with modifications in latency, recruitment order and/or EMG activity. However, it is not clear if pain and/or joint instability can account for these observed changes. The aim of this investigation was to analyze the effect of glenohumeral instability on the latencies and recruitment order of the superficial muscles of the glenohumeral and scapulothoracic joints in swimmers without pain symptom. Eight individuals with and eight without history of Shoulder Impingement Syndrome performed bilateral and simultaneous shoulder elevations at three different distances. The shoulder kinematics and EMG activities of glenohumeral and scapulothoracic muscles were registered. Results showed that subjects of both groups performed the task with similar latencies and recruitment order of the muscle activities. We conclude that shoulder instability does not necessarily affect the latencies and recruitment order of the shoulder muscles during the elevation of the shoulder in the scapular plane. Pain and other factors may be involved in the kinematics and electromyographic alterations demonstrated in other experiments.

Unloading reaction to electrical stimulation at neutral and supinated ankle positions.

An unloading reaction has been characterized as a modified flexor reflex (FR), in which the standing subjects decrease the load on the stimulated foot and increase the load on the contralateral side, but, without withdrawal of the stimulated foot. Different behavioral circumstances have been shown to modulate this reflex. It is not known whether unloading reactions can be modulated with a loaded supinated ankle position, which, in excess, may result in an ankle sprain injury. Since ankle sprain depends on the load applied to a supinated foot, our premise is that unloading reactions may protect the ankle from a sprain injury. Therefore, this study investigated how the unloading reactions were modulated during a loaded supinated ankle condition. We delivered non-nociceptive and nociceptive electrical stimulations on the lateral aspect of the ankle in standing subjects with the foot in neutral and in a supinated position. The magnitude and latencies of reflex responses were registered using kinetic and kinematic analyses and subsequently compared among the conditions. The analysis demonstrated greater reactions for the supinated ankle condition. The individuals also moved their whole body downwards and shifted the body weight to the non-stimulated foot. Therefore, this study suggested that a modified type of the classic flexion reflex, i.e., unloading reaction, may be used as a strategy to unload a supinated ankle and potentially minimize the risk of ankle sprain injuries.

The effects of ankle bracing on motion of the knee and the hip joint during trunk rotation tasks.

BACKGROUND: The use of prophylactic ankle braces is common during athletic activities since the ankle is one of the most commonly injured joints. Past studies have focused on the effects of ankle braces on ankle movement restriction, preventing injuries, proprioception, balance and athletic performance. However, the influence of ankle restriction on other joints has not been studied. The constraint of ankle movement may lead to an increased loading on the knee joint, which could be a potential risk of knee injuries during athletic activities. The primary goal of the current study was to determine quantitatively the effect of an ankle brace on the knee axial rotation during two different trunk turning tasks. METHODS: Ten healthy subjects performed trunk turning movements while standing on one leg: turning sideways to catch a ball and turning sideways to touch a target with the shoulder. The tasks were performed with and without an ankle brace worn on the supporting leg. The trunk axial rotation in reference to the floor and three dimensional joint angular motions of the ankle, knee and hip were determined. FINDINGS: The use of an ankle brace resulted in reduced trunk axial rotation during the ball catching tasks, and increased knee axial rotation during the target touching tasks. INTERPRETATION: The results of this study showed that the effect of the ankle brace on the knee axial rotation depended on the context of the tasks performed. Under situations that required forceful trunk turning movement while standing on a single leg, the ankle braces may cause an increase in the knee axial rotation indicating higher risk of knee injury.