Investigation of the effect of tonus on the change in postural control strategy using musculoskeletal simulation

BackgroundHumans use different postural control strategies depending on perturbations. The shift from an ankle strategy to a hip strategy occurs at different perturbation magnitudes for different individuals. Although such differences relate to the differences in body parameters such as muscle strength, the parameter changes that affect the strategy shift are unclear. The relationship between tonus and strategy is especially unclear, but humans control tonus, which contributes to body stability. The objective of this study was to investigate the influence of tonus on postural control strategy.Research questionIs there a trend toward the use of a hip strategy with a decrease in the magnitude of tonus?MethodsPredictive simulations were performed for changing parameters of muscle weakness and increased sensory noise, which are considered the causes of different strategies and decreased tonus. A musculoskeletal model with 70 muscles and 15 degrees of freedom of joints was controlled using a neural controller model, and the support surface was translated backward to introduce perturbations. The parameters of the musculoskeletal and neural controller models were changed for 48 conditions of different muscle strengths, sensory noise, and tonus. The control parameters were optimized for each condition. Simulations were performed with the optimized control parameters to calculate an evaluation index to show the difference in postural control strategies (peak hip angle), and the relationship between the index and parameters was analyzed using analysis of variance and multiple regression.ResultsThe main effects of muscle weakness and decreased tonus and their interaction were confirmed. The results recognized a positive response to the research question.SignificanceThe study emphasizes the importance of considering tonus while investigating the postural control strategy. Furthermore, it was suggested that when the magnitude of tonus was larger than a threshold, only the ankle strategy was used, regardless of muscle strength.     

Postural strategies assessed with inertial sensors in healthy and parkinsonian subjects

The present study introduces a novel instrumented method to characterize postural movement strategies to maintain balance during stance (ankle and hip strategy), by means of inertial sensors, positioned on the legs and on the trunk.We evaluated postural strategies in subjects with 2 types of Parkinsonism: idiopathic Parkinson's disease (PD) and Progressive Supranuclear Palsy (PSP), and in age-matched control subjects standing under perturbed conditions implemented by the Sensory Organization Test (SOT). Coordination between the upper and lower segments of the body during postural sway was measured using a covariance index over time, by a sliding-window algorithm. Afterwards, a postural strategy index was computed. We also measured the amount of postural sway, as adjunctive information to characterize balance, by the root mean square of the horizontal trunk acceleration signal (RMS).Resultsshowed that control subjects were able to change their postural strategy, whilst PSP and PD subjects persisted in use of an ankle strategy in all conditions. PD subjects had RMS values similar to control subjects even without changing postural strategy appropriately, whereas PSP subjects showed much larger RMS values than controls, resulting in several falls during the most challenging SOT conditions (5 and 6). Results are in accordance with the corresponding clinical literature describing postural behavior in the same kind of subjects.The proposed strategy index, based on the use of inertial sensors on the upper and lower body segments, is a promising and unobtrusive tool to characterize postural strategies performed to attain balance.     

Lower limb joint-specific contributions to standing postural sway in persons with unilateral lower limb loss

BackgroundIndividuals with lower limb loss are at an increased risk for falls, likely due to impaired balance control. Standing balance is typically explained by double- or single-inverted pendulum models of the hip and/or ankle, neglecting the knee joint. However, recent work suggests knee joint motion contributes toward stabilizing center-of-mass kinematics during standing balance.Research QuestionTo what extent do hip, knee, and ankle joint motions contribute to postural sway in standing among individuals with lower limb loss?MethodsForty-two individuals (25 m/17f) with unilateral lower limb loss (30 transtibial, 12 transfemoral) stood quietly with eyes open and eyes closed, for 30 s each, while wearing accelerometers on the pelvis, thigh, shank, and foot. Triaxial inertial measurement units were transformed to inertial anterior-posterior components and sway parameters were computed: ellipse area, root-mean-square, and jerk. A state-space model with a Kalman filter calculated hip, knee, and ankle joint flexion-extension angles and ranges of motion. Multiple linear regression predicted postural sway parameters from intact limb joint ranges of motion, with BMI as a covariate (p < 0.05).ResultsWith eyes open, intact limb hip flexion predicted larger sway ellipse area, whereas hip flexion and knee extension predicted larger sway root-mean-square, and hip flexion, knee extension, and ankle plantarflexion predicted larger sway jerk. With eyes closed, intact limb hip flexion remained the predictor of sway ellipse area; no other joint motions influenced sway parameters in this condition.SignificanceHip, knee, and ankle motions influence postural sway during standing balance among individuals with lower limb loss. Specifically, increasing intact-side hip flexion, knee extension, and ankle plantarflexion motion increased postural sway. With vision removed, a re-weighting of lower limb joint sensory mechanisms may control postural sway, such that increasing sway may be regulated by proximal coordination strategies and vestibular responses, with implications for fall risk.     

The control of upright stance in young, elderly and persons with Parkinson's disease

The aims of the present study are twofold: (1) to compare the postural control mechanisms of young and elderly people as well as in Parkinson's disease (PD) patients during quiet standing and (2) to assess the impact of a stooped posture on these mechanisms. All subjects were required to maintain both a side-by-side and a 45 degrees foot position. Elderly subjects performed a third condition where they were requested to mimic the stooped posture as adopted by PD subjects. The net centre of pressure (COP(net)) and centre of mass (COM) profiles in the anterior/posterior (A/P) and medial/lateral (M/L) planes were analyzed. The COP(net) signal was recorded from two force plates and was categorized in two mechanisms: an ankle mechanism (COP(c)) and a load/unload hip mechanism (COP(v)). The results showed similar postural control mechanisms in young, elderly and PD subjects. When the feet were side-by-side, the COP(net) was controlled by the ankle plantar/dorsiflexors (COP(c)) in the A/P direction, while by the hip abductor/adductors (COP(v)) controlled in the M/L direction. When the feet were in the 45 degrees position, both the ankle and hip mechanisms contributed to the COP(net). However, the PD subjects showed significant smaller RMS amplitudes compared to the elderly people in the 45 degrees foot position and in the stooped posture. These findings suggest that PD subjects resort to a stiffening strategy to control their balance in postural tasks that imply a mixed control (ankle and hip mechanisms) but have adapted to their stooped posture.     

Disrupted somatosensory input alters postural control strategies during the Star Excursion Balance Test (SEBT) in healthy people

BackgroundChronic adaptations, including persistent sensorimotor deficits, remain in individuals with a history of ankle instability, resulting in altered postural control strategies during functional tasks such as gait, running, or landing. However, we do not know the contribution of the altered somatosensory input on postural control strategies during a dynamic balance task such as the Star Excursion Balance Test (SEBT).Research questionThe purpose of this study was to characterize postural control strategies with and without disrupted somatosensory input during a dynamic balance task in people without chronic ankle sprain.MethodsThis study was a crossover study design. Twenty healthy young adults (10 men, 10 women; age = 23.9 ± 3.0 years, height = 174.2 ± 7.4 cm, mass = 71.2 ± 16.7 kg) performed the posteromedial reach test during the SEBT while standing on the ground and on foam. We measured the maximum reach distance (MRD); joint angles of the ankle, knee, hip, and trunk in the sagittal, frontal, and transverse planes; and position and displacement of the center of mass (COM) and center of pressure (COP) during the posteromedial reach task.ResultsThe MRD was shorter when standing on the foam than on the ground (p < 0.001). There was a condition by phase interaction for ankle dorsiflexion; tibia internal rotation; and trunk flexion (p < 0.001; p = 0.03; p = 0.01, respectively). The COM and COP were positioned more laterally on the foam (p < 0.001). The COM and COP anterior-posterior displacements were more anterior during the foam condition (p = 0.017).SignificanceBy using a foam pad to disrupt somatosensory information, participants demonstrated altered strategies to control the joint kinematics, COM, and COP, as a function of posteromedial distance. Ankle and trunk movement strategies may influence the posteromedial reach distance. This model may simulate changes that occur with chronic ankle instability.     

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.     

The effects of attentional focus and cognitive tasks on postural sway may be the result of automaticity

Research reveals improvements in postural control when focus is placed on movement effects rather than movement production, and further improvements during the performance of a concurrent cognitive task. It has yet to be determined if these changes are due to the use of an ankle stiffening strategy or to the use of more automatic postural control processes. The objectives of the present study were to replicate the effect of attentional focus and cognitive tasks on postural control and to test that no change occurs in lower leg muscle activity in these conditions. Twenty five healthy young adults (20.7 ± 2.76 years, 10 male) were asked to stand still while performing various tasks: baseline standing, internally focusing on minimizing movement of the ankles, externally focusing on minimizing movement of an apparatus placed on their ankle joint, and two cognitive tasks consisting of counting and simultaneously summing one or two single digits in a series of three-digit numbers. Compared to baseline and internal focus, sway decreased in external focus conditions and decreased further in cognitive task conditions. Furthermore, sway velocity increased in cognitive task conditions and sway frequency increased in the medial-lateral direction in the more difficult cognitive task. Finally, no effect of condition was found on muscle activity around the ankle joint. Collectively, the findings lend support to the hypothesis that changes in postural control were the result of an automatic type of postural control rather than due to stiffening occurring at the ankle joint.     

Effect of age on anticipatory postural adjustments in unilateral arm movement

Aging is frequently accompanied by a deterioration in postural control. Accordingly, the elderly adopt postural strategies in order to maintain balance. The purpose of this study was to compare anticipatory postural adjustments in (healthy) 10 young and 10 elderly subjects using electromyography (EMG) and biomechanical parameters. While standing on a force platform, subjects performed voluntary, arm-raising movements under five conditions: self-paced at three different velocities, self-paced with load and an externally triggered, both at maximal velocity. The force platform provided information on vertical torque (T(z)) and center of pressure anteroposterior displacements (COP). EMG activity was recorded from the biceps femoris, quadriceps, tibialis anterior and soleus muscles. Voluntary movements were associated with an early COP backward shift and an anticipatory T(z). At low velocity, elderly subjects did not show any impairment in stability. At maximal velocity, T(z) was delayed in all conditions in the elderly group, whereas COP latency was reduced only in the self-paced condition without load. Despite this decrease in anticipation, the movement was performed at the same velocity as in younger subjects. The elderly adopted various muscle strategies in order to perform the same movement with less stability. In the self-paced condition, elderly subjects used a hip strategy, whereas young subjects used an ankle strategy. In the triggered condition, the strategy corresponded to increased activation of certain thigh muscles, rather than a sequence modification. Hence, local muscle strategies were used to counteract the overall delay in postural preparation revealed by biomechanical parameters.     

Auditory cues behind congenitally blind subjects improve their balance control in bipedal upright posture

BackgroundCongenitally blind subjects developed postural adaptations improving somatosensory and vestibular systems to maintain upright stability and auditory skills to orient them in environment. However, the influence of auditory cues on upright stability in congenitally blind subjects stays unknown.Research questionThe aim of this study is to define the influence of an auditory cue in congenitally blind subjects back space on their balance posture.MethodsEleven sighted subjects and eleven congenitally blind subjects performed upright bipedal and unipedal quiet stances on a force plate with two conditions of auditory cue played by a loudspeaker placed 2 m behind them. Mean CoP velocity were recorded. Student test was used to compare significant difference between blind and sighted subjects bipedal and unipedal postures stability in both conditions of auditory cue.Results and significanceResults showed that congenitally blind subjects had no significant difference in mean sway velocity compared to sighted subjects in bipedal upright posture in auditory signal condition. However, blind subjects had significant lower mean sway velocity than sighted subjects in bipedal upright posture without sound. Blind subjects had significant increased mean sway velocity during unipedal quiet standing in both auditory cue conditions (with and without sound). The results showed that congenitally blind subjects used auditory cues placed behind them in order to improve their balance control in bipedal upright posture. In this case, blind subjects could better use compensatory mechanisms to perform quiet standing as sighted subjects. Without sound or in unipedal upright posture, congenitally blind subjects probably have sensory perturbations or limitations that impose them adaptations in order to avoid falling risk. Auditory cues should be study in the aim to better understand the compensatory mechanisms used by congenitally blind subjects to perform postural balance in usual environment.     

Balance dysfunction in hereditary and spontaneous spastic paraparesis

ObjectiveTo determine how postural sway is affected in people with spastic paraparesis (pwSP) and the impact of different impairments.MethodsIn 20 pwSP and 18 matched healthy controls standing postural sway was measured with eyes open and closed. Vibration threshold, isometric ankle and hip muscle strength and ankle stiffness with the participant at rest or preactivating the muscle was measured.ResultsAntero-posterior (AP) and medio-lateral (ML) sway was higher in pwSP. Muscle strength was reduced and ankle stiffness increased in pwSP. Increased vibratory threshold was seen in 35% of participants. Higher total ankle stiffness (R² = 0.44) was associated with lower AP sway with eyes open whilst hip abductor weakness was associated with increased ML sway with eyes open (R² = 0.36) or closed (R² = 0.47) or AP sway with the eyes closed (R² = 0.48).ConclusionsThe degree of postural sway was related to muscle paresis of the hip abductors particularly in the ML direction and under conditions of reduced sensory input. People with higher total ankle stiffness have less AP sway suggesting that this may help to stabilise the body.     

The effect of trunk muscle fatigue on postural control of upright stance: A systematic review

BackgroundFatigability and postural control deficits are both serious concerns in a variety of chronic musculoskeletal conditions. Research has shown that muscle fatigue may adversely affect postural control. This is while the evidence on the relevance of fatigue to postural control has never been summarized nor critically appraised.Research questionIs there sufficient and strong enough evidence to accept trunk muscle fatigue as a contributing factor to postural control alterations during upright standing posture?.MethodsEMBASE, Scopus, ELSEVIER, PubMed, ProQuest, Google scholar and reference lists of the relevant articles were searched through April 2018. Studies having investigated the trunk muscle fatigue effect on postural control in asymptomatic individuals were included in the study. Only those studies having assessed postural control in terms of center of pressure driven variables were included.ResultsTwelve studies (218 asymptomatic participants) matched the inclusion criteria of this systematic review. Their results supported the hypothesis that fatigue has a significant effect on postural control in terms of the time domain variables. Sway velocity was consistently found to be affected by fatigue. The results were inconsistent in the frequency domain. The only study on the structural dynamics of center of pressure displacements also confirmed such a relationship.SignificanceThe present review indicates that postural control is altered in asymptomatic individuals following trunk muscle fatigue. This may suggest that trunk muscle endurance training is crucial to address postural impairment in chronic spine musculoskeletal conditions.     

Does passive hip stiffness or range of motion correlate with spinal curvature and posture during quiet standing?

BackgroundIt has been postulated that hip muscle stiffness can be inferred from postural assessment, and that predictable relationships exist between passive stiffness of opposing muscles. Despite a lack of evidence to demonstrate such relationships, manual therapy textbooks continue to direct clinicians to treat hip muscles following postural analysis.Research QuestionDo significant correlations exist between standing posture, and hip muscle stiffness and range of motion?Methods20 participants volunteered for this observational study. Passive hip stiffness was calculated as the derivative of the moment-angle curve measured during leg raising in a variety of lying postures. Stiffness was measured during hip flexion, extension, abduction and adduction. Stiffness data was obtained during the first degree and at ROM, and at the highest common angle achieved by participants in each posture Spinal curvature and other postural components were measured using a motion tracking device. Spearman rank coefficients were determined to assess any correlations between passive stiffness, range of motion, spinal curvature and other postural components.ResultsConsistent relationships were found between maximum range of motion and hip stiffness at the common angles (P < 0.01), but not at maximum stiffness (P > 0.05). Consistent correlations were found between abductor stiffness and Q-angle (P < 0.05). Hip extensor range of motion correlated with lumbar lordosis (r = -0.472, P = 0.036). Other correlations were reported, but typically lacked consistency between left and right sides. Spinal curvature did not correlate with hip stiffness in frontal or sagittal planes (P > 0.05).SignificanceSignificant correlations were few compared with where correlations lacked statistical significance. This study demonstrates that overly simplistic assumptions about spinal posture and hip stiffness cannot be supported. Hip muscle stiffness should only be targeted in treatments where testing has demonstrated a direct need, and not assumed based upon postural assessment alone.     

Relationship between postural sway on an unstable platform and ankle plantar flexor force steadiness in community-dwelling older women

BackgroundForce steadiness is evaluated as force variability during constant force exertion around a target level. Ankle plantar flexor force steadiness is reported to be related to postural sway on an unstable platform in healthy young adults; however, this relationship in older adults is unclear.Research questionThis study aimed to investigate whether ankle plantar flexor force steadiness was related to postural sway on stable and unstable platforms in older adults.MethodsTwenty-six community-dwelling older women participated in this study (72 ± 6 years). Maximal isometric strength and force steadiness at 5%, 20 %, and 50 % of the maximal strength of ankle plantar flexion were assessed. Postural sway in the anteroposterior direction during bipedal standing was measured on stable and unstable platforms.ResultsThe results showed that force steadiness at any intensity level and maximal isometric strength were not related to postural sway on the stable platform. Force steadiness at 20 % of maximal strength alone was significantly correlated with postural sway on the unstable platform (ρ = 0.441, p < 0.05).SignificanceThese results indicate that the ability to control muscle force could be important for postural stability on an unstable platform in older adults.     

Voluntary control of forward leaning posture relates to low-frequency neural inputs to the medial gastrocnemius muscle

BackgroundVariability is an inherent feature of the motor output. Although low-frequency oscillations (<0.5 Hz) are the most important contributor to the variability of force during single-joint isolated force tasks, it remains unclear whether they contribute to the variability of a more complex task, such as a voluntary postural task.Research questionDo low-frequency oscillations contribute to postural sway (center of pressure (COP) variability) when participants attempt to voluntarily maintain posture in a forward leaning position?MethodsFourteen healthy young adults performed two tasks: 1) stand quietly (control condition); 2) leaned their body forward to 60% of their maximum lean distance by dorsiflexing the ankle joint. We recorded the COP and electromyographic (EMG) activity from the medial gastrocnemius (MG) and soleus (SL) muscles. We quantified the following: 1) COP variability as the standard deviation (SD) of anteroposterior COP displacements; 2) modulation of COP as the power in COP displacements from 0 to 2 Hz; 3) modulation of EMG bursting as the power in the rectified and smoothed EMG from 0 to 2 Hz; 4) modulation of the interference EMG as the power in the EMG from 10 to 35 and 35–60 Hz.ResultsThe SD of COP displacements related to the COP oscillations <0.5 Hz in both quiet standing and lean tasks. However, only for the lean task, the <0.5 Hz COP oscillations related to the EMG burst oscillations <0.5 Hz of the MG muscle. The EMG burst oscillations <0.5 Hz of the MG muscle further related to the interference EMG oscillations from 35 to 60 Hz for the lean task.SignificanceVoluntary control of forward leaning posture relates to low-frequency neural inputs to the MG muscle.     

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.     

Intrinsic foot muscle hardness is related to dynamic postural stability after landing in healthy young men

BackgroundThe human foot has competent mechanisms for supporting weight and adapting movement to various surfaces; in particular, the toe flexor muscles aid in supporting the foot arches and may be important contributors to postural stability. However, the role of intrinsic foot muscle morphology and structure in the postural control system remains unclear, and the relationship between them is not well known.Research questionAre intrinsic foot muscle morphology and toe flexor strength related to static and dynamic postural stability in healthy young men?.MethodsA total of 27 healthy men aged 19–27 years participated in this study. intrinsic foot muscle morphology included muscle hardness and thickness. Cross-sectional area was measured by ultrasonography at an ankle dorsiflexion angle of 0°. The hardness of the abductor hallucis (AbH), flexor hallucis brevis, and flexor digitorum brevis (FDB) muscles was measured using ultrasound real-time tissue elastography. Static postural stability during single-leg standing on a single force platform with closed eyes was assessed for the right leg. In the assessment of dynamic postural stability, the subjects jumped and landed on single-leg onto a force platform and the dynamic postural stability index (DPSI) was measured.ResultsFDB muscle thickness showed a positive correlation with anteroposterior stability index (APSI) (r = 0.398, p = 0.040). AbH muscle hardness was negatively correlated with APSI (r = −0.407, p = 0.035); whereas FDB muscle hardness was positively correlated with DPSI (r = 0.534, p = 0.004), vertical stability index (r = 0.545, p = 0.003), and maximum vertical ground reaction force (r = 0.447, p = 0.020). Multiple regression with forced entry revealed that only DPSI was significantly correlated with FDB muscle hardness (p = 0.003).SignificanceThe results indicated that intrinsic foot muscle hardness plays an important role in dynamic postural control among healthy young men, which may enable a more rapid muscular response to changes in condition during jump landing and better performance in balance tasks.     

Postural adaptation to walking on inclined surfaces: I. Normal strategies.

This study investigated the postural strategies to adapt to uphill and downhill treadmill inclination (0, 5 and 10%) during walking and standing in eight healthy subjects. Increasing the treadmill grade from 0 to 10% induced an increasingly flexed posture of the hip, knee and ankle at initial foot contact as well as a progressive forward tilt of pelvis and trunk. These postural changes were accompanied by a progressive decrease in pelvic lateral drop toward the swinging limb and a gradual increase in stride length as the uphill slope became steeper. Decreasing the treadmill grade from 0 to -10% lead to a decreasingly flexed posture of the hip at initial foot contact as well as an increase in knee flexion during weight acceptance and late stance. These changes were accompanied by a gradual decrease in stride length, a progressive backward tilt of trunk and pelvis and an increase in pelvic lateral drop toward the swinging limb as downhill slope became steeper. Changes in trunk and pelvic postural alignment in the sagittal plane might be used to facilitate power generation or absorption in adapting to slope changes during walking. During quiet standing, however, the trunk and pelvis remained aligned with respect to earth's vertical at any surface inclination. These results showed that postural adaptations are task-specific and the control requirements are different between standing and walking on an inclined surface.     

Effects of initial foot position on postural responses to lateral standing surface perturbations in younger and older adults

BackgroundAn age-related decline in standing balance control in the medio-lateral direction is associated with increased risk of falls. A potential approach to improve postural stability is to change initial foot position (IFP).Research questionsIn response to a lateral surface perturbation, how are lower extremity muscle activation levels different and what are the effects of different IFPs on muscle activation patterns and postural stability in younger versus older adults?MethodsTen younger and ten older healthy adults participated in this study. Three IFPs were tested [Reference (REF): feet were placed parallel, shoulder-width apart; Toes-out with heels together (TOHT): heels together with toes pointing outward; Modified Semi-Tandem (M-ST): the heel of the anterior foot was placed by the big toe of the posterior foot]. Unexpected lateral translations of the standing surface were applied. Electromyographic (EMG) activity of the lower extremity muscles, standard deviation (SD) of the body’s CoM acceleration (SD of CoMAccel), and center of pressure (CoP) sway area were compared across IFPs and age.ResultsActivation levels of the muscles serving the ankle and gluteus medius were greater than for the knee joint muscles and gluteus maximus in the loaded leg across all IFPs in both groups. TOHT showed greater EMG peak amplitude of the soleus and fibularis longus compared to REF, and had smaller SD of CoMAccel and CoP sway area than M-ST. Compared to younger adults, older adults demonstrated lower EMG peak amplitude and delayed peak timing of the fibularis longus and greater SD of CoMAccel and CoP sway area in all IFPs during balance recovery.SignificanceDuring standing balance recovery, ankle muscles and gluteus medius are important active responders to unexpected lateral surface perturbations and a toes-out IFP could be a viable option to enhance ankle muscle activation that diminishes with age to improve postural stability.     

Acute effects of ankle plantar flexor force-matching exercises on postural strategy during single leg standing in healthy adults

BackgroundAnkle plantar flexor force steadiness, assessed by measuring the fluctuation of the force around the submaximal target torque, has been associated with postural stability.Research questionTo investigate whether a force-matching exercise, where submaximal steady torque is maintained at the target torque, can modulate postural strategy immediately.MethodsTwenty-eight healthy young adults performed ankle plantar flexor force-matching exercises at target torques of 5%, 20%, and 50% of maximum voluntary contraction (MVC), in a randomized crossover trial. Participants with their ankle in a neutral position were instructed to maintain isometric contraction at each target torque, as measured by a dynamometer, for 20 s with 3 sets of 5 contractions. Before and after the force-matching exercises, the anterior-posterior velocities and standard deviation of the center of pressure (COP) on the stable platform and the tilt angle of the unstable platform during 20-seconds single-leg standing were measured. The velocities and standard deviations of the COP and tilt angle before and after the exercises were compared using paired t-tests.ResultsThe tilt angle velocity of an unstable platform significantly decreased after the force-matching exercise at a target torque of 5% MVC (p = 0.029), whereas it was unchanged after the exercises at target torques of 20% and 50% MVC. The standard deviations of the tilt angle of unstable platform test did not change significantly after any exercise. Furthermore, no significant differences were observed in the COP velocities or standard deviations on the stable platform test after any exercise.SignificanceOur findings suggest that repeated exertion training at low-intensity contractions can affect postural stability in an unstable condition. Particularly, force-matching exercise at very low-intensity torque, such as 5% of MVC, may be an effective method to improve postural control in the unstable condition, but not in a stable condition.