Influence of wearing an unstable shoe construction on compensatory control of posture

This study investigated the influence of wearing unstable shoe construction (WUS) on compensatory postural adjustments (CPA) associated with external perturbations. Thirty-two subjects stood on a force platform resisting an anterior-posterior horizontal force applied to a pelvic belt via a cable, which was suddenly released. They stood under two conditions: barefoot and WUS. The electromyographic (EMG) activity of gastrocnemius medialis, tibialis anterior, rectus femoris, biceps femoris, rectus abdominis, and erector spinae muscles and the center of pressure (CoP) displacement were acquired to study CPA. The EMG signal was used to assess individual muscle activity and latency, antagonist co-activation and reciprocal activation at joint and muscle group levels. Compared to barefoot, WUS led to: (1) increased gastrocnemius medialis activity, (2) increased total agonist activity, (3) decreased antagonist co-activation at the ankle joint and muscle group levels, (4) increased reciprocal activation at the ankle joint and muscle group levels, and (5) decrease in all muscle latencies. No differences were observed in CoP displacement between conditions. These findings demonstrate that WUS led to a reorganization of the postural control system associated to improved performance of some components of postural control responses.     

The organization of spontaneous leg movements in newborn infants

Spontaneous, supine kicking in newborn (2- and 4-week-old) infants is described in terms of its temporal structure, interjoint coordination, and muscle activation characteristics as measured by surface electromyography. Phasic kick movements shoed a constrained temporal organization in the movement, but not the pause phases. Hip, knee, and ankle joints moved in temporal and spatial synchrony, and all three joints showed a rhythmical or periodic organization over time. EMGs revealed antagonist coactivation at the initiation of the flexor movement, but little or not extensor activity. The dorsal muscles, the gastrocnemius and hamstrings, showed less activity than the ventral pair, tibialis anterior and quadriceps. Burst and onset-to-peak durations were also constrained. As a result of neural mechanisms and biomechanical forces, newborn leg movements are structured muscle synergies. This organization has implications both for newborn functioning and for later development.     

Interaction between transient change in local muscle structure and segmented leg motor responses

We investigated the interaction between transient change in the local structure of gastrocnemius muscle fibers and the segmented leg motor responses elicited by toes-up rotation of the support surface. The gastrocnemius muscle in healthy young subjects underwent a sustained 2-min, isometric stretch (Prestretch group), followed immediately by the rotations. Analysis showed that the prestretch affected the amplitude of the monosynaptic but not the automatic components of the medial gastrocnemius muscle responses (monosynaptic response: 93 +/- 20% of the mean of the last three rotations in the Prestretch group compared to 174 +/- 42% in the Control group; automatic response: 128 +/- 18% in the Prestretch group versus 123 +/- 31% in the Control group). The shortened tibialis anterior muscle response and the onset latencies of responses were not affected by prestretching the gastrocnemius muscle. The prestretch effect on the amplitude of the monosynaptic (mediated by group Ia afferents) but not the automatic gastrocnemius muscle response (mediated by group II afferents) suggests that the automatic component of the muscle response to platform rotations has a stronger supraspinal influence than the monosynaptic response and is thus less affected by local events such as prolonged stretching.     

Characteristics of saccadic eye movements in attentionally handicapped patients.

Some characteristics of saccadic eye movements were investigated electro-oculographically in hemiplegic attentionally handicapped patients and normal subjects under stationary and moving target situations. The velocity of saccadic eye movements was greater and the duration shorter in hemiplegic than in normal subjects. Greater variability of angular velocity and amplitude functions, and of duration and amplitude functions was found among hemiplegic subjects than among normal. The results were explained, in part, by possible differences in the strength of extra-ocular muscle contraction, variables associated with inhibitory control and learning variables.     

Perturbations of human posture: influence of impulse modality on EMG responses and cerebral evoked potentials

Leg muscle EMG responses and cerebral evoked potentials (CP), elicited by perturbations of stance while on a treadmill with split belts, were analyzed in order to study the relationship between compensatory leg muscle responses and afferent input to supraspinal centers. Various conditions of perturbation were used to establish the extent to which compensatory EMG responses and CPs show congruent behavior. Four different treadmill acceleration rates were applied in three different conditions (unilateral perturbation, directed forward or backward; bilateral perturbation, directed forward or backward; and opposing bilateral perturbation). EMG responses and CPs showed parallel increases in amplitude with increasing displacement velocity. The EMG responses showed distinct differences, predominantly in the response amplitude, between the different perturbation conditions, whereas the CPs were affected only to a minor degree. Tibialis anterior EMG responses were more closely related to the CP following forward perturbation than the corresponding gastrocnemius responses were to the CP following backward perturbation. We conclude that the EMG responses are more closely related than the CPs to displacement parameters and suggest that this is due to the further spinal processing of the afferent input needed to generate an appropriate EMG response. The closer relationship between the tibialis anterior response and CP may reflect a predominant central representation and control of tibialis anterior activation in the regulation of posture. The functional implications of these findings are discussed.     

Selective activation of lower leg muscles during maximum voluntary isometric contractions

The pronators and supinators play a key role in the medio-lateral stability of the ankle joint complex (i.e. talo-crural and subtalar joints). We hypothesized that each shank muscle has a specific activation pattern determined by its anatomical course around the axes of the subtalar and talo-crural joints. A secondary objective was to examine the effect of foot posture on these activation patterns. Forty-nine young adults (25 normal-arched feet, 24 flat-arched feet) performed maximum voluntary isometric contractions against manual resistance in four movement directions: plantarflexion (PF), dorsiflexion (DF), pronation (PRO) and supination (SUP). Electromyographic activity was recorded from tibialis posterior (TP) and peroneus longus (PL) with intramuscular electrodes, and gastrocnemius medialis (GM) and tibialis anterior (TA) with surface electrodes. When compared to their agonist function, all muscles were co-activated at significantly lower levels in their synergistic function (GM: 23% during SUP, TA: 72% during SUP; TP: 42% during PF, PL: 52% during PF) (p < 0.001). A significant interaction between foot posture and contraction type was evident for TA. During isometric contractions, the electromyographic activity of the shank muscles is geared to their biomechanical advantage according to their position relative to the subtalar and talo-crural joint axes.     

Perturbations of Human Posture

Leg muscle EMG responses and cerebral evoked potentials (CP), elicited by perturbations of stance while on a treadmill with split belts, were analyzed in order to study the relationship between compensatory leg muscle responses and afferent input to supraspinal centers. Various conditions of perturbation were used to establish the extent to which compensatory EMG responses and CPs show congruent behavior. Four different treadmill acceleration rates were applied in three different conditions (unilateral perturbation, directed forward or backward; bilateral perturbation, directed forward or backward; and opposing bilateral perturbation). EMG responses and CPs showed parallel increases in amplitude with increasing displacement velocity. The EMG responses showed distinct differences, predominantly in the response amplitude, between the different perturbation conditions, whereas the CPs were affected only to a minor degree. Tibialis anterior EMG responses were more closely related to the CP following forward perturbation than the corresponding gastrocnemius responses were to the CP following backward perturbation.

We conclude that the EMG responses are more closely related than the CPs to displacement parameters and suggest that this is due to the further spinal processing of the afferent input needed to generate an appropriate EMG response. The closer relationship between the tibialis anterior response and CP may reflect a predominant central representation and control of tibialis anterior activation in the regulation of posture. The functional implications of these findings are discussed.     

The growth of stability: postural control from a development perspective

This study compared central nervous system organizational processes underlying balance in children of three age groups: 15-31 months, 4-6 years, and 7-10 years, using a movable platform capable of antero-posterior (A-P) displacements or dorsi-plantar flexing rotations of the ankle joint. A servo system capable of linking platform rotations to A-P sway angle allowed disruption of ankle joint inputs, to test the effects of incongruent sensory inputs on response patterns. Surface electromyography was used to quantify latency and response patterns. Surface electromyography was used to quantify latency and amplitude of the gastrocnemius, hamstrings, tibialis anterior, and quadriceps muscle responses. Cinematography provided biomechanical analysis of the sway motion. Results demonstrated that while directionally specific response synergies are present in children under the age of six, structured organization of the synergies is not yet fully developed since variability in timing and amplitude relationships between proximal and distal muscles is high. Transition from immature to mature response patterns was not linear but stage-like with greatest variability in the 4- to 6- year-old children. Results from balance tests under altered sensory conditions (eyes closed and/or ankle joint inputs altered) suggested that: (a) with development a shift in controlling inputs to posture from visual dependence to more adult-like dependence on a combination of ankle joint and visual inputs occurred in the 4- to 6-year-old, and reached adult form in the 7- to 10-year-old age group. It is proposed that the age 4-6 is a transition period in the development of posture control. At this time the nervous system (a) uses visual-vestibular inputs to fine tune ankle-joint proprioception in preparation for its increased importance in posture control and (b) fine tunes the structural organization of the postural synergies themselves.     

Functions and recruitment patterns of one- and two-joint muscles under isometric and walking conditions

Hypotheses advanced concerning the functions and advantages of the two-joint (and multi-joint) muscles in the lower limb include transferring energy, ease of control, muscle bulk reduction and decreased velocity of contraction. The aim of this investigation was to assess quantitatively the generality of one such suggestion seen in the literature. It was hypothesized that two-joint muscles would be recruited preferentially when they produced appropriate moments at the joints they crossed. This organizing strategy was used to partition the sagittal plane joint moment at the hip, knee and ankle between the one- and two-joint muscles crossing those joints. If the conditions of the strategy were not met, the moment was considered to be producted by one-joint muscles only. Ten representative muscles were modelled: tibialis anterior, soleus, gastrocnemius, short head of biceps femoris, vasti, rectus femoris, long head of biceps femoris, sartorius, gluteus maximus and iliopsoas. A number of static loading and walking conditions were recorded and then compared to simultaneously measured linear envelope EMG records of each activity. The joint moments were determined from a sagittal plane kinetic analysis using cinematography and measurements of the ground reaction force. Overall, the strategy partitioned the moment between the one- and two-joint muscles in accordance with the EMG records. The strategy tended to underestimate the contributions of the one-joint musculature, implying the existence of other important control strategies, such as cocontraction of antagonists for joint stability, or of synergistic activation to share the joint moment. It was, however, observed that predicted activity of two-joint musculature did agree well with recorded EMG activity.     

Contributions of trunk muscles to anticipatory postural control in children with and without developmental coordination disorder

Current evidence suggests that movement quality is impacted by postural adjustments made in advance of planned movement. The trunk inevitably plays a key role in these adjustments, by creating a stable foundation for limb movement. The purpose of this study was to examine anticipatory trunk muscle activity during functional tasks in children with and without developmental coordination disorder (DCD). Eleven children with DCD (age 7 to 14 years) and 11 age-matched, typically-developing children performed three tasks: kicking a ball, climbing stairs, and single leg balance. Surface electromyography (EMG) was used to examine the neuromuscular activity of bilateral transversus abdominis/internal oblique, external oblique and L3/4 erector spinae, as well as the right tibialis anterior and rectus femoris muscles. Onset latencies for each muscle were calculated relative to the onset of rectus femoris activity. In comparison to the children with DCD, the typically-developing children demonstrated earlier onsets for right tibialis anterior, bilateral external oblique, and right transversus abdominis/internal oblique muscles. These results suggest that anticipatory postural adjustments may be associated with movement problems in children with DCD, and that timing of both proximal and distal muscles should be considered when designing intervention programs for children with DCD.     

Principal postural acceleration and myoelectric activity: Interrelationship and relevance for characterizing neuromuscular function in postural control

One approach to investigating sensorimotor control is to assess the accelerations that produce changes in the kinematic state of the system. When assessing complex whole-body movements, structuring the multi-segmental accelerations is important. A useful structuring can be achieved through a principal component analysis (PCA) performed on segment positions followed by double-differentiation to obtain “principal accelerations” (PAs). In past research PAs have proven sensitive to altered motor control strategies, however, the interrelationship between PAs and muscle activation (surface electromyography, sEMG) have never been determined. The purpose of the current study was therefore to assess the relationship between PAs and sEMG signals recorded from muscles controlling the ankle joint during one-leg standing trials. It was hypothesized that medium correlation should be observed when accounting for neurophysiologic latencies (electro-mechanical delay). Unipedal balancing on a level-rigid ground was performed by 25 volunteers. sEMG activities were recorded from the tibialis anterior, peroneus longus, gastrocnemius medialis, and soleus muscles of the stance leg. The first eight PA-time series were determined from kinematic marker data. Then, a cross-correlation analysis was performed between sEMG and PA time series. We found that peak correlation coefficients for many participants aligned at time delays between 0.116 and 0.362 s and were typically in the range small to medium (|r| = 0.1 to 0.6). Thus, the current study confirmed a direct association between many principal accelerations PA(t) and muscle activation signals recorded from four muscles crossing the ankle joint complex. The combined analysis of PA and sEMG signals allowed exploring the neuromuscular function of each muscle in different postural movement components.     

Can Attention Deficit Hyperactivity Disorder and Fetal Alcohol Spectrum Disorder be differentiated by motor and balance deficits?

There is an ongoing debate regarding the diagnostic overlap between Attention Deficit Hyperactivity Disorder (ADHD) and Fetal Alcohol Spectrum Disorder (FASD). Differential diagnosis is important because of treatment implications. Children aged 7–10 years (47 ADHD, 30 FASD, 39 controls) participated. The Movement Assessment Battery for Children (M-ABC) and the Clinical Observations of Motor and Postural Skills (COMPS) were used. Force plate and electromyography data were collected during static balance and balance perturbation. On the M-ABC both children with ADHD and FASD had more motor problems than controls. The ADHD-Combined and the ADHD-Predominantly Inattentive subgroups were similarly affected in their fine motor skills. On the COMPS, the majority of children in both groups performed in the normal range, but for those children clinically affected, it was the children with ADHD who were more likely to be severely impaired. The children with ADHD were characterized by early onset latencies of the tibialis anterior muscles and increased amplitudes of the gastrocnemius muscles. Difficulty scaling muscle force reflecting medial cerebellar involvement may be the key problem in ADHD. Cerebellar involvement in the postural instability in FASD awaits further study.     

Selection of movement patterns during functional tasks in humans

These experiments examine the role of vision and step height in the selection of a simple binary choice of movement pattern by human subjects. The subjects selected a heel strike movement pattern (HS) (as used during level surface locomotion) or a toe strike movement pattern (TS) (as used during stair descent). The functional task involved descending a step of adjustable height followed by level surface walking under vision and nonvision conditions. Triceps surae and tibialis anterior electromyographic (EMG) activity, ankle angle position, and vertical force were examined. As step height was increased, there was an indistinct threshold at which subjects switched from landing with a HS movement pattern to a TS movement pattern. The tibialis anterior and triceps surae precontact EMG burst and subsequent ankle movement for HS and TS trials appear to be part of preprogrammed movement patterns, which are presumably of central origin. The particular mixture of voluntary, stereotypic, and reflex actions for any specified movement is based on the intent or functional outcome desired. The switching to the TS movement pattern as step height increased presumably results in the most efficient and stable movement.     

Selection of Movement Patterns During Functional Tasks in Humans

These experiments examine the role of vision and step height in the selection of a simple binary choice of movement pattern by human subjects. The subjects selected a heel strike movement pattern (HS) (as used during level surface locomotion) or a toe strike movement pattern (TS) (as used during stair descent). The functional task involved descending a step of adjustable height followed by level surface walking under vision and nonvision conditions. Triceps surae and tibialis anterior electromyographic (EMG) activity, ankle angle position, and vertical force were examined. As step height was increased, there was an indistinct threshold at which subjects switched from landing with a HS movement pattern to a TS movement pattern. The tibialis anterior and triceps surae precontact EMG bursts and subsequent ankle movement for HS and TS trials appear to be part of preprogrammed movement patterns, which are presumably of central origin. The particular mixture of voluntary, stereotypic, and reflex actions for any specified movement is based on the intent or functional outcome desired. The switching to the TS movement pattern as step height increased presumably results in the most efficient and stable movement.     

Early changes in muscle activation patterns of toddlers during walking

Early locomotor behavior has been the focus of considerable attention by developmentalists over several decades. Few studies have addressed explicitly patterns of muscle activity that underlie this coordination pattern. Our purposes were to illustrate a method to determine objectively the onset and offset of muscle firings during early walking and to investigate the emergence of patterns of activation of the core locomotor muscles. We tested eight toddlers as they walked overground at walking onset (max. of 3-6 independent steps) and after three months of walking experience. Surface electrodes monitored activity of the gastrocnemius, tibialis anterior, quadriceps, and hamstrings. We reduced EMG signals to a frame-by-frame designation of "on-off," followed by muscle state and co-contraction analyses, and probability distributions for each muscle's activity across multiple cycles. Our results clearly show that at walking onset muscle activity was highly variable with few, if any, muscles showing recurring patterns of behavior, within or among toddlers. Variability and co-activation decreased with walking experience but remained inconsistent, in contrast to the significant increase in stability shown for joint coordination and endpoint (foot placement) parameters. We propose this trend emerges because of the high number of options (muscle combinations) available. Toddlers learn first to marshal sufficient force to balance and make forward progress but slowly discover how to optimize these resources.     

Functioning of peripheral Ia pathways in leg muscles of newly walking toddlers

Monosynaptic and polysynaptic spinal level reflexes in the leg muscles of infants show significant dispersion across muscles, high variability, and no change in response patterns over the first 10 months. Here we tested the hypothesized relation between early walking experience and the tuning of these responses in three primary gait muscles of participants in four subgroups: cruisers (n = 7) and toddlers with one (n = 5), two (n = 5), or three (n = 5) months of walking experience. Reflex responses in multiple Ia pathways – tendon reflex (T-reflex), vibration-induced inhibition of the T-reflex (VIM-T-reflex), and tonic vibration-induced reflex (VIR), were elicited by mechanical stimuli applied to the distal tendons of the quadriceps, gastrocnemius-soleus, and tibialis anterior of both legs. Walking skill was assessed via a GAITRite mat. Generally, walking experience seemed to be related to slowly emerging improvements and, depending on muscle tested and pathway, progress was quite varied. Amplitude and latency of reflex responses were more clearly impacted by age or leg length while the ratio or distribution pattern of reflex response among antagonist pairs of muscles was impacted by walking experience and skill. As walking experience increased, the ratio of reflex responses tended to increase for the stimulated and decrease for the antagonist reflex loops with distribution of the pattern shifting gradually toward a single type of reflex response in all tested muscles. The very slow tuning of these reflexes may underlie the many missteps and falls reported to occur during early walking and suggest that subsequent studies should continue to follow the developmental trajectory through the first year of walking experience.     

Tuning of human vestibulospinal reflexes by leg rotation

Changing the foot position modifies the mechanical action exerted by the ankle extensor and flexor muscles over the body. We verified, in two groups of healthy subjects standing with the heels touching or apart, whether a 90° external rotation of the right leg and foot also changes the pattern of vestibulospinal reflexes elicited by electrical stimulation of the labyrinth. With the head oriented forward, leg rotation did not modify the labyrinthine-driven displacements of the center of pressure (CoP). When the head was rotated in the horizontal plane, either to the right or to the left, the CoP displacement increased along the y axis in all subjects. Changes in the x component in most instances appropriate to preserve unmodified the direction of body sway elicited by the stimulus were observed. Right leg rotation increased the basal EMG activity of ankle extensors and flexors on the left side, while the right side activity was unaffected. The EMG responses to labyrinthine stimulation were modified only on the left side, in a way appropriate to correct the effects of the altered torque pattern exerted on the body by right leg muscles. It appears, therefore, that somatosensory signals related to leg rotation and/or copy of the corresponding voluntary motor commands modify the pattern of vestibulospinal reflexes and maintain the postural response appropriate to counteract a body sway in the direction inferred by labyrinthine signals.     

Ankle Intrinsic Stiffness in Subcortical Poststroke Subjects

The authors' purpose was to evaluate bilateral ankle intrinsic stiffness in subcortical poststroke subjects. Ten subcortical poststroke subjects and 10 healthy controls participated in this study. The ankle passive stiffness at 3 different speeds and the electromyographic activity of the soleus, the gastrocnemius, and the tibialis anterior muscles of poststroke contralesional (CONTRA) and ipsilesional (IPSI) limbs and of one limb of healthy subjects were assessed. Ankle electromyographic activity was collected to ensure that reflexive or voluntary muscle activity was not being elicited during the passive movements. A significant interaction was observed between the effects of the limb (IPSI vs. CONTRA vs. control) and ankle position, F(4, 28) = 3.285, p = .025, and between the effects of the limb and the velocity of stretch, F(2, 14) = 4.209, p = .037. While increased intrinsic stiffness was observed in the CONTRA limb of poststroke subjects at ankle neutral position when the passive stretch was applied with a velocity of 1°/s (p = .021), the IPSI limb of poststroke subjects presented increased stiffness at 20º of plantar flexion when the stretch was applied with a velocity of 5°/s (p = .009) when compared to healthy group. Subcortical poststroke subjects present increased intrinsic stiffness in both the CONTRA and IPSI limbs in specific ankle amplitudes.     

The effect upon monaural sensitivity of continuous stimulation of the opposite ear

The threshold to a 1,000 c./sec. tone presented to the left ear was measured whilst the right ear was under continuous stimulation by a 400 c./sec. tone. Observations were made, on different groups of subjects, under three stimulus conditions and two conditions of attention. Thresholds were found to increase with increasing intensities of the continuous tone. Attention to that part of the field associated with the continuous stimulus produced no significant change, whether the continuous stimulus was present or not. There were no significant after-effects during ten minutes following the end of the continuous stimulus, though the results suggested a slow decrease in threshold.

Several explanations of the phenomenon are considered. Cross-hearing and the reflex contraction of the middle-ear muscles can be virtually excluded. Central inhibition or the central control of sensory end-organs can account for the results. A statistical hypothesis is also tenable.     

Electromyographic traces of motor unit synchronization of fatigued lower limb muscles during gait

BackgroundThe study of the signal in the frequency domain has shown to be a good tool to identify muscular fatigue. Previous research has shown that the low frequency band and 40 Hz frequency band increase their relative intensity with the onset of fatigue. These findings were obtained in rectus femoris, but the behaviours of other muscles of the lower limb are unknown. In this article we explored the changes in the low frequency and 40 Hz frequency band of lower limb muscles with respect to fatigue.MethodsThirty healthy subjects were recruited to analyse the electromyography (EMG) of biceps femoris, tibialis anterior and gastrocnemius medialis and lateralis of both legs during gait. Four two-minutes walks at a self-selected speed were recorded, the first two walks with a normal muscular function and the last two walks after a fatigue protocol. All the signals were decomposed using wavelet transformations. The signals were normalized in time and spectral intensities normalized to the sum of intensities in the frequency domain. Two frequency bands were studied in each walk: the 40-Hz (34–53 Hz) and the low frequency (< 25 Hz) bands. A ratio of the spectral intensities of those frequency bands at each walk was obtained by dividing the 40-Hz frequency band spectral intensity by the low frequency band spectral intensity. Statistical parametric mapping techniques were used to compare the ratios of the prefatigue walks against the postfatigue walks.ResultsThe results of the Statistical Non-Parametric Mapping (SnPM) analysis of all muscles depict a higher relative spectral intensity in the low frequency band in the comparison of fatigue versus prefatigue recordings except for the right gastrocnemius lateralis. The critical thresholds F* were exceeded by multiple suprathreshold clusters with p values <0.05, showing that the low frequency band increased its relative spectral intensity in the case of fatigue.ConclusionThe obtained results suggest that the low frequency band increases its relative spectral intensity in all the studied muscles when fatigue onsets. This increase in relative spectral intensity may be linked to an increase in motor unit synchronization promoted by the central nervous system to ensure good motor control.