Human postural responses to different frequency vibrations of lower leg muscles

We analyzed human postural responses to muscle vibration applied at four different frequencies to lower leg muscles, the lateral gastrocnemius (GA) or tibialis anterior (TA) muscles. The muscle vibrations induced changes in postural orientation characterized by the center of pressure (CoP) on the force platform surface on which the subjects were standing. Unilateral vibratory stimulation of TA induced body leaning forward and in the direction of the stimulated leg. Unilateral vibration of GA muscles induced body tilting backwards and in the opposite direction of the stimulated leg. The time course of postural responses was similar and started within 1 s after the onset of vibration by a gradual body tilt. When a new slope of the body position was reached, oscillations of body alignment occurred. When the vibrations were discontinued, this was followed by rapid recovery of the initial body position. The relationship between the magnitude of the postural response and frequency of vibration differed between TA and GA. While the magnitude of postural responses to TA vibration increased approximately linearly in the 60-100 Hz range of vibration frequency, the magnitude of response to GA vibration increased linearly only at lower frequencies of 40-60 Hz. The direction of body tilt induced by muscle vibration did not depend on the vibration frequency.     

Velocity of body lean evoked by leg muscle vibration potentiate the effects of vestibular stimulation on posture

To investigate the vestibular and somatosensory interaction in human postural control, a galvanic vestibular stimulation of cosine bell shape resulting in a small forward or backward body lean was paired with three vibrations of both soleus muscles. The induced body lean was registered by the position of the center of foot pressure (CoP). During a quiet stance with eyes closed the vibration of both soleus muscles with frequency (of) 40 Hz, 60 Hz and 80 Hz resulted in the body lean backward with velocities related to the vibration frequencies. The vestibular galvanic stimulation with the head turned to the right caused forward or backward modification of CoP backward response to the soleus muscles vibration and peaked at 1.5-2 s following the onset of the vibration. The effect of the paired stimulation was larger than the summation of the vestibular stimulation during the quiet stance and a leg muscle vibration alone. The enhancement of the galvanic stimulation was related to the velocity of body lean induced by the leg muscle vibration. The galvanic vestibular stimulation during a faster body movement had larger effects than during a slow body lean or the quiet stance. The results suggest that velocity of a body postural movement or incoming proprioceptive signal from postural muscles potentiate the effects of simultaneous vestibular stimulations on posture.     

Human postural response to lower leg muscle vibration of different duration

Body lean response to bilateral vibrations of soleus muscles were investigated in order to understand the influence of proprioceptive input from lower leg in human stance control. Proprioceptive stimulation was applied to 17 healthy subjects by two vibrators placed on the soleus muscles. Frequency and amplitude of vibration were 60 Hz and 1 mm, respectively. Vibration was applied after a 30 s of baseline. The vibration duration of 10, 20, 30 s respectively was used with following 30 s rest. Subjects stood on the force platform with eyes closed. Postural responses were characterized by center of pressure (CoP) displacements in the anterior-posterior (AP) direction. The CoP-AP shifts as well as their amplitudes and velocities were analyzed before, during and after vibration. Vibration of soleus muscles gradually increased backward body tilts. There was a clear dependence of the magnitude of final CoP shift on the duration of vibration. The amplitude and velocity of body sway increased during vibration and amplitude was significantly modulated by duration of vibration as well. Comparison of amplitude and velocity of body sway before and after vibration showed significant post-effects. Presented findings showed that somatosensory stimulation has a long-term, direction-specific influence on the control of postural orientation during stance. Further, the proprioceptive input altered by soleus muscles vibration showed significant changes in postural equilibrium during period of vibration with interesting post-effects also.     

Somatosensory influence on postural response to galvanic vestibular stimulation

We investigated how postural responses to galvanic vestibular stimulation were affected by standing on a translating support surface and by somatosensory loss due to diabetic neuropathy. We tested the hypothesis that an unstable surface and somatosensory loss can result in an increase of vestibulospinal sensitivity. Bipolar galvanic vestibular stimulation was applied to subjects who were standing on a force platform, either on a hard, stationary surface or during a backward platform translation (9 cm, 4.2 cm/s). The intensity of the galvanic stimulus was varied from 0.25 to 1 mA. The amplitude of the peak body CoP displacement in response to the galvanic stimulus was plotted as a function of stimulus intensity for each individual. A larger increase in CoP displacement to a given increase in galvanic current was interpreted as an increase of vestibulospinal sensitivity. Subjects with somatosensory loss in the feet due to diabetes showed higher vestibulospinal sensitivity than healthy subjects when tested on a stationary support surface. Control subjects and patients with somatosensory loss standing on translating surface also showed increased galvanic response gains compared to stance on a stationary surface. The severity of the somatosensory loss in the feet correlated with the increased postural sensitivity to galvanic vestibular stimulation. These results showed that postural responses to galvanic vestibular stimulus were modified by somatosensory information from the surface. Somatosensory loss due to diabetic neuropathy and alteration of somatosensory input during stance on translating support surface resulted in increased vestibulospinal sensitivity.     

Is the attenuation effect on the ankle muscles activity from the EMG biofeedback generalized to – or compensated by – other lower limb muscles during standing?

Biofeedback based on electromyograms (EMGs) has been recently proposed to reduce exaggerated postural activity. Whether the effect of EMG biofeedback on the targeted muscles generalizes to – or is compensated by – other muscles is still an open question we address here. Fourteen young individuals were tested in three 60 s standing trials, without and with EMG-audio feedback: (i) collectively from soleus and medial gastrocnemius and (ii) from medial gastrocnemii. The Root Mean Square (RMS) of bipolar EMGs sampled from postural muscles bilaterally was computed to assess the degree of activity and postural sway was assessed from the center of pressure (CoP). In relation to standing at naturally, EMG-audio feedback from soleus and medial gastrocnemii decreased plantar flexors’ activity (∼10 %) but at the cost of increased amplitude of tibialis anterior (∼5%) and vasti muscles (∼20 %) accompanied by a posterior shift of the mean CoP position. However, EMG-audio feedback from medial gastrocnemii reduced only plantar flexors’ activity (∼5%) when compared to standing at naturally. Current results suggest the EMG biofeedback has the potential to reduce calf muscles’ activity without loading other postural muscles especially when using medial gastrocnemii as feedback source, with implications on postural training aimed at assisting individuals in activating more efficiently postural muscles during standing.     

Combined effects of aging and obesity on postural control,muscle activity and maximal voluntary force of muscles mobilizing ankle joint

ObjectiveThe aim of the study was to investigate the influence of age and/or obesity on postural control, ankle muscle activities during balance testing and force production capacities.Materials and methods4 groups; control group (CG; n = 25; age = 31.8 ± 7.5 years; BMI = 21.4 ± 2.5 kg/m²), obese group (OG; n = 25; age = 34.4 ± 9.5 years; BMI = 39.6 ± 5.4 kg/m²), elderly group (EG; n = 15; age = 77.1 ± 8.4 years; BMI = 24.4 ± 1.3 kg/m²) and obese elderly group (ObEG; n = 12; age = 78.6 ± 6.6 years; BMI = 34.5 ± 3.1 kg/m²) performed maximal voluntary contraction (MVC) before testing to calculate the maximal relative force of ankle plantar flexor (PF) and dorsal flexor (DF) muscles. Center of pressure (CoP) parameters and the electromyography (EMG) activity of PF and DF muscles were collected during MVC, quiet standing and limit of stability (LoS) testing along antero-posterior and medio-lateral axes.ResultsMaximal relative force was higher in EG and ObEG than CG and OG, respectively (p < 0.001). CoP parameters, distance traveled along the antero-posterior axis and EMG activity of PF were higher in OG, EG and ObEG compared to CG (p < 0.001) and in EG compared to ObEG (p < 0.05).The EMG activity of PF was positively correlated with CoP parameters in OG and ObEG (r > 0.6; p < 0.05). Maximal relative force of PF (r > −0.6; p < 0.05) was negatively correlated with CoP parameters in ObEG and EG.ConclusionObesity-related postural control alteration is associated with increased activity of PF. This neuromuscular adaptation may reflect deteriorations of the proprioceptive system and is likely additional to age-related muscular impairments. This may be a mechanism by which obesity increases postural control alterations in elderly.     

Aging effects on posture-related modulation of stretch reflex excitability in the ankle muscles in humans

The purpose of this study was to examine the effects of aging on posture-related changes of the stretch reflex excitability in the ankle extensor, soleus (SOL), and flexor, tibialis anterior (TA) muscles. Fourteen neurologically normal elderly (mean 68 ± 6 years) and 12 young (mean 27 ± 3 years) subjects participated. Under two postural conditions, upright standing (STD) and sitting (SIT), stretch reflex electromyographic (EMG) responses in the SOL/TA muscle were elicited by imposing rapid ankle dorsi-/plantar-flexion. Under the SIT condition, subjects were asked to keep the SOL background EMG level, which is identical to that under the STD condition. In the SOL muscle, both groups showed significant enhancement of the short-latency stretch reflex (SLR) response when the posture changed from SIT to STD. In the TA muscle, the young group showed significant enhancement of the middle- (MLR) and long-latency stretch reflex (LLR) when the posture changed from SIT to STD; no such modulation was observed in the elderly group. Since the TA stretch reflex responses under the STD condition were comparable in the young and elderly groups, the lack of posture-related modulation of the TA muscle in the elderly group might be explained by augmented stretch reflex excitability under the SIT condition. The present results suggest that the (1) SOL SLR responses are modulated both in the young and elderly subjects when the posture is changed from SIT to STD, (2) TA MLR and LLR responses are not modulated in the elderly subjects when the posture is changed from SIT to STD, while each response is same between the young and elderly in STD, and (3) the effect of aging on the posture-related stretch reflex differs in the SOL and TA muscles.     

Age-related changes of human balance during quiet stance

Certain aspects of balance control change with age, resulting in a slight postural instability. We examined healthy subjects between 20-82 years of age during the quiet stance under static conditions: at stance on a firm surface and/or on a compliant surface with eyes either open or closed. Body sway was evaluated from centre of foot pressure (CoP) positions during a 50 sec interval. The seven CoP parameters were evaluated to assess quiet stance and were analyzed in three age groups: juniors, middle-aged and seniors. The regression analysis showed evident increase of body sway over 60 years of age. We found that CoP parameters were significantly different when comparing juniors and seniors in all static conditions. The most sensitive view on postural steadiness during quiet stance was provided by CoP amplitude and velocity in AP direction and root mean square (RMS) of statokinesigram. New physiological ranges of RMS parameter in each condition for each age group of healthy subjects were determined. Our results showed that CoP data from force platform in quiet stance may indicate small balance impairment due to age. The determined physiological ranges of RMS will be useful for better distinguishing between small postural instability due to aging in contrast to pathological processes in the human postural control.     

Muscle co-activation around the knee in drop jumping using the co-contraction index.

The purpose of this study was to examine the co-activation of the rectus femoris (RF) and biceps femoris (BF) during drop jumping exercises using the co-contraction index (CI). Ten trained male long jumpers performed drop jumps from 20 cm (DJ20), 40 cm (DJ40) and 60 cm (DJ60) on a force platform. Surface electromyographic (EMG) activity of the RF and BF, vertical ground reaction force data and knee joint angular displacement and angular velocities were recorded and normalized as percentage of maximum isometric values. The CI was calculated for the pre-contact, braking and propulsive phases of the jump using four methods: (1) by dividing the double integrated antagonistic activity by the sum of the RF and BF EMG; (2) by finding the amount of overlap between the linear envelopes of the agonist and antagonist muscles and dividing by the number of data points; (3) by calculating the co-contraction at any instant point of time; and (4) by dividing the BF integrated activity by the total registered muscle activity around the knee. The CI ranged from 13.03+/-9.33 to 70.80+/-25.81%, depending on the estimation method used. A two-way analysis of variance (ANOVA) indicated that the CI was not affected by drop jumping height (p>0.05) while it was significantly higher (p<0.05) in the pre-contact phase compared to the braking and propulsion phases. The CI can be useful when examining muscle co-activation using EMG measurements in drop jumps. However, the conclusions on muscle co-activation depend on the equation used to estimate CI and therefore a commonly accepted method is necessary.     

Effects of pulse lung inflation on chest wall expiratory motor activity.

The effects of pulse lung inflation (LI) on expiratory muscle activity and phase duration (Te) were determined in anesthetized, spontaneously breathing dogs (n = 20). A volume syringe was used to inflate the lungs at various times during the expiratory phase. The magnitude of lung volume was assessed by the corresponding change in airway pressure (Paw; range 2-20 cmH(2)O). Electromyographic (EMG) activities were recorded from both thoracic and abdominal muscles. Parasternal muscle EMG was used to record inspiratory activity. Expiratory activity was assessed from the triangularis sterni (TS), internal intercostal (IIC), and transversus abdominis (TA) muscles. Lung inflations <7 cmH(2)O consistently inhibited TS activity but had variable effects on TA and IIC activity and expiratory duration. Lung inflations resulting in Paw values >7 cmH(2)O, however, inhibited expiratory EMG activity of each of the expiratory muscles and lengthened Te in all animals. The responses of expiratory EMG and Te were directly related to the magnitude of the lung inflation. The inhibition of expiratory motor activity was independent of the timing of pulse lung inflation during the expiratory phase. The inhibitory effects of lung inflation were eliminated by bilateral vagotomy and could be reproduced by electrical stimulation of the vagus nerve. We conclude that pulse lung inflation resulting in Paw between 7 and 20 cmH(2)O produces a vagally mediated inhibition of expiratory muscle activity that is directly related to the magnitude of the inflation. Lower inflation pressures produce variable effects that are muscle specific.     

Direction-dependent neck and trunk postural reactions during sitting

Postural reactions in healthy individuals in the seated position have previously been described and have been shown to depend on the direction of the perturbation; however the neck response following forward and backward translations has not been compared. The overall objective of the present study was to compare neck and trunk kinematic, kinetic and electromyographic (EMG) stabilization patterns of seated healthy individuals to forward and backward translations. Ten healthy individuals, seated on a chair fixed onto a movable platform, were exposed to forward and backward translations (distance = 0.15 m, peak acceleration = 1.2 m/s²). The head and trunk kinematics as well as the EMG activity of 16 neck and trunk muscles were recorded. Neck and trunk angular displacements were computed in the sagittal plane. The centers of mass (COMs) of the head (HEAD), upper thorax (UPTX), lower thorax (LOWTX) and abdomen (ABDO) segments were also computed. Moments of force at the C7-T1 and L5-S1 levels were calculated using a top-down, inverse dynamics approach. Forward translations provoked greater overall COM peak displacements. The first peak of moment of force was also reached earlier following forward translations which may have played a role in preventing the trunk from leaning backwards. These responses can be explained by the higher postural threat imposed by a forward translation.     

Unilateral lower limb muscle fatigue induces bilateral effects on undisturbed stance and muscle EMG activities

The study investigated the effects of an unilateral ankle muscle fatigue onto independent postural control parameters including the trajectories of the estimated resultant CoP (CoPres) and his components: the centre of gravity (CG) and CoP–CG trajectories.Nine healthy men realized series of 10 toe-lift immediately followed by 10 knee flexions until exhaustion with one (Ex) leg. Maximal isometric voluntary contractions, postural sway measures of each leg, and muscular activities of the ankle muscles were recorded before and immediately after the fatiguing exercise.As expected, the latter induced a decrease in maximal voluntary peak force associated with a greater variability of the relative contribution of each leg on the CoPres, enhanced all postural parameters of the non-exercised leg. A significant decreased of the tibialis anterior EMG activity for the Ex leg and an increased one for the NoEx leg. Finally, following unilateral fatigue, the body sway destabilisation seemed to occur only along the medio-lateral (ML) axis.The enhanced and greater variability of the variance along ML axis might be explained by the recourse at the loading–unloading strategy choice and suggests a central attempt to compensate for pain sensation.     

Synergistic co-activation in multi-directional postural control in humans

To examine the muscle synergies of multi-directional postural control, we calculated the target-directed variance fraction (η) and net action direction of each muscle using the electromyogram-weighted averaging (EWA) method. Subjects stood barefoot on a force platform and maintained their posture by producing a center of pressure (COP) in twelve target directions. Surface electromyograms were recorded from 6 right-sided muscles: tibialis anterior (TA), soleus (SOL), lateral gastrocnemius (LG), medial gastrocnemius (MG), fibularis longus (FL), and gluteus medius (GM). η was calculated from COP with duration of 20-s, during which the COP was relatively constant. The EWA method was applied to the EMG and the two COP components to estimate the net action direction of each muscle. The results showed that η values in all directions did not cross the 0.8 threshold. This suggests that human postural control is achieved by synergistic co-activation. The EWA revealed that the net action directions of TA, SOL, LG, MG, and GM were 277.6°, 71.1°, 87.7°, 94.0°, and 2.2°, respectively. This suggests that postural maintenance by muscle synergy can be attributed to the relevant muscles having various action directions. These results demonstrate that muscle synergies can be investigated using COP fluctuations.     

Effect of exercise-induced fatigue on postural control of the knee

Muscle fatigue is associated with reduced power output and work capacity of the skeletal muscle. Fatigue-induced impairments in muscle function are believed to be a potential cause of increased injury rates during the latter stages of athletic competition and often occur during unexpected perturbations. However the effect of fatigue on functionally relevant, full body destabilizing perturbations has not been investigated. This study examines the effect of muscle fatigue on the activation of the quadriceps and hamstrings to fast, full body perturbations evoked by a moveable platform. Surface electromyographic (EMG) signals were recorded from the knee extensor (vastus medialis, rectus femoris, and vastus lateralis) and flexor muscles (biceps femoris and semitendinosus) of the right leg in nine healthy men during full body perturbations performed at baseline and immediately following high intensity exercise performed on a bicycle ergometer. In each condition, participants stood on a moveable platform during which 16 randomized postural perturbations (eight repetitions of two perturbation types: 8 cm forward slides, 8 cm backward slides) with varying inter-perturbation time intervals were performed over a period of 2-3 min. Maximal voluntary knee extension force was measured before and after the high intensity exercise protocol to confirm the presence of fatigue. Immediately after exercise, the maximal force decreased by 63% and 66% for knee extensors and flexors, respectively (P<0.0001). During the post-exercise postural perturbations, the EMG average rectified value (ARV) was significantly lower than the baseline condition for both the knee extensors (average across all muscles; baseline: 19.7±25.4μV, post exercise: 16.2±19.4 μV) and flexors (baseline: 24.3±20.9 μV, post exercise: 13.8±11.0 μV) (both P<0.05). Moreover the EMG onset was significantly delayed for both the knee extensors (baseline: 132.7±32.9 ms, post exercise: 170.8±22.9 ms) and flexors (baseline: 139.1±38.8 ms, post exercise: 179.3±50.9 ms) (both P<0.05). A significant correlation (R(2)=0.53; P<0.05) was identified between the percent reduction of knee extension MVC and the percent change in onset time of the knee extensors post exercise. This study shows that muscle fatigue induces a reduction and delay in the activation of both the quadriceps and hamstring muscles in response to rapid destabilizing perturbations potentially reducing the stability around the knee.     

Muscle activation during maximal voluntary eccentric and concentric knee extension

The aim of this investigation was to study the relationships among movement velocity, torque output and electromyographic (EMG) activity of the knee extensor muscles under eccentric and concentric loading. Fourteen male subjects performed maximal voluntary eccentric and concentric constant-velocity knee extensions at 45, 90, 180 and 360 degrees.s-1. Myoelectric signals were recorded, using surface electrodes, from the vastus medialis, vastus lateralis and rectus femoris muscles. For comparison, torque and full-wave rectified EMG signals were amplitude-averaged through the central half (30 degrees-70 degrees) of the range of motion. For each test velocity, eccentric torque was greater than concentric torque (range of mean differences: 20%-146%, P less than 0.05). In contrast, EMG activity for all muscles was lower under eccentric loading than velocity-matched concentric loading (7%-31%, P less than 0.05). Neither torque output nor EMG activity for the three muscles changed across eccentric test velocities (P greater than 0.05). While concentric torque increased with decreasing velocity, EMG activity for all muscles decreased with decreasing velocity (P less than 0.05). These data suggest that under certain high-tension loading conditions (especially during eccentric muscle actions), the neural drive to the agonist muscles was reduced, despite maximal voluntary effort. This may protect the musculoskeletal system from an injury that could result if the muscle was to become fully activated under these conditions.     

Biomechanical reorganisation of stepping initiation during acute dorsiflexor fatigue

During voluntary step initiation (SI), propulsive forces are generated during anticipatory postural adjustments (APA) which displace the centre-of-gravity (CoG) in the desired direction. These propulsive forces are implemented by ankle synergy, bilateral soleus inhibition followed by activation of tibialis anterior (TA). The aim of this study was to investigate the effect of fatigue applied to ankle dorsiflexors on APA associated with SI and on related motor performance. Eight young healthy participants initiated stepping before and after a protocol designed to generate fatigue in ankle dorsiflexors. Fatigue was induced by series of high-level isometric contractions performed until exhaustion. Results showed that, with fatigue, the level of TA activation during APA, anticipatory postural dynamics (backward centre-of-pressure displacement and forward CoG velocity) and related motor performance (peak of CoG velocity) were attenuated, while APA duration and total SI duration increased. These changes were interpreted as reflecting a protective strategy aiming to preserve the integrity of the fatigued muscles, rather than an impairment associated with muscle weakness.     

Technique for interpretation of electromyography for concentric and eccentric contractions in gait

We present a technique to combine muscle shortening and lengthening velocity information with electromyographic (EMG) profiles during gait. A biomechanical model was developed so that each muscle's length could be readily calculated over time as a function of angles of the joints it crossed. The velocity of shortening and lengthening of the muscle fiber was then calculated, and with computer graphics this information was overlaid on the EMG profiles. Thus, researchers and clinicians were not only able to interpret the processed EMG signal as level of activity (tension) but also to gain insight as to the muscles' role as generators (muscle shortening) or absorbers (muscle lengthening) of energy. Six common muscles are documented, using database profiles; soleus (SOL), medial gastrocnemius (MG), tibialis anterior (TA), vastus lateralis (VL), rectus femoris (RF), and semitendinosus (ST). The protocol thus demonstrates a relatively simple technique for calculating muscle fiber velocity and for combining that velocity information with EMG activity profiles.     

The effects of normalization method on antagonistic activity patterns during eccentric and concentric isokinetic knee extension and flexion

The purpose of this study was to compare different normalization methods of electromyographic (EMG) activity of antagonists during isokinetic eccentric and concentric knee movements. Twelve women performed three maximum knee extensions and flexions isometrically and at isokinetic concentric and eccentric angular velocities of 30 °·s⁻¹, 90 °·s⁻¹, 120 °·s⁻¹ and 150 °·s⁻¹. The EMG activity of the vastus lateralis, rectus femoris, vastus medialis and hamstrings was recorded. The antagonist integrated IEMG values were normalized relative to the EMG of the same muscle during an isometric maximal action (static method). The values were also expressed as a percentage of the EMG activity of the same muscle, at the same angle, angular velocity and muscle action (dynamic method) when the muscle was acting as an agonist. Three-way analysis of variance (ANOVA) designs indicated significantly greater IEMG normalized with the dynamic method compared to the EMG derived using the static method (P < 0.05). These differences were more evident at concentric angular velocities and at the first and last 20 ° of the movement. The present findings demonstrate that the method of normalization significantly influences the conclusions on antagonistic activity during isokinetic maximum voluntary efforts. The dynamic method of normalization is more appropriate because it considers the effects of muscle action, muscle length and angular velocity on antagonist IEMG.