Responses in knee joint muscle activation patterns to different perturbations during gait in healthy subjects

PurposeTo compare the responses in knee joint muscle activation patterns to different perturbations during gait in healthy subjects.ScopeNine healthy participants were subjected to perturbed walking on a split-belt treadmill. Four perturbation types were applied, each at five intensities. The activations of seven muscles surrounding the knee were measured using surface EMG. The responses in muscle activation were expressed by calculating mean, peak, co-contraction (CCI) and perturbation responses (PR) values. PR captures the responses relative to unperturbed gait. Statistical parametric mapping analysis was used to compare the muscle activation patterns between conditions.ResultsPerturbations evoked only small responses in muscle activation, though higher perturbation intensities yielded a higher mean activation in five muscles, as well as higher PR. Different types of perturbation led to different responses in the rectus femoris, medial gastrocnemius and lateral gastrocnemius. The participants had lower CCI just before perturbation compared to the same phase of unperturbed gait.ConclusionsHealthy participants respond to different perturbations during gait with small adaptations in their knee joint muscle activation patterns. This study provides insights in how the muscles are activated to stabilize the knee when challenged. Furthermore it could guide future studies in determining aberrant muscle activation in patients with knee disorders.     

Effect of long-duration spaceflight on postural control during self-generated perturbations.

This report is the first systematic evaluation of the effects of prolonged weightlessness on the bipedal postural control processes during self-generated perturbations produced by voluntary upper limb movements. Spaceflight impacts humans in a variety of ways, one of which is compromised postflight postural control. We examined the neuromuscular activation characteristics and center of pressure (COP) motion associated with arm movement of eight subjects who experienced long-duration spaceflight (3--6 mo) aboard the Mir space station. Surface electromyography, arm acceleration, and COP motion were collected while astronauts performed rapid unilateral shoulder flexions before and after spaceflight. Subjects generally displayed compromised postural control after flight, as evidenced by modified COP peak-to-peak anterior-posterior and mediolateral excursion, and pathlength relative to preflight values. These changes were associated with disrupted neuromuscular activation characteristics, particularly after the completion of arm acceleration (i.e., when subjects were attempting to maintain upright posture in response to self-generated perturbations). These findings suggest that, although the subjects were able to assemble coordination modes that enabled them to generate rapid arm movements, the subtle control necessary to maintain bipedal equilibrium evident in their preflight performance is compromised after long-duration spaceflight.     

Interaction between acoustic startle and habituated neck postural responses in seated subjects.

Postural and startle responses rapidly habituate with repeated exposures to the same stimulus, and the first exposure to a seated forward acceleration elicits a startle response in the neck muscles. Our goal was to examine how the acoustic startle response is integrated with the habituated neck postural response elicited by forward accelerations of seated subjects. In experiment 1, 14 subjects underwent 11 sequential forward accelerations followed by 5 additional sled accelerations combined with a startling tone (124-dB sound pressure level) initiated 18 ms after sled acceleration onset. During the acceleration-only trials, changes consistent with habituation occurred in the root-mean-square amplitude of the neck muscles and in the peak amplitude of five head and torso kinematic variables. The subsequent addition of the startling tone restored the amplitude of the neck muscles and four of the five kinematic variables but shortened onset of muscle activity by 9-12 ms. These shortened onset times were further explored in experiment 2, wherein 16 subjects underwent 11 acceleration-only trials followed by 15 combined acceleration-tone trials with interstimulus delays of 0, 13, 18, 23, and 28 ms. Onset times shortened further for the 0- and 13-ms delays but did not lengthen for the 23- and 28-ms delays. These temporal and spatial changes in EMG can be explained by a summation of the excitatory drive converging at or before the neck muscle motoneurons. The present observations suggest that habituation to repeated sled accelerations involves extinguishing the startle response and tuning the postural response to the whole body disturbance.     

Trunk muscle reflexes are elicited by small continuous perturbations in healthy subjects and patients with low-back pain

Low-back pain (LBP) has been recognized as the leading cause of disability worldwide. Lumbar instability has been considered as an important mechanism of LBP and one potential contributor to lumbar stability is trunk muscle reflex activity. However, due to the differences in experimental paradigms used to quantify trunk mechanics and trunk reflexes it remains unclear as to what extent the reflex pathway contributes to overall lumbar stability. The goal of this work was to determine to what extent reflexes of various trunk muscles were elicited by the small continuous perturbations normally used to quantify trunk mechanics. Electromyographic (EMG) activity was measured bilaterally from 3 trunk extensor muscles and 3 trunk flexor muscles at four epochs: 25–50 ms, 50–75 ms, 75–100 ms and 100–125 ms following each perturbation. Reflex activity was seen in all muscles as 34 of the 48 muscle-epoch combinations showed a significant reflex response to either perturbations in the forward or backward direction. However, the reflex EMG activity did not correlate with mechanical estimates of the reflex response. Thus, even though reflexes are indeed elicited by the small perturbations used to quantify trunk mechanics, their exact contribution to overall lumbar stability remains unknown.     

Anticipatory postural adjustments associated with lateral and rotational perturbations during standing.

We studied the role of different leg and trunk muscle groups in the generation of anticipatory postural adjustments (APAs) prior to lateral and rotational perturbations associated with predictable and self-triggered postural perturbations during standing. Postural perturbations were induced by a variety of manipulations including catching and releasing a load with the right hand extended either in front of the body or to the right side, performing bilateral fast shoulder movements in different directions, and applying brief force pulses with a hand against the wall. Perturbations in a frontal plane ("lateral perturbations") were associated with significant asymmetries in APAs seen in the right and left distal (soleus and tibialis anterior) muscles; these asymmetries dependent on the direction of the perturbation. Rotational perturbations about the vertical axis of the body generated by fast movements of the two shoulders in the opposite directions were also associated with direction-dependent asymmetries in the APAs in soleus muscles. However, rotational perturbations generated by an off-body-midline force pulse application were accompanied by direction-dependent asymmetries in proximal muscle groups, but not in the distal muscles. We conclude that muscles controlling the ankle joint play an important role in the compensation of lateral and rotational perturbations. The abundance of muscles participating in maintaining vertical posture allows the control system to use different task-dependent strategies during the generation of APAs in anticipation of rotational perturbation.     

Rapid neck muscle adaptation alters the head kinematics of aware and unaware subjects undergoing multiple whiplash-like perturbations

To examine whether habituation confounds the study of whiplash injury using human subjects, we quantified changes in the magnitude and temporal development of the neck muscle electromyogram and peak linear and angular head/torso kinematics of subjects exposed to sequential whiplash-like perturbations. Forty-four seated subjects (23F, 21M) underwent 11 consecutive forward horizontal perturbations (peak sled acceleration=1.5 g). Electromyographic (EMG) activity was recorded over the sternocleidomastoid (SCM) and cervical paraspinal (PARA) muscles with surface electrodes, and head and torso kinematics were measured using linear and angular accelerometers and a 3D motion analysis system. EMG onset occurred at reflex latencies (67-75 ms in SCM) and did not vary with repeated perturbations. EMG amplitude was significantly attenuated by the second perturbation in PARA muscles and by the third perturbation in SCM muscles. The mean decrement in EMG amplitude between the first trial and the mean of the last five trials was between 41% and 64%. Related kinematic changes ranged from a 21% increase in head extension angle to a 29% decrease in forward acceleration at the forehead, and were also significantly different by the second exposure in some variables. Although a wider range of perturbation intensities and inter-perturbation intervals need to be studied, the significant changes observed in both muscle and kinematic variables by the second perturbation indicated that habituation was a potential confounder of whiplash injury studies using repeated perturbations of human subjects.     

Influence of electrode type on neuromuscular activation patterns during walking in healthy subjects

The role of muscle activation in both pathological and spastic populations is of interest for understanding central nervous system function. Muscle activation patterns may provide insight into pathological changes compared to healthy controls. To gain a better understanding of surgical interventions, gait muscle activation patterns are studied before and after surgery. Previous studies using surface electromyography have indicated that muscle activation onset, time to peak, and peak amplitude may be helpful in assessing the neuromuscular control strategy that underlies pathological populations. Geometric artifact may influence electromyographic variables as recorded by different electrode types and electrode placement. The purpose of this investigation was to compare surface and fine-wire activation patterns during gait to elucidate the influence electrode type has on electromyographic variables. Lower leg surface and fine-wire electromyographic activity was recorded simultaneously during gait to assess if electrode type (fine-wire vs. surface) affects muscle onset, time to peak, peak amplitude, and activation patterns. No significant differences were recorded between surface and fine-wire electrodes for muscle onset or time to peak activation. Activation patterns revealed similarity between electrodes. Some significant differences were detected in peak amplitude. Non-invasive surface electrodes provide an adequate representation of timing variables for primary ankle muscles during gait.     

Small,movement dependent perturbations substantially alter postural control strategy in healthy young adults

Postural control is commonly investigated by observing responses to perturbations. We developed a perturbation paradigm mimicking self-generated errors in weight shifting, which are a common cause of falling among older adults. Our aim was to determine the effects of this small, but complex, perturbation on postural sway of healthy young adults and evaluate the role of vision and cognition during movement dependent perturbations. Fifteen participants stood hip-width apart with their eyes open, closed and while performing two different cognitive tasks. Participants were continuously perturbed by medial-lateral (ML) support surface translations corresponding to, and hence doubling, their own center of mass sway. We analyzed the standard deviation (SD), root mean square (RMS), range, and mean power frequency (MPF) of center of pressure displacements. ML postural sway increased due to the perturbation (SD p ≤ .001, range p < .001, RMS p ≤ .001, MPF p < .001). Cognitive load increased the ML sway range (p = .048). Lack of vision increased ML MPF (p = .001) and anterior-posterior (AP) range (p < .001), SD (p < .001), and RMS (p = .001). Significant interaction of vision with the perturbation was found for the ML range (p = .045) and AP SD (p = .018). The perturbation specifically affected ML postural sway. Increased MPF is indicative of a postural control strategy change, which was insufficient for fully controlling the increased sway. Despite being small, this type of perturbation appears to be challenging for young adults.     

Reduced lipoprotein lipase activity in postural skeletal muscle during aging.

Lipoprotein lipase (LPL) is a key enzyme for fatty acid and lipoprotein metabolism in muscle. However, the effect of aging on LPL regulation in skeletal muscle is unknown. We report the effect of aging on LPL regulation in the soleus (red oxidative postural) muscle and the tibialis anterior (white glycolytic non-weight-bearing) muscle in 4- and 24-mo-old Fischer 344 rats and 18- and 31-mo-old Fischer 344 x Brown-Norway F1 (F-344 x BN F1) rats. Total and heparin-releasable LPL (HR-LPL) activities were decreased 38% (P < 0.01) and 52% (P < 0.05), respectively, in the soleus muscle of the older Fischer 344 rats. There was a 32% reduction (P < 0.05) of total LPL protein mass in the soleus muscle with aging. The results were confirmed in another strain. A decrease of total LPL activity (-50%, P < 0.05) was also found in the soleus muscle between 18- and 31-mo-old F-344 x BN F1 rats. LPL mRNA concentration in the soleus muscle was not different between ages. Total LPL protein mass was reduced by 46% (P < 0.05) in the soleus muscle of the 31-mo-old F-344 x BN F1 rats. In the tibialis anterior muscle, neither LPL activity nor mRNA concentration was affected by age in either strain. In conclusion, LPL regulation in a non-weight-bearing muscle was not affected by aging. However, there was a pronounced reduction in LPL activity and LPL protein mass in postural muscle with aging.     

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.     

Voluntary activation of the trapezius muscle in cases with neck/shoulder pain compared to healthy controls

Subjects reporting neck/shoulder pain have been shown to generate less force during maximal voluntary isometric contractions (MVC) of the shoulder muscles compared to healthy controls. This has been suggested to be caused by a pain-related decrease in voluntary activation (VA) rather than lack of muscle mass. The aim of the present study was to investigate VA of the trapezius muscle during MVCs in subjects with and without neck/shoulder pain by use of the twitch interpolation technique.Ten cases suffering from pain and ten age and gender matched, healthy controls were included in the study. Upper trapezius muscle thickness was measured using ultrasonography and pain intensity was measured on a 100 mm visual analog scale (VAS). VA was calculated from five maximal muscle activation attempts. Superimposed stimuli were delivered to the accessory nerve at peak force and during a 2% MVC following the maximal contraction.Presented as mean ± SD for cases and controls, respectively: VAS; 16.0 ± 14.4 mm and 2.1 ± 4.1 mm (P = 0.004), MVC; 545 ± 161 N and 664 ± 195 N (P = 0.016), upper trapezius muscle thickness; 10.9 ± 1.9 mm and 10.4 ± 1.5 mm (P = 0.20), VA; 93.6 ± 14.2% and 96.3 ± 6.0% (P = 0.29).In spite of significantly eight-fold higher pain intensity and ∼20% lower MVC for cases compared to controls, no difference was found in VA. Possible explanations for the reduction in MVC could be differences in co-activation of antagonists and synergists as well as muscle quality.     

Anticipatory postural adjustments during sitting reach movement in post-stroke subjects

The study assessed the effect of velocity of arm movement on anticipatory postural adjustments (APAs) generation in the contralateral and ipsilateral muscles of individuals with stroke in seating. Ten healthy and eight post-stroke subjects were studied in sitting. The task consisted in reaching an object placed at scapular plane and mid-sternum height at self-selected and fast velocities. Electromyography was recorded from anterior deltoid (AD), upper (UT) and lower trapezius (LT) and latissimus dorsi (LD). While kinematic analysis was used to assess peak velocity and trunk displacement. Differences were found between the timing of APAs on ipsi and contralateral LD and LT in both movement speeds and in ipsilateral UT during movement of the non-affected arm at a self-selected velocity. A delay on the contralateral LD to reach movement with the non-affected arm at fast velocity was also observed. The trunk displacement was greater in post-stroke subjects. Individuals with stroke demonstrated a delay of APAs in the muscles on both sides of the body compared to healthy subjects. The delay was observed during performance of the reaching task with the fast and self-selected velocity.     

Adrenergic airway vascular smooth muscle responsiveness in healthy and asthmatic subjects.

The purpose of the present study was to determine the responsiveness of airway vascular smooth muscle (AVSM) as assessed by airway mucosal blood flow (Qaw) to inhaled methoxamine (alpha(1)-agonist; 0.6-2.3 mg) and albuterol (beta(2)-agonist; 0.2-1.2 mg) in healthy [n = 11; forced expiratory volume in 1 s, 92 +/- 4 (SE) % of predicted] and asthmatic (n = 11, mean forced expiratory volume in 1 s, 81 +/- 5%) adults. Mean baseline values for Qaw were 43.8 +/- 0.7 and 54.3 +/- 0.8 microl. min(-1). ml(-1) of anatomic dead space in healthy and asthmatic subjects, respectively (P < 0.05). After methoxamine inhalation, the maximal mean change in Qaw was -13.5 +/- 1.0 microl. min(-1). ml(-1) in asthmatic and -7.1 +/- 2.1 microl. min(-1). ml(-1) in healthy subjects (P < 0.05). After albuterol, the mean maximal change in Qaw was 3.0 +/- 0.8 microl. min(-1). ml(-1) in asthmatic and 14.0 +/- 1.1 microl. min(-1). ml(-1) in healthy subjects (P < 0.05). These results demonstrate that the contractile response of AVSM to alpha(1)-adrenoceptor activation is enhanced and the dilator response of AVSM to beta(2)-adrenoceptor activation is blunted in asthmatic subjects.     

Increased probability of repetitive spinal motoneuron activation by transcranial magnetic stimulation after muscle fatigue in healthy subjects

Triple stimulation technique (TST) has previously shown that transcranial magnetic stimulation (TMS) fails to activate a proportion of spinal motoneurons (MNs) during motor fatigue. The depression in size of the TST response, but no attenuation of the conventional motor-evoked potential, suggested increased probability of repetitive spinal MN activation during exercise, even if some MNs failed to discharge by the brain stimulus. Here we used a modified TST [quadruple stimulation (QuadS) and quintuple stimulation (QuintS)] to examine the influence of fatiguing exercise on second and third MN discharges after a single TMS in healthy subjects. This method allows an estimation of the percentage of double and triple discharging MNs. Following a sustained contraction of the abductor digiti minimi muscle at 50% maximal force maintained to exhaustion, the size of QuadS and QuintS responses increased markedly, reflecting that a greater proportion of spinal MNs was activated two or three times by the transcranial stimulus. The size of QuadS responses did not return to precontraction levels during 10-min observation time, indicating long-lasting increase in excitatory input to spinal MNs. In addition, the postexercise behavior of QuadS responses was related to the duration of the contraction, pointing to a correlation between repeated activation of MNs and the subject's ability to maintain force. In conclusion, the study confirmed that an increased fraction of spinal MNs fire more than once in response to TMS when the muscle is fatigued. Repetitive MN firing may provide an adaptive mechanism to maintain motor unit activation and task performance during sustained voluntary activity.     

Decreased fibularis reflex response during inversion perturbations in FAI subjects

Investigate reflex responses in muscles throughout the lower limb and low back during sudden inversion perturbations in individuals with and without Functional Ankle Instability (FAI) while walking. Forty subjects participated in the study. Surface electromyogram recordings were obtained from the fibularis (FIB), gluteus medius (GM), erector spinae (ES), and sternocleidomastoid (SCM) of the injured/matched side as well as the uninjured/matched contralateral side (FIB_CLS, GM_CLS, or ES_CLS). Latency and amplitude data were collected while subjects were walking on a custom-built perturbation walkway. The onset of the short-latency stretch reflex of the FIB was significantly later in the injured side of the FAI individuals when compared to the control group (P = 0.009). Both the short and long latency reflex amplitude was significantly smaller in the FIB muscle in the FAI group than in the control group (P < 0.008). No significant differences in latency or amplitude reflex responses were identified between the two groups in the GM, ES, FIB_CLS, GM_CLS, or ES_CLS (P > .05). Interpretation of these results indicate that during a dynamic perturbation task individuals with FAI demonstrate longer fibularis muscle latencies on the injured side while no significant changes in the proximal muscle groups. Additionally, short and long latency reflex amplitude was significantly decreased in FAI individuals.     

Alae nasi activation and nasal resistance in healthy subjects

To investigate the effect of alae nasi (AN) activation on nasal resistance, we monitored AN electromyographic (EMG) activity in 17 healthy subjects using surface electrodes placed on either side of the external nares and measured inspiratory nasal resistance utilizing the method of posterior rhinometry. With CO2 inhalation (6 subj), AN EMG activity increased as nasal resistance fell 23 +/- 5% (P less than 0.01). In the same subjects, voluntary flaring of the external nares also increased AN EMG and decreased nasal resistance by 29 +/- 5% (P less than 0.01). Nasal resistance was altered by nasal flaring and CO2 inhalation even after administration of a topical nasal vasoconstrictive spray (8 subj). In six subjects, voluntary nasal flaring or inhibition with the mouth closed produced a 21 +/- 12% change (P less than 0.01) in total airway resistance as measured by body plethysmography. We conclude that activation of the alae nasi will decrease nasal and total airway resistance during voluntary nasal flaring and during CO2 inhalation and thus should be considered in any studies of upper airway resistance.     

Abdominal muscle activity during breathing with and without inspiratory and expiratory loads in healthy subjects

Central Nervous System modulates the motor activities of all trunk muscles to concurrently regulate the intra-abdominal and intra-thoracic pressures. The study aims to evaluate the effect of inspiratory and expiratory loads on abdominal muscle activity during breathing in healthy subjects. Twenty-three higher education students (21.09 ± 1.56 years; 8 males) breathed at a same rhythm (inspiration: two seconds; expiration: four seconds) without load and with 10% of the maximal inspiratory or expiratory pressures, in standing. Surface electromyography was performed to assess the activation intensity of rectus abdominis, external oblique and transversus abdominis/internal oblique muscles, during inspiration and expiration. During inspiration, transversus abdominis/internal oblique activation intensity was significantly lower with inspiratory load when compared to without load (p = 0.009) and expiratory load (p = 0.002). During expiration, the activation intensity of all abdominal muscles was significantly higher with expiratory load when compared to without load (p < 0.05). The activation intensity of external oblique (p = 0.036) and transversus abdominis/internal oblique (p = 0.022) was significantly higher with inspiratory load when compared to without load. Transversus abdominis/internal oblique activation intensity was significantly higher with expiratory load when compared to inspiratory load (p < 0.001).Transversus abdominis/internal oblique seems to be the most relevant muscle to modulate the intra-abdominal pressure for the breathing mechanics.     

Changes in intra-abdominal pressure during postural and respiratory activation of the human diaphragm.

In humans, when the stability of the trunk is challenged in a controlled manner by repetitive movement of a limb, activity of the diaphragm becomes tonic but is also modulated at the frequency of limb movement. In addition, the tonic activity is modulated by respiration. This study investigated the mechanical output of these components of diaphragm activity. Recordings were made of costal diaphragm, abdominal, and erector spinae muscle electromyographic activity; intra-abdominal, intrathoracic, and transdiaphragmatic pressures; and motion of the rib cage, abdomen, and arm. During limb movement the diaphragm and transversus abdominis were tonically active with added phasic modulation at the frequencies of both respiration and limb movement. Activity of the other trunk muscles was not modulated by respiration. Intra-abdominal pressure was increased during the period of limb movement in proportion to the reactive forces from the movement. These results show that coactivation of the diaphragm and abdominal muscles causes a sustained increase in intra-abdominal pressure, whereas inspiration and expiration are controlled by opposing activity of the diaphragm and abdominal muscles to vary the shape of the pressurized abdominal cavity.