The effects of slow breathing on postural muscles during standing perturbations in young adults.

Maintaining standing balance is vital to completing activities in daily living. Recent findings suggest an interaction between cardiovascular and postural control systems. Volitional slow breathing can modulate the cardiovascular response and affect postural control during quiet standing. However, the effects of slow breathing during threats to standing balance have not been studied. The study examined the effect of slow breathing on the latency and amplitude of postural muscle responses to perturbations of the base of support in healthy, young adults. Twenty-seven participants completed two balance perturbation tasks in standing on an instrumented split-belt treadmill while breathing spontaneously and breathing at 6 breaths per minute. Each perturbation task consisted of 25 posteriorly directed translations of the treadmill belts every 8-12 s. Muscle latency and muscle burst amplitude were measured using surface electromyography from the right limb for the quadriceps (QUADS), medial hamstring (MH), gastrocnemii (GASTROC), soleus (SOL), and tibialis anterior (TA) muscle groups, while a respiratory belt was used to record respiratory rate. Results indicated that during the slow breathing task both muscle latency (p = 0.022) and muscle burst amplitude (p = 0.011) decreased compared to spontaneous breathing. The EMG pre-perturbation activation was not significantly different in any muscle group between conditions (p > 0.167). The study found that reducing respiratory rate to approximately 6 breaths per minute affects the neuromuscular responses in the lower limb muscles to perturbations.

Postural control in response to unilateral and bilateral external perturbations in older adults.

BACKGROUND: Balance is an important determinant of physical function and falls risk. The ability to withstand external perturbations is important when walking on icy or uneven surfaces, whether the perturbations are bilateral or unilateral. RESEARCH QUESTION: This study sought to determine the effect of unilateral and bilateral standing perturbations on leg muscle activity in healthy older adults. METHODS: Participants experienced unilateral and bilateral standing perturbations of the treadmill. Surface electromyography (EMG) from lower limb muscles was recorded unilaterally. EMG onset latency and root mean square (RMS) amplitude of the muscle bursts were calculated. RESULTS: Older adults demonstrated a combined ankle/hip strategy, along with pre-activation and co-contraction of muscles in response to unilateral and bilateral stance perturbations. As well, older adults demonstrated higher levels of EMG, but no difference in the latency of burst onset, in bilateral than unilateral perturbation types. SIGNIFICANCE: When the stance limb was perturbed in the bilateral condition, the older adults responded with a Gastrocs EMG burst nearly 100% of the maximum EMG. The high level of EMG used, especially in the Gastrocs, during the bilateral perturbations may reduce the safety factor for falls in older adults. Older adults responded to the different perturbation demands by modulating EMG amplitude as opposed to the onset timing of EMG.

Regional modulation of the ankle plantarflexor muscles associated with standing external perturbations across different directions.

Maintenance of upright standing posture has often been explained using the inverted pendulum model. This model considers the ankle plantarflexors to act as a single synergistic group. There are differences in muscle properties among the medial and lateral gastrocnemius (MG and LG, respectively) and the soleus that may affect their activation. Twelve volunteers participated in an investigation to determine whether the activation of the ankle plantarflexor muscles was modulated according to perturbation direction during unilateral standing perturbations of 1% body mass. High-density surface electromyography (HDS-EMG) was used to determine the amplitude and barycenter of the muscle activation and kinematic analysis was used to evaluate ankle, knee, and hip joint movement. The HDS-EMG amplitude and barycenter of MG and LG were modulated with the perturbation direction (MG p < 0.05; LG p < 0.01; one-way repeated-measures ANOVA). In soleus, the HDS-EMG barycenter modulated across the perturbation direction (p < 0.01 for X&Y coordinates), but the HDS-EMG amplitude did not change. A repeated-measures correlation was used to interpret the HDS-EMG pattern in the context of the kinematics. The relative contribution of MG activation compared to the total gastrocnemii activation was significantly associated with ankle dorsi/plantarflexion (rrm = 0.620), knee flexion/extension and abduction/adduction (rrm = 0.622 and rrm = 0.547, respectively), and hip flexion/extension and abduction/adduction (rrm = 0.653 and rrm = 0.432, respectively). The findings suggest that the central nervous system activates motor units within different regions of MG, LG and SOL in response to standing perturbations in different directions.

Effect of standing posture on inhibitory postsynaptic potentials in gastrocnemius motoneurons.

This study examined the task dependence of sensory inputs on motoneuron excitability by comparing the inhibitory postsynaptic potential (IPSP) evoked by stimulation of the sural nerve between a standing postural task (Free Standing) and a comparable voluntary isometric contraction performed in a supine position (Lying Supine). We hypothesized that there would be a smaller IPSP in standing than in the supine position, based on the task dependence of the ankle plantarflexor activity on the standing task. Ten healthy participants participated in a total of 15 experiments. Single motor unit (MU) firings were recorded with both intramuscular fine-wire electrodes and high-density surface electromyography. Participants maintained the MU discharge at 6-8 Hz in Free Standing or Lying Supine while the right sural nerve was stimulated at random intervals between 1 and 3 s. To evaluate the reflex response, the firing times of the discriminated MUs were used to construct peristimulus time histograms and peristimulus frequencygrams. The sural nerve stimulation resulted in weaker inhibition in Free Standing than in Lying Supine. This finding is discussed in relation to the putative activation of persistent inward currents in standing posture and the task-dependent advantages of overriding inhibitory synaptic inputs to the plantarflexors to maintain the standing posture. NEW & NOTEWORTHY The task-dependent modulation of sensory inputs on motoneuron excitability in standing is not well understood. Evoking an inhibitory postsynaptic potential (IPSP) resulted in a smaller IPSP in gastrocnemius motoneurons in standing than in the supine position. Mildly painful sensory inputs produced weaker motoneuron inhibition in standing, suggesting an imperative to maintain ankle plantarflexion activity for the task of upright stance.

Physiological arousal accompanying postural responses to external perturbations after stroke.

OBJECTIVE: The purpose of this study was to examine simultaneously the level of physiological arousal and the postural response to external perturbations in people post-stroke compared to age-matched controls to build a more comprehensive understanding of the effect of stroke on postural control and balance self-efficacy. METHODS: Participants stood with each foot on separate force platforms. Ten applications of loads of 2% body weight at the hips perturbed the participant anteriorly under two conditions: investigator-triggered or self-triggered (total 20). Electrodermal activity (EDA; measurement of physiological arousal), electromyography (EMG) of the ankle plantarflexor muscles and anterior-posterior center of pressure measurements were taken pre-perturbation (anticipatory) and post-perturbation (response) and compared between the initial (first two) and final (last two) perturbations. RESULTS: Participants post-stroke demonstrated significantly higher levels of anticipatory EDA and anticipatory paretic plantarflexor EMG during both self- and investigator-triggered conditions compared to controls. Anticipatory EDA levels were higher in the final perturbations in participants post-stroke in both conditions, but not in controls. Habituation of the EDA responses post-perturbation was exhibited in the self-triggered perturbations in controls, but not in participants post-stroke. CONCLUSIONS: Physiological arousal and postural control strategies of controls revealed habituation in response to self-triggered perturbations, whereas this was not seen in participants post-stroke. SIGNIFICANCE: Understanding the physiological arousal response to challenges to standing balance post-stroke furthers our understanding of postural control mechanisms post-stroke.

Between-day reliability of triceps surae responses to standing perturbations in people post-stroke and healthy controls: A high-density surface EMG investigation.

The reliability of triceps surae electromyographic responses to standing perturbations in people after stroke and healthy controls is unknown. High-Density surface Electromyography (HDsEMG) is a technique that records electromyographic signals from different locations over a muscle, overcoming limitations of traditional surface EMG such as between-day differences in electrode placement. In this study, HDsEMG was used to measure responses from soleus (SOL, 18 channels) and medial and lateral gastrocnemius (MG and LG, 16 channels each) in 10 people after stroke and 10 controls. Timing and amplitude of the response were estimated for each channel of the grids. Intraclass Correlation Coefficient (ICC) and normalized Standard Error of Measurement (SEM%) were calculated for each channel individually (single-channel configuration) and on the median of each grid (all-channels configuration). Both timing (single-channel: ICC=0.75-0.96, SEM%=5.0-9.1; all-channels: ICC=0.85-0.97; SEM%=3.5-6.2%) and amplitude (single-channel: ICC=0.60-0.91, SEM%=25.1-46.6; ICC=0.73-0.95, SEM%=19.3-42.1) showed good-to-excellent reliability. HDsEMG provides reliable estimates of EMG responses to perturbations both in individuals after stroke and in healthy controls; reliability was marginally better for the all-channels compared to the single-channel configuration.

Patterns of muscle coordination during stepping responses post-stroke.

This study compared self-induced stepping reactions of seventeen participants after stroke and seventeen controls. Surface electromyographic (EMG) signals were recorded bilaterally from the soleus (SOL), tibialis anterior (TA), biceps femoris (BF) and rectus femoris (RF) muscles. Principal component analysis (PCA) was used to reduce the data into muscle activation patterns and examine group differences (paretic, non-paretic, control leg). The first principal component (PC1) explained 46.7% of the EMG signal of the stepping leg. Two PCs revealed distinct activation features for the stepping paretic leg: earlier TA onset at step initiation and earlier BF and SOL onset at mid-step. For the stance leg, PC1 explained 44.4% of the EMG signal and significant differences were found in the non-paretic leg compared to paretic (p < 0.001) and control (p < 0.001). In PC1, at step onset the BF and SOL EMG and the RF and TA EMG were increased over the latter half of the step. No PC loadings were distinct for the paretic leg during stance, however differences were found in the non-paretic leg: earlier TA burst and increased BF and SOL EMG at step initiation. The results suggest impairments in the paretic leg when stepping and compensatory strategies in the non-paretic stance leg.

Behavior of medial gastrocnemius motor units during postural reactions to external perturbations after stroke.

OBJECTIVE: This study investigated the behavior of medial gastrocnemius (GM) motor units (MU) during external perturbations in standing in people with chronic stroke. METHODS: GM MUs were recorded in standing while anteriorly-directed perturbations were introduced by applying loads of 1% body mass (BM) at the pelvis every 25-40s until 5% BM was maintained. Joint kinematics, surface electromyography (EMG), and force platform measurements were assessed. RESULTS: Although external loads caused a forward progression of the anterior-posterior centre of pressure (APCOP), people with stroke decreased APCOP velocity and centre of mass (COM) velocity immediately following the highest perturbations, thereby limiting movement velocity in response to perturbations. MU firing rate did not increase with loading but the GM EMG magnitude increased, reflecting MU recruitment. MU inter spike interval (ISI) during the dynamic response was negatively correlated with COM velocity and hip angular velocity. CONCLUSIONS: The GM utilized primarily MU recruitment to maintain standing during external perturbations. The lack of MU firing rate modulation occurred with a change in postural central set. However, the relationship of MU firing rate with kinematic variables suggests underlying long-loop responses may be somewhat intact after stroke. SIGNIFICANCE: People with stroke demonstrate alterations in postural control strategies which may explain MU behavior with external perturbations.

Motor unit recruitment and firing rate in medial gastrocnemius muscles during external perturbations in standing in humans.

There is limited investigation of the interaction between motor unit recruitment and rate coding for modulating force during standing or responding to external perturbations. Fifty-seven motor units were recorded from the medial gastrocnemius muscle with intramuscular electrodes in response to external perturbations in standing. Anteriorly directed perturbations were generated by applying loads in 0.45-kg increments at the pelvis every 25-40 s until 2.25 kg was maintained. Motor unit firing rate was calculated for the initial recruitment load and all subsequent loads during two epochs: 1) dynamic response to perturbation directly following each load drop and 2) maintenance of steady state between perturbations. Joint kinematics and surface electromyography (EMG) from lower extremities and force platform measurements were assessed. Application of the external loads resulted in a significant forward progression of the anterior-posterior center of pressure (AP COP) that was accompanied by modest changes in joint angles (<3°). Surface EMG increased more in medial gastrocnemius than in the other recorded muscles. At initial recruitment, motor unit firing rate immediately after the load drop was significantly lower than during subsequent load drops or during the steady state at the same load. There was a modest increase in motor unit firing rate immediately after the load drop on subsequent load drops associated with regaining balance. There was no effect of maintaining balance with increased load and forward progression of the AP COP on steady-state motor unit firing rate. The medial gastrocnemius utilized primarily motor unit recruitment to achieve the increased levels of activation necessary to maintain standing in the presence of external loads.

Changes in kinematics and trunk electromyography during a 2000 m race simulation in elite female rowers.

Achieving excellence in rowing requires optimization of technique to maximize efficiency and force production. Investigation of the kinematics of the trunk, upper and lower extremity, together with muscle activity of the trunk, provides an insight into the motor control strategies utilized over a typical race. Nine elite female rowers performed a 2000 m race simulation. Kinematic data of the trunk and extremities, together with electromyography (EMG) activity of spinal and pelvic extensor and flexor muscles, were compared at 250 and 1500 m. At 1500 m, there was greater dissociation in the timing of leg extension and arm flexion and delayed trunk extension. Also at 1500 m, the spine demonstrated a delayed peak extension angular velocity of the T4-T7 and L3-S1 spinal segments in the early drive along with delayed and increased peak extension angular velocity of T10-L1 and L1-L3 spinal segments during the late drive. Trunk muscle fatigue was not evident; however, the abdominals demonstrated larger EMG burst areas at 1500 m. Alterations in trunk kinematics suggest that the trunk acts as a less stiff lever on which to transfer the forces of the legs to the arms and handle. Increased abdominal activity may reflect increased demand to control the trunk, given the altered coordination between the legs, trunk and arms.

The origins of neuromuscular fatigue post-stroke.

Fatigue post-stroke is a disabling and persistent symptom affecting many stroke survivors. Despite its high prevalence, the pathophysiology underlying this phenomenon remains obscure. The aim of the present study was to investigate the origins of neuromuscular fatigue post-stroke. Ten chronic stroke survivors and 10 controls sustained an isometric contraction at 30% of maximal voluntary contraction (MVC) with the ankle dorsiflexors. Motor evoked potential (MEP), cortical silent period (SP), voluntary activation, M wave and contractile properties were evaluated before, during and after fatigue among the paretic, non-paretic and control limbs. The pattern of response to fatigue in the non-paretic and control limbs was comparable; therefore, results are presented between the paretic and non-paretic limbs. Before fatigue, reduced MVC peak torque and MEP amplitude were observed on the paretic side in comparison with the non-paretic side. During fatigue, the cortical SP duration increased significantly in both limbs, whereas the MEP amplitude significantly increased only in the non-paretic limb. After fatigue, MVC peak torque decreased significantly in both limbs. Significant reductions in M wave and twitch peak torque were observed in both limbs, pointing to the development of peripheral fatigue. However, central fatigue, evident by a significant reduction in voluntary activation, was greater in the paretic than in the non-paretic limb. After stroke, an inability to increase central excitability in response to an increased cortical inhibition associated with the fatiguing contraction may contribute to central fatigue observed in the paretic limb, which may also be linked to increased self-reported fatigue during activities of daily living. These findings advance our understanding of the neuromuscular basis of fatigue post-stroke.

Factors affecting the common modulation of bilateral motor unit discharge in human soleus muscles.

The purpose of this study was to determine the factors that influence the co-modulation of motor unit discharge rate in soleus muscles of both legs during upright standing. Single motor units were recorded from the left and right soleus muscles under three experimental conditions: standing quietly with the eyes open and closed, standing with the eyes closed while vibration was applied to one Achilles tendon, and swaying voluntarily or producing variable low-force isometric contractions at a frequency of 0.05 Hz. Correlations in motor unit discharge rate between left and right soleus motor units were assessed using common drive analysis. The results showed that common drive to motoneurons of the two muscles did not differ between standing with the eyes open or closed, but there was an order effect with the second task having significantly lower common drive than the first. Common drive was also significantly lower when vibration was applied to one leg compared with when no vibration was applied. Common drive was higher as subjects swayed anteriorly as compared with when they swayed posteriorly. There were no significant differences in common drive across phases of the variable isometric force contraction. Common drive was higher during voluntary sway than during variable force production; both of these values were significantly lower than those derived from the quiet standing task. These results suggest that proprioceptive and sub-cortical inputs contribute to the co-modulation of the firing rate of soleus motor unit pairs of the left and right leg during standing posture.

Low-frequency common modulation of soleus motor unit discharge is enhanced during postural control in humans.

The maintenance of quiet stance requires the activation of muscles bilaterally. The soleus muscles in each leg share a common function in standing; that is, each muscle acts to control antero-posterior (AP) sway on its own side. We sought to determine the extent to which oscillations in motor unit discharge were related in motor unit pairs of the soleus muscles during postural and voluntary isometric tasks, both within and between legs. Subjects stood quietly for 5 min or performed a voluntary isometric plantarflexion contraction in a seated position. During the postural tasks, the excursions of AP sway between legs were highly correlated (rho = 0.86 +/- 0.06). The strength of common modulation of motor unit discharge rates was assessed using time- and frequency-domain analyses. The time-domain common drive analysis revealed that the strongest correlation in motor unit discharge modulation occurred in the postural task with unilateral pairs (rho = 0.71 +/- 0.13) being more strongly correlated than bilateral pairs (rho = 0.50 +/- 0.16). Common modulation of motor unit discharge was lowest for the voluntary tasks, with rho = 0.38 +/- 0.11 and 0.16 +/- 0.08 for unilateral and bilateral pairs, respectively. Similarly, the frequency-domain coherence analysis demonstrated an identical ordering effect, with the largest maximum pooled coherence occurring during standing posture in unilateral (0.070 at 1.6 Hz) and bilateral (0.055 at 1.6 Hz) recordings, whereas minimal coherence was observed in the voluntary task in both unilateral and bilateral recordings within the 0-5 Hz range. These results indicate that in the soleus muscle, common modulation of motor unit discharge is greater during postural tasks than during voluntary isometric tasks and can be observed in both bilateral and unilateral motor unit pairs. Differences in the extent of co-modulation of motor unit discharge between tasks may be attributed to either differences in the descending control or differences in the proprioceptive input between postural and isometric tasks.

Synchronization of motor units in human soleus muscle during standing postural tasks.

During standing posture, the soleus muscles acts to control sway in the anteroposterior (AP) direction. The soleus muscles bilaterally share a common function during standing tasks. We sought to determine whether common descending inputs, as evidenced by the synchronization of bilateral motor unit pairs, were employed as a strategy to control this common function. Single motor units were recorded from the soleus muscles in subjects who stood on adjacent force platforms for 5 min with their eyes open or closed. While standing with the eyes open, only 4/39 bilateral motor unit pairs showed significant synchronization. Similarly, only 3/36 motor unit pairs were significantly synchronized during the eyes closed task. The low incidence of synchronization was observed despite a high correlation in the amount of sway in the AP direction between legs in both the eyes open and eyes closed tasks (rho = 0.80 and rho = 0.83, respectively). When the extent of synchronization was assessed between pairs of motor units within the same leg with the eyes open, 10/12 pairs were synchronized. Furthermore, when pairs of soleus motor units were recorded both bilaterally and unilaterally during voluntary isometric ankle plantarflexion, only 4/30 bilateral pairs showed significant synchronization, whereas 19/24 unilateral pairs had significant synchronization. In this study, there was little evidence of the existence of synchronization between bilateral soleus motor unit pairs in either postural tasks or voluntary isometric contractions. In cases in which bilateral synchronization was observed, it was considerably weaker than the synchronization of motor units within a single soleus muscle. The results of this study reveal that it is rather uncommon for bilateral soleus motoneurons to receive common descending synaptic inputs, whereas two motoneurons within a single soleus muscle do.

Modulation of motor unit discharge rate and H-reflex amplitude during submaximal fatigue of the human soleus muscle.

Declining motor unit discharge rates and H-reflex amplitude have been observed in separate experiments during fatiguing submaximal contractions in humans. The purpose of this experiment was to investigate motor unit discharge rate, H-reflex amplitude, and twitch contractile properties concurrently during a fatiguing submaximal isometric contraction of the ankle plantarflexors. Eleven healthy subjects performed fatiguing contractions of low force (25% maximal voluntary contraction (MVC)) or high force (42-66% MVC). Hoffmann (H)-reflexes, muscle compound action potentials (M-waves), twitch contractile properties, and motor unit discharges were recorded from the soleus muscle. In the low-force fatigue task, motor unit firing rate increased gradually over time, whereas the resting H-reflex was significantly depressed at 15% of endurance time and remained quasiconstant for the rest of the task. This suggests that the processes mediating the resting H-reflex depression are relatively independent of those modulating the motor unit firing rate during a low-force fatigue task. In the high-force fatigue task, a decline in the average motor unit discharge rate was accompanied by a decrease in the resting H-reflex amplitude and a prolongation of the twitch half-relaxation time (HRT) at the completion of the fatigue task. Overall, motor unit firing rate was modulated in parallel with changes in the twitch HRT, consistent with the muscle wisdom hypothesis.

Postural muscle activity during bilateral and unilateral arm movements at different speeds.

The muscle activation patterns in anterior and posterior leg muscles were investigated with two types of perturbations to standing balance. Subjects stood with each foot on adjacent force platforms and performed arm flexion movements to shoulder height. Nine subjects performed ten repetitions unilaterally and bilaterally at 100, 75, 50, 25, and 12.5% of maximal acceleration as measured by an accelerometer placed on the dominant hand. Four subjects also performed the fastest movements while leaning forwards and backwards. The area and latency of the EMG activity from the quadriceps (QUAD), hamstrings (BF), soleus (SOL), and tibialis anterior (TA) were measured bilaterally, along with the excursions of the center of pressure (COP) during each movement. In both unilateral and bilateral tasks, subjects showed a scaling of EMG area and COP excursion with the acceleration of the arm movement. Prior to movement onset, significant scaling of EMG area with movement speed occurred in both unilateral and bilateral tasks in most muscles. Following movement onset, EMG areas scaled significantly to movement speed in only the anterior musculature, with the exception of the left BF. The latency of BF was consistent for the four fastest movements. Only the slowest movements resulted in a significant rightward shift of the BF EMG latency. During the unilateral task, the ipsilateral hamstrings were activated significantly earlier than in the bilateral task and the contralateral hamstrings were activated significantly later. It was also observed that subjects utilized one of two different strategies to maintain balance. Five individuals displayed simultaneous anterior/posterior muscle activation while the other four displayed a reciprocal pattern of activation. Regardless of the initial standing position (leaning forwards or backwards), subjects used the same simultaneous or reciprocal activation strategy. The results indicate that muscle activation patterns change with different tasks, but remain the same during variations of the same task.

Changes in motor unit discharge rate are not associated with the amount of twitch potentiation in old men.

This study examined, in nine old men (82 +/- 4 yr), whether there is an association between the magnitude of change in motor unit discharge rate and the amount of twitch potentiation after a conditioning contraction (CC). The evoked twitch force and motor unit discharge rate during isometric ramp-and-hold contractions (10-18 s) of the triceps brachii muscle at 10, 20, and 30% of the maximal voluntary contraction were determined before and 10 s, 2 min, 6 min, and 11 min after a 5-s CC at 75% maximal voluntary contraction. After the CC, there was a potentiation of twitch force (approximately twofold), and the discharge rate of the 47 sampled motor units declined (P < 0.05) an average of 1 Hz 10 s after the CC, compared with the control condition. The CC had no effect on the variability (coefficient of variation) of both force and discharge rate, as well as the electromyographic activity recorded over the triceps brachii and biceps brachii muscles. In contrast to our earlier study of young men (Klein CS, Ivanova TD, Rice CL, and Garland SJ, Neurosci Lett 316: 153-156, 2001), the magnitude of the reduction in discharge rate after the CC was not associated (r = 0.06) with the amount of twitch potentiation. These findings suggest an age-related alteration in the neural strategy for adjusting motor output to a muscle after a CC.

Motor unit discharge rate following twitch potentiation in human triceps brachii muscle.

It has been proposed that during brief voluntary contractions, twitch potentiation may sustain force output despite a decline in motor unit discharge rate. This study examined the evoked twitch force and motor unit discharge rates during submaximal voluntary contractions of the triceps brachii muscle before and after a 5 s conditioning contraction (CC) at 75% of maximal voluntary force. After the CC, twitch force potentiated ( approximately 1.3-2-fold), and the discharge rate in 33 of 35 motor units declined significantly by 1-6 Hz. The increase in twitch force was significantly correlated with the decline in discharge rate (r=-0.74). These findings suggest that the extent of the decrease in motor unit discharge rate following a CC is associated with the magnitude of twitch potentiation.