Regional recruitment and differential behavior of motor units during postural control in older adults

Aging is associated with neuromuscular system changes that may have implications for the recruitment and firing behaviors of motor units (MUs). In previous studies, we observed that young adults recruit subpopulations of triceps surae MUs during tasks that involved leaning in five directions: common units that were active during different leaning directions and unique units that were active in only one leaning direction. Furthermore, the MU subpopulation firing behaviors [average firing rate (AFR), coefficient of variation (CoVISI), and intermittent firing] modulated with leaning direction. The purpose of this study was to examine whether older adults exhibited this regional recruitment of MUs and firing behaviors. Seventeen older adults (aged 74.8 ± 5.3 yr) stood on a force platform and maintained their center of pressure leaning in five directions. High-density surface electromyography recordings from the triceps surae were decomposed into single MU action potentials. A MU tracking analysis identified groups of MUs as being common or unique across the leaning directions. Although leaning in different directions did not affect the AFR and CoVISI of common units (P > 0.05), the unique units responded to the leaning directions by increasing AFR and CoVISI, albeit modestly (F = 18.51, P < 0.001). The unique units increased their intermittency with forward leaning (F = 9.22, P = 0.003). The mediolateral barycenter positions of MU activity in both subpopulations were found in similar locations for all leaning directions (P > 0.05). These neuromuscular changes may contribute to the reduced balance performance seen in older adults.NEW & NOTEWORTHY In this study, we observed differences in motor unit recruitment and firing behaviors of distinct subpopulations of motor units in the older adult triceps surae muscle from those observed in the young adult. Our results suggest that the older adult central nervous system may partially lose the ability to regionally recruit and differentially control motor units. This finding may be an underlying cause of balance difficulties in older adults during directionally challenging leaning tasks.

Cardiovascular response to anticipatory and reactionary postural perturbations in young adults

NEW FINDINGS

What is the central question of this study? It has been suggested that the cardiovascular responses to a postural perturbation are centrally mediated and reflex mediated. We wanted to know the extent to which the cardiovascular responses to external perturbations could be executed in a feedforward manner, in anticipation of the perturbation. What is the main finding and its importance? We found no anticipatory component driving heart rate and systolic blood pressure responses, suggesting that reflexive mechanisms dominate cardiovascular regulation after a postural perturbation in young adults.

ABSTRACT

Cardiovascular responses to postural perturbations have been reported, but whether the cardiovascular responses to external perturbations could be executed in anticipation of the perturbation is unknown. The purpose of this study was to determine the effect of anticipated and reactionary perturbations on heart rate (HR) and systolic blood pressure (SBP) responses in healthy young adults. A secondary aim was to determine whether perceived state anxiety scores were correlated with the change in HR response during postural perturbation. Twenty healthy young adults stood on a treadmill and experienced two perturbation conditions (anticipatory vs. reactionary), each with two intensity levels (Step vs. No Step). The HR and SBP were collected continuously. Two-way repeated-measures statistical non-parametric mapping tests were used to compare HR and SBP responses to the perturbations over time (from -3 to +8 s). The results indicated that HR was significantly elevated in the higher intensity perturbations [Step vs. No Step, at 0.56-1.32 s (P < 0.0001) and 1.92-3.44 s (P < 0.0001) post-perturbation], while there were no differences in HR between perturbation types (anticipatory vs. reactionary) or in SBP between perturbation types and intensity levels. The perceived state anxiety scores did not differ between perturbation types and intensity levels but were correlated with the change in HR post-perturbation (P = 0.013). We suggest that reflexive mechanisms dominate cardiovascular regulation after anticipatory and reactionary perturbations. The data highlight the cardiovascular mechanism(s) associated with perturbations that should be considered when assessing postural stability in populations with poor balance performance.

Differential behavior of distinct motoneuron pools that innervate the triceps surae

It has been shown that when humans lean in various directions, the central nervous system (CNS) recruits different motoneuron pools for task completion; common units that are active during different leaning directions, and unique units that are active in only one leaning direction. We used high-density surface electromyography (HD-sEMG) to examine if motor unit (MU) firing behavior was dependent on leaning direction, muscle (medial and lateral gastrocnemius; soleus), limits of stability, or whether a MU is considered common or unique. Fourteen healthy participants stood on a force platform and maintained their center of pressure in five different leaning directions. HD-sEMG recordings were decomposed into MU action potentials and the average firing rate (AFR), coefficient of variation (CoV_ISI), and firing intermittency were calculated on the MU spike trains. During the 30°-90° leaning directions both unique units and common units had higher firing rates (F = 31.31, P < 0.0001). However, the unique units achieved higher firing rates compared with the common units (mean estimate difference = 3.48 Hz, P < 0.0001). The CoV_ISI increased across directions for the unique units but not for the common units (F = 23.65, P < 0.0001). Finally, intermittent activation of MUs was dependent on the leaning direction (F = 11.15, P < 0.0001), with less intermittent activity occurring during diagonal and forward-leaning directions. These results provide evidence that the CNS can preferentially control separate motoneuron pools within the ankle plantarflexors during voluntary leaning tasks for the maintenance of standing balance.NEW & NOTEWORTHY In this study, we demonstrate that the different subpopulations of motor units within the three muscles comprising the ankle plantarflexors behave differently during multidirectional leaning. Our results suggest that the central nervous system has the capability to control distinct subpopulations of motor units to meet the force requirements necessary for leaning. This may allow for a precise, efficient, and flexible control strategy for the maintenance of standing balance.

Cardiovascular response to postural perturbations of different intensities in healthy young adults

The ability to regain control of balance is vital in limiting falls and injuries. Little is known regarding how the autonomic nervous system responds during recovery from balance perturbations of different intensities. The purpose of this study was to examine the cardiovascular response following a standing balance perturbation of varying intensities, quantify cardiac baroreflex sensitivity (cBRS) during standing perturbations, and to establish the stability of the cardiac baroreflex during quiet standing before and after balance disturbances. Twenty healthy participants experienced three different perturbation intensity conditions that each included 25 brief posteriorly-directed perturbations, 8-10 s apart. Three perturbation intensity conditions (low, medium, high) were given in random order. Physiological data were collected in quiet stance for 5 min before testing (Baseline) and again after the perturbation conditions (Recovery) to examine baroreflex stability. Beat-to-beat heart rate (HR) and systolic blood pressure (SBP) analysis post-perturbation indicated an immediate acceleration of the HR for 1-2 s, with elevated SBP 4-5 s post-perturbation. Heart rate changes were greatest in the medium (p = 0.035) and high (p = 0.012) intensities compared to low, while there were no intensity-dependent changes in SBP. The cBRS was not intensity-dependent (p = 0.402) but when perturbation conditions were combined, cBRS was elevated compared to Baseline (p = 0.046). The stability of baseline cBRS was excellent (ICC = 0.896) between quiet standing conditions. In summary, HR, but not SBP or cBRS were intensity-specific during postural perturbations. This was the first study to examine cardiovascular response and cBRS to postural perturbations.

Does the stimulus provoking a stepping reaction correlate with step characteristics and clinical measures of balance and mobility post-stroke?

BACKGROUND

Retraining stepping reactions in people post-stroke is vital. However, the relationship between the stimulus and resulting stepping performance in people post-stroke is unknown. We explored relationships between stepping stimulus and stepping reactions initiated by either paretic or non-paretic legs of people post-stroke and controls. Relationships were examined in the context of clinical measures of balance.

METHODS

Centre of mass dynamics were measured during self-initiated destabilizing leaning stimuli that required stepping reactions by paretic and non-paretic legs of people post-stroke (n = 10) and controls (n = 10) to recover balance. Step characteristics of the first two steps of stepping reactions were measured. Correlations were calculated between clinical measures of balance and mobility and the centre of mass and step characteristics.

FINDINGS

Steps were shorter and slower with decreased centre of mass fore-aft and downward displacement and velocity when initiated by paretic and non-paretic legs compared with controls. However, increase in centre of mass displacement and velocity in the fore-aft and downward direction tended to be associated with a greater increase in step length and speed when stepping reactions were initiated by the paretic and non-paretic legs compared with controls. Time to step initiation in response to onset of falling stimulus did not differ between groups. Strong positive correlations were found between clinical balance and mobility scores and centre of mass and step dynamics in fore-aft and vertical directions.

INTERPRETATION

These results support objective measurement of centre of mass to quantify the stimulus influencing step dynamics and stepping performance during retraining interventions following stroke.

Relationships Between Stepping-Reaction Movement Patterns and Clinical Measures of Balance, Motor Impairment, and Step Characteristics After Stroke

OBJECTIVE

Successful stepping reactions, led by either the paretic or nonparetic leg, in response to a loss of balance are critical to safe mobility poststroke. The purpose of this study was to measure sagittal plane hip, knee, ankle, and trunk kinematics during 2-step stepping reactions initiated by paretic and nonparetic legs of people who had stroke and members of a control group.

METHODS

Principal component analysis (PCA) was used to reduce the data into movement patterns explaining interlimb coordination of the stepping and stance legs. Correlations among principal components loading scores and clinical measures of balance ability (as measured on the Community Balance and Mobility scale), motor impairment (as measured on the foot and leg sections of the Chedoke-McMaster Stroke Assessment), and step characteristics (length and velocity) were used to examine the effect of stroke on stepping reaction movement patterns.

RESULTS

The first 5 principal components explained 95.9% of the movement pattern of stepping reactions and differentiated between stepping reactions initiated by paretic legs, nonparetic legs, or the legs of controls. Moderate-strong associations (ρ/r > 0.50) between specific principal component loading scores and clinical measures and step characteristics were dependent on the initiating leg. Lower levels of motor impairment, higher levels of balance ability, and faster and longer steps were associated with stepping reactions initiated by the paretic leg that comprised paretic leg flexion and nonparetic leg extension. Step initiation with the nonparetic leg showed associations between higher scores on clinical measures and movement patterns of flexion in both paretic and nonparetic legs.

CONCLUSIONS

Movement patterns of stepping reactions poststroke were influenced by the initiating leg. After stroke, specific movement patterns showed associations with clinical measures depending on the initiating leg, suggesting that these movement patterns are important to retraining of stepping reactions. Specifically, use of flexion patterning and assessment of between-leg pattern differentiation may be important aspects to consider during retraining of stepping reactions poststroke.

IMPACT

Evidence-based interventions targeting balance reactions are still in their infancy. This investigation of stepping reactions poststroke addresses a major gap in research.

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.

Symmetry of cortical planning for initiating stepping in sub-acute stroke

OBJECTIVE

This study examined motor planning for stepping when the paretic leg was either stepping or standing (to step with the non-paretic leg), to understand whether difficulty with balance and walking post-stroke could be attributed to poor motor planning.

METHODS

Individuals with stroke performed self-initiated stepping. Amplitude and duration of the movement-related cortical potential (MRCP) was measured from Cz. Electromyography (EMG) of biceps femoris (BF) and rectus femoris (RF) were collected.

RESULTS

There were no differences between legs in stepping speed, MRCP or EMG parameters. The MRCPs when stepping with the paretic leg and the non-paretic leg were correlated. When the paretic leg was stepping, the MRCP amplitude correlated with MRCP duration, indicating a longer planning time was accompanied by higher cognitive effort. Slow steppers had larger MRCP amplitudes stepping with the paretic leg and longer MRCP durations stepping with the non-paretic leg.

CONCLUSIONS

MRCP measures suggest that motor planning for initiating stepping are similar regardless of which limb is stepping. Individuals who stepped slowly had greater MRCP amplitudes and durations for planning.

SIGNIFICANCE

Individuals who step slowly may require more time and effort to plan a movement, which may compromise their safety in the community.

Regionalization of the stretch reflex in the human vastus medialis

KEY POINTS

Regionalization of the stretch reflex, i.e. the notion that the activation of 1a afferents from a muscle region influences only the activation of motor units in the same region, has been demonstrated previously in animals but not in humans. Mechanical stretches applied to regions of vastus medialis as close as 10 mm apart resulted in recruitment of motor units localized topographically with respect to the location of the mechanical stretch. Stretch reflexes are regionalized in the human vastus medialis. The human spinal cord has the neuromuscular circuitry to preferentially activate motoneurones innervating muscle fibres located in different regions of the vastus medialis.

ABSTRACT

The localization of motor unit territories provides an anatomical basis to suggest that the CNS may have more independence in motor unit recruitment and control strategies than what was previously thought. In this study, we investigated whether the human spinal cord has the neuromuscular circuitry to independently activate motor units located in different regions of the vastus medialis. Mechanical taps were applied to multiple locations in the vastus medialis (VM) in nine healthy individuals. Regional responses within the muscle were observed using a grid of 5 × 13 surface EMG electrodes. The EMG amplitude was quantified for each channel, and a cluster of channels showing the largest activation was identified. The spatial location of the EMG response was quantified as the position of the channels in the cluster. In a subset of three participants, intramuscular recordings were performed simultaneously with the surface EMG recordings. Mechanical taps resulted in localized, discrete responses for each participant. The spatial location of the elicited responses was dependent on the location of the tap (P < 0.001). Recordings with intramuscular electrodes confirmed the regional activation of the VM for different tap locations. Selective stimulation of 1a afferents localized in a region of the VM results in reflex recruitment of motor units in the same region. These findings suggest that the human spinal cord has the neuromuscular circuitry to modulate spatially the motoneuronal output to vastus medialis regions, which is a neuroanatomical prerequisite for regional activation.

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.

Regional activation within the vastus medialis in stimulated and voluntary contractions

This study examined the contribution of muscle fiber orientation at different knee angles to regional activation identified with high-density surface electromyography (HDsEMG). Monopolar HDsEMG signals were collected using a grid of 13 × 5 electrodes placed over the vastus medialis (VM). Intramuscular electrical stimulation was used to selectively activate two regions within VM. The distribution of EMG responses to stimulation was obtained by calculating the amplitude of the compound action potential for each channel; the position of the peak amplitude was tracked across knee angles to describe shifts of the active muscle regions under the electrodes. In a separate experiment, regional activation was investigated in 10 knee flexion-extension movements against a fixed resistance. Intramuscular stimulation of different VM regions resulted in clear differences in amplitude distribution along the columns of the electrode grid (P < 0.001); changes in knee angle resulted in consistent shifts along the rows (P < 0.01) and negligible shifts along the columns of the electrode grid. Regional VM activation was identified in dynamic movement, with distal shifts of the EMG distribution in the eccentric phase of the movement (P < 0.05) and at more flexed knee angles (P < 0.05). HDsEMG was used to describe regional activation across the VM that was not attributable to anatomic factors. Changes in muscle fiber orientation associated with knee joint angle mainly influence the amplitude distribution along the fiber direction. Future studies are needed to understand possible functional roles for regional activation within the VM in dynamic tasks.

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.

Motor Planning for Loading During Gait in Subacute Stroke

OBJECTIVES

To determine the characteristics of motor planning surrounding initial contact during gait through examination of thigh muscle timing, amplitude, and co-contraction of the paretic and nonparetic limbs in people poststroke, and to investigate whether muscle timing, amplitude, and clinical performance measures of balance and mobility differ based on the level of co-contraction.

DESIGN

Observational study.

SETTING

University-based research laboratory.

PARTICIPANTS

Individuals (n=27) in the subacute phase after stroke and healthy controls (n=8) (N=35).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Timing (onset and offset) and normalized amplitude (percent electromyography maximum) of the biceps femoris (BF) and rectus femoris (RF) muscles were measured during terminal swing and early stance. A co-contraction index (CCI) was calculated for the BF and RF muscle activity. Individuals with CCI values equal to or below the mean of the healthy group were in the low CCI group, whereas those with values above the mean were in the high CCI group. Functional balance and mobility evaluation used the Community Balance and Mobility Scale (CB&M).

RESULTS

For the paretic and nonparetic limbs, measures of timing, amplitude, and co-contraction were similar for both limbs. Compared with the healthy group, the high CCI group had lower CB&M scores, longer durations, and higher levels of RF and BF muscle activity, whereas the low CCI group had electromyographic measures statistically similar to healthy controls.

CONCLUSIONS

The motor control of gait after subacute stroke is characterized by symmetry of timing and amplitude of muscle recruitment at the knee. High co-contraction levels surrounding the knee were associated with lower functional balance and mobility. These findings suggest a compensatory strategy of increased co-contraction in those with more impairment while maintaining symmetry of lower-limb biomechanics between limbs.

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.

Is the Recovery of Functional Balance and Mobility Accompanied by Physiological Recovery in People With Severe Impairments After Stroke?

Background. Rehabilitation after severe stroke is often limited because of impairments in sensorimotor function. Functional and physiological recovery after severe stroke is poorly understood and has not been studied extensively. Objective. This study’s purpose was to examine functional and physiological recovery of standing balance during inpatient rehabilitation in people with severe impairments after stroke. Methods. A total of 10 participants with severe impairments after stroke were evaluated monthly in a stroke rehabilitation unit with the following functional outcome measures: Berg Balance Scale (BBS), Clinical Outcome Variables Scale (COVS), and Chedoke McMaster Stroke Assessment (CMSA). Weight bearing (WB), center of pressure (COP) velocity, and electromyography (EMG) data were collected during quiet standing and during internal perturbation with a rapid nonparetic arm raise. Results. Cross-sectionally, there were moderate to strong correlations for EMG area and WB with CMSA and COVS. Additionally, the BBS was correlated with WB on the paretic side. Longitudinally, statistically significant improvement was found for functional measures but not for physiological measures. The mean BBS and COVS improved by 23 and 21 points, respectively. COP velocity decreased by 60.1% on the paretic leg but not significantly. Conclusions. During stroke rehabilitation, all participants improved functionally. Some patients improved physiologically, though near discharge, all participants remained very impaired. Future studies with larger sample sizes are needed to explore the capacity for physiological recovery in this population.

Effects of fast functional exercise on muscle activity after stroke

BACKGROUND

In stroke rehabilitation, considerable emphasis is placed on improving muscle strength with less focus on the speed of movement. Muscle power (product of force and velocity) is essential for balance and mobility but velocity of movement is impaired after stroke.

OBJECTIVE

The purpose of this efficacy study is to determine if a single session of fast functional movements can increase muscle activation and the speed of movement in participants with a subacute stroke.

METHODS

In total, 32 individuals poststroke and 32 age- and sex-matched controls performed a single session of 50 fast squats and steps. Electromyographic (EMG) activity was measured bilaterally in the rectus femoris (RF), biceps femoris (BF), tibialis anterior (TA), and soleus muscles. The average EMG area and the movement speed were calculated over 10 trials. The effect of exercise was determined as the change from the second set (Start) to the last set (End) of 10 trials.

RESULTS

The stroke group had significant increases in EMG area of the TA, BF, and RF during the squatting exercise. There was an increase in EMG area of the RF and BF when the paretic leg was stepping. Improvements in EMG area of the soleus and RF when the paretic leg was in stance accompanied increases in EMG area when the nonparetic leg was stepping. There was a trend for improved movement speed for both exercises.

CONCLUSION

A single session of exercises emphasizing speed of movement can be used to improve muscle activation in persons with mild to moderately severe strokes.

Control of fast squatting movements after stroke

OBJECTIVE

Little is known about how residual motor impairments after stroke affect the motor control of fast movements, particularly those that combine postural control and limb movement. The purpose of this study was to examine the influence of stroke on the motor control of fast squatting movements.

METHODS

Seventeen individuals with hemiparesis and seventeen age- and sex-matched controls performed fast squatting movements. Force platform data, knee acceleration, and electromyographic activity from rectus femoris, biceps femoris, tibialis anterior, soleus, were collected.

RESULTS

Subjects after stroke performed the squats asymmetrically, with reduced velocity and acceleration compared to controls. Subjects with low motor recovery depended on the non-paretic leg to compensate for poor paretic muscle activation whereas subjects with high motor recovery activated muscles in the paretic leg in an adaptive manner, making the movement more symmetrical. Difficulty with postural control was evident by reduced coupling of the timing of the knee movement with the center of pressure excursion.

CONCLUSIONS

Slow performance of squatting movements was accompanied by altered muscle activation, coupled with impaired postural control.

SIGNIFICANCE

Fast squatting movements in standing require appropriate muscle activation and postural control, the latter of which can be measured easily with force platform and accelerometer data.

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.

Changes in the Estimated Time Course of the Motoneuron Afterhyperpolarization Induced by Tendon Vibration

Group Ia afferents are activated vigorously with high-frequency tendon vibration and provide excitatory input to the agonist muscle and inhibitory input to the antagonist muscle group via inhibitory interneurons. The purpose of this experiment was to determine whether the afterhyperpolarization (AHP) time course in humans is altered in response to tendon vibration. The AHP time course is estimated using the interval death rate (IDR) analysis, a transform of the motor unit action potential train. Single motor units from tibialis anterior (TA) were recorded as subjects held low force dorsiflexor contractions for 600 s with and without vibration. The vibratory stimulus was superimposed on the low force contraction either to the tendon of the TA or the antagonist Achilles tendon. During TA tendon vibration, the time course of the AHP, as expressed by its time constant (τ), decreased from 35.5 ms in the previbration control condition to 31.3 ms during the vibration ( P = 0.003) and returned to 36.3 ms after the vibration was removed ( P = 0.002). The AHP τ during vibration of the antagonist Achilles tendon (38.6 ms) was greater than the previbration control condition (33.6 ms; P = 0.001). It is speculated that the reduction in AHP time constant with TA vibration may have resulted alone or in combination with a modulation of motoneuron gain, an alteration of persistent inward currents and/or the restructuring of synaptic noise. A decrease in firing probability, possibly reflecting Ia reciprocal inhibition, may have been responsible for the larger AHP time constant.

Moments and muscle activity after high tibial osteotomy and anterior cruciate ligament reconstruction

PURPOSES

To evaluate the effects of simultaneous high tibial osteotomy (HTO) and anterior cruciate ligament (ACL) reconstruction on 1) the external knee adduction moment, 2) the external knee flexion and extension moments, and 3) the quadriceps, hamstrings, and gastrocnemius muscle activity during walking.

METHODS

Twenty-one patients with varus malalignment of the lower limb, medial compartment knee osteoarthritis, and concomitant anterior cruciate ligament (ACL) deficiency were tested before and 1 yr after undergoing simultaneous medial opening wedge high tibial osteotomy (HTO) and ACL reconstruction during a single operation. Three-dimensional kinetic and kinematic data were used to calculate external coronal and sagittal moments about the knee. EMG data from the quadriceps, hamstrings, and gastrocnemius were used to determine coactivation ratio and activation patterns.

RESULTS

Neutral alignment and knee stability were achieved in all patients after surgery. The peak knee adduction moment decreased from 2.88 +/- 0.57 to 1.71 +/- 0.56%BW x Ht (P < 0.001). The early stance knee flexion moment decreased from 1.95 +/- 1.89 to 0.88 +/- 1.17%BW x Ht (P < 0.01). The late stance knee extension moment increased from 1.83 +/- 1.53 to 2.76 +/- 1.22%BW x Ht (P < 0.001). There were no significant differences in muscle coactivation or muscle activation patterns (P > 0.05).

CONCLUSIONS

Improving lower limb alignment and knee stability significantly alters the coronal and the sagittal moments about the knee during walking, without apparent changes in muscle activation patterns.

Afterhyperpolarization time-course and minimal discharge rate in low threshold motor units in humans

The alpha-motoneurone afterhyperpolarization (AHP) duration correlates with a number of its muscle unit properties in animal preparations. In humans, the interval death rate (IDR) analysis has been used to estimate the time course of human motoneurone AHP based on the pattern of motor unit firing. The purpose of this experiment was first, to examine the relationship between estimated AHP time course and the minimal firing rate of the motor unit and second, to examine the relationship between the AHP and motor unit contractile properties in the tibialis anterior (TA) muscle. Motor unit data were obtained from the TA muscle during low force isometric contractions lasting 600 s. Muscle unit twitch characteristics were determined using spike-triggered averaging (STA) and the motoneurone AHP time course was estimated using the IDR analysis. Minimal discharge rate and derecruitment threshold torque were determined for 2 s preceding motor unit derecruitment. The AHP time constant and minimal discharge rate were negatively correlated, whereas the derecruitment threshold torque was not associated with the AHP time constant. The estimated AHP duration, however, is considerably shorter than the mean ISI of the minimal discharge rate suggesting that synaptic noise and AHP duration are important factors in dictating the minimal discharge rate in low force voluntary contractions in humans. The AHP time constant did not vary significantly with motor unit twitch amplitude; however, significant positive relationships were found between the AHP time constant and the temporal properties of the motor unit twitch. The calculated AHP time course using the IDR analysis, therefore, is a reasonable estimate and coupled with motor unit properties attained with STA, it provides a powerful method to describe low-threshold motor units.

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.

Recovery of standing balance and health-related quality of life after mild or moderately severe stroke

OBJECTIVE

To examine the physiologic and functional recovery of standing balance and health-related quality of life (HRQOL) in people after mild and moderate stroke.

DESIGN

Inception cohort study with evaluations at 1 month and 3 months poststroke.

SETTING

Laboratory.

PARTICIPANTS

Twenty-nine volunteers who had sustained a stroke. Subjects were categorized into mild and moderate groups.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Functional balance was assessed (Clinical Outcome Variables Scale [COVS]) and physiologic measures (electromyography, postural sway) were taken when subjects stood quietly on a force platform and when they performed a rapid unilateral arm-raise perturbation. The Medical Outcomes Study 36-Item Short-Form Health Survey (SF-36) was administered to evaluate HRQOL.

RESULTS

Subjects in the mild group were approaching maximal scores on the COVS (87.7+/-4.1/91) at 3 months poststroke, yet had significant impairment in paretic muscle activation patterns when compared with healthy subjects. Subjects in the moderate group had increased paretic muscle activation over the 2 months, accompanied by significant increases of 10.7+/-5.9 points on the COVS. For both groups, there was significantly less postural sway on the paretic than the nonparetic leg and significant improvements in the SF-36 (physical component) over time.

CONCLUSIONS

Subjects recovering from a stroke showed a significant improvement in physical HRQOL and functional and physiologic balance, yet the physiologic balance recovery was not complete even in the mild group.

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.

The time course of the motoneurone afterhyperpolarization is related to motor unit twitch speed in human skeletal muscle

The relationship between the electrophysiological properties of motoneurones and their muscle units has been established in animal models. A functionally significant relationship exists whereby motoneurones with long post-spike afterhyperpolarizations (AHPs) innervate slow contracting muscle units. The purpose of this study was to determine whether the time course of the AHP as measured by its time constant is associated with the contractile properties of its muscle unit in humans. Using an intramuscular fine wire electrode, 46 motor units were recorded in eight subjects as they held a low force contraction of the first dorsal interosseus muscle for approximately 10 min. By applying a recently validated transform to the interspike interval histogram, the mean voltage versus time trajectory of the motoneurone AHP was determined. Spike-triggered averaging was used to extract the muscle unit twitch from the whole muscle force with strict control over force variability and motor unit discharge rate (interspike intervals between 120 and 200 ms). The AHP time constant was positively correlated to the time to half-force decay (rho = 0.36, P < 0.05) and twitch duration (rho = 0.57, P < 0.001); however, time to peak force failed to reach significance (rho = 0.27, P < 0.07). These results suggest that a similar functional relationship exists in humans between the motoneurone AHP and the muscle unit contractile properties.

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.

Ischemia sensitivity and motoneuron afterhyperpolarization in human motor units

The purpose of this study was to examine whether motor units, grouped by speed of contraction in the human first dorsal interosseous muscle, differed in their sensitivity to ischemia and motoneuron afterhyperpolarization (AHP) time-course. Motor units were recorded while subjects held an abduction force for approximately 10 min. Subsequently, subjects abducted for 4-5 min under ischemic conditions. Motor unit twitches derived using spike-triggered averaging were allocated into "fast" or "slow" contracting groups based on twitch time to peak (TTP) force. Motor units in the "slow" group had a greater sensitivity to ischemia than the "fast" group. When upper and lower quartiles of TTP were compared, motor units with slow TTP had long AHP time-constants (as estimated by an interspike interval histogram transform). Thus, motor units grouped by speed of contraction differed in both their sensitivity to ischemia and motoneuron AHP time-course. This provides preliminary evidence that the estimated AHP time-constant may be used to deduce motor-unit type in humans.

Recovery of standing balance and functional mobility after stroke11No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the author(s) or upon any organization with which the author(s) is/are associated

OBJECTIVE

To examine the extent to which recovery of functional balance and mobility is accompanied by change in a few specific physiologic measures of postural control.

DESIGN

Longitudinal prospective study.

SETTING

Laboratory setting in Ontario.

PARTICIPANTS

Twenty-seven volunteers (age, 64.2+/-13.7y) undergoing 4 weeks of rehabilitation after stroke participated. At initial testing, patients were 32.7+/-18.4 days poststroke and exhibited a moderate level of motor recovery (lower-extremity and postural control, stages 3-4 on the Chedoke-McMaster Stroke Assessment Impairment Inventory).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Three functional measures (Berg Balance Scale, Clinical Outcome Variables Scale, gait speed) were assessed. Three physiologic measures (electromyographic data of hamstrings and soleus muscles bilaterally, postural sway, arm acceleration) were taken while subjects stood quietly on a force platform and while they performed a rapid shoulder flexion movement of the nonparetic upper extremity.

RESULTS

After 1 month of rehabilitation, there was an overall significant improvement in all outcome measures (functional, physiologic). However, 10 patients failed to show any improvement in the electromyographic activation of hamstrings muscle on the paretic side in response to the rapid arm movement. These patients compensated by increasing the anticipatory activation of the nonparetic hamstrings.

CONCLUSION

After stroke, patients showed improvement in both physiologic and functional measures of balance and mobility over a 1-month period. We have identified some patients who may be using compensatory strategies to increase function. The factors that may predict those patients who are likely to use compensatory strategies awaits further study.

Factors affecting the stability of the spike-triggered averaged force in the human first dorsal interosseus muscle

The reproducibility of motor unit twitches obtained using spike-triggered averaging (STA) was examined in the human first dorsal interosseus. For each motor unit (30 total) a series of STA twitches was derived using a 30 s averaging window. Within each averaging window, eight independent measures characterizing motor unit discharge and whole muscle force properties were recorded. These included the mean and standard deviation (S.D.) of the interspike interval (ISI), the mean and S.D. of pre and post-trigger ISIs used in averaging, and mean and S.D. of whole muscle force. To determine the relative importance of the independent variables on twitch reproducibility, the variables were used in a multiple regression analysis performed on STA twitch peak force (PF), time to peak force (TTP) and time of half-force decay (HFD). It was found that PF was significantly correlated to the mean and S.D. of whole muscle force, and mean post-trigger ISI. TTP was significantly correlated to the S.D. of the post-trigger ISI and mean whole muscle force while HFD was related to the mean and S.D. of the pre-trigger ISI and the mean post-trigger ISI. It was concluded that by minimizing whole muscle force variability and the mean and S.D. of acceptable ISIs used in the STA process, the reproducibility of the STA twitch is improved.

Effect of force level and training status on contractile properties following fatigue

This study examined the effects of fatigue on the contractile properties of the twitch contraction evoked by the H-reflex and the maximal M-wave. Untrained subjects demonstrated more pronounced slowing of half relaxation time (HRT) in the H-reflex twitch than in the M-wave twitch and also more slowing of HRT in high force vs. low force contractions. Endurance-trained subjects, however, demonstrated a shortening of the H-reflex twitch. The twitch contractile properties following fatigue are dependent on force level and training status.

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.

Reflex inhibition during muscle fatigue in endurance-trained and sedentary individuals

Reflex inhibition of the motoneuron pool following fatiguing contractions may be mediated by the build-up of byproducts of fatigue. Endurance training is accompanied by neuromuscular adaptations that would alter the production and/or clearance of metabolic substrates. The purpose of the study was to determine the extent of reflex inhibition during and after fatigue in endurance-trained individuals compared to sedentary controls. Subjects produced isometric ankle plantarflexion contractions at 30% of maximal voluntary contraction (MVC) until their MVC torque declined by 30%. H-reflexes were measured during a brief rest period every 3 min as well as superimposed upon the contraction every minute. Both groups of subjects experienced a similar amount of reflex inhibition by the end of the fatiguing protocol, although the endurance time was twice as long for the endurance-trained subjects. The endurance-trained subjects showed a greater reduction in H-reflex amplitude early in the fatiguing protocol compared to the sedentary subjects. These experiments have demonstrated that the neuromuscular processes associated with fatigue-related reflex inhibition must be multi-faceted and cannot be explained solely by small-diameter afferents responding to the byproducts of muscle contraction.

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.