Effect of a reduced base of support in standing and balance training on the soleus H-reflex

Virtual reality does not fool the brain only: spinal excitability changes during virtually simulated falling

Virtual reality (VR) is known to induce substantial activation of brain's motor regions. It remains unclear to what extent virtual reality can trigger the sensorimotor system, and more particularly, whether it can affect lower nervous levels. In this study, we aimed to assess whether VR simulation of challenging and stressful postural situations (Richie's plank experience) could interfere with spinal excitability of postural muscles in 15 healthy young participants. The H-reflex of the triceps surae muscles was elicited with electrical nerve stimulation while participants were standing and wearing a VR headset. Participants went through several conditions, during which stimulations were evoked: standing still (noVR), standing in VR on the ground (groundVR), standing on the edge of a building (plankVR), and falling from the building (fallingVR). Myoelectrical activity of the triceps surae muscles was measured throughout the experiment. Leg and head movements were also measured by means of accelerometers to account for body oscillations. First, no differences in head rotations and myoelectrical activity were to be noted between conditions. Second, triceps H-reflex (H_MAX/M_MAX) was not affected from noVR to groundVR and plankVR. The most significant finding was a drastic decrease in H-reflex during falling (-47 ± 26.9% between noVR and fallingVR, P = 0.015). It is suggested that experiencing a postural threat in VR efficiently modulates spinal excitability, despite remaining in a quiet standing posture. This study suggests that simulated falling mimics the neural adjustments observed during actual postural challenge tasks.NEW & NOTEWORTHY The present study showed a modulation of spinal excitability induced by virtual reality (VR). In the standing position, soleus H-reflex was downmodulated during a simulated falling, in the absence of apparent changes in body oscillations. Since the same behavior is usually observed during real falling, it was suggested that the visual cues provided by VR were sufficiently strong to lead the neuromuscular system to mimic the actual modulation.

Reliability of transcranial magnetic stimulation and H-reflex measurement during balance perturbation tasks

Following ankle movement, posterior balance perturbation evokes short- (SLR ∼30–50 ms), medium- (MLR ∼50–60 ms), and long-latency responses (LLR ∼70–90 ms) in soleus muscle before voluntary muscle contraction. Transcranial magnetic stimulation (TMS) and Hoffmann-reflex (H-reflex) measurements can provide insight into the contributions of corticospinal and spinal mechanisms to each response. Motor evoked potential (MEP) and H-reflex responses have shown good reliability in some dynamic muscle contraction tasks. However, it is still unclear how reliable these methods are in dynamic balance perturbation and corticospinal modulation during long amplitude balance perturbation tasks. 14 subjects completed two test sessions in this study to evaluate the reliability of MEPs, H-reflex, and corticospinal modulation during balance perturbation. In each session, the balance perturbation system operated at 0.25 m/s, accelerating at 2.5 m/s² over 0.3 m displacement. MEPs and H-reflexes were elicited in the right leg soleus muscle at four delays after ankle movement (10 ms, 40 ms, 80 ms, and 140 ms), respectively. Test-retest reliability of MEP and H-reflex amplitudes were assessed via intraclass correlation coefficients (ICC) both between- and within-session. Between-session test-retest reliability for MEPs was excellent (ICC = 0.928–0.947), while H-reflex demonstrated moderate-to-good reliability (ICC = 0.626–0.887). Within-session reliability for both MEPs and H-reflex was excellent (ICC = 0.927–0.983). TMS and H-reflex measurements were reliable at different delays after perturbation between- and within-sessions, which indicated that these methods can be used to measure corticospinal excitability during balance perturbation.

The Reduced Adaptability of H-Reflex Parameters to Postural Change With Deficiency of Foot Plantar Sensitivity

Purpose: The project was to examine the influence of peripheral neuropathy (PN) severity on the relationship between Hoffmann-reflex (H-reflex) and postures.

Methods: A total of 34 participants were recruited. H-reflex (H/M ratio and H-index) during prone, standing, and the heel-contact phase of walking was tested, along with foot sole sensitivity.

Results: The participants were divided into three groups based on the severity of the foot sole sensitivity deficit: control, less (LA), and more (MA) affected with both feet 5.07 monofilament test scores ranging 10, 0–5, and 6–9, respectively. A significant group by the posture interaction was observed in the H/M ratio (F_{3.0, 41.9} = 2.904, p = 0.046, η_p² = 0.172). In the control group, the H/M ratio of prone (22 ± 7%) was greater than that of the standing (13 ± 3%, p = 0.013) and heel-contact phase (10 ± 2%, p = 0.004). In the MA group, the H/M ratio of standing (13 ± 3%) was greater than that of the heel-contact phase (8 ± 2%, p = 0.011). The H-index was significantly different among groups (F_2,28 = 5.711, p = 0.008, and η_p²= 0.290). Post hoc analysis showed that the H-index of the control group (80.6 ± 11.3) was greater than that of the LA (69.8 ± 12.1, p = 0.021) and MA groups (62.0 ± 10.6, p = 0.003).

Conclusion: In a non-PN population, the plantar sensory input plays an important role in maintaining standing postural control, while as for the PN population with foot sole sensitivity deficiency, type Ⅰ afferent fibers reflex loop (H-reflex) contributes more to the standing postural control. The H-index parameter is an excellent method to recognize the people with and without PN but not to distinguish the severity of PN with impaired foot sole sensitivity.

The underlying mechanisms of improved balance after one and ten sessions of balance training in older adults

Training improves balance control in older adults, but the time course and neural mechanisms underlying these improvements are unclear. We studied balance robustness and performance, H-reflex gains, paired reflex depression, and co-contraction duration in ankle muscles after one and ten training sessions in 22 older adults (+65 yrs). Mediolateral balance robustness, time to balance loss in unipedal standing on a platform with decreasing rotational stiffness, improved (33%) after one session, with no further improvement after ten sessions. Balance performance, absolute mediolateral center of mass velocity, improved (18.75%) after one session in perturbed unipedal standing and (18.18%) after ten sessions in unperturbed unipedal standing. Co-contraction duration of soleus/tibialis anterior increased (16%) after ten sessions. H-reflex gain and paired reflex depression excitability did not change. H-reflex gains were lower, and soleus/tibialis anterior co-contraction duration was higher in participants with more robust balance after ten sessions, and co-contraction duration was higher in participants with better balance performance at several time-points. Changes in robustness and performance were uncorrelated with changes in co-contraction duration, H-reflex gain, or paired reflex depression. In older adults, balance robustness improved over a single session, while performance improved gradually over multiple sessions. Changes in co-contraction and excitability of ankle muscles were not exclusive causes of improved balance.

Effect of Incorporating Short-Foot Exercises in the Balance Rehabilitation of Flat Foot: A Randomized Controlled Trial

Effective balance rehabilitation is essential to address flat foot (pes planus) which is closely associated with reduced postural stability. Although sensorimotor training (SMT) and short-foot exercise (SFE) have been effective for improving postural stability, the combined effects of SMT with SFE have not been evaluated in previous studies. The aim of this study was to compare the lone versus combined effects of SMT with SFE on postural stability among participants with flat foot. This was a single-blinded, randomized controlled trial. A total of 32 flat-footed participants were included in the study (14 males and 18 females) and assigned to the SMT combined with SFE group and SMT alone group. All participants underwent 18 sessions of the SMT program three times a week for six weeks. Static balance, dynamic balance, and the H_max/M_max ratio were compared before and after the interventions. Static and dynamic balance significantly increased in the SMT combined with SFE group compared with the SMT alone group. However, the H_max/M_max ratio was not significantly different between the two groups. Therefore, this study confirms that the combination of SMT and SFE is superior to SMT alone to improve postural balance control in flat-footed patients in clinical settings.

Exercise of Dynamic Stability in the Presence of Perturbations Elicit Fast Improvements of Simulated Fall Recovery and Strength in Older Adults: A Randomized Controlled Trial

Age-related impairments of reactive motor responses to postural threats and reduced muscular capacities of the legs are key factors for the higher risk of falling in older people. It has been evidenced that a training of dynamic stability in the presence of perturbations has the potential to improve these deficits. However, the time course of training effects during such interventions is poorly understood. The purpose of this parallel-group study was to investigate the temporal adaptation dynamics of the balance recovery performance and leg strength during a dynamic stability training. Forty-two healthy older adults (65–85 years) were randomly assigned to a training (n = 27, analyzed n = 18) or control group (n = 15, n = 14). The training was conducted in a group setting for 6 weeks (3×/week, 45 min). The exercises focused on the mechanism of stability control (i.e., modulation of the base of support and segment counter-rotations around the center of mass) during standing, stepping, and jumping on unstable surfaces with a high balance intensity. Before, after 3 and after 6 weeks, the maximum plantar flexion moment and the knee extension moment were assessed. The recovery performance was evaluated by a simulated forward fall (lean-and-release test) and the margin of stability concept. The margin of stability at release decreased significantly after 3 weeks of training (34%, effect size g = 0.79), which indicates fast improvements of balance recovery performance. The margin of stability further decreased after week 6 (53%, g = 1.21), yet the difference between weeks 3 and 6 was not significant. Furthermore, the training led to significant increases in the plantar flexion moment after weeks 3 (12%, g = 0.72) and 6 (13%, g = 0.75) with no significant difference between weeks. For the knee extension moment, a significant increase was found only after week 6 (11%, g = 1.07). The control group did not show any significant changes. This study provides evidence that a challenging training of dynamic stability in the presence of perturbations can improve balance recovery performance and leg strength of older adults already after a few weeks. Therefore, short-term training interventions using this paradigm may be an effective strategy for fall prevention in the elderly population, particularly when intervention time is limited.

Anticipatory coadaptation of ankle stiffness and sensorimotor gain for standing balance

External perturbation forces may compromise standing balance. The nervous system can intervene only after a delay greater than 100 ms, during which the body falls freely. With ageing, sensorimotor delays are prolonged, posing a critical threat to balance. We study a generic model of stabilisation with neural delays to understand how the organism should adapt to challenging balance conditions. The model suggests that ankle stiffness should be increased in anticipation of perturbations, for example by muscle co-contraction, so as to slow down body fall during the neural response delay. Increased ankle muscle co-contraction is indeed observed in young adults when standing in challenging balance conditions, and in older relative to young adults during normal stance. In parallel, the analysis of the model shows that increases in either stiffness or neural delay must be coordinated with decreases in spinal sensorimotor gains, otherwise the feedback itself becomes destabilizing. Accordingly, a decrease in spinal feedback is observed in challenging conditions, and with age-related increases in neural delay. These observations have been previously interpreted as indicating an increased reliance on cortical rather than spinal control of balance, despite the fact that cortical responses have a longer latency. Our analysis challenges this interpretation by showing that these observations are consistent with a functional coadaptation of spinal feedback gains to functional changes in stiffness and neural delay.

Being able to stand still can be difficult when faced with an unexpected push. It takes the nervous system more than a tenth of a second to respond to such a perturbation, and during this delay the body falls under the influence of its own weight. By co-contracting their ankle muscles in anticipation of a perturbation, subjects can increase their ankle stiffness, which slows down their fall during the neural delay. Young subjects indeed adopt this strategy when they need to remain particularly still (for example when they stand in front of a cliff). Older subjects adopt this strategy even during normal standing. We present a model of standing balance that shows that this postural strategy provides partial compensation for the increase in neural delays with ageing. According to our model, increasing ankle stiffness only improves balance if it is accompanied by a decrease in sensorimotor gain. This provides a novel and functional interpretation for the decrease in spinal feedback observed during ageing, and observed in young subjects when they stand in challenging balance conditions.

Exploiting cervicolumbar connections enhances short-term spinal cord plasticity induced by rhythmic movement

Arm cycling causes suppression of soleus (SOL) Hoffmann (H-) reflex that outlasts the activity period. Arm cycling presumably activates propriospinal networks that modulate Ia presynaptic inhibition. Interlimb pathways are thought to relate to the control of quadrupedal locomotion, allowing for smooth, coordinated movement of the arms and legs. We examined whether the number of active limb pairs affects the amount and duration of activity-dependent plasticity of the SOL H-reflex. On separate days, 14 participants completed 4 randomly ordered 30 min experimental sessions: (1) quiet sitting (CTRL); (2) arm cycling (ARM); (3) leg cycling (LEG); and (4) arm and leg cycling (A&L) on an ergometer. SOL H-reflex and M-wave were evoked via electrical stimulation of the tibial nerve. M-wave and H-reflex recruitment curves were recorded, while the participants sat quietly prior to, 10 and 20 min into, immediately after, and at 2.5, 5, 7.5, 10, 15, 20, 25, and 30 min after each experimental session. Normalized maximal H-reflexes were unchanged in CTRL, but were suppressed by > 30% during the ARM, LEG, and A&L. H-reflex suppression outlasted activity duration for ARM (≤ 2.5 mins), LEG (≤ 5 mins), and A&L (≤ 30 mins). The duration of reflex suppression after A&L was greater than the algebraic summation of ARM and LEG. This non-linear summation suggests that using the arms and legs simultaneously-as in typical locomotor synergies-amplifies networks responsible for the short-term plasticity of lumbar spinal cord excitability. Enhanced activity of spinal networks may have important implications for the implementation of locomotor training for targeted rehabilitation.

Selective effect of static stretching, concentric contractions, and a one-leg balance task on ankle motion sense in young and older adults

BACKGROUND

Being aware of ankle movement and motor control has a critical role in maintaining balance during functional activities such as standing, walking, and running. Since the somatosensory system declines with aging, this is even more important for older adults.

RESEARCH QUESTION

How do different exercise modalities (static stretching, one-leg balance task, concentric contractions, and control) acutely influence ankle motion sense in young and older adults?

METHODS

Seventeen young and fifteen older participants performed four different intervention protocols (static stretching, one-leg balance task, concentric contractions, and control) in random order. Each session comprised measurements of ankle motion sense in plantar flexion (PF) and dorsal flexion (DF) directions prior to and after an intervention protocol. Average threshold levels (in degrees) of motion sense detection were calculated from three trials in each direction (PF/DF).

RESULTS

A lower threshold of motion ankle sense was observed for young adults compared to older adults regardless of the exercise modality and the direction of the movement (p < 0.001). However, the changes in PF and DF ankle motion senses followed a similar trend in both groups during the three exercise modalities: static stretching increased ankle motion sense threshold (PF: 14% and 5%; DF: 19% and 11% in young and older adults, respectively), concentric contractions decreased ankle motion sense threshold (PF: -24% and -14%; DF: -19% and -21% in young and older adults, respectively), and the one-leg balance task did not significantly influence the ankle motion sense threshold (PF: -1% and -2%; DF: 6% and 1% in young and older adults, respectively).

SIGNIFICANCE

Based on these results, static stretching should not be performed before ankle activities that require a good balance, precision, and coordination. Concentric contractions could be recommended before activities that challenge our postural stability.

H-reflex in abductor hallucis and postural performance between flexible flatfoot and normal foot

OBJECTIVE

Morphological changes of the abductor hallucis muscle (AbH) in flexible flatfoot (FF) individuals influence regulations of the medial longitudinal arch (MLA). Prolonged and repeated stretching of AbH in flexible flatfoot may cause changes in muscle reflex properties and further influence postural performance. However, AbH muscle reflex under different postural conditions have never been examined. The purpose of this study was to investigate differences in AbH H-reflex and postural performance between individuals with normal foot (NF) alignment and FF under prone, double-leg stance (DLS), and single-leg stance (SLS) conditions.

DESIGN

Cross-sectional study.

SETTING

University laboratory.

PARTICIPANTS

Individuals with FF (n = 12) and NF (n = 12).

MAIN OUTCOME MEASURES

AbH H-reflex, AbH EMG and center of pressure (CoP) displacement.

RESULTS

Under all postural conditions, AbH H-reflex was significantly lower in the FF group (P < .05). Under the SLS condition, AbH EMG was significantly higher in the FF group (P < .05), and CoP displacement for the medial-lateral and anterior-posterior directions were significantly higher in the FF group (P < .05).

CONCLUSIONS

With increased postural demand, FF individuals maintained their postural stability by recruiting greater AbH activities than through automatic stretch reflex, but FF individuals still showed inferior posture stability.

Examining Neural Plasticity for Slip-Perturbation Training: An fMRI Study

Perturbation-based balance training has shown to induce adaptation of reactive balance responses that can significantly reduce longer-term fall risk in older adults. While specific cortical and subcortical areas in control of posture and locomotion have been identified, little is known about the training-induced plasticity occurring in neural substrates for challenging tasks involving reactive balance control. The purpose of this study was to use functional neuroimaging to examine and determine the neural substrates, if any, involved in inducing adaptation to slip-like perturbations experienced during walking over 3 consecutive training days. We used a mental imagery task to examine the neural changes accompanied by treadmill-slip perturbation training. Ten healthy young adults were exposed to increasing magnitude of displacements during slip-like perturbations while walking, with an acceleration of 6 m/s² on a motorized treadmill for 3 consecutive days. Brain activity was recorded through MRI while performing imagined slipping and imagined walking tasks before and after the perturbation training. The number of compensatory steps and center of mass state stability at compensatory step touchdown were recorded. As compared with day 1 (first trial), on day 3 (last trial) there was a significant reduction in number of compensatory steps and increase in stability at compensatory step touchdown on the mid and highest perturbation intensities. Before perturbation training, imagined slipping showed increased activity in the SMA, parietal regions, parahippocampal gyrus, and cingulate gyrus compared with rest. After perturbation training, imagined slipping showed increased activation in DLPFC, superior parietal lobule, inferior occipital gyrus, and lingual gyrus. Perturbation training was not associated with decline in activity in any of the brain regions. This study provides evidence for learning-related changes in cortical structures while adapting to slip-like perturbations while walking. The findings reflect that higher-level processing is required for timing and sequencing of movements to execute an effective balance response to perturbations. Specifically, the CNS relies on DLPFC along with motor, parietal, and occipital cortices for adapting to postural tasks posing a significant threat to balance.

Selective effect of static stretching, concentric contractions, and a balance task on ankle force sense

Proper ankle motor control is critical for balance in the human body during functional activities such as standing, walking, and running. Different exercise modalities are often performed during the same training session where earlier activities may influence later ones. The purpose of the current study was to determine the acute effects of different exercise modalities on ankle force sense. Seventeen subjects performed four different intervention protocols (static stretching, balance task, concentric contractions, and control) in random order. Each session comprised measurements before and after the intervention protocol of the force sense of the ankle plantar flexors (PF) and dorsal flexors (DF) at 10% and 30% of maximal voluntary isometric contraction (MVC). Absolute errors (AE) were calculated separately for each force level and muscle group. An overall PF error (PF-SUM = PF at 10%MVC + PF at 30%MVC), DF error (DF-SUM = DF at 10%MVC + DF at 30%MVC) and ankle error (PF-DF-SUM = PF-SUM + DF-SUM) were also calculated. The main effect of time generally revealed that ankle force sense was significantly reduced after static stretching (PF-DF-SUM: Pre: 6.11±2.17 Nm, Post: 8.03±3.28 Nm; p < 0.05), but no significant differences were observed for the concentric contractions (PF-DF-SUM: Pre: 6.01±1.97 Nm, Post: 6.50±2.28 Nm) and the balance task (PF-DF-SUM: Pre: 5.25±1.97 Nm, Post: 5.50±1.26 Nm). The only significant interaction was observed for the PF-DF-SUM (F = 4.48, p = 0.008) due to greater error scores after stretching (+31.4%) compared to the concentric (+8.2%), balance (+4.8%), and control (-3.5%) conditions. Based on these results, static stretching should not be performed before activities that require a high ankle force sense such as balance, coordination, and precision tasks.

The center of pressure and ankle muscle co-contraction in response to anterior-posterior perturbations

Though both contraction of agonist muscles and co-contraction of antagonistic muscle pairs across the ankle joint are essential to postural stability, they are perceived to operate independently of each other, In an antagonistic setup, agonist muscles contract generating moment about the joint, while antagonist muscles contract generating stiffness across the joint. While both work together in maintaining robustness in the face of external perturbations, contractions of agonist muscles and co-contractions of antagonistic muscle pairs across the ankle joint play different roles in responding to and adapting to external perturbations. To determine their respective roles, we exposed participants to repeated perturbations in both large and small magnitudes. The center of pressure (COP) and a co-contraction index (CCI) were used to quantify the activation of agonist muscles and antagonistic muscle pairs across the ankle joint. Our results found that participants generated moment of a large magnitude across the ankle joint—a large deviation in the COP curve—in response to perturbations of a large magnitude (p <0.05), whereas the same participants generated higher stiffness about the ankle—a larger value in CCI—in response to perturbations of a small magnitude (p <0.05). These results indicate that participants use different postural strategies pertaining to circumstances. Further, the moment across the ankle decreased with repetitions of the same perturbation (p <0.05), and CCI tended to remain unchanged even in response to a different perturbation following repetition of the same perturbation (p <0.05). These findings suggest that ankle muscle contraction and co-contraction play different roles in regaining and maintaining postural stability. This study demonstrates that ankle moment and stiffness are not correlated in response to external perturbations.

Relationships Between Hoffman Reflex Parameters, Trait Stress, and Athletic Performance

This study evaluated the relationships between trait stress, Hoffman reflex, and performance among 36 healthy amateur male athletes. We first obtained a trait stress questionnaire from participants and then assigned them to high- and low-stress groups. We next recorded Hoffman reflex data from the soleus and lateral gastrocnemius muscles and then examined their athletic performance on testing protocols separated by a 72-hour washout period. Performance testing utilized vertical jump height, 20 -m sprint time, and standing stork tests. There were significant correlations between (a) the standing stork test, vertical jump height, and trait stress and (b) H_max/ M_max ratios, threshold intensity ( H_th), the intensity of the H_max, and the intensity of the H_last. H_th, the intensity of H_max, and the intensity of H_last were significantly higher among the low-stress compared with the high-stress participant groups ( p < .05), despite participants' similar training history. We suggest that self-perceived psychological stress affects performance through neural adaptation.

Learning to balance on a slackline: Development of coordinated multi-joint synergies

Previous research has investigated synergies involved in locomotion and balance reactions; however, there is limited insight into the emergence of skilled balance control with practice of challenging tasks. We explored motor learning of tandem and single leg stance on an unstable surface-a slackline. Balance was tested in 10 naïve healthy adults at four time points: baseline, after one slackline practice session, after 1 week of practice, and 1 week following the final practice session. We recorded kinematics of the upper and lower arms bilaterally, trunk, and thigh and foot unilaterally while participants balanced in tandem and single leg stance on a slackline and narrow rigid beam (transfer task). When participants first attempted to stand on the slackline, they exhibited fast and frequent movements across all joints with actions along the frontal plane (particularly the hip) and fell after a short period (~3 seconds). Performance improved rapidly (fewer falls), and this was accompanied by dampened trunk and foot oscillations and the development of coordinated movement patterns with a progressive emphasis on more distal upper body segments. Continuous relative phase angles between joint pairs began to cluster around either 0° (indicating in-phase movement) or 180° (indicating anti-phase movement). Participants also began to demonstrate coordinated upper body synergies and performance improvements (fewer falls) on the transfer task, while a control group (n = 10) did not exhibit similar synergies or performance improvements. Our findings describe the emergence of coordinated movement synergies involving the upper body as healthy adults learn a challenging balance task.

Postactivation depression of soleus H-reflex increase with recovery of lower extremities motor functions in patients with subacute stroke

[Purpose] The soleus H-reflex is depressed at stimulation rates greater than 0.1 Hz. This reflex depression is referred to as postactivation depression. Postactivation depression reflects the reduced efficacy of the Ia-motoneurons synapses when they are evaluated after a previous activation. The aim of this study was to determine whether the recovery of motor functions in the lower extremities affects the PAD of the soleus H-reflex in patients with subacute stroke undergoing rehabilitation. [Subjects and Methods] Eight patients with subacute stroke patients were recruited. Postactivation depression, Fugl-Meyer score (lower-limb portion), walking velocity, the Modified Ashworth Scale, and center of pressure sway during standing were measured within three days of admission to rehabilitation and 50 days later. [Results] After rehabilitation, Fugl-Meyer scores, center of pressure path length, and walking velocity were significantly improved, and postactivation depression had significantly increased. There was a significant positive correlation between the rates of change of postactivation depression and center of pressure path length. [Conclusion] The results demonstrated that postactivation depression is partially normalized after rehabilitation in patients with subacute stroke, and suggested that the recovery in lower extremity function after stroke particularly standing stability is affected by spinal synaptic plasticity.

Learning to balance on one leg: motor strategy and sensory weighting

We investigated motor and sensory changes underlying learning of a balance task. Fourteen participants practiced balancing on one leg on a board that could freely rotate in the frontal plane. They performed six, 16-s trials standing on one leg on a stable surface (2 trials without manipulation, 2 with vestibular, and 2 with visual stimulation) and six trials on the balance board before and after a 30-min training. Center of mass (COM) movement, segment, and total angular momenta and board angles were determined. Trials on stable surface were compared with trials after training to assess effects of surface conditions. Trials pretraining and posttraining were compared to assess rapid (between trials pretraining) and slower (before and after training) learning, and sensory manipulation trials were compared with unperturbed trials to assess sensory weighting. COM excursions were larger on the unstable surface but decreased with practice, with the largest improvement over the pretraining trials. Changes in angular momentum contributed more to COM acceleration on the balance board, but with practice this decreased. Visual stimulation increased sway similarly in both surface conditions, while vestibular stimulation increased sway less on the balance board. With practice, the effects of visual and vestibular stimulation increased rapidly. Initially, oscillations of the balance board occurred at 3.5 Hz, which decreased with practice. The initial decrease in sway with practice was associated with upweighting of visual information, while later changes were associated with suppression of oscillations that we suggest are due to too high proprioceptive feedback gains.

Slackline training and neuromuscular performance in seniors: A randomized controlled trial

Slackline training (balancing on nylon ribbons) has been shown to improve neuromuscular performance in children and adults. Comparable studies in seniors are lacking. Thus, 32 seniors were randomly assigned [strata: age, gender, physical activity (PA)] to an intervention [INT; n = 16, age: 65 ± 4 years, PA: 9 ± 5 h/week] or control [CON, n = 16, age: 63 ± 4 years, PA: 8 ± 4 h/week] group. Slackline training was given for 6 weeks (3 times per week, attendance 97%). Static and slackline standing balance performance, force development, and maximal strength of the ankle muscles were assessed before and after slackline training. Muscle activity (lower limb and trunk) was recorded during balance testing. Moderate to large group × time interactions (0.02 < P < 0.04, 0.11 < ηp (2)  < 0.17) in favor of INT were found for slackline standing times (INT: left, +278%, P = 0.02; right, +328%, P = 0.03; tandem, +94%, P = 0.007) and muscle activity during single-limb slackline standing [INT: right: rectus abdominis (RA), P = 0.003, -15%; multifidus (MF), P = 0.01, -15%; left: tibialis anterior (TIB), P = 0.03, -12%; soleus (SOL), P = 0.006, -18%; RA, P = 0.04, -11%; MF, P = 0.01, -16%; gastrocnemius medialis (GM), P = 0.02, -19%]. Static balance performance, ankle strength, and power were not affected. Slackline training induced large task-specific improvements of slackline standing performance accompanied with reductions of lower limb and trunk muscle activity. Transfer effects to static balance and strength measures seem limited.

The inflow of sensory information for the control of standing is graded and bidirectional

The control of upright standing is accomplished through the integration of different sources of sensory information and by providing an appropriate motor program to control both expected and unexpected perturbations imposed on the system. However, the dynamic characteristics of postural sway and its interplay with the regulation of Ia sensory information within the spinal cord are largely unknown. Here, using a stochastic technique for analyzing the dynamics of upright standing, we demonstrate that the changes in the dynamics of postural sway were accompanied by modulation of the soleus H-reflex during quiet standing. While the causality of this relation was not established, the results showed that these changes were independent of the sway of the center of pressure and were bidirectional and purposeful. With this novel perspective, the appropriate reflex gain, which is important for balance control, can be predicted from the dynamic characteristics of postural sway. Our current findings provide the first human behavioral evidence to suggest the contribution of the spinal cord in fulfilling the desired motor programming of a complex task. This contribution is, by conventional guess, carried out through interneuronal adjustments, which are under the control of different brain areas.

Massage Therapy Produces Short-term Improvements in Balance, Neurological, and Cardiovascular Measures in Older Persons

Background:

Falls are the primary cause of accidental death in older persons, producing increased morbidity, decreased independence, and billions in medical costs annually. Massage therapy (MT) may produce adaptations that decrease risk of falling. If MT can improve stability in older persons, it may provide a new intervention for this issue.

Purpose:

Determine the acute effects of a 60-minute MT treatment on static and functional balance, neurological measures, heart rate, and blood pressure in healthy, older individuals.

Setting:

Laboratory

Research Design:

A 2 by 4 (treatment by time) mixed factorial experimental design for the cardiovascular and postural control variables; independent variables were treatment with two levels (control, MT) and time with four levels (pretreatment baseline, immediate post-treatment, 20-minute post-treatment, 60-minute post-treatment). Neurological measures utilized a 2 by 2 mixed design, with testing conducted pre- and 60-minutes post-treatment.

Participants:

Thirty-five healthy, older volunteers (19 male and 16 female; ages 62.9 ± 4.6).

Intervention:

A 60-minute full-body therapeutic massage. The control group rested quietly in the treatment room.

Main Outcome Measures:

Static (double-legged) and functional (single-legged) postural control with eyes-open and eyes-closed; Hoffmann-reflex measures; heart rate, and systolic and diastolic blood pressure.

Results:

MT significantly decreased rectangular displacement area in both the eyes-open and eyes-closed, double-legged stance conditions (p < 0.05); displacement velocity in both eyes-open conditions (p < .05); and systolic and diastolic blood pressure (p < .05), while increasing heart rate (p < .05). MT also significantly lowered H_max/M_max ratios compared to controls (p = .002). Decreased H_max/M_max measures were correlated to improved stability.

Conclusions:

A single, 60-minute, full-body massage therapy treatment was shown to have a stabilizing effect on measures of static and dynamic balance and physiological factors related to stability in older adults. MT should be investigated as a potential intervention to decrease falls in older individuals.

Activity-dependent plasticity of spinal circuits in the developing and mature spinal cord

Part of the development and maturation of the central nervous system (CNS) occurs through interactions with the environment. Through physical activities and interactions with the world, an animal receives considerable sensory information from various sources. These sources can be internally (proprioceptive) or externally (such as touch and pressure) generated senses. Ample evidence exists to demonstrate that the sensory information originating from large diameter afferents (Ia fibers) have an important role in inducing essential functional and morphological changes for the maturation of both the brain and the spinal cord. The Ia fibers transmit sensory information generated by muscle activity and movement. Such use or activity-dependent plastic changes occur throughout life and are one reason for the ability to acquire new skills and learn new movements. However, the extent and particularly the mechanisms of activity-dependent changes are markedly different between a developing nervous system and a mature nervous system. Understanding these mechanisms is an important step to develop strategies for regaining motor function after different injuries to the CNS. Plastic changes induced by activity occur both in the brain and spinal cord. This paper reviews the activity-dependent changes in the spinal cord neural circuits during both the developmental stages of the CNS and in adulthood.

Effects of Long-Term Tai Chi Practice on Balance and H-Reflex Characteristics

The purpose of the present study was to examine the effects of long-term Tai Chi practice on postural balance and H-reflex. Sixteen healthy volunteers, eight with three or more years of experience in Tai Chi training (Tai Chi Group-TCG), and eight with no experience in Tai Chi training (Control Group-CG) participated in the study. Postural sways were measured under four experimental conditions: (1) Standing still with eyes open (EO); (2) Standing still with eyes closed (EC); (3) Standing and turning head to left and right with eyes open (EOT); and (4) Standing and turning head to left and right with eyes closed (ECT). Paired reflex depression (PRD) of the soleus muscle was measured under two conditions: supine and standing. Less significant postural sway was observed in the TCG than in the CG under four conditions including EO, EC, EOT, and ECT (p < 0.01). The TCG demonstrated 14.1%, 30.6%, 33.3% and 22.7% less postural sway, respectively. Significant PRD change from a supine to standing position was observed between TCG and CG (p < 0.05). A significant correlation between PRD change (from supine to standing) and years of Tai Chi practice was observed (r = 0.80, p < 0.05). The findings of this study support the positive effects of Tai Chi exercise on balance control under different conditions. Long-term Tai Chi exercisers also demonstrated different reflex modulation from a supine to standing position, and long-term Tai Chi practice may lead to a change of PRD modulation as neuroadaptation.

Contributions of feed-forward and feedback strategies at the human ankle during control of unstable loads

The nervous system can regulate the mechanical properties of the human ankle through feed-forward mechanisms such as co-contraction and rapid feedback mechanisms such as stretch reflexes. Though each of these strategies may contribute to joint stability, it is unclear how their relative contribution varies when ankle stability is threatened. We addressed this question by characterizing co-contraction and stretch reflexes during balance of an inverted pendulum simulated by a rotary motor configured as an admittance servo. The stability of this haptic environment was manipulated by varying the stiffness of a virtual spring supporting the pendulum. We hypothesized that co-contraction and stretch reflex amplitude would increase as the stability of the haptic load attached to the ankle was reduced. Electromyographic activity in soleus, medial and lateral gastrocnemius, and tibialis anterior was used to characterize co-contraction patterns and stretch reflex amplitude as subjects stabilized the haptic load. Our results revealed that co-contraction was heightened as stability was reduced, but that the resulting joint stiffness was not sufficient to fully counteract the imposed instability. Reflex amplitude, in comparison, was attenuated as load stability was reduced, contrary to results from upper limb studies using similar paradigms. Together these findings suggest that the nervous system utilizes feed-forward co-contraction rather than rapid involuntary feedback to increase ankle stability during simple balance tasks. Furthermore, since the stiffness generated through co-contraction was not sufficient to fully balance the haptic load, our results suggest an important role for slower, volitional feedback in the control of ankle stability during balancing tasks.

Soleus H-reflex and its relation to static postural control

The Hoffmann reflex (H-reflex) test has been extensively used to investigate the responsiveness of Ia afferent spinal loop in animal and human studies. The H-reflex response is influenced by multiple neural pathways and the assessment of H-reflex variation is a useful tool in understanding the neural mechanisms in control of movement. Recently, several studies have examined the relationship between the H-reflex modulation and postural stability. For example, it has been reported that the amplitude of soleus (SOL) H-reflex is depressed in relation to increased body sway during upright standing on a soft surface compared to that on a solid surface. It has been suggested that the SOL H-reflex modulation under such condition is predominately affected by the presynaptic inhibitory mechanisms for avoiding oversaturation of the spinal motoneurons. It has also been reported that after balance training, the SOL H-reflex amplitude is down-modulated in parallel with improvement in balance control, suggesting a functional adaptation at the supraspinal levels. The aim of this review is to examine the current literature on the relationship between H-reflex modulation and postural control for a better understanding of the physiological mechanisms involved in control of posture in humans.

Changes in spinal reflex excitability associated with motor sequence learning

There is ample evidence that motor sequence learning is mediated by changes in brain activity. Yet the question of whether this form of learning elicits changes detectable at the spinal cord level has not been addressed. To date, studies in humans have revealed that spinal reflex activity may be altered during the acquisition of various motor skills, but a link between motor sequence learning and changes in spinal excitability has not been demonstrated. To address this issue, we studied the modulation of H-reflex amplitude evoked in the flexor carpi radialis muscle of 14 healthy individuals between blocks of movements that involved the implicit acquisition of a sequence versus other movements that did not require learning. Each participant performed the task in three conditions: "sequence"-externally triggered, repeating and sequential movements, "random"-similar movements, but performed in an arbitrary order, and "simple"- involving alternating movements in a left-right or up-down direction only. When controlling for background muscular activity, H-reflex amplitude was significantly more reduced in the sequence (43.8 +/- 1.47%. mean +/- SE) compared with the random (38.2 +/- 1.60%) and simple (31.5 +/- 1.82%) conditions, while the M-response was not different across conditions. Furthermore, H-reflex changes were observed from the beginning of the learning process up to when subjects reached asymptotic performance on the motor task. Changes also persisted for >60 s after motor activity ceased. Such findings suggest that the excitability in some spinal reflex circuits is altered during the implicit learning process of a new motor sequence.

Modulation of soleus H reflex due to stance pattern and haptic stabilization of posture

The quiet stance is a complicated motor act requiring sophisticated sensorimotor integration to balance an artificial inverted pendulum with the ankle musculature. The objective of this study was to characterize the effects of stance pattern (bilateral stance vs. unilateral stance) and directional influence of light finger touch (medial-lateral vs. anterior-posterior) in unilateral stance upon responsiveness of the soleus H reflex. Sixteen healthy volunteers (mean age, 24.25+/-1.77 years) participated in four postural tasks with the eyes open, including the bilateral stance (BS), the unilateral stance without finger touch (USNT), and with finger touch in the medial-lateral direction (USML) and anterior-posterior direction (USAP). Meanwhile, the soleus H reflex, the pre-stimulus background activity of ankle antagonist pairs, and center of pressure (CoP) sway were measured. In reference to the BS, the USNT resulted in a significant stance effect on suppression of the soleus H reflex (H/M(max)) associated with enhancement of CoP sway. Among the conditions of unilateral stance, there was a marked directional effect of finger touch on modulation of the H/M(max). A greater disinhibition of the H/M(max) in consequence to light touch in the ML direction than in the AP direction was noted (H/M(max): USML>USAP>USNT). This directional modulation of the soleus H reflex concurred with haptic stabilization of posture in unilateral stance, showing a more pronounced reduction in CoP sway in the USML condition than in the USAP condition. However, alteration in postural sway and modulation of the soleus H reflex were not mutually correlated when stance pattern or touch vector varied. In conclusion, gating of the soleus H reflex indicated adaptation of an ankle strategy to stance pattern and haptic stabilization of posture. Relative to bilateral stance, postural maintenance in unilateral stance relied less on reflexive correction of the soleus. When finger touch was provided in line with prevailing postural threat in the lateral direction, postural stability in unilateral stance was better secured than finger touch in anterior-posterior direction, resulting in more pronounced disinhibition of the monosynaptic reflex pathway.

Role of vision and task complexity on soleus H-reflex gain

There exists extensive evidence supporting the presence of reflex modulation in humans during a variety of motor tasks. The soleus H-reflex has been shown to be modulated during static and dynamic balance conditions as well as during various motor tasks. The purpose of this study was to examine the effects of two different stance positions and visual conditions on soleus H-reflex gain in 15 apparently healthy adults (mean age=30.27+/-6.92 yrs). The soleus H-reflexes were examined in two experimental stance conditions: two-legged (stable) and one-leg (unstable), and two visual conditions: eyes open and eyes closed. To assess the reflex gain, subjects performed ten trials under each of the four conditions and a soleus H-reflex was elicited during the performance of each trial. For each condition the peak-to-peak amplitude of the H-reflex and the EMG activity 50 ms prior to the stimulus was recorded. Differences in the peak-to-peak amplitudes of the soleus H-reflex for the experimental conditions were compared with a 2x2 (Stance x Vision) repeated measures ANOVA. The level of significance was p<0.05. Results demonstrated significant differences in reflex gain for both the vision (F(l,15)=4.87, p<0.05) and the stance condition (F(l,15)=14.86, p<0.05). Although both the stance condition and vision significantly affected the H-reflex gain, there was no interaction between these two variables (F(l,15)=0.17). From these results, we conclude that H-reflex gain was decreased both as stance complexity increased and as visual inputs were removed. Consistent with previous reports, it may be speculated that changes in presynaptic inhibition to the soleus Ia fibers regulate these gain changes. We propose that vision and stability of stance affect soleus H-reflex gain, but do so without any interactive effects.

Spinal and supraspinal adaptations associated with balance training and their functional relevance

Traditionally, balance training has been used to rehabilitate ankle injuries and postural deficits. Prospective studies have shown preventive effects with respect to ankle and knee joint injuries. Presently, balance training is not only applied for rehabilitation and prevention but also for improving motor performance, especially muscle power. The recent application of noninvasive electrophysiological and brain imaging techniques revealed insights into the central control of posture and the adaptations induced by balance training. This information is important for our understanding of the basic control and adaptation mechanisms and to conceptualize appropriate training programmes for athletes, elderly people and patients. The present review presents neurophysiological adaptations induced by balance training and their influence on motor behaviour. It emphasizes the plasticity of the sensorimotor system, particularly the spinal and supraspinal structures. The relevance of balance training is highlighted with respect to athletic performance, postural control within elderly people as well as injury prevention and rehabilitation.

Central nervous adaptations following 1 wk of wrist and hand immobilization

Plastic neural changes have been documented in relation to different types of physical activity, but little is known about central nervous system plasticity accompanying reduced physical activity and immobilization. In the present study we investigated whether plastic neural changes occur in relation to 1 wk of immobilization of the nondominant wrist and hand and a corresponding period of recovery in 10 able-bodied volunteers. After immobilization, maximal voluntary contraction torque decreased and the variability of submaximal static contractions increased significantly without evidence of changes in muscle contractile properties. Hoffmann (H)-reflex amplitudes and the ratios of H-slope to M-slope increased significantly in flexor carpi radialis and abductor pollicis brevis at rest and during contraction without changes in corticospinal excitability, estimated from motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation. Corticomuscular coherence measures were derived from EEG and EMG obtained during static contractions. After immobilization, corticomuscular coherence in the 15- to 35-Hz range associated with maximum negative cumulant values at lags corresponding to MEP latencies decreased. One week after cast removal, all measurements returned to preimmobilization levels. The increased H-reflex amplitudes without changes in MEPs may suggest that presynaptic inhibition or postactivation depression of Ia afferents is reduced following immobilization. Reduced corticomuscular coherence may be caused by changes in afferent input at spinal and cortical levels or by changes in the descending drive from motor cortex. Further studies are needed to elucidate the mechanisms underlying the observed increased spinal excitability and reduced coupling between motor cortex and spinal motoneuronal activity following immobilization.

Effect of inversion and ankle bracing on peroneus longus Hoffmann reflex

This study examined peroneus longus (PL) Hoffmann reflex (H-reflex) during sudden inversion perturbation of the ankle/foot complex under an ankle brace and non-brace condition. Ten healthy subjects volunteered. H-reflexes were tested on the up-sloping portion of the recruitment curve, utilizing a control trial M-wave above motor threshold to maintain consistency between subjects and conditions. The PL H/maximum M-wave (M(max)) ratio was established using the PL H-reflex and PL M(max) peak-to-peak measures. The mean ratio across five trials for each subject under each ankle brace (brace, no brace) and surface (flat, inversion) conditions was utilized for analysis. The 1 x 4 repeated measures ANOVA revealed a significant main effect for treatment condition (P<0.0001). The PL H/M(max) ratio significantly increased during sudden inversion-no ankle brace condition compared with the flat surface no-ankle brace condition (P=0.04). Application of an ankle brace had no effect on PL H/M(max) ratio during inversion (P=0.78). During this study PL H/M(max) ratios increased during an inversion perturbation in healthy ankles. This is believed to occur due to heightened sensorimotor demand placed on the nervous system during this motion. Moreover, application of an ankle brace during inversion does not appear to affect PL H/M(max) ratio.

Cortical and spinal adaptations induced by balance training: correlation between stance stability and corticospinal activation

AIM

To determine the sites of adaptation responsible for improved stance stability after balance (=sensorimotor) training, changes in corticospinal and spinal excitability were investigated in 23 healthy subjects.

METHODS

Neural adaptations were assessed by means of H-reflex stimulation, transcranial magnetic stimulation (TMS) and conditioning of the H-reflex by TMS (Hcond) before and after 4 weeks of balance training. All measurements were performed during stance perturbation on a treadmill. Fast posterior translations induced short- (SLR), medium- and long-latency responses (LLR) in the soleus muscle. Motor-evoked potential- (MEP) and Hcond-amplitudes as well as Hmax/Mmax ratios were determined at SLR and LLR. Postural stability was measured during perturbation on the treadmill.

RESULTS

Balance training improved postural stability. Hmax/Mmax ratios were significantly decreased at LLR. MEPs and Hcond revealed significantly reduced facilitation at LLR following training. A negative correlation between adaptations of Hcond and changes in stance stability was observed (r = -0.87; P < 0.01) while no correlation was found between stance stability and changes in Hmax/Mmax ratio. No changes in any parameter occurred at the spinally organized SLR and in the control group.

CONCLUSION

The decrease in MEP- and Hcond-facilitation implies reduced corticospinal and cortical excitability at the transcortically mediated LLR. Changes in cortical excitability were directly related to improvements in stance stability as shown by correlation of these parameters. The absence of such a correlation between Hmax/Mmax ratios and stance stability suggests that mainly supraspinal adaptations contributed to improved balance performance following training.

Field of view and base of support width influence postural responses to visual stimuli during quiet stance

We explored the destabilizing effect of visual field motion as the base of support (BOS) and the field of view (FOV) were narrowed. Visual field motion was achieved using an immersive virtual environment (scene) that moved realistically with head motion (natural motion) and translated sinusoidally at 0.1Hz in the fore-aft direction (augmented motion). Natural motion was presented in stereo while augmented motion was presented in both stereo and non-stereo. Subjects viewed scene motion under wide (90 degrees and 55 degrees in the horizontal and vertical directions) and narrow (25 degrees in both directions) FOV conditions while standing flatfooted (100% BOS) and on two blocks (45% and 35% BOS). Head and whole body center of mass (COM) and ankle angle root mean square (RMS) were determined as were head, whole body, and shank COM FFTs. During natural motion, the primary effect emerged in the head RMS which was significantly smaller with a 35% BOS and the wide FOV compared to the narrow FOV. However, the primary effect of augmented motion emerged in the power analysis of head and whole body COM which significantly increased with the wide FOV for a 35% BOS compared to 100% BOS. Statistical analysis indicated an effect of BOS on depth perception for head and whole body RMS; however, post hoc comparisons revealed no significant differences between stereo and non-stereo augmented motion. We conclude that reducing the BOS increased reliance on peripheral visual information to stabilize the head in space even when the augmented visual motion promoted postural instability.

Training-Specific Adaptations of H- and Stretch Reflexes in Human Soleus Muscle

The authors investigated the effect of physical exercise on reflex excitability in a controlled intervention study. Healthy participants (N = 21) performed 4 weeks of either power training (ballistic strength training) or balance training (sensorimotor training [SMT]). Both training regimens enhanced balance control and rate of force development, whereas reductions in peak-to-peak amplitudes of stretch reflexes and in the ratio of the maximum Hoffman reflex to the maximum efferent motor response (Hmax:Mmax) measured at rest were limited to SMT. The differences in reflex excitability between the training regimens indicated different underlying neural mechanisms of adaptation. The reduced reflex excitability following SMT was most likely induced by supraspinal influence. The authors discuss an overall increase in presynaptic inhibition of Ia afferent fibers as a possible mechanism.

Modulation of soleus H-reflex by presynaptic spinal mechanisms during varying surface and ankle brace conditions

AIMS

Reflex excitability is modulated in part by presynaptic spinal mechanisms. Presynaptic inhibition may prevent an over-response of the motoneuron pool to afferent information. A paired-reflex depression (PRD) conditioning protocol can be used to monitor reflex plasticity. Manipulation of stance, surface, and external bracing are common methods of rehabilitating and treating lower extremity musculoskeletal injuries. The intent of this study was to evaluate changes in PRD of the soleus H-reflex during single-leg stance under varying stability conditions.

METHODS

Seven trials were completed for each condition in ten healthy volunteers (age=23+/-1.8 yr, weight 65.0+/-11.3 kg, height=168.7+/-28.0 cm). The conditioning stimuli were composed of soleus H-reflex pairs evoked 80 ms apart at an equal intensity. The mean percent decrease of the second H-reflex relative to the first represented PRD.

RESULTS

A 2 x 2 repeated measures ANOVA (P<0.05) was used to evaluate influence of surface (foam, no foam) and support (semi-rigid ankle brace, no ankle brace) on PRD. Main effects testing revealed a significantly greater soleus PRD (P=.034) for the foam surface (62.5%) compared the flat surface (57.5%). Ankle brace application did not influence soleus PRD (P=0.63).

CONCLUSION

The increase in soleus PRD during the foam condition suggests depression of the motoneuron pool. This may lessen postural over-corrections while maintaining upright stance during less stable conditions. No change in PRD during the ankle brace condition suggests that mechanical reinforcement provided an increase in ankle stability, decreasing the demand on the motoneuron pool.

Effects of postural anxiety on the soleus H-reflex

Previous research has proposed that spinal reflex modulation may mediate anxiety-related changes in postural control. This study investigated how soleus H-reflex amplitude was influenced by standing at heights that induced different levels of anxiety. H-reflexes were elicited in 15 participants standing at the center and edge of a platform raised from a low to a high height (with and without vision). Increased skin conductance confirmed the anxiety effect of elevated surface heights. When standing at the edge of the platform with vision, H-reflex amplitude was attenuated in the high compared to low height condition. Changes in background muscle activity could not explain observed H-reflex changes, suggesting the potential involvement of pre-synaptic inhibition or fusimotor drive on anxiety-related changes in reflex modulation. This study reveals that healthy participants reduce spinal reflex excitability in the presence of increased postural anxiety and a postural threat imposed by standing at the edge of a raised platform. These findings have implications for understanding control of standing balance in individuals with postural instability and/or fear of falling, such as the elderly or stroke.

Influence of posture and stimulus parameters on post-activation depression of the soleus H-reflex in individuals with chronic spinal cord injury

In non-disabled (ND) individuals, reflexes are modulated by influences related to physiologic state (e.g., posture, joint position, load) and activation history. Repeated activation of the H-reflex results in post-activation depression (PAD) of the response amplitude. The modulation associated with physiologic state and activation history is suppressed or abolished in individuals with spinal cord injury (SCI). While posture is known to affect H-reflex amplitude and PAD in non-disabled individuals, the effect of posture on PAD in SCI individuals is not known. Further, while the amount of PAD is also known to be influenced by the stimulus rate and by the amplitude of the evoked reflex, the interaction of posture with stimulus parameters has not been previously investigated in either group. We investigated differences in PAD of the soleus H-reflex between SCI subjects and ND subjects during sitting versus supported standing. Subjects were tested using paired conditioning-test stimulus pulses of 2.5s and 5s interpulse intervals (ISI) and with stimulus intensity adjusted to evoke reflex responses of 20% and 40% of the maximum motor response. We found standing posture to be associated with significantly less PAD in SCI subjects compared to ND subjects. In both groups, shorter ISIs and smaller reflex amplitudes were associated with greater PAD of the H-reflex. These results indicate that postural influences on post-activation modulation, while present, are impaired in individuals with chronic incomplete SCI.

Training-induced adaptive plasticity in human somatosensory reflex pathways

This paper reviews evidence supporting adaptive plasticity in muscle and cutaneous afferent reflex pathways induced by training and rehabilitative interventions. The perspective is advanced that the behavioral and functional relevance of any intervention and the reflex pathway under study should be considered when evaluating both adaptation and transfer. A cornerstone of this concept can be found in acute task-dependent reflex modulation. Because the nervous system allows the expression of a given reflex according to the motor task, an attempt to evaluate the training adaptation should also be evoked under the same conditions as training bearing in mind the functional role of the pathway under study. Within this framework, considerable evidence supports extensive adaptive plasticity in human muscle afferent pathways in the form of operant conditioning, strength training, skill training, and locomotor training or retraining. Directly comparable evidence for chronic adaptation in cutaneous reflex pathways is lacking. However, activity-dependent plasticity in cutaneous pathways is documented particularly in approaches to neurological rehabilitation. Overall, the adaptive range for human muscle afferent reflexes appears bidirectional (that is, increased or reduced amplitudes) and on the order of 25-50%. The adaptive range for cutaneous pathways is currently uncertain.

Plastic changes in the human H-reflex pathway at rest following skillful cycling training

OBJECTIVE

The spinal cord is capable of activity-dependent plasticity, but the extent of its participation in human motor learning is not known. Here, we tested the hypothesis that acquisition of a locomotor-related skill modulates the pathway of the H-reflex, a measure of spinal cord excitability that is susceptible to plastic changes.

METHODS

Subjects were tested on their ability to establish a constant cycling speed on a recumbent bike despite frequent changes in pedal resistance. The coefficient of variation of speed (CV(speed)) measured their ability to acquire this skill (decreasing CV(speed) with training reflects performance improvements). Soleus H-reflexes were taken at rest before and after cycling.

RESULTS

Ability to establish a target speed increased and H-reflex size decreased more after cycling training involving frequent changes in pedal resistance that required calibrated locomotor compensatory action than with training involving constant pedal resistances and lesser compensation. The degree of performance improvement correlated with the reduction in the amplitude of the H-reflex.

CONCLUSIONS

Skillful establishment of a constant cycling speed despite changing pedal resistances is associated with persistent modulation of activity in spinal pathways.

SIGNIFICANCE

Recalibration of activity in the H-reflex pathway may be part of the control strategy required for locomotor-related skill acquisition.

Neural Adaptations to Resistive Exercise

It is generally accepted that neural factors play an important role in muscle strength gains. This article reviews the neural adaptations in strength, with the goal of laying the foundations for practical applications in sports medicine and rehabilitation. An increase in muscular strength without noticeable hypertrophy is the first line of evidence for neural involvement in acquisition of muscular strength. The use of surface electromyographic (SEMG) techniques reveal that strength gains in the early phase of a training regimen are associated with an increase in the amplitude of SEMG activity. This has been interpreted as an increase in neural drive, which denotes the magnitude of efferent neural output from the CNS to active muscle fibres. However, SEMG activity is a global measure of muscle activity. Underlying alterations in SEMG activity are changes in motor unit firing patterns as measured by indwelling (wire or needle) electrodes. Some studies have reported a transient increase in motor unit firing rate. Training-related increases in the rate of tension development have also been linked with an increased probability of doublet firing in individual motor units. A doublet is a very short interspike interval in a motor unit train, and usually occurs at the onset of a muscular contraction. Motor unit synchronisation is another possible mechanism for increases in muscle strength, but has yet to be definitely demonstrated. There are several lines of evidence for central control of training-related adaptation to resistive exercise. Mental practice using imagined contractions has been shown to increase the excitability of the cortical areas involved in movement and motion planning. However, training using imagined contractions is unlikely to be as effective as physical training, and it may be more applicable to rehabilitation. Retention of strength gains after dissipation of physiological effects demonstrates a strong practice effect. Bilateral contractions are associated with lower SEMG and strength compared with unilateral contractions of the same muscle group. SEMG magnitude is lower for eccentric contractions than for concentric contractions. However, resistive training can reverse these trends. The last line of evidence presented involves the notion that unilateral resistive exercise of a specific limb will also result in training effects in the unexercised contralateral limb (cross-transfer or cross-education). Peripheral involvement in training-related strength increases is much more uncertain. Changes in the sensory receptors (i.e. Golgi tendon organs) may lead to disinhibition and an increased expression of muscular force. Agonist muscle activity results in limb movement in the desired direction, while antagonist activity opposes that motion. Both decreases and increases in co-activation of the antagonist have been demonstrated. A reduction in antagonist co-activation would allow increased expression of agonist muscle force, while an increase in antagonist co-activation is important for maintaining the integrity of the joint. Thus far, it is not clear what the CNS will optimise: force production or joint integrity. The following recommendations are made by the authors based on the existing literature. Motor learning theory and imagined contractions should be incorporated into strength-training practice. Static contractions at greater muscle lengths will transfer across more joint angles. Submaximal eccentric contractions should be used when there are issues of muscle pain, detraining or limb immobilisation. The reversal of antagonists (antagonist-to-agonist) proprioceptive neuromuscular facilitation contraction pattern would be useful to increase the rate of tension development in older adults, thus serving as an important prophylactic in preventing falls. When evaluating the neural changes induced by strength training using EMG recording, antagonist EMG activity should always be measured and evaluated.

Aging, resistance training, and motor unit discharge behavior

Researchers have alluded the existence of "neural factors" in the expression and development of muscular strength. Candidate neural factors including motor unit recruitment, rate coding, double firing, and motor unit synchronization are discussed in this review. Aging is generally accompanied by lower motor unit discharge rates. However, both young and older adults exhibit rapid changes in muscular strength with repeated strength testing. These strength changes occur with concomitant albeit transient increase in motor unit discharge rate. These and other neural factors may contribute to the initial increase in muscular strength observed during the early phases of resistance exercise training.

Presynaptic control of group Ia afferents in relation to acquisition of a visuo-motor skill in healthy humans

Sensory information continuously converges on the spinal cord during a variety of motor behaviours. Here, we examined presynaptic control of group Ia afferents in relation to acquisition of a novel motor skill. We tested whether repetition of two motor tasks with different degrees of difficulty, a novel visuo-motor task involving the ankle muscles, and a control task involving simple voluntary ankle movements, would induce changes in the size of the soleus H-reflex. The slope of the H-reflex recruitment curve and the H-max/M-max ratio were depressed after repetition of the visuo-motor skill task and returned to baseline after 10 min. No changes were observed after the control task. To elucidate the mechanisms contributing to the H-reflex depression, we measured the size of the long-latency depression of the soleus H-reflex evoked by peroneal nerve stimulation (D1 inhibition) and the size of the monosynaptic Ia facilitation of the soleus H-reflex evoked by femoral nerve stimulation. The D1 inhibition was increased and the femoral nerve facilitation was decreased following the visuo-motor skill task, suggesting an increase in presynaptic inhibition of Ia afferents. No changes were observed in the disynaptic reciprocal Ia inhibition. Somatosensory evoked potentials (SEPs) evoked by stimulation of the tibial nerve (TN) were also unchanged, suggesting that transmission in ascending pathways was unaltered following the visuo-motor skill task. Together these observations suggest that a selective presynaptic control of Ia afferents contributes to the modulation of sensory inputs during acquisition of a novel visuo-motor skill in healthy humans.

Responsiveness of the H reflex to loading and posture in patients following stroke

The objective of the research was to examine the effects of loading and posture on motoneuronal excitability of the triceps surae (TS) for patients with hemiplegia. Twelve healthy subjects and 12 patient subjects with post-stroke hemiparesis (onset period: 3-60 months) were enrolled in this study. The subjects were instructed to remain in quiet sitting with the test knee straight and three standing conditions of different superincumbent loads by shifting body weight to the test leg (10%, 50%, and 90% of body weight), while the H reflexes and M waves of the TS were measured. The results clearly indicated that H reflex amplitudes were not affected by different loading conditions in standing for both healthy subjects and patients who had a previous stroke. In addition, the H reflex amplitude in quiet standing for healthy subjects was significantly downward modulated relative to that in relaxed sitting with the test knee straight, but this posturally driven modulation was impaired in patients following stroke. Current electrophysiological findings imply that body weight as a means for rehabilitation facilitation had little immediate effect on paretic TS, and absence in postural gating of reflex excitability appeared to be an incentive for postural instability resulting from post-stroke hemiparesis.

Neuromuscular characteristics of endurance- and power-trained athletes

In response to chronic physical training, the human neuromuscular system undergoes significant and specific adaptations. More importantly, these influences are the result of the type and quantity of physical activity. One of the simplest neuromuscular mechanisms is the spinal stretch reflex. The reflex system was previously viewed as inflexible, with a relatively fixed response that could vary only slightly. However, more recent data have identified an adaptive plasticity in the reflex system. In this respect, the reflex system can be used to assess training and aging adaptations of the human neuromuscular system. Due to their methodological simplicity, both the tendon-tap reflex and the electrically evoked Hoffmann reflex (H-reflex) can be used to assess training adaptations of the human neuromuscular system. The purpose of this paper is to review briefly the tendon-tap and H-reflex paradigms and delineate the research findings pertaining to changes in the reflex system with physical training. For purposes of clarity, this discussion will be divided into the following: (a) differences observed in the tendon-tap reflex, (b) differences observed in the H-reflex, and (c) role of interneurons in mediating these changes.

Down training of the elderly soleus H reflex with the use of a spinally induced balance perturbation

The purpose of this study was to determine the ability of the elderly central nervous system to modulate spinal reflex output to functionally decrease a spinally induced balance perturbation. In this case, the soleus H reflex was used as the source of perturbation. Therefore, decreasing (down training) of the soleus H reflex was necessary to counteract this perturbation and to better maintain postural control. In addition to assessing the effect of this perturbation on the H reflex, static postural stability was measured to evaluate possible functional effects. Ten healthy young subjects (age: 27.0 +/- 4.6 yr) and 10 healthy elderly subjects (age: 71.4 +/- 5.1 yr) participated in this study. Subjects underwent balance perturbation on 2 consecutive days. On day 1 of perturbation, significant down training of the soleus H reflex was demonstrated in both young (-20.4%) and elderly (-18.7%) subjects. On day 2 of perturbation, significant down training of the soleus H reflex was again demonstrated in both young (-24.6%) and elderly (-21.0%) subjects. Analysis of static stability after the 2 days of balance perturbation revealed a significant 10.1% decrease in the area of sway in elderly subjects. In conclusion, this study demonstrated that healthy, elderly subjects compared with young subjects were equally capable of down training the soleus H reflex in response to a balance perturbation. Furthermore, the improvement in static stability through balance training may provide further evidence that balance can be retrained and rehabilitated in subjects with decreased reflex function.