Postural displacement induced by electrical stimulation; A new approach to examine postural recovery.

BACKGROUND: Controlling upright posture entails acute adjustments by the neuromuscular system to keep the center of mass (COM) within the limits of a relatively small base of support. Sudden displacement of the COM triggers several strategies and balance recovery mechanisms to prevent excessive COM displacement. NEW METHOD: We have examined and quantified a new approach to induce an internal neuromuscular perturbation in standing posture on 15 healthy individuals to provide an insight into the mechanism of loss of balance (LOB). The method comprises eliciting an H-reflex protocol while subjects are standing which produces a contraction in soleus and gastrocnemius muscles. We have also defined analytical techniques to provide biomarkers of balance control during perturbation. We used M-Max unilaterally or bilaterally and induced a forward or sideway perturbation. The vector analysis and the Equilibrium Point calculations defined here can quantify the amplitude, direction, and evolution of the perturbation. RESULTS: Clear patterns of loss of balance due to stimulation was observed. Compared to quiet standing, the density of the EPs substantially increased in the perturbation phase. Leftward stimulation produced significantly higher number of EPs compared to the bilateral stimulation condition which could be due to the fact that the left leg was the nondominant side in all our subjects. COMPARISON AND CONCLUSION: In this study we provide a proof-of-concept technique for examining recovery from perturbation. The advantage of this technique is that it provides a safe perturbation, is internally induced at the spinal cord level, and is free from other factors that might complicate the recovery analysis (e.g., locomotion and the integration of the spinal pattern generator and cutaneous pathways in mediating changes). We have shown that the perturbation induced by this method can be quantified as vectors. We have also shown that the density of instantaneous equilibrium points (EPs) could be a good biomarker for defining and examining the perturbation phase. Thus, this protocol and analysis provides a unique individual assessment of recovery which can be used to assess interventions. Finally, given that the maximal motor response is used as the perturbation (e.g., M-max) it is highly reliable and reproducible within an individual patient.

Return to Learn: Preferences of College Educators When Receiving Concussion Medical Notes.

The aim of this work is to uncover the preferences and perspectives of college educators as they interpret medical documentation outlining medically requested return-to-learn (RTL) instructions. Participants were recruited from five colleges across campus at a large Midwest public university. They each engaged in a private, one-on-one, audio-recorded interview. All recordings were transcribed and inductively analyzed using a grounded theory approach and two-coder system. All codes and themes were finalized once agreement was reached by both coders. Resultant themes from axial coding had to represent the voices of at least 80% of participants. Three characteristics emerged as being desired by college educators: brevity, clarity, and direction. Educators also expressed considerably less utility with medical documentation designed for pediatric students with concussion. College educators desire medical notes that are brief, clear, and provide straightforward direction, in addition to documentation that is tailored for the college setting.

Measuring movement in health and disease.

Evaluating and quantifying the many aspects of movement - from open-field locomotion and stepping patterns in rodent models to stride trajectory and postural sway in human patients - are key to understanding brain function. Various experimental approaches have been used in applying these lines of research to investigate the brain mechanisms underlying neurodegenerative disease. Although valuable, data on movement are often limited by the shortcomings inherent in the data collection process itself. Steve Fowler and his research group have been instrumental in pioneering a technology that both minimizes these pitfalls in studies of rodent behavior and has applications to research on human patients. At the center of this technology is the force-plate actometer, developed by the Fowler group to assess multiple aspects of movement in rodent models. Our review highlights how use of the actometer and related behavioral measurements provides valuable insight into Huntington's disease (HD), an autosomal dominant condition of progressively deteriorating behavioral control. HD typically emerges in mid-life and has been replicated in multiple genetically engineered mouse models. The actometer also can be a valuable addition to cutting-edge neuronal and synaptic technologies that are now increasingly applied to studies of behaving animals. In short, the impact of the Fowler contribution to the neuroscience of movement is both meaningful and ongoing.

Educator perspectives on concussion management in the college classroom: a grounded theory introduction to collegiate return-to-learn.

OBJECTIVES: To gather the perspectives of collegiate instructors regarding how concussion is managed within the college classroom. To introduce the themes surrounding collegiate return-to-learn (RTL) and the classroom management of students with concussion. DESIGN: Qualitative grounded theory. SETTING: Large, public university in the Midwest. PARTICIPANTS: Twenty-three college instructors participated in a private, semistructured, audio-recorded, one-on-one interview. Participants included 12 males and 11 females. Interview recordings were transcribed verbatim, followed by an iterative process of open-coding and axial-coding, performed by two researchers. RESULTS: Three themes emerged from the coded data: (1) awareness-external knowledge of concussion and previous experiences, (2) legitimacy-medical note provided and no note provided and (3) accommodating the student-instructor's role and feasibility of the accommodation. Psychosocial factors such as small class sizes, graduate-level students and an instructor's empathy appeared to influence an instructor's decision making when accommodating a student recovering from concussion. CONCLUSION: These novel data provide foundational evidence regarding how college instructors perceive and subsequently manage concussion within the classroom, while also offering accuracy to aims of subsequent collegiate RTL investigations ARTICLE SUMMARY: RTL is an emerging field within concussion management, yet is grossly underexplored within the college setting. By utilising a grounded theory approach, this article introduces the themes that dictate the landscape of RTL for a college student.

Respiratory Muscle Fatigue Alters Cycling Performance and Locomotor Muscle Fatigue.

PURPOSE: This study aimed to determine if preexisting respiratory muscle fatigue (RMF) alters motoneuronal output, locomotor muscle fatigue, and cycling performance. METHODS: Eight trained male cyclists performed 5-km cycling time trials after a resistive breathing task that induced RMF and under control conditions (CON). Motoneuronal output was estimated using vastus lateralis surface electromyography, and locomotor muscle fatigue was quantified as the change in potentiated quadriceps twitch force from preexercise to postexercise. RESULTS: Time to complete the time trial was 1.9% ± 0.9% longer in RMF compared with CON (P < 0.001). Estimated motoneuronal output was lower in RMF compared with CON during 1 km (45% ± 11% vs 53% ± 13%, P = 0.004) and 2 km (45% ± 14% vs 51% ± 14%, P = 0.008), but was not different thereafter. Ventilation was lower in RMF compared with CON during 1 km (114 ± 19 vs 135 ± 24 L·min, P = 0.003) and 2 km (136 ± 23 vs 152 ± 31 L·min, P = 0.009); however, ratings of dyspnea were similar. After the 5-km time trial, locomotor muscle fatigue was attenuated in RMF compared with CON (-22% ± 6%, vs -28% ± 7%, P = 0.02). CONCLUSIONS: Alterations to dyspnea for a given ventilation seem to have constrained power output during cycling exercise, thereby limiting the development of locomotor muscle fatigue. These findings indicate that the respiratory system is an integral component in a global feedback loop that regulates exercise performance and the development of locomotor muscle fatigue.

Modulation of cutaneous reflexes during sidestepping in adult humans.

A common neural control mechanism coordinates various types of rhythmic locomotion performed in the sagittal plane, but it is unclear whether frontal plane movements show similar neural patterning in adult humans. The purpose of this study was to compare cutaneous reflex modulation patterns evoked during sagittal and frontal plane rhythmic movements. Eight healthy, neurologically intact adults (three males, five females) walked and sidestepped on a treadmill at approximately 1 Hz. The sural nerve of the dominant (and lead) limb was stimulated randomly every 3-7 steps at eight phases of each gait cycle. Ipsilateral electromyographic recordings from four lower leg muscles and kinematic data from the ankle were collected continuously throughout both tasks. Data from unstimulated gait cycles were used as control trials to calculate middle-latency reflex responses (80-120 ms) and kinematic changes (140-220 ms) following electrical stimulation. Results show that the cutaneous reflex modulation patterns were similar across both tasks despite significant differences in background EMG activity. However, increased reflex amplitudes were observed during the late swing and early stance phases of sidestepping, which directly altered ankle kinematics. These results suggest that the neural control mechanisms responsible for coordinating sagittal locomotion are flexibly modified to coordinate frontal plane activities even with very different foot landing mechanics.

Combined Aerobic and Resistance Training Increases Stretch- Shortening Cycle Potentiation and Walking Economy in Postmenopausal Women.

PURPOSE: Secondary analyses were performed to test whether combined aerobic and resistance training altered walking economy (i.e., net oxygen uptake) and/or stretch-shortening cycle potentiation (SSCP). A further objective was to determine if walking economy and SSCP were related before or after training. METHODS: Ninety-two postmenopausal women were enrolled wherein 76 completed 16 weeks of supervised aerobic and resistance training. Participants were randomized to one of three training groups based on frequencies: (a) 1 d⋅wk-1 (n = 23); (b) 2 d⋅wk-1 (n = 30) or; (c) 3 d⋅wk-1 (n = 23). Following assessments were performed at baseline and post-training. Indirect calorimetry was used to measure maximal oxygen uptake () and walking economy (submaximal - resting = net ) during a graded exercise test and steady-state treadmill task, respectively. SSCP was determined by measuring the difference between a concentric (CO) and counter-movement (CM) leg press throw. RESULTS: , walking economy, CO and CM velocity were significantly improved (p < 0.05) for all training groups, however; no time by group interactions were observed. Paired t-tests revealed participants exercise training 2 d⋅wk-1 exhibited a significant time effect for SSCP (+0.04 ± 0.09 ms-1; p = 0.03). At baseline, multiple linear regression showed a negative relationship between walking net and SSCP (r = -0.22; p < 0.04) adjusted for relative proportion of . No such relationship was found post-training. CONCLUSION: Among older postmenopausal women, our results indicate that irrespective of frequency of training, 16 weeks of combined aerobic and resistance exercise training increased ease of walking and economy. Additionally, only participants exercising 2 d⋅wk-1 exhibited significant improvement in SSCP.

Residential Proximity to Agricultural Fields and Neurological and Mental Health Outcomes in Rural Adults in Matlab, Bangladesh.

Pesticide exposure is an important rural public health concern that is linked to a spectrum of health outcomes in farmers. However, little is known about these effects on residents living in close proximity to agricultural fields and who are not involved in regular farming. This paper compared the effects of residential proximity to farming lands on a number of neurological and mental health outcomes in adults. A cross-sectional study was performed on 57 adults involved in farming only occasionally in rural Matlab in Bangladesh. A health and demographic surveillance system (HDSS) and geocoding were used to define proximity to the agricultural field. Neurological health was measured using the trail making test, vibrotactile threshold measurement, and dominant ulnar nerve conduction velocity (NCV) amplitude. An adapted Center for Epidemiological Studies Depression scale (CES-D) questionnaire was used to evaluate mental health. Results indicated that respondents living near agricultural fields had significantly higher vibrotactile threshold in big toes (p < 0.004) and needed a longer time to complete the trail making test (p < 0.004) than those living far from fields after accounting for the covariates. Results of this pilot study suggest further investigations to establish the impact of pesticide exposure among occasional and non-farmers on neurological health outcomes.

Effects of chronic ankle instability on cutaneous reflex modulation during walking.

Chronic ankle instability (CAI) is characterized by persistent giving way at the ankle following an acute lateral ankle sprain and is associated with an early onset of osteoarthritis. Researchers have reported that the cutaneous afferent pathway from certain leg muscles is modified in people with CAI while in a seated position. However, we do not know if these reflex modulations persist during functional activities. The purpose of this study was to further explore sensorimotor function in patients with CAI by analyzing cutaneous reflex modulation during gait. CAI (n = 11) and uninjured control (n = 11) subjects walked on a treadmill at 4 km/h and received non-noxious sural nerve stimulations at eight different time points during the gait cycle. Net electromyographic responses from four lower leg muscles were quantified 80-120 ms after stimulation for each phase of the gait cycle and compared between groups. We found that cutaneous reflex responses between groups were largely similar from the late stance to late swing phases, but uninjured control subjects, and not CAI subjects, experienced significant suppression in the medial gastrocnemius and lateral gastrocnemius muscles during the early stance phase of the gait cycle. Our results indicate that people with CAI lack a protective unloading response in the triceps surae following high-intensity sural nerve stimulation during the early stance phase of the gait cycle. Evaluating cutaneous reflex modulations may help to identify neural alterations in the reflex pathways that contribute to functional deficits in those with CAI.

Effect of carbohydrate ingestion on central fatigue during prolonged running exercise in moderate hypoxia.

To determine whether acute exposure to moderate hypoxia alters central and peripheral fatigue and to test whether carbohydrate ingestion impacts fatigue characteristics, 12 trained runners completed three running trials lasting 1 h each at 65% of normoxic maximum oxygen uptake. The first trial was performed in normoxia [inspired O2 fraction ( FiO2 ) = 0.21], and the last two trials were completed in hypoxia ( FiO2 = 0.15). Participants ingested a placebo drink in normoxia (NORM-PLA), a placebo drink in hypoxia (HYP-PLA), or a carbohydrate solution in hypoxia (HYP-CHO). HYP conditions were randomized. Peripheral [change in potentiated quadriceps twitch force (ΔQtw,pot)] and central [change in voluntary activation (ΔVA)] fatigue were assessed via preexercise-to-postexercise changes in magnetically evoked quadriceps twitch. In HYP, blood was drawn to determine the ratio of free-tryptophan (f-TRP) to branched-chain amino acids (BCAA). After exercise, peripheral fatigue was reduced to a similar degree in normoxia and hypoxia (ΔQtw,pot = -4.5 ± 1.3% and -4.0 ± 1.5% in NORM-PLA and HYP-PLA, respectively; P = 0.61). Central fatigue was present after normoxic and hypoxic exercise but to a greater degree in HYP-PLA compared with NORM-PLA (ΔVA: -4.7 ± 0.9% vs. -1.9 ± 0.7%; P < 0.01). Carbohydrate ingestion did not influence central fatigue (ΔVA in HYP-CHO: -5.7 ± 1.2%; P = 0.51 vs. HYP-PLA). After exercise, no differences were observed in the ratio of f-TRP to BCAA between HYP-PLA and HYP-CHO ( P = 0.67). Central fatigue increased during prolonged running exercise in moderate hypoxia although the ratio of f-TRP to BCAA remained unchanged. Ingesting carbohydrates while running in hypoxia did not influence fatigue development. NEW & NOTEWORTHY Hypoxic exposure influences the origin of exercise-induced fatigue and the rate of fatigue development depending on the severity of hypoxia. Our data suggest that moderate hypoxia increases central, but not peripheral, fatigue in trained runners exercising at 65% of normoxic maximum oxygen uptake. The increase in central fatigue was unaffected by carbohydrate intake and occurred although the ratio of free tryptophan to branched-chain amino acids remained unchanged.

Acute hypercapnia does not alter voluntary drive to the diaphragm in healthy humans.

Although systemic hypercapnia is a common outcome of pulmonary disease, the relationship between hypercapnia and voluntary diaphragmatic activation (VAdi) is unclear. To examine whether hypercapnia independent of ventilatory work contributes to reduced central motor drive to the diaphragm in healthy humans, 14 subjects spontaneously breathed room air (NN) or a hypercapnic gas mixture (HH; 7% CO2 with air) while at rest. Thereafter, subjects volitionally hyperventilated room air (NH) matching the minute ventilation recorded during HH while maintained at eucapnic levels. Twitch interpolation with bilateral magnetic stimulation of phrenic nerves at functional residual capacity was used to assess VAdi during the three trials. Although PETCO2 was elevated during HH compared with NN and NH (52 vs 36 mmHg), VAdi was not altered across the trials (HH = 93.3 ±â€¯7.0%, NN = 94.4 ±â€¯5.0%, NH = 94.9 ±â€¯4.6%, p = 0.48). Our findings indicate that the magnitude of hypercapnia acutely imposed may not be effective in inhibiting voluntary neural drives to the diaphragm in normal resting individuals.

The role of anterior cruciate ligament in the control of posture; possible neural contribution.

The anterior cruciate ligament (ACL) is not only a mechanical structure for knee joint stability but is also a source of sensory information which could be used in the control of standing posture. It has been shown that the center of pressure (COP) time series during normal standing may be decomposed into two components which are hypothetically governed by different neural mechanisms, namely rambling and trembling. The aim of the present study was to investigate to what extent an injury to the ACL structure would affect these two control mechanisms. In this study the balance of a group of ACL deficient (ACLD) patients during double and single leg standing was examined and compared with that of a group of healthy individuals. We not only calculated the traditional measures of COP, but also decomposed this complex signal to investigate if ACL deficiency would affect the rambling and trembling components differently. The results showed that rambling was not significantly different between the two groups; however the trembling component was significantly greater for the ACLD group in both the single leg and the double leg condition. Further, there was also a component (rambling/trembling) by direction (anterior-posterior/mediolateral) interaction for both groups, indicating that the rambling component exhibited differences between directions of sway whereas the trembling component did not. This study provided evidence that the two components of postural control are differently affected by ACL deficiency, and that the rambling component is influenced by direction of sway.

Static otolithic drive alters presynaptic inhibition in soleus motor pool.

The vestibular system has both direct and indirect connections to the soleus motor pool via the vestibulospinal and reticulospinal tracts. The exact nature of how this vestibular information is integrated within the spinal cord is largely unknown. The purpose of this study was to identify whether changes in static otolithic drive altered the amount of presynaptic inhibition in the soleus H-reflex pathway. Changes in static otolithic drive were investigated in sixteen healthy participants using a tilt table. Two presynaptic pathways (common peroneal and femoral) to the soleus H-reflex were tested in three weight conditions (supine, non-weight bearing, and weight bearing). The dependent variable was the peak-to-peak amplitude of the soleus H-reflex. Inhibition to the soleus motor pool through the common peroneal nerve pathway differed significantly during weight conditions and tilt. During tilt and non-weight bearing there was greater inhibition of the soleus H-reflex compared to supine, however, this effect was reversed during tilt and weight bearing. Facilitation from the femoral nerve pathway was reduced by tilt compared to supine, but this reduction was unaffected by weight condition. This supports a role of the vestibular system as providing complex, task-dependent presynaptic input to motoneurons in the lower limbs.

Contralateral conditioning to the soleus H-reflex as a function of age and physical activity.

Coordination between ipsilateral and contralateral muscles of the lower limbs has a critical role in movement control. However, the roles that aging and physical activity have in maintaining bilateral coordination are understudied. The aim of this study was to examine the roles of physical activity and age on pathways between the soleus and tibialis anterior muscles. Fourteen young and 14 older subjects (7 active, 7 sedentary) participated. A Hoffmann (H)-reflex was elicited in the ipsilateral soleus following a conditioning stimulus to the contralateral common peroneal nerve at differing intervals (25-300 ms). Significant H-reflex facilitation from the control value was observed for the sedentary group at the 50-ms (28.7 %), 75-ms (24.5 %), and 150-ms (34.0 %) intervals when compared with the physically active group. There were also significant differences between the young and older groups. Results demonstrated differences in soleus H-reflex excitability as a result of contralateral conditioning and highlighted the influence of age and physical activity in maintaining these neural pathways.

Characteristics of preceding Ia activity on postactivation depression in health and disease.

Previous activation of the soleus Ia afferents causes a depression in the amplitude of the H-reflex. This mechanism is referred to as postactivation depression (PAD) and is suggested to be presynaptically mediated. With the use of a paired reflex depression paradigm (eliciting two H-reflexes with conditioning-test intervals from 80 ms to 300 ms), PAD was examined in a group of healthy individuals and a group of hemiplegic patients. Healthy individuals showed substantial depression of the test H-reflex at all intervals. Although the patient group showed substantially less depression at all intervals, increasing the interval between the two reflexes sharply reduced the depression. In a separate experiment, we varied the size of the conditioning H-reflex against a constant test H-reflex. In healthy individuals, by increasing the size of the conditioning H-reflex, the amplitude of the test H-reflex exponentially decreased. In the patient group, however, this pattern was dependent on the conditioning-test interval; increasing the size of the conditioning H-reflex caused an exponential decrease in the size of the test reflex at intervals shorter than 150 ms. This pattern was similar to that of healthy individuals. However, conducting the same protocol at a longer interval (300 ms) in these patients resulted in an abnormal pattern (instead of an exponential decrease in the size of the test reflex, exaggerated responses were observed). Fisher discriminant analysis suggested that these two patterns (which differed only in the timing between the two stimuli) were substantially different from each other. Therefore, it is suggested that the abnormal pattern of PAD in hemiplegic stroke patients could be a contributing factor for the pathophysiology of spasticity.

Post-activation potentiation: The neural effects of post-activation depression.

INTRODUCTION: Our knowledge of the neurophysiology of post-activation potentiation (PAP) is limited. The purpose of this study was to examine the effect of PAP on twitch torque and H-reflex amplitude after a 10-s maximal voluntary contraction (MVC). METHODS: PAP measurements were assessed with the plantarflexors in a relaxed state and during a tonic contraction at 10% MVC. RESULTS: The H-reflex/maximum M-wave ratio (H/M) decreased significantly (P<0.05) and returned to baseline levels after 1 min. The decrement in H/M was depressed when the plantarflexors were active at 10% MVC, and the depression was more obvious in the lateral gastrocnemius than in the soleus muscle. CONCLUSIONS: The inhibition induced immediately after contraction could be attributed to post-activation depression. We conclude that PAP after a 10-s MVC cannot be attributed to increased motor neuron excitability through the reflex pathway as assessed by the H-reflex technique.

Novel balance rehabilitation and training apparatus to improve functional balance.

A new balance rehabilitation and training apparatus has been developed to allow a balance-impaired person to cope with his or her fear of falling while safely and independently performing exercises necessary to improve functional balance. The apparatus consists of a stable platform where the user stands and a vertical structure that supports free-floating handles that the user holds with both hands while performing various exercises. The purpose of study 1 was to determine whether this new apparatus significantly alters the biological postural control system, and the purpose of study 2 was to document the benefits of balance training using the apparatus. Study 1 was a randomized repeated-measures design with six healthy adult subjects (mean age = 35.5 yr), and study 2 was a 4 wk intervention case study with a generally healthy 63-yr-old individual. The results suggest that postural sway characteristics and the cortical and proprioceptive feedback were not limited when using the apparatus. We also observed improvements in balance control and postural stability with 4 wk of training with the apparatus. These results support that the apparatus could be an effective tool to help individuals safely and independently perform balance exercises while potentially preventing falls and minimizing fear of falling.

Independent segmental inhibitory modulation of synaptic efficacy of the soleus H-reflex.

Synaptic efficacy associated with muscle spindle feedback is partly regulated via depression at the Ia-motorneuron synapse through paired reflex depression (PRD) and presynaptic inhibition (PI). The purpose of this study was to examine PRD and PI of the soleus H-reflex at rest and with a background voluntary muscle contraction. The experiment was conducted on 10 healthy males with no history of neurological deficits. Soleus H-reflex and M-wave curves were elicited in three conditions: unconditioned, PRD (two consecutive H-reflexes with 100 ms interval), and PI (1.2 × MT to tibialis anterior 100 ms prior to soleus H-reflex). Each condition was tested at rest and with a 10% soleus contraction. PRD and PI both produced a pronounced inhibition to the soleus motor pool at rest, with a significant difference observed between threshold values (78.9, 89.3, and 90.4% for unconditioned, PRD, and PI reflexes, respectively). During the voluntary contraction the threshold for both inhibitory mechanisms was significantly reduced, and were not different from the unconditioned H-reflex (74.5, 78.9, and 77.0% for unconditioned, PRD, and PI reflexes, respectively). The slope of PI and the PI Hmax/Mmax ratio were significantly altered during contraction whereas no differences were observed for PRD. The results suggest these inhibitory mechanisms depend on the interaction between background voluntary activation and stimulus intensity. This behavior of these inhibitory mechanisms underscores the specificity of spinal circuitry in the control of motor behaviors.

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.

Differential control of H-reflex amplitude in different weight-bearing conditions in young and elderly subjects.

OBJECTIVE: This study measured the modulation of conditioned (femoral nerve, paired-stimuli) and unconditioned soleus H-reflexes in young and elderly subjects when changing weight-bearing (WB) requirements and body position. METHODS: Conditioned and unconditioned H-reflexes were examined in 14 elderly subjects and 11 young subjects during six different WB conditions: (1) lying supine with no WB, (2) supine position inclined by 30° with 50% WB, (3) standing with 50%, (4) 75%, (5) 100% and (6) 125% WB. RESULTS: The elderly subjects had consistently higher background soleus EMG activity across the WB conditions compared to the young. Femoral nerve conditioning caused facilitation of the H-reflex that changed across WB conditions in the young subjects, but not in the elderly subjects. Finally, elderly subjects had less depression with paired-stimulation (PRD) across WB conditions, which was not observed in the young subjects. CONCLUSIONS: The elderly may have more direct activation of motoneurons from descending pathways, coupled with less segmental spinal control of inhibitory interneurons, as evidenced by the increased background soleus activity, H/M-max ratios and the lack of modulatory control observed when conditioning the H-reflex. SIGNIFICANCE: There was an age-specific response from descending and segmental pathways during conditions that involved either different WB requirements or changes in body position.