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School of Biological Sciences

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College of Sciences

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Center for the Study of Systems Biology

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Now showing 1 - 10 of 17

Spatiotemporal patterns of parietofrontal activity and eye movements underlying the visual perception of complex human tool use

(Georgia Institute of Technology, 2015-11-16) Natraj, Nikhilesh

When watching a child learning to use a spoon, a mother is immediately able to recognize the error when the child grabs the bowl rather than the stem, or when the child uses the spoon to try and scoop paper. Recognizing proper tool grasp-postures and use-contexts is an ability vital for daily life and can be lost due to brain injury. A better understanding of how the brain encodes contextual and grasp-specific tool-use not only furthers basic neuroscience, but also has strong relevance to deficits arising from neural pathologies. However, the majority of research till date has studied the neural response to viewing tools in isolation or viewing simple tool-grasps. These studies have shown that the recognition of tools to be a complex visuomotor process, as not only was the visual cortex engaged but also parietal and frontal regions that underlie actual tool-use. The recognition of tools therefore involves automatically recalling their motor information (graspability and manipulability) via activation of parietofrontal motor regions, a property called action affordances. Yet, it is still unclear how parietofrontal regions encode the combination of contextual and grasp-specific tool-use scenes. In addition, parietofrontal regions are multifaceted and also underlie visuospatial attention and eye movements. It is possible a relationship might exist between eye movements, attention and tool-use understanding over parietofrontal regions. Therefore the overall goal of this thesis was to understand the spatiotemporal patterns of parietofrontal activity and eye movements underlying the perceptual of contextual and grasp-specific static tool use images. Electroencephalography (EEG) was used to measure neural activity, combined with eye tracking to measure fixation and saccades. Overall, results from this thesis present evidence that the affordances of non-functional grasp-postures perturbed an observer from understanding the contextual uses of tools, with corresponding unique patterns of parietofrontal activity and eye movements. This effect was most robust when the tool was placed in contexts that afforded a certain degree of tool-use. Results also revealed a relationship between attention, eye movements and action perception over parietofrontal regions. Specifically, saccades perturbed activity over frontal regions during the perception of non-functional grasp postures and in addition, there was greater engagement of the left precuneus in the superior parietal lobe if the observer had to quickly parse the scene information using peripheral vision and rely on short term memory. In contrast, there was greater engagement of the left middle temporal gyrus if the observer had the ability to parse scene information continuously using foveal attention. Results in this thesis shed light on the neural and visual mechanisms in understanding the affordances of non-functional grasp postures, and the relation between the two mechanisms. The automatic sensitivity in understanding the intent of non-functional grasp-postures may correspond to a lifetime of learning the affordances of grasp-specific action outcomes with tools. Such cognitive motor knowledge may be vital in navigating a human environment almost entirely constructed on advanced tool-use knowledge and findings from this thesis have many potential applications in the field of neuro-rehabilitation.
Neuromechanics of locomotion: Insights from the walk-to-run transition in amputees and pedaling in able-bodied individuals

(Georgia Institute of Technology, 2015-10-16) Norman, Tracy L.

Afferent feedback is important for modulating locomotion and maintaining stability. Studying locomotor extremes and applying perturbations to normal locomotion allows us to probe the effects of afferent feedback on the control of normal gait. Investigating the walk-to-run gait transition specifically provides a unique locomotor event to investigate the fundamental determinants of legged locomotion (walking or running) and identify the sensory inputs important to the ongoing neuromuscular control of walking and running. The first goal of this dissertation was to investigate the contributions of plantarflexor muscles during stance (Aim 1) and flexor muscles during swing (Aim 2) to the walk-to-run transition. To accomplish this I used unilateral, transtibial amputee subjects as a means to assess the affects of unilaterally eliminating plantarflexor propulsive force production and below-knee flexor activation on the walk-to-run transition speed. The main objective of Aim 1 was to determine the preferred gait transition speeds of unilateral, transtibial amputee subjects, and the influence of kinetics on the walk-to-run gait transition speed. Unilateral, transtibial amputee subjects transition between gaits at a lower speed than able-bodied controls and are still able to generate higher propulsive forces walking at speeds above their preferred gait transition speed. This finding indicates that their walk-to-run transition is not likely dictated by the force-length-velocity characteristics of the intact plantarflexor muscles. Thus, as an experimental model, unilateral, transtibial amputee subjects can provide unique insights for decoupling the previously identified performance limit of plantarflexor muscles from the preferred gait transition speed in order to probe other potential determinants. The main objective of Aim 2 was to quantify the muscle activation during walking and running gaits relative to the walk-to-run gait transition speed for unilateral, transtibial amputee subjects. The swing phase tibialis anterior muscle activation is a major determinant of the walk-to-run transitions in unilateral, transtibial amputee subjects. Swing phase dorsiflexion moments alone do not explain these results and additional work is necessary to probe potential mechanical and neural explanations. Furthermore, in unilateral, transtibial amputee subjects, swing-phase rectus femoris and biceps femoris long head activations and their respective joint moments are a function of changes in absolute speed and thus not indicative of their significantly lower gait transition speed. The second goal of this dissertation was to probe the potential contributions of afferent feedback to the underlying neuromuscular mechanism ultimately responsible for the transition (Aim 3). The main objective of Aim 3 was to evaluate the effects of contralateral sensory loss on the motor output of the ipsilateral leg. Unilateral below-knee, ischemic deafferentation has significant effects on both inter- and intra- limb motor output. The net effect of contralateral sensory loss below the knee is a significant decrease in ipsilateral flexor muscle activations during the transition from flexion to extension in pedaling (Q1). Due to the rapid time course of these responses, I speculate either i) contralateral below-knee afferents (most likely Ia and/or cutaneous) have a net excitatory effect on the ipsilateral flexor muscles or ii) contralateral above knee afferents (most likely Ib) have an inhibitory effect on the ipsilateral flexor muscles.
Altered intermuscular force feedback after spinal cord injury in cat

(Georgia Institute of Technology, 2015-07-24) Niazi, Irrum Fawad

Bipeds and quadrupeds are inherently unstable and their bodies sway during quiet stance and require complex patterns of muscle activation to produce direction-specific forces to control the body’s center of mass. The relative strength of length and force feedback within and across muscles collectively regulates the mechanical properties of the limb as a whole during standing and locomotion (Bonasera and Nichols 1994; Ross and Nichols 2009). Loss of posture control following spinal cord injury (SCI) is a major clinical challenge. While much is known about intermuscular force feedback during crossed extension reflex (XER) and locomotion in decerebrate cats, these have not been well characterized in animals with spinal cord injury. In this study, we mapped the distribution of heterogenic force feedback in hindlimb ankle extensor muscles using muscle stretch (natural stimulation) in intercollicular, non-locomoting, decerebrate cats with chronic lateral spinal hemisection (LSH). We also, determined the time of onset of redistribution of heterogenic force feedback following LSH by collecting force feedback data from cats with acute sci. In addition we revisited heterogenic force feedback between ankle extensors in decerebrate non-locomoting cats during mid-stance to ascertain whether these cats with intact spinal cord depict a certain pattern of force feedback. The goal was to ascertain whether the patterns and strength of feedback was different between the two states (cats with intact spinal cord and cats with SCI). We found that heterogenic feedback pathways remained inhibitory in non-locomoting decerebrate cats in two states. The latencies of inhibition also corresponded to those observed for force feedback from Golgi tendon organs. We observed variable patterns of force feedback between ankle extensors in decerebrate/control cats. On the other hand we observed consistent results in cats with chronic LSH exhibiting very strong distal to proximal pattern of inhibition from 2 weeks to 20 weeks following chronic LSH. The same results were obtained in acute LSH cats suggest that the change in neuromuscular system appears immediately after SCI and persists even after the animal start walking following SCI. The observed altered pattern of force feedback after spinal cord injury suggests either presence of a pattern intrinsic to the spinal cord or a unique pattern exhibited by the damaged spinal cord. The results are important clinically because even with vigorous rehabilitation attempts patients do not regain posture control after SCI even though they regain ability to walk. Therefore, to effectively administer treatment and therapy for patients with compromised posture control, a complete understanding of the circuitry is required.
Influencing motor behavior through constraint of lower limb movement

(Georgia Institute of Technology, 2015-04-29) Hovorka, Christopher Francis

Limited knowledge of the neuromechanical response to use of an ankle foot orthosis-footwear combination (AFO-FC) has created a lack of consensus in understanding orthotic motion control as a therapeutic treatment. Lack of consensus may hinder the clinician’s ability to target the motion control needs of persons with movement impairment (e.g., peripheral nerve injury, stroke, etc.). Some evidence suggests a proportional relationship between joint motion and neuromuscular activity based on the notion that use of lower limb orthoses that constrain joint motion may invoke motor slacking and decreasing levels of muscle activity. Use of AFO-FCs likely alters the biomechanical and neuromuscular output as the central control system gradually forms new movement patterns. If there is proportional relationship between muscle activation and joint motion, then it could be examined by quantifying joint motion and subsequent neuromuscular output. Considering principles of neuromechanical adjustment, my general hypothesis examines whether orthotic control of lower limb motion alters neuromuscular output in proportion to the biomechanical output as a representation of the limb’s dynamics are updated by the neural control system. The rationale for this approach is that reference knowledge of the neuromechanical response is needed to inform clinicians about how a person responds to walking with motion controlling devices such as ankle foot orthoses combined with footwear. In the first line of research, I hypothesize that a newly developed AFO which maximizes leverage and stiffness will constrain the talocrural joint and alter joint kinematics and ground reaction force patterns. To answer the hypothesis, I sampled kinematics and kinetics of healthy subjects’ treadmill walking using an AFO-FC in a STOP condition and confirmed that the AFO substantially limited the range of talocrural plantarflexion and dorsiflexion motion to 3.7° and in a FREE condition maintained talocrural motion to 24.2° compared to 27.7° in a CONTROL (no AFO) condition. A follow up controlled static loading study sampled kinematics of matched healthy subjects limbs and cadaveric limbs in the AFO STOP and FREE conditions. Findings revealed healthy and cadaveric limbs in the AFO STOP condition substantially limited their limb segment motion similar to matched healthy subjects walking in the STOP condition and in the AFO FREE condition healthy and cadaveric limbs maintained similar limb segment motion to matched healthy subjects walking in the FREE condition. In a second line of research, I hypothesize that flexibility of a newly developed footwear system will allow normal walking kinetics due to the shape and flexibility of the footwear. To answer the hypothesis, I utilized a curved-flexible footwear system integrated with an AFO in a STOP condition and sampled kinematics and kinetics of healthy subjects during treadmill walking. Results revealed subjects elicited similar cadence, stance and swing duration and effective leg-ankle-foot roll over radius compared to walking in the curved-flexible footwear integrated with the AFO in a FREE condition and a CONTROL (no AFO) condition. To validate rollover dynamics of the curved-flexible footwear system, a follow up study of healthy subjects’ treadmill walking in newly developed flat-rigid footwear system integrated with the AFO in a STOP condition revealed interrupted leg-ankle-foot rollover compared to walking in curved-flexible footwear in STOP, FREE and CONTROL conditions. In a third line of research, I hypothesize that use of an AFO that limits talocrural motion in a STOP condition will proportionally reduce activation of Tibialis Anterior, Soleus, Medial and Lateral Gastrocnemii muscles compared to a FREE and CONTROL condition due to alterations in length dependent representation of the limb’s dynamics undergoing updates to the central control system that modify the pattern of motor output. To answer the question, the same subjects and AFO-footwear presented in the first two lines of research were used in a treadmill walking protocol in STOP, FREE, and CONTROL conditions. Findings revealed the same subjects and ipsilateral AFO-footwear system presented in Aim 1 exhibited an immediate yet moderate 30% decline in EMG activity of ipsilateral Soleus (SOL), Medial Gastrocnemius (MG) and Lateral Gastrocnemius (LG) muscles in the STOP condition compared to the CONTROL condition. The reduction in EMG activity in ipsilateral SOL, MG and LG muscles continued to gradually decline during 15 minutes of treadmill walking. On the contralateral leg, there was an immediate yet small increase of 1% to 14% in EMG activity in SOL, MG, LG muscles above baseline. After 10 minutes of walking, the EMG activity in contralateral SOL, MG and LG declined to a baseline level similar to the EMG activity in the contralateral CONTROL condition. These collective findings provide compelling evidence that the moderate 30% reduction in muscle activation exhibited by subjects as they experience substantial (85%) constraint of total talocrural motion in the AFO STOP condition is not proportionally equivalent. Further, the immediate decrease in muscle activation may be due to a reactive feedback mechanism whereas the continued decline may in part be explained by a feedforward mechanism. The clinical relevance of these findings suggests that short term use of orthotic constraint of talocrural motion in healthy subjects does not substantially reduce muscle activation. These preliminary findings could be used to inform the development of orthoses and footwear as therapeutic motion control treatments in the development of motor rehabilitation protocols.
Understanding the neurophysiology of action interpretation in right and left-handed individuals

(Georgia Institute of Technology, 2015-04-08) Kelly, Rachel Louise

Investigating the neurophysiology behind our action encoding system offers a way of probing the underlying mechanisms regarding how we understand seen action. The ability to mentally simulate action (motor simulation) is a strong proposal to understand how we interpret others’ actions. The process of how we generate accurate motor simulations is proposed to be reliant on the context of the movement and sensory feedback from the limb. However, the neurophysiological mechanisms behind motor simulation are not yet understood. Known motor physiology for right-handed individuals show there is a left parietal-frontal network for the mental simulation of skilled movements; however, it remains unclear whether this is due to right limb dominance of the observer’s motor system because action simulation research has been focused primarily on right-handed individuals. The goal of this dissertation is to understand the underlying neurophysiology of the motor simulation process during action encoding. Generally, we propose different strategies of action simulation between right and left handed individuals. More specifically, we propose that right-handed individuals rely on their motor dominant left hemisphere for action encoding and motor simulation, while left-handed individuals will rely on their motor dominant right hemisphere. We will test this by evaluating neurobehavioral patterns of potential symmetry and asymmetry of motor simulation and action encoding based on patterns of limb dominance. We will also evaluate how impaired sensory feedback affects motor simulations, which can reveal how limb state affects the simulation process. The results of this series of studies will fill a void in our basic understanding of the motor simulation process and may generalize to populations with upper limb functional loss. Specifically, those with different hand dominance may require different rehabilitation programs in order to retrain an affected limb.
Fatigue mechanisms in sedentary and endurance trained adults: effects of nutritional countermeasures

(Georgia Institute of Technology, 2015-04-08) O'Dea, Namrita Kumar

Physical inactivity is a major risk factor for chronic disease; yet the majority of the population does not meet physical activity recommendations, with fatigue being a primary underlying reason. Common nutritional supplements such as caffeine (CAF) and carbohydrate (CHO) are frequently taken prior to and during exercise and can delay fatigue, particularly in trained athletes; but, whether these same reductions in fatigue translate to habitually sedentary individuals is less clear. The main goal of this research was to investigate the influence of nutritional aids: CAF, low-calorie CHO, and their combination CAF+CHO to delay physical and mental fatigue in healthy but sedentary men and women compared to endurance-trained counterparts. The minimum amount of CHO that could elicit a benefit without eliciting a metabolic response when ingested remains unclear. Thus, our first aim was to evaluate glycemic response after ingestion of CHO drinks ranging from 0-6% CHO; and, to determine effects of two methods of CHO administration (ingest and oral rinse without ingestion) on sustained attention during a mentally fatiguing cognitive task. In agreement with evidence that “low calorie beverages” do not appear to alter blood glucose, a 0.4% CHO solution (<2 g CHO) elicited the most similar response to artificially sweetened placebo and was evaluated further. Compared to oral rinse of a small amount (<2 g) of CHO or control (0 g CHO), ingestion of an equivalent “dose” (<2 g CHO) in the 0.4% CHO solution maintained sustained attention, providing evidence that ingesting a low-CHO drink may be more effective than simply rinsing, in a fasted state. Oral rinse of CHO did not, however, provide any benefit compared to rinsing a control solution. Our second aim was to evaluate the efficacy of ingesting a moderate dose of CAF (3 mg/kg), low-CHO, and the combination of CAF+CHO on exercise capacity in trained (ET) versus sedentary (SED) groups. As expected, CAF reduced perceived effort during exercise and increased endurance capacity (longer total time to volitional fatigue and longer duration before rating vigorous intensity exercise as “very hard”), for ET and SED. However, addition of <2 g CHO did not provide further improvement with CAF; or any benefit when ingested alone, compared to placebo. CAF and CHO did not influence blood glucose but CAF resulted in higher blood lactate compared to no-CAF. As expected, ET had higher fat oxidation than SED; and, CAF increased CHO oxidation but not fat oxidation. CAF tended to maintain maximal voluntary contractile (MVC) strength in the quadriceps after exercise; but, CAF did not influence voluntary muscle activation or appear to have a direct effect on skeletal muscle since electrically evoked strength was not altered with CAF. Since acute exercise also benefits cognition independent of CAF or CHO, our third aim was to determine: (1) the effect of moderate intensity exercise (MOD-EX) on sustained attention in comparison to seated rest; and 2) whether CAF provides additional benefit to sustained attention and perceptual measures when combined with exercise. As expected, compared to an equivalent duration of rest, MOD-EX improved sustained attention following mental fatigue in both groups; and, when coupled with CAF, provided greater benefit to sustained attention and perceived mental energy. Although CAF’s beneficial effect on sustained attention persisted after exercise to volitional fatigue, it did not improve perceptual measures of reduced mental energy and increased mental fatigue at the point of physical fatigue. Our research investigated nutritional aids that are not only beneficial for athletes, but also widely consumed by the general population despite their lack of regular physical activity. In response to public health recommendations for reducing sugar intake, low-sugar/low-calorie drinks have been heavily marketed to the population despite much research to support their efficacy. Our findings suggest that low-CHO can be ergogenic during a sedentary mental task in a fasted state, perhaps through central mechanisms. Although CHO mouth rinse is evidenced to be efficacious for exercise performance without ingestion, we did not find that ingestion of low-CHO provides any significant benefit during moderate to vigorous exercise following a small meal. However, a moderate dose of CAF was ergogenic for both ET and SED. Future work should not only investigate additional exercise interventions; but also different CHO and CAF doses and administration protocols, and their relative metabolic implications.
Adaptation of locomotor control in able and impaired human walking

(Georgia Institute of Technology, 2014-06-13) Toney, Megan

Extensive research has documented the stereotypical kinematic and kinetic patterns in healthy human walking, but we have a limited understanding of the neuromechanical control principles that contribute to their execution. Furthermore, the strategies used to adapt human walking to morphological or environmental constraints are poorly understood. After a traumatic injury, like amputation, regaining independent mobility is a primary goal of rehabilitation. Without a clear understanding of the neuromechanical principles governing locomotion, monitoring and quantitatively improving gait rehabilitation outcomes is challenging. The purpose of this doctoral work was to identify controlled variables in able and impaired human walking and to compare the control strategies used to adapt to a novel walking environment both with and without amputation. I apply an uncontrolled manifold (UCM) analysis to test whether likely goal variables of human walking are selectively stabilized through step-to-step variability structure. I found that both able-bodied subjects and subjects with an amputation maintain consistent whole body dynamics and leg power production by exploiting inherent motor abundance. Consistent leg power production is accomplished primarily through step-to-step leg force corrections that are driven by variable timing of ankle torque production. Covariance between ankle and knee torques enable robust motor control in able-bodied individuals, but this stabilizing mechanism is absent in individuals with a transtibial amputation. This coordinated joint torque control also appears to assist able-bodied short-term adaptation, invoked by split-belt treadmill walking. However, loss of ankle motor control and distal sensory feedback due to amputation appears to limit reactive, feedback driven adaptation patterns in subjects with an amputation. Ultimately, this work highlights the role of intact distal sensorimotor function in locomotor control and adaptation. The major findings I present have substantial implications for gait rehabilitation and prosthetic design.
Promoting enhanced motor planning in prosthesis users via matched limb imitation

(Georgia Institute of Technology, 2014-03-13) Cusack, William Fitzpatrick

As of 2005, there were over 1.5 million amputees living in the United States, more than 548,000 of them with upper extremity involvement. The total number of amputees is projected to rise to at least 2.2 million by 2020. Unfortunately, full functional use of upper extremity prosthetic devices is low. Knowledge gained regarding the cortical systems active in amputees performing motor tasks may reveal atypical motor control strategies that contribute to these issues. Substantial evidence demonstrates a strong dependence on left parietofrontal cortical areas to successfully plan and execute tool-use movements and pantomimes. It was previously unclear how this network functioned in users of prostheses. The hypothesis of this dissertation is that in order to optimally engage the typical parietofrontal network during action imitation with a prosthetic device, the action being imitated should be performed by a matching prosthesis. Also, that greater engagement of the parietofrontal network will result in increased ability to perform tool-use movements. First, this dissertation showed that when imitating motor tasks performed by intact actors, prosthesis users exhibit lower engagement of the parietofrontal action encoding system. This network is crucial for motor adaptation. Left parietofrontal engagement was only observed when prosthesis users imitated matched limb prosthesis demonstrations, which suggests that matched limb imitation may be optimal to establish motor representations. Next, intact subjects donned a fictive amputee model system (FAMS) to simulate the limb movement that transradial amputees experience. Matched limb imitation in FAMS users yielded better movement technique compared to mismatched imitation. Finally, the longitudinal effects of a matched limb training paradigm on the cortical action encoding activity and motor behavior in FAMS users were investigated. Matched limb imitation subjects showed greater engagement of the parietofrontal network and better movement technique compared to those trained with mismatched limb. This dissertation has clinical relevance as it supports the notion that matched limb imitation could play an important role in the performance of motor tasks using a prosthetic device. These findings could be used to inform the development of improved rehabilitation protocols that may lead to greater functional adaptation of prosthetic devices into the lives of amputees.
The acute effects of physical activity on the stiffness of the plantar skin of people with and without diabetes

(Georgia Institute of Technology, 2013-11-18) Wendland, Deborah Michael

Diabetes affects 25.8 million Americans. Complications related to this growing disease impact public health. One secondary complication of diabetes is changes in skin that can contribute to an increased risk for ulceration. Skin of people with diabetes has not been characterized over time nor has the skin’s acute response to exercise been assessed. The objective of this project was to establish the changes in skin properties over time, within different ambient environments, and after acute exercise. This objective sought to address the central hypothesis that skin will demonstrate decreased stiffness and increased elasticity as a result of acute physical activity. Skin stiffness, compliance, and thickness measurements of the plantar foot were compared across time and environment. Skin stiffness and compliance were also compared before and after treadmill walking. First, three devices were validated. Accuracy of the StepWatch was validated for people using assistive devices. The tissue interrogation device (TID), a novel device that measures tangential skin stiffness, and the myotonometer, which measures skin compliance, were validated using elastomer phantoms. Both were found suitable to measure plantar skin properties. Second, skin properties of 16 persons with and without diabetes were measured over time and environmental condition. Skin was variable across subjects over time, but was stable within subjects over a month, supporting the use of a repeated measures approach to interventional study on the plantar skin in people with diabetes. Previous findings for general skin characteristics were supported including the tendency for persons with diabetes to have a thinner epidermis and a thicker dermis than persons without diabetes. Tangential skin stiffness was determined to be less stiff in people with diabetes when measured in a medial-lateral direction. People with diabetes had lower tissue compliance than those without. Skin properties varied across environmental condition, supporting the consideration of testing environment when evaluating skin. Finally, changes in skin properties were evaluated in 32 persons with diabetes before and after treadmill (TM) walking. Using the TID, skin stiffness (tangential) at the great toe of people with diabetes (663.705±4.796 N/m) and without (647.753±5.328 N/m) were different (p=0.040). Stiffness immediately following TM walking did not differ from pre-walking stiffness, but subsequent trials had increased stiffness. Similar, but not significant responses were noted at the first metatarsal head. Compliance using normal loading increased after walking with statistical differences lasting 30-60 minutes.
Task dependent effects of baroreceptor unloading on motor cortical and corticospinal pathways

(Georgia Institute of Technology, 2013-08-27) Buharin, Vasiliy E.

Corticospinal and intracortical excitability are excitability measures of the central nervous system responsible for motor generation, and are studied for their contribution to fine motor skill execution and learning. Since the need for proper execution of fine motor skills is ever-present and necessary for everyday life, identification of physiological pathways that may disrupt or enhance corticospinal and intracortical excitability is an important research topic. This thesis investigates the effects of baroreceptor unloading on corticospinal and intracortical excitability during various motor tasks. Baroreceptor unloading is a physiological response to common hemodynamic stress (e.g. hypovolemia and orthostasis). The motor tasks investigated are complete muscular relaxation, individual isometric low-force contraction of a muscle, and an isometric co-contraction of a muscle in a joint-stabilizing task. The effects of baroreceptor unloading on corticospinal and intracortical excitability appear to be very task specific. The results are discussed in view of available pharmacological and physiological research, and potential neural pathways for the observed effects are suggested. The overall conclusion is that baroreceptor unloading increases corticospinal excitability and decreases intracortical inhibition in a resting muscle, does not produce any observable effects during individual muscle activity, and decreases corticospinal excitability during joint-stabilizing co-contraction.