Case Studies in Neuroscience: A dissociation of balance and posture demonstrated by camptocormia.

Upright stance in humans requires an intricate exchange between the neural mechanisms that control balance and those that control posture; however, the distinction between these control systems is hard to discern in healthy subjects. By studying balance and postural control of a participant with camptocormia - an involuntary flexion of the trunk during standing that resolves when supine - a divergence between balance and postural control was revealed. A kinematic and kinetic investigation of standing and walking showed a stereotyped flexion of the upper body by almost 80° over a few minutes, and yet the participant's ability to control center of mass within the base of support and to compensate for external perturbations remained intact. This unique case also revealed the involvement of automatic, tonic control of the paraspinal muscles during standing and the effects of attention. Although strength was reduced and MRI showed a reduction in muscle mass, there was sufficient strength to maintain an upright posture under voluntary control and when using geste antagoniste maneuvers or "sensory tricks" from visual, auditory, and haptic biofeedback. Dual tasks that either increased or decreased the attention given to postural alignment would decrease or increase the postural flexion, respectively. The custom-made "twister" device that measured axial resistance to slow passive rotation revealed abnormalities in axial muscle tone distribution during standing. The results suggest that the disorder in this case was due to a disruption in the automatic, tonic drive to the postural muscles and that myogenic changes were secondary. NEW & NOTEWORTHY By studying an idiopathic camptocormia case with a detailed biomechanical and sensorimotor approach, we have demonstrated unique insights into the neural control of human bipedalism 1) balance and postural control cannot be considered the same neural process, as there is a stereotyped abnormal flexed posture, without balance deficits, associated with camptocormia, and 2) posture during standing is controlled by automatic axial tone but "sensory tricks" involving sensory biofeedback to direct voluntary attention to postural alignment can override, when required.

Human cervical spinal cord circuitry activated by tonic input can generate rhythmic arm movements.

The coordination between arms and legs during human locomotion shares many features with that in quadrupeds, yet there is limited evidence for the central pattern generator for the upper limbs in humans. Here we investigated whether different types of tonic stimulation, previously used for eliciting stepping-like leg movements, may evoke nonvoluntary rhythmic arm movements. Twenty healthy subjects participated in this study. The subject was lying on the side, the trunk was fixed, and all four limbs were suspended in a gravity neutral position, allowing unrestricted low-friction limb movements in the horizontal plane. The results showed that peripheral sensory stimulation (continuous muscle vibration) and central tonic activation (postcontraction state of neuronal networks following a long-lasting isometric voluntary effort, Kohnstamm phenomenon) could evoke nonvoluntary rhythmic arm movements in most subjects. In ∼40% of subjects, tonic stimulation elicited nonvoluntary rhythmic arm movements together with rhythmic movements of suspended legs. The fact that not all participants exhibited nonvoluntary limb oscillations may reflect interindividual differences in responsiveness of spinal pattern generation circuitry to its activation. The occurrence and the characteristics of induced movements highlight the rhythmogenesis capacity of cervical neuronal circuitries, complementing the growing body of work on the quadrupedal nature of human gait.

Trunk orientation, stability, and quadrupedalism.

Interesting cases of human quadrupedalism described by Tan and Colleagues (2005-2012) have attracted the attention of geneticists, neurologists, and anthropologists. Since his first publications in 2005, the main attention has focused on the genetic aspects of disorders that lead to quadrupedalism within an evolutionary framework. In recent years this area has undergone a convincing critique (Downey, 2010) and ended with a call "… to move in a different direction … away from thinking solely in terms of genetic abnormality and evolutionary atavism." We consider quadrupedalism as a "natural experiment" that may contribute to our knowledge of the physiological mechanisms underlying our balance system and our tendency toward normal (upright) posture. Bipedalism necessitates a number of characteristics that distinguish us from our ancestors and present-day mammals, including: size and shape of the bones of the foot, structure of the axial and proximal musculature, and the orientation of the human body and head. In this review we address the results of experimental studies on the mechanisms that stabilize the body in healthy people, as well as how these mechanisms may be disturbed in various forms of clinical pathology. These disturbances are related primarily to automatic rather than voluntary control of posture and suggest that human quadrupedalism is a behavior that can result from adaptive processes triggered by disorders in postural tone and environmental cues. These results will serve as a starting point for comparing and contrasting bi- and quadrupedalism.

Lack of non-voluntary stepping responses in Parkinson's disease.

The majority of research and therapeutic actions in Parkinson's disease (PD) focus on the encephalic areas, however, the potential involvement of the spinal cord in its genesis has received little attention. Here we examined spinal locomotor circuitry activation in patients with PD using various types of central and peripheral tonic stimulation and compared results to those of age-matched controls. Subjects lay on their sides with both legs suspended, allowing low-friction horizontal rotation of the limb joints. Air-stepping can be used as a unique and important model for investigating human rhythmogenesis since its manifestation is largely facilitated by the absence of external resistance. In contrast to the frequent occurrence of non-voluntary stepping responses in healthy subjects, both peripheral (muscle vibration) and central (Jendrassik maneuver, mental task, Kohnstamm phenomenon) tonic influences had little if any effect on rhythmic leg responses in PD. On the other hand, a remarkable feature of voluntary air-stepping movements in patients was a significantly higher frequency of leg oscillations than in age-matched controls. A lack of non-voluntary stepping responses was also observed after dopaminergic treatment despite the presence of prominent shortening reactions (SRs) to passive movements. We argue that the state and the rhythmogenesis capacity of the spinal circuitry are impaired in patients with PD. In particular, the results suggest impaired central pattern generator (CPG) access by sensory and central activations.

Foot anatomy specialization for postural sensation and control.

Anthropological and biomechanical research suggests that the human foot evolved a unique design for propulsion and support. In theory, the arch and toes must play an important role, however, many postural studies tend to focus on the simple hinge action of the ankle joint. To investigate further the role of foot anatomy and sensorimotor control of posture, we quantified the deformation of the foot arch and studied the effects of local perturbations applied to the toes (TOE) or 1st/2nd metatarsals (MT) while standing. In sitting position, loading and lifting a 10-kg weight on the knee respectively lowered and raised the foot arch between 1 and 1.5 mm. Less than 50% of this change could be accounted for by plantar surface skin compression. During quiet standing, the foot arch probe and shin sway revealed a significant correlation, which shows that as the tibia tilts forward, the foot arch flattens and vice versa. During TOE and MT perturbations (a 2- to 6-mm upward shift of an appropriate part of the foot at 2.5 mm/s), electromyogram (EMG) measures of the tibialis anterior and gastrocnemius revealed notable changes, and the root-mean-square (RMS) variability of shin sway increased significantly, these increments being greater in the MT condition. The slow return of RMS to baseline level (>30 s) suggested that a very small perturbation changes the surface reference frame, which then takes time to reestablish. These findings show that rather than serving as a rigid base of support, the foot is compliant, in an active state, and sensitive to minute deformations. In conclusion, the architecture and physiology of the foot appear to contribute to the task of bipedal postural control with great sensitivity.

[Optimization of human walk speed at minimum energy expenditure--the function of leg own oscillation frequency on the transfer phase].

Equations were developed to determine the work of transferring the body gravity center for a unit mass in a path unit, and to estimate walk speed at a minimum energy expenditure. It was demonstrated that minimum energy expenditure can be achieved by walking at the pace that corresponds to leg own oscillation frequency.

Increased dynamic regulation of postural tone through Alexander Technique training.

Gurfinkel and colleagues (2006) recently found that healthy adults dynamically modulate postural muscle tone in the body axis during anti-gravity postural maintenance and that this modulation is inversely correlated with axial stiffness. Our objective in the present study was to investigate whether dynamic modulation of axial postural tone can change through training. We examined whether teachers of the Alexander Technique (AT), who undergo "long-term" (3-year) training, have greater modulation of axial postural tone than matched control subjects. In addition, we performed a longitudinal study on the effect of "short-term" (10-week) AT training on the axial postural tone of individuals with low back pain (LBP), since short term AT training has previously been shown to reduce LBP. Axial postural tone was quantified by measuring the resistance of the neck, trunk and hips to small (±10°), slow (1°/s) torsional rotation during stance. Modulation of tone was determined by the torsional resistance to rotation (peak-to-peak, phase-advance, and variability of torque) and axial muscle activity (EMG). Peak-to-peak torque was lower (∼50%), while phase-advance and cycle-to-cycle variability were enhanced for AT teachers compared to matched control subjects at all levels of the axis. In addition, LBP subjects decreased trunk and hip stiffness following short-term AT training compared to a control intervention. While changes in static levels of postural tone may have contributed to the reduced stiffness observed with the AT, our results suggest that dynamic modulation of postural tone can be enhanced through long-term training in the AT, which may constitute an important direction for therapeutic intervention.

Haptic touch reduces sway by increasing axial tone.

It is unclear how haptic touch with a stable surface reduces postural sway. We hypothesized that haptic input enhances postural stability due to alterations in axial postural tone. We measured the influence of heavy and light touch (LT) of the hands on a stable bar on axial postural tone and postural sway during stance in 14 healthy adults. A unique "Twister" device measured hip torque by fixing the upper body in space while oscillating the surface in yaw ±10 at 1 deg/s. Subjects were tested while: (1) standing quietly with their arms at their sides, (2) lightly touching a rigid bar in front of them and (3) firmly gripping the bar. Horizontal and vertical sway was not restricted by the device's yaw fixation, therefore, the subjects remained in a state of active postural control during the three touch conditions. Haptic touch significantly increased hip postural tone by 44% during light touch, from 2.5±0.9 to 3.6±1.0 Nm (P=0.005), and by 40% during firm grip to 3.5±0.8 Nm (P=0.005). Increases in hip postural tone were associated with a reduction in postural sway (r=-0.55, P=0.001). This is the first study showing that axial postural tone can be modified by remote somatosensory input and provides a potential explanation for how light touch improves postural stability. Changes in subjects' perception from trunk to surface rotation when changing from no touch (NT) to haptic touch, suggests that the CNS changes from using a global, to a local, trunk reference frame for control of posture during touch. The increase of hip postural tone during touching and gripping can be explained as a suppression of hip muscle shortening reactions that normally assist axial rotation.

[Biochemical profile of human-spacesuit interaction].

The article discusses and analyzes the issues of optimizing energy, kinematic and dynamic structures of the process of human-spacesuit system movement. Recommendations concerning system stabilization during posture acquisition and motion are made; the biomechanic requirement for spacesuit R&D is that joints with preset frequencies must be designed.

Axial kinesthesia is impaired in Parkinson's disease: effects of levodopa.

Integration of sensory and motor inputs has been shown to be impaired in appendicular muscles and joints of Parkinson's disease (PD) patients. As PD advances, axial symptoms such as gait and balance impairments appear, which often progresses to complete inability stand or walk unaided. The current study evaluates kinesthesia in the axial musculature of PD patients during active postural control to determine whether impairments similar to those found in the appendages are also present in the hip and trunk. Using axial twisting, we quantified the detection threshold and directional accuracy of the hip relative to the feet (i.e. Hip Kinesthesia) and the hip relative to the shoulders (i.e. Trunk Kinesthesia). The relation of kinesthetic threshold to disease progression as measured by UPDRS and the effect of levodopa treatment on kinesthesia were assessed in 12 PD compared to age-matched controls. Subjects stood unaided while passively twisted at a very low constant rotational velocity (1 degrees /s). The results showed that accuracy in determining the direction of axial twisting was reduced in PD relative to healthy control subjects in the hip (PD-ON: 81%; PD-OFF: 91%; CTL=96%) and trunk (PD-ON: 81%; PD-OFF: 88%; CTL=95%). Thresholds for perception of axial twisting were increased when PD subjects were ON levodopa versus OFF in both the hip (p<0.01) and the trunk (p<0.05). The magnitude of decrease in sensitivity due to being ON levodopa was significantly correlated with the increase in UPDRS motor scores (Hip: r=0.90, p<0.01 and Trunk: r=0.60, p<0.05). This effect was not significantly correlated with equivalent levodopa dosage. PD subjects with disease onset on the left side of their body showed significantly higher axial thresholds than subjects with right PD onset (p<0.05). In conclusion, deficits in axial kinesthesia seem to contribute to the functional impairments of posture and locomotion in PD. Although levodopa has been shown to improve appendicular kinesthesia, we observed the opposite in the body axis. These findings underscore the dissociable neurophysiological circuits and dopaminergic pathways that are known to innervate these functionally distinct muscle groups.

Tonic central and sensory stimuli facilitate involuntary air-stepping in humans.

Air-stepping can be used as a model for investigating rhythmogenesis and its interaction with sensory input. Here we show that it is possible to entrain involuntary rhythmic movement patterns in healthy humans by using different kinds of stimulation techniques. The subjects lay on their sides with one or both legs suspended, allowing low-friction horizontal rotation of the limb joints. To evoke involuntary stepping of the suspended leg, either we used continuous muscle vibration, electrical stimulation of the superficial peroneal or sural nerves, the Jendrassik maneuver, or we exploited the postcontraction state of neuronal networks (Kohnstamm phenomenon). The common feature across all stimulations was that they were tonic. Air-stepping could be elicited by most techniques in about 50% of subjects and involved prominent movements at the hip and the knee joint (approximately 40-70 degrees). Typically, however, the ankle joint was not involved. Minimal loading forces (4-25 N) applied constantly to the sole (using a long elastic cord) induced noticeable (approximately 5-20 degrees) ankle-joint-angle movements. The aftereffect of a voluntary long-lasting (30-s) contraction in the leg muscles featured alternating rhythmic leg movements that lasted for about 20-40 s, corresponding roughly to a typical duration of the postcontraction activity in static conditions. The Jendrassik maneuver per se did not evoke air-stepping. Nevertheless, it significantly prolonged rhythmic leg movements initiated manually by an experimenter or by a short (5-s) period of muscle vibration. Air-stepping of one leg could be evoked in both forward and backward directions with frequent spontaneous transitions, whereas involuntary alternating two-legged movements were more stable (no transitions). The hypothetical role of tonic influences, contact forces, and bilateral coordination in rhythmogenesis is discussed. The results overall demonstrated that nonspecific tonic drive may cause air-stepping and the characteristics and stability of the evoked pattern depended on the sensory input.

[Temporal pattern of walking on various training facilities under the conditions of the earth's and simulated lunar gravity].

Eight test-subjects participated in 120 treadmill tests (drive power of 10 and 85 kW) aimed to compare the walking patterns at 1 and reduced gravity. The temporal pattern of steps was noted to change significantly on the low-power treadmill. On the strength of convergence of calculated and experimental data the suggestion has been made that the leg transfer movement follows the pattern of spontaneous oscillations.

[Human walk in spacesuit as a self-oscillating process].

A series of 40 biomechanic and physiological tests of semi-rigid and flexible spacesuits as possible candidates for Moon explorations purposes were conducted with involvement of 20 volunteered subjects. Ability to walk in the spacesuits with the internal positive pressure of 0.4 kg/cm2 in the normal gravity was assessed simultaneously with energy expenditure for moving over preset distances. Also, mating of the leg movements with the spacesuit shell was investigated The longest distance test elicited the fact of acquisition of stable motor skills in the unusual circumstances. The acquired motor skills bring about restructuring of step kinematics and make equal knee flexures during leg transfer and stepping on platform (matching the angular movement of the spacesuit knee joint) to an accuracy of tenths of degree. This phenomenon is used by the authors as the ground for proposing a reasoned optimization of the walk pattern in spacesuits as a self-oscillating process.

Axial hypertonicity in Parkinson's disease: direct measurements of trunk and hip torque.

A cardinal feature of Parkinson's disease (PD) is muscle hypertonicity, i.e. rigidity. Little is known about the axial tone in PD or the relation of hypertonia to functional impairment. We quantified axial rigidity to assess its relation to motor symptoms as measured by UPDRS and determine whether rigidity is affected by levodopa treatment. Axial rigidity was measured in 12 PD and 14 age-matched controls by directly measuring torsional resistance of the longitudinal axis to twisting (+/-10 degrees ). Feet were rotated relative to fixed hips (Hip Tone) or feet and hips were rotated relative to fixed shoulders (Trunk Tone). To assess tonic activity only, low constant velocity rotation (1 degrees /s) and low acceleration (<12 degrees /s(2)) were used to avoid eliciting phasic sensorimotor responses. Subjects stood during testing without changing body orientation relative to gravity. Body parts fixed against rotation could translate laterally within the boundaries of normal postural sway, but could not rotate. PD OFF-medication had higher axial rigidity (p<0.05) in hips (5.07 N m) and trunk (5.30 N m) than controls (3.51 N m and 4.46 N m, respectively), which did not change with levodopa (p>0.10). Hip-to-trunk torque ratio was greater in PD than controls (p<0.05) and unchanged by levodopa (p=0.28). UPDRS scores were significantly correlated with hip rigidity for PD OFF-medication (r values=0.73, p<0.05). Torsional resistance to clockwise versus counter-clockwise axial rotation was more asymmetrical in PD than controls (p<0.05), however, there was no correspondence between direction of axial asymmetry and side of disease onset. In conclusion, these findings concerning hypertonicity may underlie functional impairments of posture and locomotion in PD. The absence of a levodopa effect on axial tone suggests that axial and appendicular tones are controlled by separate neural circuits.

Interaction of involuntary post-contraction activity with locomotor movements.

Involuntary post-contraction muscle activity may occur after performing a strong long-lasting (about 30 s) isometric muscle contraction (Kohnstamm phenomenon). Here we examined how this putative excitatory state may interact with a locomotor movement. The subjects stood upright and were asked to oppose a rotational force applied to the pelvis for about 30 s either in the clockwise or in the counterclockwise direction. After that, they were asked to perform various motor tasks with the eyes closed. During quiet standing, we observed an involuntary post-contraction torsion of the trunk. During walking, the post-contraction facilitatory effect of body torsion was not overridden by the voluntary activity, but instead significantly influenced the forward locomotor program such that subjects walked along a curved trajectory in the direction of the preceding torsion. In contrast, we did not observe any rotational component when subjects were asked to step in place. We conclude that the post-contraction rotational aftereffect does not transfer to just any motor task but apparently manifests itself in those movements that incorporate the activated axial muscle synergy or rotational component. We argue that central excitability changes following the voluntary effort may contribute to the phenomenon and highlight the role of tonic influences in fine-tuning of the spinal cord.

Effect of slow, small movement on the vibration-evoked kinesthetic illusion.

The study reported in this paper investigated how vibration-evoked illusions of joint rotation are influenced by slow (0.3 degrees /s), small (2-4 degrees ) passive rotation of the joint. Normal human adults (n=15) matched the perceived position of the left ("reference") arm with the right ("matching") arm while vibration (50 pps, 0.5 mm) was applied for 30 s to the relaxed triceps brachii of the reference arm. Both arms were constrained to rotate horizontally at the elbow. Three experimental conditions were investigated: (1) vibration of the stationary reference arm, (2) slow, small passive extension or flexion of the reference arm during vibration, and (3) slow, small passive extension or flexion of the reference arm without vibration. Triceps brachii vibration at 50 pps induced an illusion of elbow flexion. The movement illusion began after several seconds, relatively fast to begin with and gradually slowing down to a stop. On average, triceps vibration produced illusory motion at an average latency of 6.3 s, amplitude of 9.7 degrees , velocity of 0.6 degrees /s, and duration of 16.4 s. During vibration, slow, small ( approximately 0.3 degrees /s, 1.3 degrees ) passive rotations of the joint dramatically enhanced, stopped, or reversed the direction of illusory movement, depending on the direction of the passive joint rotation. However, the subjects' perceptions of these passive elbow rotations were exaggerated: 2-3 times the size of the actual movement. In the absence of vibration, the subjects accurately reproduced these passive joint rotations. We discuss whether the exaggerated perception of slow, small movement during vibration is better explained by contributions of non muscle spindle Ia afferents or by changes in the mechanical transmission of vibration to the receptor.

The effects of moving sound images on postural responses and the head rotation illusion in humans.

The results of pilot studies on the effects of sound images moving in the horizontal plane on poststimulus responses and the head rotation illusion are presented. These phenomena are demonstrated to occur.

The sit-up: complex kinematics and muscle activity in voluntary axial movement.

This paper describes the kinematics and muscle activity associated with the standard sit-up, as a first step in the investigation of complex motor coordination. Eight normal human subjects lay on a force table and performed at least 15 sit-ups, with the arms across the chest and the legs straight and unconstrained. Several subjects also performed sit-ups with an additional weight added to the head. Support surface forces were recorded to calculate the location of the center of pressure and center of gravity; conventional motion analysis was used to measure segmental positions; and surface EMG was recorded from eight muscles. While the sit-up consists of two serial components, 'trunk curling' and 'footward pelvic rotation', it can be further subdivided into five phases, based on the kinematics. Phases I and II comprise trunk curling. Phase I consists of neck and upper trunk flexion, and phase II consists of lumbar trunk lifting. Phase II corresponds to the point of peak muscle contraction and maximum postural instability, the 'critical point' of the sit-up. Phases III-V comprise footward pelvic rotation. Phase III begins with pelvic rotation towards the feet, phase IV with leg lowering, and phase V with contact between the legs and the support surface. The overall pattern of muscle activity was complex with times of EMG onset, peak activity, offset, and duration differing for different muscles. This complex pattern changed qualitatively from one phase to the next, suggesting that the roles of different muscles and, as a consequence, the overall form of coordination, change during the sit-up.