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.

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.

[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.

[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.

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.

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.

Kinesthetic reference for human orthograde posture.

Humans with occluded vision were subjected to superslow tilts of the supporting platform, producing the inclination of the subject's body in the sagittal plane, but subthreshold for the most vestibular and proprioceptive phasic reactions. Two types of perturbation were used: sinusoidal tilts (frequency 0.007 Hz, amplitude 1.5 degrees) and ramps (amplitude 1.0 and 0.25 degrees, angular velocity 0.04 degrees/s). During slow sinusoidal tilts of the platform, the ankle angle and body position undergo periodical changes, but these changes have significant phase lead relative to the platform movement: 119 +/- 26 for ankle angle and 55 +/- 19 degrees for body sway. Gains were about 0.9 for both parameters. Large phase shift (tens of seconds) indicated a long delay in compensation of body inclination by ankle joint. The ramp tilt produced an initial body deviation followed by a slow (seconds or tens of seconds) approach of body position to a new steady level after the termination of ramp. Large slow body movements were superimposed with small irregular oscillations (about 10% of the amplitude of large displacements) of higher frequency. These oscillations resembled normal stabilograms on a stationary support. Thus, the usual process of stabilization of body gravity center was continued, though not around a fixed set-point but relative to a slowly changing position. Data obtained support the hypothesis that, besides operative control assigned to compensate deviations from a reference position, the system of postural control includes at least one additional level, which elaborates this reference using information about mutual position of body links, muscular torques and interaction with the support on the basis of criteria taking into account the energy cost of standing and demands for stability and security.

Muscle resistance to slow ramp weakly depends on activation level.

The mechanical response of human m. flexor pollicis longus to slow (3.2 degrees/s) linear stretch by 5.5 degrees was measured during sustained (45-60 s, 9-13.5 p.p.s.) unfused tetanus evoked by electrical stimulation. The stiffness increased during unfused tetanus. At the late phase of unfused tetanus it was 1.8 +/- 0.2 (mean +/- S.D.) times greater than at the early phase. The sensitivity of the isometric tension level to a short change in a stimulation frequency also increased. At the late phase of unfused tetanus force oscillations increased 1.2 +/- 0.2-fold during slow stretch or shortening and immediately reached a smaller amplitude after the cessation of length change. This was probably related to the friction and thixotropy in muscles. Muscle resistance to slow ramp depended only weakly on activation level. In the late phase of unfused tetanus the stiffness per unit force was 1.5 +/- 0.4 times greater at 9-13.5 p.p.s. than at 20-25 p.p.s. Thus, the relative value of muscle stiffness was greater for smaller activation levels typical for maintenance of posture. The enhancement of muscle stiffness during sustained unfused tetanus and a weak stiffness dependence on the activation level indicated a non-additivity of processes occurring in active muscle.

The influence of head rotation on human upright posture during balanced bilateral vibration.

During a balanced bilateral vibration of tensor fasciae latae muscles evoking no evident postural changes in a standing human the slow head turns resulted in a pronounced lateral common gravity centre displacements to the 'occipital' side. Neck influences were most prominent at intermediate tonic background and were lacking both during very weak and strong vibratory stimulation in contrast to usual tonic vibratory reflex which becomes stronger with the increase of frequency. Body sway was induced not only by actual head turns but also by illusory ones evoked by neck muscle vibration. The neck influences on standing posture are therefore present in healthy adults, but they require a definite level of tonic readiness for their manifestation and are incorporated in the whole system of body scheme mechanisms.

Response to transient disturbances during intentional forearm flexion in man.

Steps of torque were applied to the human forearm during intentional flexion movements with peak velocities of 40-500 degrees/sec and during ballistically initiated flexion movements. The question asked was whether or not the motor ouput to both agonist and antagonist muscles is independent of sensory input data or is accessible and influenced by these inputs. The data obtained demonstrate unequivocally that the applied perturbation affects the motor output, irrespective of the speed of the movement. The time of onset of the reflex changes in motor output and their maxima are related to the movement's parameters. Data are presented which indicate that the source for the reflex changes observed are the muscle spindles.

Anticipatory neck muscle activity associated with rapid arm movements.

This study reveals the existence of a backward acceleration of the head prior to the onset of voluntary raising movements of the upper limb. This backward acceleration is produced by the displacement of the head-trunk as a whole. The anticipatory head movement is organized according to a sequence of activation and desactivation of the neck muscles, time locked with the anticipatory leg muscle activity. These findings highlight the existence of a complex postural behavior selected in advance of movement. It is proposed that the feedforward type of neural control of neck muscles should not be interpreted as a compensation to postural perturbation. This anticipatory process might play an important role in the widespread postural fixation of the cervical and dorsal spine.

Role of Ia muscle spindle afferents in post-contraction and post-vibration motor effect genesis.

Experiments carried out on 14 human subjects showed that long-lasting involuntary tonic motor responses occurred after the offset of muscle vibration (70 Hz, 0.5 mm, duration 30 s). These post-vibratory biceps and triceps brachii motor responses were compared with the motor responses observed in the same subjects after performing an isometric contraction of the same duration, i.e., post-contraction responses, or the so-called 'Kohnstamm phenomenon'. The results show the existence of close similarities between these two types of motor after-effect, particularly as regards the muscle sites where they develop, their amplitudes and their temporal patterns (latencies and offset times). Neither type of excitatory post-effect can be elicited by co-contracting or co-stimulating two antagonist muscles at the same frequency. Lastly, visual stimulation can cause both types of motor response to switch from one muscle to its antagonist. Comparative analysis of the spindle proprioceptive activities recorded in response to either vibration or isometric contractions suggests that these motor after-effects may both result from the fact that the spindle afferents from agonist and antagonist muscles are asymmetrically activated in these two particular situations.

Muscular after-contraction and ongoing postural reactions in standing and sitting humans.

Muscular after-contraction (MA-C) and the ongoing postural reaction of the body was studied in standing and sitting subjects in two visual situations: eyes open or closed. EMG of trapezius and latissimus dorsalis and 3D kinematic recordings of the left scapula were analysed. The release of the long-lasting sustained isometric contraction at the level of the scapula produced a muscular after-contraction consisting of involuntary muscular contraction associated with a trunk movement similar to the unroll of a spiral. The unroll of the spiral is in the opposite direction when we compare the standing and sitting situations. We suppose that the muscular after-contraction reveals the activity of central tonigenic structures in evoking involuntary trunk movements in humans and stresses the importance of the initial postural situation (standing or sitting) and the visual condition in the characteristics of these involuntary movements.

Human equilibrium on unstable support: the importance of feet-support interaction.

Healthy humans maintained equilibrium on rocking supports (seesaw) of different curvatures and heights. We recorded platform tilt, horizontal displacements of the upper body, ankle joint angle and activity of ankle joint muscles. Subjects maintained balance by making seesaw rotations placing the support under the body's centre-of-gravity. Forward displacement was balanced by compensatory plantariflexion: thus the relation between muscle activity and ankle joint angle differed from that on a rigid floor. Mechanical analysis of stability showed that standing on low seesaws requires ankle torque increase during forward body shift (as on a rigid floor) and torque decrease on high seesaws (when the seesaw height exceeded its radius). In the latter case, balancing was impossible with eyes closed. The results suggest that directionally specific torque changes in response to centre-of-gravity shifts provide important information for maintenance of orthograde posture.

Some properties of linear relaxation in unfused tetanus of human muscle.

The linear relaxation (LR) was studied in isometric unfused tetanus (UT) of the human flexor digitorum sublimis muscle. With a decrease of the force level, the shoulder on the relaxation curve, as measured from the last stimulus, shifted to the right. The length of the linear portion itself weakly depended on activation level. When steady force changed from 100 to 40-50% of the maximum, the slope of LR decreased only by 15 +/- 4%. At smaller force levels the slope began to increase. LR can probably also be hidden in the twitch. With increased tetanus duration, LR becomes longer and slower at all force levels. LR was markedly diminished in contraction on the steep part of the exponential relaxation after a smooth tetanus. Its full recovery needed a train of 4-5 pulses (near 1 s) at the new stationary level. The form of the response to the additional pulse given during relaxation remained approximately constant during the most of LR portion and differed markedly before and after it. LR did not have direct relation to fatigue: in the first UT LR was always slower and longer than in subsequent ones.

The contribution of foot deformation to the changes of muscular length and angle in the ankle joint during standing in man.

This study was designed to evaluate the extent of foot deformation in healthy subjects during standing on an immobile support and during slow tilts of the support platform by 1 deg. The angle in ankle joint was evaluated by two methods: as an angle of skin inclination relative to the platform and as an angle, calculated on the basis of recording of the projective length of the soleus muscle. It was shown that the real changes of the angle in ankle joint during standing on an immobile platform were up to 2 times smaller than the changes of angular position of the shin relative to vertical axis. However, considerable intersubject variability was observed in this respect. During slow tilts of the support platform a marked divergence was observed in the shape of recordings of two "ankle angles" in subjects with high foot compliance. The vertical displacements of the calcaneus recorded by means of a clamp rigidly fixed at the heel were 0.5 +/- 0.3 mm (the range 0.1-1 mm) for each degree of body deviation in the forward or backward direction. In 12 subjects, the average foot compliance was 0.04 +/- 0.03 deg/Nm (maximal value 0.1 deg/Nm). It can be assumed that the mechanical properties of the foot can appreciably influence the afferent outflow during maintenance of orthograde posture in man.

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.