Biomedical subjects
Y P Ivanenko
Publications and source records attributed to Y P Ivanenko.
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.
The contribution of otoliths and semicircular canals to the perception of two-dimensional passive whole-body motion in humans.
1. Perception of two-dimensional (2-D) whole-body passive motion in the horizontal plane was studied in twelve blindfolded healthy volunteers: pure rotation in place (180 deg), linear motion (4.5 m) and a semicircular trajectory (radius, 1.5 m; angular acceleration, 0.2 rad s-2) were applied in random sequence by means of a remote-controlled robot equipped with a racing-car seat. The seat orientation in the horizontal plane was controlled by the experimenter, independent of the robot trajectory. Thus different degrees of otolith-canal interaction were obtained. The maximal linear acceleration during the semicircular trajectory was 0.1 g; however, the linear acceleration vector was complex as it rotated relative to the subject's head. 2. In the first of two sessions, subjects were instructed to maintain an angular pointer oriented towards a remote (15 m) previously seen target during the passive movements. In the second session they had to make a drawing of the path of the perceived trajectory, after the movement was finished. 3. The results showed that, on average, the movement of the pointer matched the dynamics of the rotatory component of the 2-D motion well. This suggests that, in the range of linear accelerations used in this study, no appreciable influence of otolith input on canal-mediated perception of angular motion occurred. 4. The curvature of the drawn paths was mostly explained by the input to the semicircular canals. Subjects' reconstruction of motion did not account for the directional dynamics of the input to the otoliths occurring during passive motion. 5. This finding proves that reconstructing trajectory in space does not imply a mathematically perfect transformation of the linear and angular motion-related inputs into a Cartesian or polar 2-D representation. Physiological constraints on the interaction between motion direction and change of heading play an important role in motion perception.
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.
Integration of somatosensory and vestibular inputs in perceiving the direction of passive whole-body motion.
We investigated the contribution of somatosensory and vestibular inputs in the detection of the direction of linear whole-body displacement (1.5 m) at low levels of linear acceleration (peak acceleration 0.2 m/s2), in normal subjects. Subjects sat on a mobile robot wearing opaque goggles and headphones. They indicated the direction of motion by using a laser pointer held by the right hand at the level of the chin. Adaptation to a long-lasting static head turn (45 degrees to the right) was used to modify the perceived head orientation relative to the trunk. After about 15 min the head and trunk were perceived to be aligned with each other. After adaptation subjects pointed in the same direction as in the control condition in spite of the change in the perception of the head orientation. Because space orientated reactions to vestibular stimuli were previously shown to be coded in the perceived head reference frame, these results indicate that somatosensory cues are also integrated in the perception of linear motion. Unexpectedly, after adaptation, trunk and head perceived orientations were attracted toward the direction of the imposed motion. This suggests that the internal representation of body configuration depends also upon available cues from the extrapersonal space.
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.
Adaptation as a sensorial profile in trait anxiety: a study with virtual reality.
Twenty-four volunteers were recruited on the basis of their trait anxiety scores (low trait anxiety [LTA] and high trait anxiety [HTA]) as assessed by the State-Trait Anxiety Inventory. Adaptation to conflicting visual-vestibular stimulation (VVS) was used to study integration of space-related multi-sensory information in trait anxiety. First, vestibular perception was assessed by rotating the blindfolded subjects about the vertical axis (horizontal plane rotations) on a remote-controlled mobile robot. The subjects were asked to indicate the perceived rotation by use of an angular pointer. Subjects were then immersed into the center of a visual virtual square room by means of a head-mounted display. They were asked to control the robot with a joystick in order to perform 90 degrees rotations in the virtual room. However, a gain of 0.5 was introduced between visual scene and robot rotation so that the subjects were submitted to a conflict situation in which the 90 degrees rotational visual input was concurrent with a 180 degrees vestibular input. After 45 min of training with the virtual reality display, subjects were tested again in total darkness in order to determine whether their vestibular system had been reset by the conflicting visual signals. We found significant differences in adaptation to VVS between HTA and LTA groups as well as between males and females. Subjects of the HTA group demonstrated larger adaptation than that of the LTA group. Males also showed a greater level of adaptation compared to females. Our results suggest greater visual dependence in HTA subjects. This might be important for understanding the mechanisms underlying pathological anxiety and particularly agoraphobia.