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  • 1
    Digitale Medien
    Digitale Medien
    Visual influence on human locomotion Modulation to changes in optic flow (1997)
    Springer
    Experimental brain research 114 (1997), S. 63-70 
    Details
    ISSN: 1432-1106
    Schlagwort(e): Key words Vision ; Locomotion ; Optic Flow Adaptation ; Human
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Medizin
    Notizen: Abstract  The effect of an optic flow pattern on human locomotion was studied in subjects walking on a self-driven treadmill. During walking an optic flow pattern was presented, which gave subjects the illusion of walking in a tunnel. Visual stimulation was achieved by a closed-loop system in which optic flow and treadmill velocity were automatically adjusted to the intended walking velocity (WV). Subjects were instructed to keep their WV constant. The presented optic flow velocity was sinusoidally varied relative to the WV with a cycle period of 2 min. The independent variable was the relative optic flow (rOF), ranging from −1, i.e., forward flow of equal velocity as the WV, and 3, i.e., backward flow 3 times faster than WV. All subjects were affected by rOF in a similar way. The results showed, firstly, an increase in stride-cycle variability that suggests a larger instability of the walking pattern than in treadmill walking without optic flow; and, secondly, a significant modulating effect of rOF on the self-chosen WV. Backward flow resulted in a decrease, whereas forward flow induced an increase of WV. Within the analyzed range, a linear relationship was found between rOF and WV. Thirdly, WV-related modulations in stride length (SL) and stride frequency (SF) were different from normal walking: whereas in the latter a change in WV is characterized by a stable linear relationship between SL and SF (i.e., an approximately constant SL to SF ratio), optic flow-induced changes in WV are closely related to a modulation of SL (i.e., a change of SL-SF ratio). Fourthly, this effect of rOF diminished by about 45% over the entire walking distance of 800 m. The results suggest that the adjustment of WV is the result of a summation of visual and leg-proprioceptive velocity informations. Visual information about ego-motion leads to an unintentional modulation of WV by affecting specifically the relationship between SL and SF. It is hypothesized that the space-related parameter (SL) is influenced by visually perceived motion information, whereas the temporal parameter (SF) remains stable. The adaptation over the entire walking distance suggests that a shift from visual to leg-proprioceptive control takes place.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/PL00005624
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  • 2
    Digitale Medien
    Digitale Medien
    Developmental aspects of stance regulation, compensation and adaptation (1992)
    Springer
    Experimental brain research 90 (1992), S. 610-619 
    Details
    ISSN: 1432-1106
    Schlagwort(e): Stance regulation ; Adaptational processes ; Developmental aspects ; Load receptors ; Human
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Medizin
    Notizen: Summary Recordings of electromyographic (EMG) leg muscle activity, head and joint movements and platform torque were taken in healthy subjects within three age groups (approximately 6, 10 and 〉22 years) standing upright upon a sinusoidally moving treadmill. The sinusoidal frequency was randomly changed between 0.5, 0.33 and 0.25 Hz, while the amplitude of the deflection was constant (±12 cm). During an adapted sinus, forward inclination of the body at the posterior turning point was associated with a slowly increasing tibialis anterior and decreasing gastrocnemius activity, while straightening of the body at the anterior turning point was associated with a sharply increasing gastrocnemius and decreasing tibialis anterior activity. The angle of forward inclination was greatest in the groups of children and was dependent upon both the sinus frequency and the child's height. The presumed programmed adjustment of the body inclination was such that the net effect of both inertial and gravitational forces acting on the body coincided approximately with the axis of the body at the posterior turning point. Changes of sinusoidal frequency were followed by compensatory responses, the amplitude of which depended upon the velocity of the body's displacement and the height of the subjects. In all three subject groups the response latencies were significantly shorter at the posterior turning point for the gastrocnemius response to a change from 0.5 to 0.25 Hz (105 ms for children and 119 ms for adults) than for the tibialis anterior response to a change from 0.25 to 0.5 Hz for which the values were 162 and 169 ms, respectively. This difference could be attributed to the forward inclination of the body at the posterior turning point which requires an earlier onset of compensatory extensor activity in order to maintain equilibrium. Adaptation to a new sinusoidal frequency occurred within 4 cycles following a change in sinus frequency. The phase shifts between treadmill position and the biomechanical and EMG signals that occurred during the adaptational process suggest that the position of the body's centre of gravity is the variable controlled by the programmed leg muscle activation. In young children the phase shifts during adaptation were absent, which may contribute to their greater instability. It is concluded that posture is continually adjusted in such a way that the resulting torque acting on the body during the treadmill movement becomes minimized. For this regulation load receptors in addition to the classical afferent impulses from visual, vestibular and muscle stretch receptors could play a major role.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00230945
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  • 3
    Digitale Medien
    Digitale Medien
    Adaptational and learning processes during human split-belt locomotion: interaction between central mechanisms and afferent input (1995)
    Springer
    Experimental brain research 106 (1995), S. 449-456 
    Details
    ISSN: 1432-1106
    Schlagwort(e): Split-belt locomotion ; Interlimb coordination ; Adaptation ; Motor learning ; Human
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Medizin
    Notizen: Abstract Split-belt locomotion (i.e., walking with unequal leg speeds) requires a rapid adaptation of biome-chanical parameters and therefore of leg muscle electromyographic (EMG) activity. This adaptational process during the first strides of asymmetric gait as well as learning effects induced by repetition were studied in 11 healthy volunteers. Subjects were switched from slow (0.5 m/s) symmetric gait to split-belt locomotion with speeds of 0.5 m/s and 1.5 m/s, respectively. All subjects were observed to adapt in a similar way: (1) during the first trial, adaptation required about 12–15 strides. This was achieved by an increase in stride cycle duration, i.e., an increase in swing duration on the fast side and an increase in support duration on the slow side. (2) Adaptation of leg extensor and flexor EMG activity paralleled the changes of biomechanical parameters. During the first strides, muscle activity was enhanced with no increase in coactivity of antagonistic leg muscles. (3) A motor learning effect was seen when the same paradigm was repeated a few minutes later — interrupted by symmetric locomotion — as adaptation to the split-belt speeds was achieved within 1–3 strides. (4) This short-time learning effect did not occur in the “mirror” condition when the slow and fast sides were inverted. In this case adaptation again required 12–15 strides. A close link between central and proprioceptive mechanisms of interlimb coordination is suggested to underlie the adaptational processes during split-belt conditions. It can be assumed that, as in quadrupedal locomotion of the cat, human bipedal locomotion involves separate locomotor generators to provide the flexibility demanded. The present results suggest that side-specific proprioceptive information regarding the dynamics of the movement is necessary to adjust the centrally generated locomotor activity for both legs to the actual needs for controlled locomotion. Although the required pattern is quickly learned, this learning effect cannot be transferred to the contralateral side.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00231067
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  • 4
    Digitale Medien
    Digitale Medien
    Human stance on a sinusoidally translating platform: balance control by feedforward and feedback mechanisms (1993)
    Springer
    Experimental brain research 93 (1993), S. 352-362 
    Details
    ISSN: 1432-1106
    Schlagwort(e): Muscle receptor ; Motor control ; Electromyogram ; Stance ; Human
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Medizin
    Notizen: Abstract With subjects standing on a treadmill moving sinusoidally backward and forward, recordings of electromyographic (EMG) leg and trunk muscle activity, head and joint movements and platform torque were made with the subjects' eyes open or closed. The sinusoidal frequency was changed, stepwise and randomly, between 0.5, 0.3 and 0.25 Hz. The amplitude of the deflection was constant at ±12 cm. During an adapted sinus cycle, the maximum leg muscle EMG activity was recorded in the tibialis anterior around the posterior turning point and in the gastrocnemius around the anterior turning point in the treadmill cycle. This activity was associated with a forward inclination of the body around the posterior point and a straightening of the body at the anterior point. Both the degree of body inclination and the corresponding EMG activity were dependent upon the sinusoidal frequency. The programmed adjustment of the body inclination was such that the result of inertial and gravitational forces acting on the body coincided with the axis of the body at the posterior turning point. At the anterior point, the adjustment was achieved mainly by strong activation of the leg extensors. The latencies of the compensatory muscle responses to a change in treadmill frequency were significantly shorter at the posterior point for the gastrocnemius than for the tibialis anterior, and at the anterior point for the tibialis anterior than for the gastrocnemius. No correlated changes were seen in the corresponding head and joint movements. The difference in latency can best be attributed to the different body postures during the sinusoid. Early activation of the gastrocnemius is required due to the forward-directed impulse to the inclined body at the posterior point, and of the tibialis anterior muscle due to the backward-directed impulse to the erect body at the anterior point. It is suggested that afferent input from extensor load receptors provides information about the position of the body's centre of gravity relative to the support surface and determines the generation of the EMG responses. Adaptation of both the EMG and biomechanical patterns to a new sinusoidal frequency of the treadmill occurred within four cycles after the change. Biomechanically, this was reflected as a change in the body posture. Vision did not significantly affect these changes. In conclusion, standing on a sinusoidally moving platform, the nervous system acts to control the position of the body's centre of gravity relative to the feet. Body posture is adjusted in such a way that the forces acting on the body during the treadmill movements become minimised. After adaptation, body equilibrium becomes predominantly controlled by positive feedback from programmed leg muscle activation.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00228405
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  • 5
    Digitale Medien
    Digitale Medien
    Gating of sensation and evoked potentials following foot stimulation during human gait (1990)
    Springer
    Experimental brain research 105 (1990), S. 423-431 
    Details
    ISSN: 1432-1106
    Schlagwort(e): Gait ; Sural nerve ; Sensory gating ; Somatosensory evoked potential ; Human
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Medizin
    Notizen: Abstract To investigate how gait influences the perceived intensity of cutaneous input from the skin of the foot, the tibial or sural nerves were stimulated at the ankle during walking or running on a treadmill. As compared to standing, the detection threshold for these stimuli was raised by more than 30% during the locomotion tasks. During walking, there was a phase-dependent modulation in perceived intensity of suprathreshold stimuli (1.5, 2, or 2.5×PT). Stimuli given just prior to footfall were perceived as significantly above average (Wilcoxon signed-rank test). In contrast there was a significant phasic decrease in sensitivity for shocks delivered immediately after ipsi- and contralateral footfall. The amplitude of somatosensory evoked potentials (P50–N80 complex), simultaneously evoked from pulse trains to the sural nerve and recorded at scalp level, was, on average, 62% of the level during standing. During gait, the amplitude of this complex was significantly smaller just after footfall than the amplitude during late swing (MANOVA). It is suggested that the reduced sensation and the decreased evoked potentials after touchdown may be due to occlusion or masking by concomitant afferent input from the feet. On the other hand, the phasic increase in sensitivity at the end of swing is thought to result from a centrally generated facilitation of sensory transmission of signals in anticipation of foot-placing.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00233042
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  • 6
    Digitale Medien
    Digitale Medien
    Quaternary Fourier stratigraphy: Orbital templates and Milankovitch anomalies (1994)
    Springer
    Mathematical geology 26 (1994), S. 769-781 
    Details
    ISSN: 1573-8868
    Schlagwort(e): cyclic sedimentation ; Fourier stratigraphy ; Milankovitch ; orbital templates ; Quaternary
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Mathematik
    Notizen: Abstract A simple template-generating algorithm using summer insolation at 65°N as input provides a timeseries for the last 2 million years that can be compared directly with the oxygen isotope record in deep-sea sediments. Discrepancies between template and record are diminished by representing both series as Fourier expansions, and importing the power spectrum of the record to the template, without changing phase. The remaining differences between the hybrid template and the record contain messages about time spans of unusual behavior of the system. The most striking anomalies in the Quaternary are the unusually cold period following the mid-Pleistocene climate shift at 900 ka (Stage 22) and much of Stage 11 near 400 ka, representing excess warming. The present interglacial also is too warm, compared with expectations. Anomalies are thought to be the result of stabilization of unusually cold periods (by albedo feedback) and unusually warm periods (by carbon dioxide feedback). It is proposed that there is a connection between surplus ice buildup (after the mid-Pleistocene climate shifi at 900 ka) on marine shelves and subsequent extra-large transgressions, which stabilize warm periods by shallow-water carbonate production (coral reef hypothesis).
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF02083116
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  • 7
    Digitale Medien
    Digitale Medien
    The 100-kyr ice-age cycle: internal oscillation or inclinational forcing? (1999)
    Springer
    International journal of earth sciences 88 (1999), S. 305-316 
    Details
    ISSN: 1437-3262
    Schlagwort(e): Key words Quaternary ; Milankovitch ; 100-kyr cycles ; Eccentricity ; Inclination ; Ice age cycles
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie
    Notizen: Abstract The ice-age hypothesis of Muller and MacDonald (1995, 1997a, 1997b) has two parts: (a) The 100-kyr cycle does not owe its existence to Milankovitch forcing; and (b) variations in inclination of Earth's orbit (i.e., the orbit's angle with the solar system invariable plane) provide the mechanism sought. In support of the first proposition, Muller and MacDonald point to the paradox that the spectrum of oxygen isotope series from deep-sea sediments contains no power for two prominent eccentricity cycles, 125 and 400 kyr. In support of the second proposition, they offer a match between the SPECMAP record (Imbrie et al. 1984) and a plot of the amplitude of orbital inclination, shifted by 33 kyr. The hypothesis of Muller and MacDonald is rejected in both parts, although an influence of inclination forcing is not precluded entirely. The paradox of the missing eccentricity cycles (125 and 400 kyr) is explained by suppression of the two longer cycles, and enhancement of the one near 96 kyr, as a result of internal oscillation. A Muller–MacDonald machine for making the 100-kyr ice-age cycles, however conceived, would have to have a memory near 30 kyr to provide for phase shift between input and output. Precisely this amount of memory is sufficient to produce the needed oscillation in Milankovitch machine here applied; thus, there is no advantage, from the point of view of either necessity or simplicity, in replacing Milankovitch forcing, with its precise phasing (despite the fuzzy physics), with inclination forcing, and with its severe problems in phasing (and, thus far, no physics at all).
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s005310050266
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