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  • 1
    Electronic Resource
    Electronic Resource
    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
    Keywords: Key words Quaternary ; Milankovitch ; 100-kyr cycles ; Eccentricity ; Inclination ; Ice age cycles
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: 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).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s005310050266
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  • 2
    Electronic Resource
    Electronic Resource
    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
    Keywords: Muscle receptor ; Motor control ; Electromyogram ; Stance ; Human
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00228405
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  • 3
    Electronic Resource
    Electronic Resource
    Developmental aspects of stance regulation, compensation and adaptation (1992)
    Springer
    Experimental brain research 90 (1992), S. 610-619 
    Details
    ISSN: 1432-1106
    Keywords: Stance regulation ; Adaptational processes ; Developmental aspects ; Load receptors ; Human
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00230945
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  • 4
    Electronic Resource
    Electronic Resource
    Afferent control of human stance and gait: evidence for blocking of group I afferents during gait (1985)
    Springer
    Experimental brain research 61 (1985), S. 153-163 
    Details
    ISSN: 1432-1106
    Keywords: Stance and gait perturbation ; Cerebral potential ; Leg muscle e.m.g. response ; Spinal and transcortical reflexes ; Motor control
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The cerebral potentials (c.p.) evoked by electrical stimulation of the tibial nerve during stance and in the various phases of gait of normal subjects were compared with the c.p. and leg muscle e.m.g. responses evoked by perturbations of stance and gait. Over the whole step cycle of gait the c.p. evoked by an electrical stimulus were of smaller amplitude (3 μV and 9 μV, respectively) than that seen in the stance condition, and appeared with a longer latency (mean times to first positive peak: 63 and 43 ms, respectively). When the electrical stimulus was applied during stance after ischaemic blockade of group I afferents, the c.p. were similar to those evoked during gait. The c.p. evoked by perturbations were larger in amplitude than those produced by the electrical stimulus, but similar in latencies in both gait and stance (mean 26 μV and 40 μV; 65 ms and 42 ms, respectively) and configurations. The large gastrocnemius e.m.g. responses evoked by the stance and gait perturbations arose with a latency of 65 to 70 ms. Only in the stance condition was a smaller, shorter latency (40 ms) response seen. It is concluded that during gait the signals of group I afferents are blocked at both segmental and supraspinal levels which was tested by tibial nerve stimulation. It is suggested that the e.m.g. responses induced in the leg by gait perturbations are evoked by group II afferents and mediated via a spinal pathway. The c.p. evoked during gait most probably reflect the processing of this group II input by supraspinal motor centres for the coordination of widespread arm and trunk muscle activation, necessary to restablish body equilibrium.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00235630
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  • 5
    Electronic Resource
    Electronic Resource
    Fast head tilt has only a minor effect on quick compensatory reactions during the regulation of stance and gait (1988)
    Springer
    Experimental brain research 73 (1988), S. 470-476 
    Details
    ISSN: 1432-1106
    Keywords: Vestibulo-spinal reflexes ; Motor control ; Regulation of stance and gait ; Head tilt ; Man
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Sudden tilts of the head to the front or rear were induced during stance, balancing, gait and during perturbations of gait. The most prominent response in the leg muscle electromyogram (e.m.g.) to head tilt occurred in the tibialis anterior muscle (latency about 55 ms) following a backward tilt induced during balancing. During stance and gait, the e.m.g. activity related to head tilt was only a minor component of the leg muscle activity normally occurring during gait. When the head tilt was induced shortly after a perturbation of gait (treadmill acceleration impulse), the compensatory reaction in the leg muscles did not significantly differ from that seen after the gait perturbation alone. In addition, the rate of acceleration of the head was tested against the compensatory e.m.g. responses: No correlation of influence could be discerned. The results indicate that sudden head tilts and the resulting head acceleration have little influence on the e.m.g. patterns that occur during gait and perturbations of gait. It is assumed that these patterns are regulated by central programs, and that the compensation for leg perturbation is achieved mainly by spinal reflex mechanisms. It is discussed whether the lack of head tilt responses is the result of an antagonistic vestibularneck interaction, or whether it indicates a reduced effectiveness of vestibulo- and cervico-spinal reflexes during gait.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00406603
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  • 6
    Electronic Resource
    Electronic Resource
    Cerebral potentials and leg muscle e.m.g. responses associated with stance perturbation (1985)
    Springer
    Experimental brain research 57 (1985), S. 348-354 
    Details
    ISSN: 1432-1106
    Keywords: Stance perturbation ; Cerebral potential ; Leg muscle e.m.g. response ; Spinal reflexes ; Motor control
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary In order to investigate the neuronal mechanisms underlying the compensatory movements following stance disturbance, leg muscle e.m.g. responses and cerebral potentials evoked by a treadmill acceleration impulse were analysed. It was found that the displacement was followed by a cerebral potential of a latency of 40–45 ms and EMG responses in the calf muscles at a latency of 65–70 ms. The e.m.g. responses represented specific compensatory reactions to the mode of perturbation (with a gastrocnemius activation following positive acceleration but a tibialis ant. activation following negative acceleration). The cerebral potentials, however, showed a common pattern to both conditions. In addition, the leg muscle e.m.g. reactions were not altered by learning effects and by forewarning of displacement onset, while the amplitude of the cerebral potentials was significantly smaller in these conditions compared to those produced in response to randomly induced perturbations. It was therefore concluded that the leg muscle e.m.g. reactions are mediated by a polysynaptic spinal reflex pathway which depends on a supraspinal control. The cerebral potentials seem to represent afferent signals which can be supposed to be subjected to modification and processing by supraspinal motor centres, according to the actual requirements.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00236540
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  • 7
    Electronic Resource
    Electronic Resource
    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
    Keywords: Gait ; Sural nerve ; Sensory gating ; Somatosensory evoked potential ; Human
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00233042
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  • 8
    Electronic Resource
    Electronic Resource
    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
    Keywords: Key words Vision ; Locomotion ; Optic Flow Adaptation ; Human
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/PL00005624
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  • 9
    Electronic Resource
    Electronic Resource
    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
    Keywords: Split-belt locomotion ; Interlimb coordination ; Adaptation ; Motor learning ; Human
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00231067
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  • 10
    Electronic Resource
    Electronic Resource
    Quaternary Fourier stratigraphy: Orbital templates and Milankovitch anomalies (1994)
    Springer
    Mathematical geology 26 (1994), S. 769-781 
    Details
    ISSN: 1573-8868
    Keywords: cyclic sedimentation ; Fourier stratigraphy ; Milankovitch ; orbital templates ; Quaternary
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Mathematics
    Notes: 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).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02083116
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