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  • 1
    Electronic Resource
    Electronic Resource
    Synchronization of human leg motor units during co-contraction in man (1994)
    Springer
    Experimental brain research 102 (1994), S. 84-94 
    Details
    ISSN: 1432-1106
    Keywords: Co-contraction ; Motor units ; Synchronization ; Voluntary movement ; Human
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Cross-correlograms were constructed from the spike trains of pairs of individual motor units recorded from the tibialis anterior (TA) and soleus (Sol) muscles. In most correlograms the synchronization consisted of a central peak with a short duration. When the motor units were recorded from the same muscle this synchronization was seen in almost 100% of the cases (TA 92%, Sol 89%) and the peak of synchronization was large (TA 0.060 extra sample spikes per trigger spike, Sol 0.030). In contrast, clear peaks of short duration synchronization was seen in only 4 out of 30 antagonistic TA and Sol motor unit pairs (0.025 extra sample spikes per trigger spike). In another 4 motor unit pairs central troughs were seen. In approximately 10% of all combinations of motor unit pairs a low-amplitude long-duration synchronization was seen. This type of synchronization was seen either as the only type of synchronization or at the same time as the short-duration synchronization. Finally, secondary peaks on both sides of the central peak were seen in a few cases. The distance between the sites of recording of TA motor unit pairs was systematically varied. The latency of the short-duration synchronization became longer and the amount of synchronization smaller the longer the distance between the recording sites. The amount of synchronization of two motor units recorded from the same muscle was shown to be larger when the muscle was activated in co-contraction rather than in an isolated agonist movement. The results are discussed in relation to the organization of the descending command eliciting co-contraction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00232441
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  • 2
    Electronic Resource
    Electronic Resource
    Segmental reflexes and ankle joint stiffness during co-contraction of antagonistic ankle muscles in man (1994)
    Springer
    Experimental brain research 102 (1994), S. 350-358 
    Details
    ISSN: 1432-1106
    Keywords: Co-contraction ; Motoneurones ; Musculus soleus ; Stiffness ; Reflex pathways ; H-reflex ; Stretch reflex ; Human
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The size of soleus H-reflexes and short-latency stretch reflexes was measured at different levels of plantar flexion or co-contraction (simultaneous activation of dorsi- and plantar flexors) in seven healthy subjects. In four of seven subjects the short-latency stretc reflex was smaller during weak co-contraction than during isolated plantar flexion at matched background electromyogram (EMG) levels in the soleus muscle. In three of these four subjects the stretch reflex was larger during strong co-contraction than during plantar flexion, whereas it had the same size during the two tasks in the last subject. In the remaining subjects the stretch reflex either had the same size or was larger at all levels of co-contraction than at similar levels of plantar flexion. In contrast, the H-reflex was found to decrease with co-contraction at all contraction levels in all subjects. The decrease in the reflexes during weak co-contraction might be caused by presynaptic inhibition of Ia afferents. It is unclear why only the H-reflex decreased during strong co-contraction. The stiffness of the ankle joint was measured from the torque increment following the stretch of the plantar flexors divided by the stretch amplitude. In all subjects the total stiffness of the ankle joint was larger during strong co-contraction than during plantar flexion of similar strength. The stiffness was smaller during weak co-contraction than during weak plantar flexion in three out of seven subjects. The medial gastrocnemius muscle was more active at a given level of soleus activity during the co-contraction task than during the isolated plantar flexion task. It is suggested that the increase in the stiffness during co-contraction as compared to isolated plantar flexion was mainly due to the mechanical contribution of the activity in the tibialis anterior and medial gastrocnemius muscles. The decrease in stiffness during weak co-contraction was, in contrast, most likely mainly caused by modulation of reflex stiffness.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00227521
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  • 3
    Electronic Resource
    Electronic Resource
    Central facilitation of Ia inhibition during tonic ankle dorsiflexion revealed after blockade of peripheral feedback (1992)
    Springer
    Experimental brain research 88 (1992), S. 651-656 
    Details
    ISSN: 1432-1106
    Keywords: Spinal cord ; Reciprocal inhibition ; Iaafferents ; Reflex pathways ; H-reflexes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Recent studies have reported that no increase of the disynaptic reciprocal inhibition can be observed during tonic voluntary dorsiflexion of the foot as compared to rest, when the size of the control H-reflex is kept constant. Other studies have, however, shown that a voluntary contraction evokes a strong and long-lasting depression of the synaptic transmission from Ia afferents to motoneurones, most likely secondary to activation of these afferents during the contraction (post-activation depression). It was thought that this effect could also interfere with the demonstration of a central facilitation of the reciprocal inhibition during movement. The amount of disynaptic Ia reciprocal inhibition from the pretibial flexors to the soleus H-reflex was therefore estimated in normal human subjects at rest and during voluntary tonic dorsiflexion before, during and after blocking the peripheral feedback from the investigated muscles. It was observed that the reciprocal inhibition measured during dorsiflexion increased during occlusion of the blood supply to the leg, reaching a maximum of inhibition after 30 min of ischaemia. After release of the ischaemia the inhibition gradually decreased to its pre-ischaemic level. It is therefore suggested that the brain facilitates transmission in the Ia disynaptic reciprocal pathway during tonic voluntary dorsiflexion of the foot, but that this facilitation is normally not observed due to a post-activation depression following the peripheral feedback activation during the movement.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00228194
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    Disorder in reciprocal innervation upon initiation of voluntary movement in patients with Parkinson's disease (1988)
    Springer
    Experimental brain research 70 (1988), S. 437-440 
    Details
    ISSN: 1432-1106
    Keywords: Reciprocal facilitation ; Reciprocal inhibition ; Parkinson's disease ; Supraspinal control ; Movement initiation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Reciprocal innervation of the soleus motoneurones upon initiation of voluntary ankle dorsiflexion was investigated in eight patients with Parkinson's disease. H-reflex and visually guided step tracking methods were used for testing moto-neurone excitability and for controlling the timing of movement initiation, respectively. While reciprocal inhibition appeared almost simultaneously with the agonist electromyographic (EMG) onset in normal subjects (Kagamihara and Tanaka 1985), facilitation appeared in the majority of patients under the same onset condition. It increased slowly, reaching a maximum at about 100 ms after the EMG onset. It then subsided slowly at around 200–300 ms, and was replaced thereafter by an inhibitory effect. No coactivation of the soleus muscle was detected electromyographically. The facilitation between the EMG onset and the onset of mechanical contraction was attributed to the direct effect of the descending command from the brain, suggesting a certain disorder in controlling the system for reciprocal innervation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00248369
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    Paper (German National Licenses)
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