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The synchronization of motor units discharge during isometric contraction as a compensation phenomenon to peripheral muscular fatigue
ABSTRACTS returned to control values at 80 o. The results indicate that different head-body positions are able to modify the bias of spinal interneurons in man. We discuss the hypothetical role of the vestibular system in producing the effects seen.
PS. The synchronization of motor units discharge during isometric contraction as a compensation phenomenon to peripheral muscular fatigue. - B. Rossi, R. Risaliti, G. Siciliano, A. Starita * and E. De Cecco (Istituto di Clinica Neurologica, Universit~ di Pisa, and * Dipartimento di Informatica, Universitl di Pisa, Pisa, Italy) Spectral analysis of the EMG allows the study of central phenomena that compensate muscular fatigue: increase of recruitment and synchronization. In 10 males and 10 females (25-45 years) biceps brachii were investigated with surface electrodes during 70% maximal isometric contraction. The following spectral parameters of EMG were evaluated in basal condition and after 60 see: frequency of maximal peak, and median, total and maximal peak powers. Total power and especially maximal peak power increased in both males (211% and 259%, respectively) and females (411% and 597%, respectively). Ratio between maximal peak power and total power changed from 0.1 to 0.13 in males, and from 0.12 to 0.15 in females. Frequency of maximal peak and of median decrease in all cases. The increase of total power can be considered a consequence of the greater recruitment that occurs to counteract the reduced muscular contractility. A further mechanism is the increase of synchronization of motor unit discharge expressed as an increase of maximal peak power. To better measure the respective contributions of these two parameters of central adaptation to peripheral fatigue, an index of synchronization can be calculated from the ratio between maximal peak power and total spectral power.
PS. Differences in mapping the scalp topography of magnetic and electric somatoscnsory EPs. - P.M. Rossini, M. Caramia*, L. Cecchi **, G.L. Romani, C. Salustri, V. P i z z e l l a * * * , L. Narici and I. M o d e n a * * * (Dept. of * Clin. Neurophysiul., * * Physiol., and * * * Physics, Ist and lind University and Institute of Electronics, N.R.C., 00173-Rome, Italy) The topography of electric somatosensory evoked potentials (SEPs) and of somatosensory magnetic evoked fields (SEFs) to median nerve stimulation were analyzed. Thirty channels were utilized for SEPs recordings with a 10 kHz/chan. sampling rate. Amplitude estimates at different scalp sites were expressed by frozen maps of 20 colors with a display resolution of 4096 pixels. Such estimates were achieved by computing the weighted mean of the closest 4 electrodes, each electrode having been weighted in a manner inversely proportional to its squared distance from the examined pixel. In a separate session SEFs were measured from up to 45 scalp positions using a 4-channel sensor with pick-up coils
$89 using second derivative gradiometers. The centers of the gradiometers were placed at the comers of a 2 cm side square. The comparative analysis was limited to the 30 msec poststimulus epoch. SEPs morphology was a complex one, characterized by a frontal P20-N24-P28 complex sometimes partly obscured by a wide N30. On the rolandic districts a localized P22 was present. In the antero-parietal sites an 'early P25' vanishing frontally in the P22 and parietally in a 'late P25' was found. More posteriorly the N20 and 'late P25 waves' were maximal. SEFs showed a simpler configuration characterized by 'mirror-image' peaks in phase opposition, with a latency corresponding to the N20-P20 and N30-'late P25' complexes.
PS. Transcranial unifocal electrical stimulation of motor cortex in humans: physiological mechanisms. - P.M. Rossini, M. Caramia and F. Zarola (Clinical Neurophysiulogy Lab., Dept. of Public Health, Elnd University of Rome, 00173 Rome, Italy) Motor evoked potentials (MEPs) were elicited through a pericranial cathode consisting of 6-12 regularly spaced, connected disks and of a stimulating anode(s) glued on the appropriate scalp region(s). Mapping of hand, shoulder and leg motor representations was carried out; these were enlarged during voluntary contraction of the target muscle (TM). Scalpto-cervical cord, -to-lumbosaeral cord and intraspinal conduction times were calculated through a formula utilizing F-wave latencies. CCTs were correlated with the body length. Amplitude-latency 'facilitation' was provoked by vibration of the TM tendon, contraction of the contralateral TM, of the homologous muscle ipsilateral to the stimulus, paired scalp stimuli pre-stimulation of the ipsilateral motor cortex and of the nerve to TM. Near-nerve recordings, collision techniques and CCTs all supported the view that unifocal scalp stimuli during voluntary phasic TM contraction sequentially engage large pyramidal cells or axons, fast pyramidal tract neurons, large alphaMNs, fast nerve motor fibers, large-phasic motor units. Appropriate experimental stimulating protocols, i.e., with relaxed TM, during slight tonic contraction, immediately before onset of EMG in the TM, allowed to elicit MEPs partly mediated by small spinal alpha-motoneurons, relatively slow nerve motor fibers and small motor units.
PS. Pre-EMG facilitation o! motor evoked potential to transcranial unifocal stimulation in man. - P.M. Rossini *, M. Caramia*, F. Zarola* and A. Starr ** (* Clin. Nem'ophysiol., lind University of Rome, Rome, Italy, and * * Dept. Neurology, University Irvine, lrvine, CA, U.S.A.) The time-course of facilitation of thenar-MEP in relation with the onset of a voluntary ballistic thumb movement in a reaction time paradigm (warning click) was studied. A clickto-stimulus interval was selected such that the scalp stimuli occurred from -100 to + 100 msec with 0 time centered on the onset of EMG activity in the thenar muscle. The intensity
PS. The synchronization of motor units discharge during isometric contraction as a compensation phenomenon to peripheral muscular fatigue. - B. Rossi, R. Risaliti, G. Siciliano, A. Starita * and E. De Cecco (Istituto di Clinica Neurologica, Universit~ di Pisa, and * Dipartimento di Informatica, Universitl di Pisa, Pisa, Italy) Spectral analysis of the EMG allows the study of central phenomena that compensate muscular fatigue: increase of recruitment and synchronization. In 10 males and 10 females (25-45 years) biceps brachii were investigated with surface electrodes during 70% maximal isometric contraction. The following spectral parameters of EMG were evaluated in basal condition and after 60 see: frequency of maximal peak, and median, total and maximal peak powers. Total power and especially maximal peak power increased in both males (211% and 259%, respectively) and females (411% and 597%, respectively). Ratio between maximal peak power and total power changed from 0.1 to 0.13 in males, and from 0.12 to 0.15 in females. Frequency of maximal peak and of median decrease in all cases. The increase of total power can be considered a consequence of the greater recruitment that occurs to counteract the reduced muscular contractility. A further mechanism is the increase of synchronization of motor unit discharge expressed as an increase of maximal peak power. To better measure the respective contributions of these two parameters of central adaptation to peripheral fatigue, an index of synchronization can be calculated from the ratio between maximal peak power and total spectral power.
PS. Differences in mapping the scalp topography of magnetic and electric somatoscnsory EPs. - P.M. Rossini, M. Caramia*, L. Cecchi **, G.L. Romani, C. Salustri, V. P i z z e l l a * * * , L. Narici and I. M o d e n a * * * (Dept. of * Clin. Neurophysiul., * * Physiol., and * * * Physics, Ist and lind University and Institute of Electronics, N.R.C., 00173-Rome, Italy) The topography of electric somatosensory evoked potentials (SEPs) and of somatosensory magnetic evoked fields (SEFs) to median nerve stimulation were analyzed. Thirty channels were utilized for SEPs recordings with a 10 kHz/chan. sampling rate. Amplitude estimates at different scalp sites were expressed by frozen maps of 20 colors with a display resolution of 4096 pixels. Such estimates were achieved by computing the weighted mean of the closest 4 electrodes, each electrode having been weighted in a manner inversely proportional to its squared distance from the examined pixel. In a separate session SEFs were measured from up to 45 scalp positions using a 4-channel sensor with pick-up coils
$89 using second derivative gradiometers. The centers of the gradiometers were placed at the comers of a 2 cm side square. The comparative analysis was limited to the 30 msec poststimulus epoch. SEPs morphology was a complex one, characterized by a frontal P20-N24-P28 complex sometimes partly obscured by a wide N30. On the rolandic districts a localized P22 was present. In the antero-parietal sites an 'early P25' vanishing frontally in the P22 and parietally in a 'late P25' was found. More posteriorly the N20 and 'late P25 waves' were maximal. SEFs showed a simpler configuration characterized by 'mirror-image' peaks in phase opposition, with a latency corresponding to the N20-P20 and N30-'late P25' complexes.
PS. Transcranial unifocal electrical stimulation of motor cortex in humans: physiological mechanisms. - P.M. Rossini, M. Caramia and F. Zarola (Clinical Neurophysiulogy Lab., Dept. of Public Health, Elnd University of Rome, 00173 Rome, Italy) Motor evoked potentials (MEPs) were elicited through a pericranial cathode consisting of 6-12 regularly spaced, connected disks and of a stimulating anode(s) glued on the appropriate scalp region(s). Mapping of hand, shoulder and leg motor representations was carried out; these were enlarged during voluntary contraction of the target muscle (TM). Scalpto-cervical cord, -to-lumbosaeral cord and intraspinal conduction times were calculated through a formula utilizing F-wave latencies. CCTs were correlated with the body length. Amplitude-latency 'facilitation' was provoked by vibration of the TM tendon, contraction of the contralateral TM, of the homologous muscle ipsilateral to the stimulus, paired scalp stimuli pre-stimulation of the ipsilateral motor cortex and of the nerve to TM. Near-nerve recordings, collision techniques and CCTs all supported the view that unifocal scalp stimuli during voluntary phasic TM contraction sequentially engage large pyramidal cells or axons, fast pyramidal tract neurons, large alphaMNs, fast nerve motor fibers, large-phasic motor units. Appropriate experimental stimulating protocols, i.e., with relaxed TM, during slight tonic contraction, immediately before onset of EMG in the TM, allowed to elicit MEPs partly mediated by small spinal alpha-motoneurons, relatively slow nerve motor fibers and small motor units.
PS. Pre-EMG facilitation o! motor evoked potential to transcranial unifocal stimulation in man. - P.M. Rossini *, M. Caramia*, F. Zarola* and A. Starr ** (* Clin. Nem'ophysiol., lind University of Rome, Rome, Italy, and * * Dept. Neurology, University Irvine, lrvine, CA, U.S.A.) The time-course of facilitation of thenar-MEP in relation with the onset of a voluntary ballistic thumb movement in a reaction time paradigm (warning click) was studied. A clickto-stimulus interval was selected such that the scalp stimuli occurred from -100 to + 100 msec with 0 time centered on the onset of EMG activity in the thenar muscle. The intensity