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larger motor units in the same muscle control a larger number of fibres.The EMG can
distinguishbetweenmotorunits,becausethesmallerunitsproduceanelectricalsignalof
lower amplitude than the larger units.Thus, a large frequencyof low amplitude motor
unitspikes,indicatesthatmanysmaller
motorunitsareoperating.
Theuseofvoluntarymusclesis initiated by excitation of neuronsinthemotorcortexof
the cerebrum.Upper motor neurons in the cerebrum have descending fibres that
terminate in the spinal cord, where they can excite the cell body of the lower motor
neuron.If
astrongcontractionofamuscleisrequired,agreateramountofexcitationis
producedinthedescendingfibres,whichresultsinthestimulationofagreaternumberof
lowermotorunits.Thesmallermotorunitsareactivatedfirst,andthenumberof these
recruited,increaseswith the desired strength of the muscular contraction.Forstronger
contractions, the larger motor units are recruited, their number increasing as effort
increases.For the greatest degree of effort, the very largest motor units are recruited.
Thus,anelectromyogramofamusclethatisputunderincreasingstrainwillshowaseries
of low amplitude motor unit spikes to begin with, and then the frequency of these will
increasebefore spikes of moderatelyhigh amplitudeare recorded.Whenthe muscle is
contracted with the greatest effort there will be a high frequency of both low and
moderatemotorspikeswithanincreasingfrequencyofthehighestamplitudespikes.
When the muscular effort is decreased gradually, the reverse situation occurs.The
highestamplitudespikesdisappearfirst,followedbythemoderatelyhighspikes,untilat
thelowestlevelofeffortjustafewofthelowestamplitudespikesremain.
ComponentsforusewiththeEKG/EMGSensor
Q‐S207EKG/EMGsensor
PowersupplyforQ‐S207
A263TabElectrodes(pkg.of100)
C610LabQuestMinidataacquisitioninterface(notincluded)
C901LoggerProDataAcquisitionSoftware(notincluded)
C404CustomizedSetupFiles(notincluded)
Manual