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George E. Carvell, PhD, PT   University of Pittsburgh
 
MOTOR UNIT RECRUITMENT: MUP Firing Rates
Fig 2 & Table 1 IN: T. Eken, Spontaneous Electromyographic Activity in Adult Rat Soleus Muscle. J Neurophysiol 80: 365-376, 1998.
Electromyography (EMG) recordings are used to measure motor unit activity. Surface EMG measures overall levels of muscle activity (see SMCs In Action) but individual motor unit potentials cannot be reliably isolated in these recordings. Special indwelling electrodes are required to see individual motor unit potentials (MUPs). During low levels of tonic activity individual MUPs can be isolated(Fig 2 and 34-12). Average firing rates tend to be < 25 Hz but there are exceptions (see Table 1 and fig 34-12). Isolating individual MUPs is difficult with phasic activities requiring rapid and/or large forces since the MUPs tend to overlap even when using indwelling electrode recordings & gradual force increments (see fig 34-12).
KS&J Fig 34-12 p 687
Eken Fig 2 p 368
Eken Table 1 p 369
1- 4 = MUPs during 4 trials of low levels of tonic activity in Rat Soleus Muscle
Single MUPs a- h
Each line = a single MUP