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The Journal of Neurophysiology Vol. 86 No. 5 November 2001, pp. 2144-2158
Copyright ©2001 by the American Physiological Society
1Department of Clinical Neurosciences, Faculty of Medicine, University of Calgary, Calgary, Alberta T2N 4N1, Canada; and 2Center for Musculoskeletal Research, National Institute for Working Life, S907 13 Umeå, Sweden
Day, Scott J. and
Manuel Hulliger.
Experimental Simulation of Cat Electromyogram: Evidence for
Algebraic Summation of Motor-Unit Action-Potential Trains. J. Neurophysiol. 86: 2144-2158, 2001. Prompted by
the observation that the slope of the relationship between average
rectified electromyography (EMG) and the ensemble activation rate of a
pool of motor units progressively decreased (showing a downward
nonlinearity), an experimental study was carried out to test the widely
held notion that the EMG is the simple algebraic sum of motor-unit
action-potential trains. The experiments were performed on the cat
soleus muscle under isometric conditions, using electrical stimulation
of 
-motor axons isolated in ventral root filaments. The EMG signals
were simulated experimentally under conditions where the activation of
nearly the entire pool of motor units or of subsets of motor units was
completely controlled by the experimenter. Sets of individual motor
units or of small groups of motor units were stimulated independently,
using stimulation profiles that were strictly repeatable between
trials. This permitted a rigorous quantitative comparison of EMGs that
were recorded during combined activation of multiple motor filaments
with EMGs that were synthesized from the algebraic summation of motor
unit action potential trains generated by individual nerve filaments. These were recorded separately by individually stimulating the same
filaments with the same activation profiles that were employed during
combined stimulation. During combined activation of up to 10 motor
filaments, experimentally recorded and computationally synthesized EMGs
were virtually identical. This indicates that EMG signals indeed are
the outcome of the simple algebraic summation of motor-unit
action-potential trains generated by concurrently active motor units.
For both recorded and synthesized EMGs, it was confirmed that EMG
magnitude increased nonlinearly with the ensemble activation rate of a
pool of motor units. The nonlinearity was largely abolished when EMG
magnitude was estimated as the sum of rectified, instead of raw,
motor-unit action-potential trains. This suggests that the downward
nonlinearity in the EMG-ensemble activation rate relation is due to
signal cancellation arising from the perfectly linear summation of
positive and negative components of action-potential waveforms. The
findings provide a much needed post hoc validation of the concept of
EMG generation by strict algebraic summation of motor unit action
potentials that is generally relied on in theoretical modeling studies
of EMG and in EMG decomposition algorithms.
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