Oct 6 (Thursday) Kevin Keenan (Mechanical and Aerospace Engineering, Cornell)

The surface electromyogram as an index of activity in a population of motor units.

Abstract:

The nervous system uses electrical signals, called action potentials, to activate muscle.  The moment-to-moment control of muscle force is graded by the nervous system by varying the rate at which motor units discharge action potentials (rate coding), and by altering the number of motor units that are active (recruitment).  A recording of this electrical activity, the electromyogram (EMG), represents the summated activity of both the recruitment and rate coding of those motor units within the detection range of the recording electrodes.  Therefore, the EMG signal represents the peripheral neural drive to the muscle, but in a manner that is not clearly understood.  Because the interactions of a motor unit population cannot be studied experimentally in humans, and the understanding of these interactions improves only slightly during reduced animal preparations, a computational approach was used to examine the relation between the EMG signal and activation of the motor unit pool.  The first study I will present indicated that the surface EMG underestimated the activation signal sent by the nervous system to muscle due to cancellation between the positive and negative phases of motor unit potentials (amplitude cancellation).  The second study examined how different motor unit properties influenced the size of potentials evoked with an electrical stimulus.  These results indicate that although the surface EMG signal is commonly used as an index of activity in populations of motor units, there are limits to the information that can be extracted from this signal.  These limitations influence our ability to interpret adaptations within the neuromuscular system from the EMG signal.