An ePIC Assessment of Quadriceps Motor Unit Firing Patterns During Isometric Knee Extension: A High-Density Surface Electromyography Study
Session Number
Project ID: BIO 07
Advisor(s)
Dr. Obaid U. Khurram; Northwestern Feinberg School of Medicine, Department of Physiology
Dr. Gregory E. Pearcey; Northwestern Feinberg School of Medicine, Department of Physiology
Dr. Charles J. Heckman; Northwestern Feinberg School of Medicine, Department of Physiology
Discipline
Biology
Start Date
22-4-2020 10:05 AM
End Date
22-4-2020 10:20 AM
Abstract
Central to understanding motor tasks is the motor unit, which consists of a motor neuron and the set of muscle fibers it innervates. Motor neurons are unique in the central nervous system in that their firing patterns can be readily measured and related directly to functional behaviors due to the one to one spike ratio between the motor neuron and innervated muscle fibers. However, these firing patterns vary between the muscles in the human body. The diversity of motor tasks, the structure of the musculoskeletal system, and the synaptic organization of motor commands are the cause for this. In human subjects, these firing patterns have historically been attained using fine-wire electrodes that are inserted directly into muscles to record muscle electrical activity (i.e. electromyography or EMG) during muscle force production. Recent advances in surface EMG array technology have made it possible to record the activity of a great population of motor units non-invasively. This is because modern-day high-density EMG technology can be used in conjunction with blind source separation techniques, which detect repetitive patterns in a signal to allow the discrimination of dozens of individual motor unit action potential trains. Using data from the EMG arrays and a paired motor unit analysis technique, which compares the onset and offset of a high-threshold motor unit to a low-threshold unit, the level of neuromodulatory drive can be estimated. The difference between the two firing rates is delta F (difference in reference unit firing frequency between test unit recruitment and derecruitment) and serves as an estimation of PIC in humans. In the arm, preliminary studies from our lab suggest there is a strong proximal to distal gradient of neuromodulatory drive, likely reflecting the varying behaviors in which the different muscles of the arm are involved. The proximal muscles in the arms have ∆F = ~4-6 spikes/s whereas the distal muscles have ∆F = ~2-3 spikes/s. However, it is unclear if the same proximal to distal gradient is present in the legs as in the arms. Our data indicate that quadricep activation generates motor unit firing patterns indicative of low neuromodulatory drive with ∆F around 1.5 spikes/s and lower leg muscles have ∆F= ~3-5 spikes/s. Thus, these findings suggest that the proximal to distal gradient found in the legs is not analogous to the ones in the arm, which may reflect the impact of task diversity on motor unit firing patterns between the arms and legs in bipeds.
An ePIC Assessment of Quadriceps Motor Unit Firing Patterns During Isometric Knee Extension: A High-Density Surface Electromyography Study
Central to understanding motor tasks is the motor unit, which consists of a motor neuron and the set of muscle fibers it innervates. Motor neurons are unique in the central nervous system in that their firing patterns can be readily measured and related directly to functional behaviors due to the one to one spike ratio between the motor neuron and innervated muscle fibers. However, these firing patterns vary between the muscles in the human body. The diversity of motor tasks, the structure of the musculoskeletal system, and the synaptic organization of motor commands are the cause for this. In human subjects, these firing patterns have historically been attained using fine-wire electrodes that are inserted directly into muscles to record muscle electrical activity (i.e. electromyography or EMG) during muscle force production. Recent advances in surface EMG array technology have made it possible to record the activity of a great population of motor units non-invasively. This is because modern-day high-density EMG technology can be used in conjunction with blind source separation techniques, which detect repetitive patterns in a signal to allow the discrimination of dozens of individual motor unit action potential trains. Using data from the EMG arrays and a paired motor unit analysis technique, which compares the onset and offset of a high-threshold motor unit to a low-threshold unit, the level of neuromodulatory drive can be estimated. The difference between the two firing rates is delta F (difference in reference unit firing frequency between test unit recruitment and derecruitment) and serves as an estimation of PIC in humans. In the arm, preliminary studies from our lab suggest there is a strong proximal to distal gradient of neuromodulatory drive, likely reflecting the varying behaviors in which the different muscles of the arm are involved. The proximal muscles in the arms have ∆F = ~4-6 spikes/s whereas the distal muscles have ∆F = ~2-3 spikes/s. However, it is unclear if the same proximal to distal gradient is present in the legs as in the arms. Our data indicate that quadricep activation generates motor unit firing patterns indicative of low neuromodulatory drive with ∆F around 1.5 spikes/s and lower leg muscles have ∆F= ~3-5 spikes/s. Thus, these findings suggest that the proximal to distal gradient found in the legs is not analogous to the ones in the arm, which may reflect the impact of task diversity on motor unit firing patterns between the arms and legs in bipeds.