EMG & Motor Neurons

Chapter 18

KINE 3301Biomechanics of Human Movement

A motor unit is one alpha motor neuron and all of the muscle fibers it innervates.

Motor Unit

The EMG amplifier takes the difference between the muscle fiber action potential from each electrode.

EMG Differential Amplifier

• Synchronization of two muscle fiber action potentials with the same polarity causes amplification.

• Synchronization of opposite polarity causes cancellation.

Synchronization Causes Amplification & Cancellation

Stimulation of a motor neuron with a single impulse causes a twitch response.

Muscle Twitch Response

• Increasing the stimulation rate (firing rate) causes a stair-stepping effect of force.

• Maximal stimulation produces a tetanic contraction.

Stimulation Rate and Tetanic Contractions

EMG, nerve conduction velocity, firing rate ranges, twitch responses and fatigue curves by motor unit type.

EMG & MU Properties

Instrumentation used to study muscle force-velocity and force-length relationship.

EMG – Force Data Collection

• In a ramp contraction motor units are recruited according to size (ST, INT, FT).

• A newly recruited unit fires slower than currently active unit.

• Motor Units are recruited and de-recruited at the same force threshold.

Ramp Contraction

Theoretically all motor units are recruited simultaneously and each fires at it’s maximal rate at the outset.

Ballistic Contraction

The subject is instructed to generate maximum force as rapidly as possible.

• On the left the subject is instructed to generate a 50% MVC as rapidly as possible.

• On the right the subject is instructed to generate a 50% MVC in 2 sec.

Effects of Rate of Force Development on Recruitment & Firing Rate

Concentric & Eccentric EMG - Torque

Concentric EccentriciEMG (mV) 685.1 660.6

Average Torque (N m)∙

48.6 61.1

Peak Torque (N m)∙

84.7 98.2

Electromechanical Delay (EMD)