KIN 313 Lecture 3 - Motor Units: Size Principle and Recruitment

September 10 2025

how an indwelling electrode EMG works

records electrical activity of muscle → shows shift in the sarcolemic potential that travels down the muscle to cause a contraction

designed so that only the tip is active. sees the impulses near the tip the best, the further you get the harder they are to see/cant see them at all

can create a compound electrical potential signal based on all the sarcolemmic potentials that came from the same AP → once amplified, can record what came out of an AP

pros and cons of a metal microelectrode

slightly flexible → low pain

can adjust length for needs

but not stable, if the fibres move, the needle does too → low confidence readings

fine wires EMG

more stable/permanent than the metal micro electrode

inserted with a needle, hooks on end grab the muscle and stay there when needle is removed

works better because they stay in place

can use an ultrasount to see where the wires are in the muscle

How motor units are identified on an EMG

same shape = same motor unit (can overlay the traces and see the average shape)

why? same shape because its the same Motor Neuron telling them what to do

a bigger shape/signal coming from a motor neuron on an EMG means

more muscle fibres close to the electrode

two ways to control muscle force

1. recruit more motor units

2. make the ones that are active fire more often

Henneman’s size principle

smaller motor units/motor neurons are recruited first and turn off last

bigger motor units are recruited last and turn off first

recruitment threshold is determined by

cell body size (of the motor neuron)

low threshold = the ____ to be recruited and ___ to be derecruited

first, last

sequential inactivation

order to who turns off first

last one on - first one off (bigger motor neurons)

why recruit in an orderly fashion (size principle)

small first = increment control

big first = spazzy incontrolable force production

pros and cons of following the size principle

PRO: brain doesnt have to recruit each guy individually, can jsut say “give me 75% contraction” and call it good → efficiency

CON: cant selectively activate motor units out of order. cant just decide to send on a big guy, you have to work your way up there

when does our body decide to recruit more motor units vs rate modulate

muscles have a percentage of their maximum force production where all motor units will be recruited (biceps brachii has all recruited at about 85% maximum)

after that point, all increases in force come from rate modulation

before that point, method depends on purpose

FOR EXAMPLE

just picking up a cup, you can turn on a few motor units and then just rate modulate as needed. no reason to continually recruit and derecruit small units in order to match movement needs that shift with every small change

what happens to the firing rate of a motor unit before another unit is recruited due to increased force demands

rate will increase via rate modulation as the nervous system realizes this guy isnt enough and we need another guy on the field