KIN 313 Lecture 5 - Motor Units: Concepts and Research 2

18 September 2025

How does motor unit cycling/rotation work

Motor units that are super similar are able to trade each other out once fatigued in order to allow the other to rest → can hold a low level contraction for longer

example:

MUa and MUb are very similar motor units, but MUa has a slightly lower threshold (higher resistance, easier to excite/faster to be recruited). It is recruited first, and is able to keep up with force production. when force demands increase too much for only MUa to sustain via rate modulation, MUb comes on to help, and they work together to keep up with force demands. When force demands go back down, MUa is fatigued, and therefore has a higher threshold than it did before (due to lactic acid buildup). even tho MUb originally had a higher threshold, it now has a comparably lower threshold than MUa. therefore, now that the muscle does not require both motor units in order to sustain force production, MUa is derecruited so that it may rest while MUb takes over. Then they can trade off again later when MUb is fatigued.

this only works because recruitment threshold IS NOT FIXED 

note that this may not work at high level contractions, but definitely holds up for low level contractions

implications of motor unit rotation/cycling

new understanding of henneman’s size principle

not just MUa, MUb, MUc, etc

but MUa1, MUa2, MUa3, etc for all of them

in theory, any of the MUa units could cycle/rotate for each other, and so on with the other groups

effect of afferent input on MotU selection/recruitment? implications?

afferent input to a Motor Neuron can change the recruitment threshold of Motor Units, increasing or decreasing the threshold → size principle can be affected by input source → afferent/sensory input to the motor neuron pool can make Motor Units more or less susceptible to voluntary control

if sensory input increases the threshold of one MU and decreases the threshold of another, they could then be recruited “out of order” (the bigger MU could be recruited before the smaller MU)

in the example, the stimulation increased the threshold of low threshold MUs and lowered it for high threshold MUs

benefits of high density surface EMGs

does a much better job representing MUs → shows compartmentalization is real

• allow us to map out exactly where a MU is in the muscle

  • how far does it go? is it compartmentalized?

• by comparing electrodes, we can tease out 20-50 different MUs in an area

• could also help us see changes in firing rates and recruitment after a period of training

increased strength after 4 weeks of training is due to…

due to improved ability to efficiently activate the Motor Neuron Pool by withdrawing inhibition from needed MUs and inhibiting the MUs that would not be helpful

NOT due to physiological changes such as hypertrophy… 4 weeks is not enough time for this. However, muscle can begin to atrophy in this time period

application of imagined training for post surgical patients

can do this mental training when they are not with the Physical Therapist and still have improvements/maintain MN recruitment efficacy