What a great question.
Unfortunately, it is incredibly hard to research a rapidly firing neuron at the electrochemical level. Let alone inside the neuron of a live subject that is exercising at peak effort.
CNS-causes of fatigue
Here are a few nervous system causes of exercise-induced fatigue:
Central fatigue reflects changes in the CNS and may involve altered input from muscle sensory nerve fibers, reduced excitatory input to motor control centers of the brain and spinal cord, and altered excitability of α and γ motor neurons. The contributions of these factors vary with the individual and with the nature of the activity.2
This explains which parts of the nervous system may play a role in causing the slow firing of the motor-unit. However, this does not explain why novices
have a harder time to keep their motor neuron frequency up.
The role of motivation
This is one possible hypothesis to your question: novices are less capable of pushing through the discomfort of training. This might be due to fear of getting hurt, not knowing if the felt pain is dangerous, not yet being mentally conditioned to push through discomfort, etc3.
Here are more relevant citations that you can dive into regarding 'motivational fatigue':
central fatigue is likely of greater importance in novice athletes and during repetitive (i.e., boring) tasks.2
... force will obviously fall if the motor drive from the CNS declines below the level required for sufficient muscle activation, as may happen if the subject simply lacks motivation or is not prepared to tolerate the increasing sensations of discomfort.1
... the rising perception of discomfort produced by exhausting exercise progressively reduces the conscious desire to over-ride this control mechanism, which, if it were to be reduced, would lead to the recruitment of more motor units4.
The effect of dopamine on fatigue
Studies in rodents have shown that increased dopaminergic activity is associated with increased performance5. And that chronic exercise induces plasticity in the dopaminergic pathways of the brain5.
Giving dopaminergic medication to human athletes that 'pace' during a prolonged exercise also leads to an increase in power output:
'pacing' represents the use of energetic resources during exercise, in a way such that all energy stores are used before finishing a race, but not so far from the end of a race that a meaningful slowdown can occur. It is believed that the pacing selected by athletes is largely dependent on the anticipated exercise duration and on the presence of an experientially developed performance template. [...] after dopamine reuptake inhibition, subjects are able to maintain a higher power output compared with placebo.6
TLDR: Given that dopamine is the 'motivation hormone', I would not be surprised that novices are on average less capable in pushing through when the going gets tough. This reduced motivation leads to a reduced effort and subsequently a lower motor-unit firing frequency.
Limitation: I was not able to find a paper where they actually measured motor-unit firing frequency changes after motivating exhausted novices. So, as per usual, more science is needed.
Citations
[1] https://pubmed.ncbi.nlm.nih.gov/6100456/
[2] Medical Physiology, Boron et al. Edition 3, page 1212
[3] https://onlinelibrary.wiley.com/doi/abs/10.1002/mus.880070902
[4] https://en.wikipedia.org/wiki/Central_governor - accessed on 18-04-2022
[5] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5649871/
[6] https://www.semanticscholar.org/paper/Neurophysiological-Determinants-of-Theoretical-and-Roelands-Koning/403b22a06c7399c1ce55ba11c0fc36026405f4a5
