Is it true that muscle fibers are a group of motor units, or is that false?

I think that only the muscle itself is a motor unit, so false?

edit: muscle fibers are motor units however the muscle itself is a motor pool

these are common warning signs of overtraining syndrome:

washed-out feeling, tired, drained, lack of energy

mild leg soreness, general aches, and pains

pain in muscles and joints

sudden drop in performance

insomnia

headaches

decreased immunity (increased number of colds, and sore throats)

decrease in training capacity/intensity

moodiness and irritability

depression

loss of enthusiasm for the sport

decreased appetite

increased incidence of injuries.

a compulsive need to exercise