Final answer:
True, motor units can cause muscles to become tense, and with factors like action potential frequency and recruitment of more motor units, they can generate enough tension to cause movement.
Step-by-step explanation:
Motor units do indeed cause muscles to become tense; this statement is true. However, it's important to clarify that motor units can produce varying levels of tension, from a low level that does not cause movement, known as muscle tone, to a level that can produce significant movement when necessary.
A motor unit consists of a motor neuron and the group of muscle fibers it innervates. When the motor neuron sends an electrical signal to these muscle fibers, they contract.
The tension a muscle produces varies depending on numerous factors, including the frequency of action potentials and the number of motor neurons activating the muscle fibers.
A single contraction, or twitch, typically does not create enough force for movement, however, when multiple twitches occur rapidly, they can summate, leading to graded muscle responses and potentially movement.
Lastly, increasing the number of activated motor units through a process called recruitment can lead to stronger muscle contractions and movement.