Thanks for this question! For determining motor function for AIS grades, we ignore levels where there is no muscle test – so the only levels that count toward that “half of key muscle functions” are C5-T1 and L2-S1.
For Practice ASIA 3, the neurologic level of injury is T10 and sensory and motor function are preserved in S4-5. Therefore, we look at the motor function in L2-S1 to determine the AIS grade. 7/10 of these levels (4/5 on the right and 3/5 on the left) are graded 3 or better. Because at least half of key muscle functions below the NLI have a muscle grade of >=3/5, this person’s injury is graded as AIS D.