Thanks for this question! This is one of the trickier concepts of the ASIA exam. Because there is no myotome testing for the trunk, we have to make assumptions about this region – and the ASIA is standardized to make conservative assumptions. In regions with no formal muscle test (T2-L1), motor function is presumed to be the same as sensory function if testable motor function above that level is also normal. So, even if motor scores are normal in all testable regions (upper and lower extremity), if sensation is impaired between T2 and L1, the motor level is the lowest level with intact sensation. This is also outlined in the “Steps in Classification” section on page 2 of the PDFs.
Let me know if you have any other questions!