In Brief:
A population-based cohort of 554 young adults was clustered according to low back symptoms and functional limitations at 18, 19, and 21 years. Low back symptom severity did not associate with paraspinal muscle atrophy, that is, multifidus fat content using Opposed-Phase magnetic resonance imaging or cross-sectional area of erector spinae and multifidus muscles, at a mean age of 21 years.