BACKGROUND: We sought to analyze whether the amount of paraspinal fatty degeneration correlates with a patient's physical fitness, and to determine if these findings on lumbar magnetic resonance imaging (MRI) scans can help predict functional outcomes. METHODS: A retrospective review was performed on 172 patients. Inclusion criteria involved being seen by a spine surgeon for low back pain, having aerobic index (AI), body mass index (BMI), Oswestry disability index (ODI), and body fat percentage measured recently, and having had a recent lumbar MRI scan. The percentage of fatty muscle degeneration was graded by three reviewers using T2-weighted axial images at L3 and L5 using a newly proposed system that was validated independently. The system is graded as follows: Grade 1: 0-24%, Grade 2: 25-49%, Grade 3: 50-74%, and Grade 4: 75-100%. An independent t-test was used for comparisons. RESULTS: The average AI was 34.87, and the cohort was divided into two groups: above-average AI (89 patients) and below-average AI (83 patients). For all paraspinal fat measurements and body fat percentage, the difference between the above- and below-average AI groups was statistically significant (P < 0.05), with the least amount of paraspinal fatty degeneration and body fat in the greater AI group. Weight alone and BMI were not found to be significantly different between those with above-average AI when compared to those with below-average AI (P = 0.491 and P = 0.122, respectively). There was a trend for lower ODI scores in the above-average AI group (41.9 vs 46.1), but this did not reach statistical significance between the two groups (P = 0.075). For all patients it was shown that there was significantly less paraspinal fat at the L3 level as compared to L5 (P < 0.001). CONCLUSIONS: We were able to show that patients with a higher AI have lower body fat percentages and lower amounts of fatty degeneration in their lumbar paraspinal musculature. The amount of paraspinal fatty degeneration, therefore, correlates with physical fitness. Patients with higher AI also showed a trend toward having a lower ODI score.