What is the most likely diagnosis for a patient with chronic low back pain and stiffness that improves with movement and worsens in the morning, following a low back injury while squatting?

Most Likely Diagnosis: Inflammatory Back Pain (Axial Spondyloarthritis)

The clinical presentation of chronic low back pain and stiffness that improves with movement and worsens in the morning following a gym injury is most consistent with inflammatory back pain, specifically early axial spondyloarthritis (including pre-radiographic ankylosing spondylitis), rather than mechanical low back pain. 1

Key Distinguishing Features

The pattern described is pathognomonic for inflammatory rather than mechanical back pain:

• Morning stiffness that improves with movement is a cardinal feature of inflammatory back pain, whereas mechanical pain typically worsens with activity 1, 2
• Duration of months indicates chronicity beyond the typical self-limited course of mechanical injury 3
• The gym injury may be coincidental rather than causative, as inflammatory back pain often has an insidious onset 2

Clinical Parameters Supporting Inflammatory Back Pain

Five clinical parameters help identify inflammatory back pain, and this patient demonstrates at least three 2:

• Improvement with exercise (pain gets better with movement) 1, 2
• Morning stiffness (worse in the morning) 1
• No improvement with rest (implied by improvement with movement) 2
• If the patient is younger than 40-45 years (typical for gym-goers with squatting injuries), this further supports the diagnosis 1

Prevalence Considerations

While mechanical low back pain is far more common overall, the specific symptom pattern dramatically shifts the probability 1:

• Ankylosing spondylitis prevalence in primary care patients with chronic low back pain ranges from 0.3% to 5% 1
• Among patients with chronic back pain presenting before age 45, the prevalence of axial spondyloarthritis is estimated at 5% when inflammatory features are present 1
• The classic triad of improvement with exercise, morning stiffness, and worsening with rest has high specificity for inflammatory causes 1, 2

Critical Diagnostic Pitfall

The most important pitfall is attributing these symptoms to the gym injury and labeling it as mechanical low back pain. 1 While trauma can occur in patients with underlying inflammatory conditions, the symptom pattern—particularly improvement with movement and morning predominance—is inconsistent with typical mechanical injury, which would worsen with activity and improve with rest 4, 3.

Recommended Next Steps

• Assess for alternating buttock pain and whether pain awakens the patient during the second part of the night 1
• Consider HLA-B27 testing and inflammatory markers (ESR, CRP), though these have limited sensitivity 1
• MRI of the sacroiliac joints can detect inflammation before radiographic changes appear, as radiological sacroiliitis may take years to develop 1
• Early diagnosis is critical because NSAIDs should be taken regularly once diagnosis is made, and TNF-blocking agents show strong efficacy, particularly in patients with disease duration less than 10 years 1

Why Not Mechanical Low Back Pain?

Mechanical low back pain from a squatting injury would be expected to 4, 3:

• Worsen with activity rather than improve
• Improve with rest rather than worsen
• Show gradual improvement over weeks to months without the persistent morning stiffness pattern
• Not demonstrate the characteristic inflammatory pattern of symptoms