Background: Prospective population studies demonstrate that poor paraspinal muscle endurance increases the risk of developing first-time LBP and many CLBP studies also document excessive paraspinal muscle fatigability. The question arises as to whether this could have predisposed to chronic symptoms, through impaired spinal instability, especially in light of the wide inter-individual variation observed in the constitutionally determined paraspinal muscle fibre-type composition, which governs contractile performance.

Objective: To determine whether CLBP-associated excessive paraspinal fatigue results from a paucity in the type I fibre content.

Design: Control comparison using male subjects.

Subjects: Thirty-five CLBP patients with Von-Korff Chronic Pain Scores of ≤ III (high level of residual function, despite pain, to negate effects of disuse atrophy), and 32 controls of similar age.

Outcome measures: Fatigue-induced median frequency (MF) declines in the surface EMG signal, monitored bilaterally at L4 level during Biering-Sorensen- and 60%MVC- isometric fatigue tests. Percutaneous para-spinal muscle biopsies permitted histomorphometric comparisons.

Results: Between-group differences were assessed using independent t-tests (p < 0.05). There were no differences for MF decline during the Biering-Sorensen -0.37(0.16) vs. -0.36(0.12), and the 60% MVC test −0.42(0.31) vs −0.51(0.29), and in the percentage number of type I fibres, 63.6% vs 64.3%, or percentage area occupied by type I fibres, 69.4% vs 67.2%, in the paraspinal muscles for patients and controls respectively (p> 0.05).

Conclusion: Impaired CLBP-associated endurance is not the result of a constitutionally ‘adverse’ fibre-type composition.