MUSCLE-FIBER SIZE AND TYPE DISTRIBUTION IN THORACIC AND LUMBAR REGIONS OF ERECTOR SPINAE IN HEALTHY-SUBJECTS WITHOUT LOW-BACK-PAIN - NORMALVALUES AND SEX-DIFFERENCES

This study sought to investigate the normal muscle fibre size and type distribution of the human erector spinae, both in thoracic and lumbar regions, in a group of 31 young healthy male (n = 17) and female (n = 14) volunteers. Two percutaneous muscle biopsy samples were obtained under local anaesthesia, from the belly of the left erector spinae, at the levels of the 10th thoracic and 3rd lumbar vertebrae. Samples wer e prepared for routine histochemistry for the identification of fibre types. Fibre size (cross-sectional area (CSA) and narrow diameter (ND) ) was quantified using computerised image analysis. The mean CSA/ND fo r each fibre type was greater in the thoracic than the lumbar region, but there was no difference between the 2 regions either for percentag e type I (i.e. percentage distribution by number), percentage type I a rea (i.e. relative area of the muscle occupied by type I fibres) or th e ratio describing the size of the type I fibre relative to that of th e type II. Men had larger fibres than women, for each fibre type and a t both sampling sites. In the men, each fibre type was of a similar me an size, whereas in the women the type I fibres were considerably larg er than both the type IIA and type IIB fibres, with no difference betw een the latter two. In both regions of the erector spinae there was no difference between men and women for the proportion (%) of a given fi bre type, but the percentage type I fibre area was significantly highe r in the women. The erector spinae display muscle fibre characteristic s which are clearly very different from those of other skeletal muscle s, and which, with their predominance of relatively large type I (slow twitch) fibres, befit their function as postural muscles. Differences between thoracic and lumbar fascicles of the muscle, and between the muscles of men and women, may reflect adaptive responses to difference s in function. In assessing the degree of any pathological change in t he muscle of patients with low back pain, it seems clear that (1) sex cannot be disregarded and (2) 'atrophied' (using the criteria from oth er muscles) type II fibres are not necessarily abnormal for the erecto r spinae, particularly in women.