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 were
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
for each fibre type was greater in the thoracic than the lumbar region, but
there was no difference between
the 2 regions either for percentage type I (i.e. percentage distribution by
number), percentage type I area (i.e.
relative area of the muscle occupied by type I fibres) or the ratio describing
the size of the type I fibre
relative to that of the type II. Men had larger fibres than women, for each
fibre type and at both sampling
sites. In the men, each fibre type was of a similar mean size, whereas in the
women the type I fibres were
considerably larger than both the type II A and type II B fibres, with no
difference between the latter two. In
both regions of the erector spinae there was no difference between men and
women for the proportion (%)
of a given fibre type, but the percentage type I fibre area was significantly
higher in the women.
The erector spinae display muscle fibre characteristics which are clearly very
different from those of other
skeletal muscles, 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 differences
in function. In assessing the
degree of any pathological change in the muscle of patients with low back
pain, it seems clear that (1) sex
cannot be disregarded and (2) ‘atrophied’ (using the criteria
from other muscles) type II fibres are not
necessarily abnormal for the erector spinae, particularly in women.