Objective. Quantification of the maximum anatomical cross-sectional area of the lumbar back
muscles as a function of torso flexion angle and development of prediction equations
as a function of torso flexion and anthropometric measures.
Background. Cross-sectional areas of the lumbar back muscles used as inputs into biomechanical
models have traditionally been derived from subjects lying in the neutral supine posture.
However, it is known that the cross-sectional area of muscle is altered as the torso
Design. Experimental design consisted of a two-factor multivariate analysis of variance
on the cross-sectional area of the lumbar torso muscle across the lumbar levels, as
a function of gender and torso angle. Hierarchical linear regression was utilized
to assess the association between cross-sectional area and individual and torso posture
Method. Axial MRI scans, through and parallel to each of the lumbar intervertebral discs
at four torso flexion positions were obtained from subjects in a lateral recumbent
posture. Cross-sectional areas were quantified and converted into anatomical cross-sectional
areas utilizing known fascicle orientations.
Results. The maximum anatomical cross-sectional area was located between the L3/L4 and L4/L5 level in the neutral posture. The anatomical cross-sectional areas at the L4/L5 and L5/S1 decreased during torso flexion, however, the percent change varied as a function
of the individual level. The majority of the anatomical cross-sectional area variability
was explained by gender and body mass. Lumbar curvature explained a larger proportion
of the anatomical cross-sectional area variability at the lower lumbar levels than
at the higher lumbar levels.
Conclusions. The maximum anatomical cross-sectional area of the lumbar back muscles occur at
the neutral torso posture and did not decrease as a function of torso flexion. When
using maximum anatomical cross-sectional area or specific lumbar level anatomical
cross-sectional areas, it appears necessary to account for gender and body mass. At
the lower lumbar levels, knowledge of spinal curvature plays an increasing role in
the estimation of the size of the lumbar torso muscle cross-sectional area.Relevance
This research indicates the lower lumbar level trunk muscle anatomical cross-sectional
area decrease as torso flexion increases, however, the maximum lumbar trunk muscle
anatomical cross-sectional area does not vary as a function of torso flexion. Accounting
for gender, body mass, torso characteristics and lumbar curvature may help increase
accuracy of anatomical cross-sectional area prediction, as well as muscle force predictions
from biomechanical models.