Civil Engineering Reference
In-Depth Information
FIGURE 10.1
Illustration of the
L4/L5 and the L5/S1 discs. The L5/S1
disc shows herniation. From the classic
drawings of Keegan (1953).
Since disc injuries occur in either the L4/L5 disc or the L5/S1 disc, the biomechanical
calculations are done for these discs. To calculate the compressive force on disc L5/S1
we make several assumptions. We assume that the individual weighs 75 kg, and that 65%
of the body mass is in the upper part of the body, denoted by the vector B (Lindh, 1980).
The length of the moment arm from the erector spinae muscle to the disc is 6 cm. The
calculations are illustrated in Figure 10.2.
Let us apply this model to the two different cases of lifting shown in Figure 10.3.
Assume that for the case of lifting with a bent back (A) the moment arms are w= 40 cm
and b=26 cm. For lifting with a straight back (B) the moment arms are somewhat
reduced: w=35 cm and b=18 cm.
Assuming that B=75 x 0.65×g=75×0.65×9.81=478 N and that W= 250 N, we can use
Equation 10.1 in Figure 10.2 to calculate that for case (A), ES= 3658 N. Assuming a
body inclination of 30°, the disc compressive force is calculated using Equation 10.2:
F=3658+478×0.89+2500.89=4306 N. Similarly, for case (B), ES is calculated to 2892 N
and, assuming a body inclination of 30°, F= 3540 N. This corresponds to a reduction in
disc compressive force by 18% for the case with bent knees (B).
