Biomechanical Basis for Ergonomics - Fundamentals and Assessment Tools for Occupational Ergonomics - page 242

10000

8000

6000

4000

2000

0

< 40 40 - 50 50-60 > 60

Age

FIGURE 11.10 Mean and range of disc compression failures by age. (Adapted from National Institute for
Occupational Safety and Health (NIOSH) Work practices guide for manual lifting, Department of Health and
Human Services (DHHS), NIOSH, Cincinnati, OH, 81-122, 1981. With permission.)

the compressive load on the spine reaches a value of 9317 N, almost all of those exposed to the loading
will experience a vertebral endplate microfracture. It should also be noted that the tolerance distribution
shifts to lower levels with increasing age (Adams et al., 2000). In addition, it should be emphasized that
this tolerance is based upon compression of the vertebral endplate alone. Shear and torsional forces in
combination with compressive loading would be expected to further lower the tolerance of the end plate.
This distribution of risk has been widely used as the tolerance limits of the spine. However, it should be
noted that others have identified different limits of vertebral endplate tolerance. Jager et al. (1991) have
reviewed 13 studies of spine compressive strength and suggested different compression value limits. Their
summary of these spine tolerance limits are shown in Table 11.2. These researchers have also been able to
describe the vertebral compressive strength based upon an analysis of 262 values collected from 120
samples. They have related the compressive strength of the lumbar spine according to a regression equation:

Compressive Strength (kN) ¼ (7 : 26 þ 1 : 88G) 0 : 494 þ 0 : 468 G) A

þ (0 : 042 þ 0 : 106 G) C 0 : 145 L 0 : 749 S,

TABLE 11.2 Investigations into Static Lumbar Compressive Strength

Strength in kN

Reference n Mean s.d.

Wyss and Ulrich, 1954 8 5.89 2.24

Brown et al., 1957 5 5.20 0.54

Perey, 1957 142 5.15 2.10

Decoulx and Rienau, 1958 9 4.41 1.14

Evans and Lissner, 1959 11 3.51 1.22

Roaf, 1960 3 4.83 2.06

Eie, 1966 16 3.70 1.60

Farfan, 1973 39 3.84 1.22

Hutton et al., 1979 23 5.35 2.67

Hansson et al., 1980 109 3.85 1.71

Hutton and Adams, 1982 33 7.83 2.87

Brinckmann and Horst, 1983 22 6.42 2.00

Brinckmann et al., 1989 87 5.35 1.76

Female 132 3.97 1.50

Male 174 5.81 2.58

Total 507 4.96 2.20

Source: Adapted from Jager, Luhman, and Laurig, Int. J. Indust. Ergo., 1991.
With permission.