Cumulative Spine Loading - Fundamentals and Assessment Tools for Occupational Ergonomics - page 308

30

26400 Cycles
(Complete Herniation)

81000 Cycles
(No Damage)

25

Specimen 1
Specimen 2

20

15

Initiation of
Herniation

10

5870
Cycles

3850
Cycles

5

0

1000

300 1000 1000

Compression Group (N)

FIGURE 13.7 Cumulative compression for two in vitro spine specimens tested in combined flexion extension

motion with static compression. Specimen 1 (300 N) exhibited no injury. The three columns representing
specimen 2 (1000 N) demonstrate the development of initiation of an injury and complete intervertebral disc
herniation (based on data from Callaghan and McGill, Clin. Biomech., 16, pp. 28-37, 2001. With permission.)

/

In summary, the idea of cumulative exposure being linked to injury is well supported by both repeti-
tive testing on isolated tissues at sub-acute levels and existing in vitro spine biomechanics research.
However, the clear influence of loading factors (i.e., load magnitude) as modifiers of cumulative
loading to failure that the spine can sustain raises the question of whether a single tolerance value can
be employed to assess a worker's risk of injury from cumulative loading exposure.

13.5 Cumulative Injury Theories

Ultimately the appeal of cumulative loading is in its ability to predict either injury or the reporting of low
back pain and thereby allow for intervention to prevent these events. One misleading outcome from the
presented in vitro work is that injury rate tends to be related to an increased magnitude of exposure
(Figure 13.8A). This response holds true for in vitro compressed time loading paradigms where exposure
is accelerated to represent weeks of loading in hours of testing. However, when biological repair processes

(a) (b)

Increased
chance of
Injury

Increased
chance of
Injury

Risk Risk

Exposure magnitude Exposure magnitude

FIGURE 13.8 Injury exposure relationships. Simple models (a) that represent increasing risk with increasing
exposure, either linearly or nonlinearly. The “J” or “U” nonlinear risk model (b), where there is an optimum
loading that results in minimal risk of injury.