Biological tissues are like all other physical materials with afinite life, and similarly subject to wear and tear. They are capable of self-repair but undergo mechanical degradation with
0 20 40 60 80 100 Fatigue in combined extension and rotation (female)
Time in percent of the task cycle
0 20 40 60 80 100
0 20 40 60 80 100
Time in percent of the task cycle 0.6
Figure 1.12 Differential fatigue of the trunk muscles in combined motions of right extension–
rotation as seen by decline in raw MF (right panel) and normalized MF (left panel) of trunk muscles.
(Muscles: RL3, right erector spinae at third lumbar vertebral level; LL3, left erector spinae at third lumbar vertebral level; RT10, right erector spinae at tenth thoracic vertebral level; LT10, left erector spinae at tenth thoracic vertebral level; REO, right external oblique; LEO, left external oblique; RLD, right latissimus dorsi; LLD, left latissimus dorsi.) (From Kumar, S., Ergonomics, 44, 17–47, 2001.)
repeated and prolonged usages. All biological tissues are viscoelastic and their prolonged loading may result in permanent deformation. Repeated load application may also result in cumulative fatigue, reducing their stress-bearing capacity. Such changes may reduce the threshold stress at which the tissues fail. Kumar (1990a,b) reported a strong association between cumulative load (biomechanical load and exposure time integral over entire work life) and LBI=pain (p < .01). Among nursing aides employed in public sector in Alberta, Kumar (1990a,b) reported that 42.8% of males and 64.6% of females were having back pain.
Data on length-of-service before the onset of thefirst episode of back pain in age, sex, body weight, height, occupational activities, and recreational activities matched samples revealed that in the pain group, the number of cumulative years worked at which pain precipitated was significantly greater than the number of cumulative years worked by the no-pain group (p < .01). The mean cumulative compression and shear loads for subjects were obtained from the biomechanical analysis of each of the job tasks performed. Each of the task cycles was biomechanically analyzed for compression and shear load on the spine at discrete time intervals dividing each task cycle into 200 ms chunks. By summing the load–time product for 200 ms chunks, the total load–time product for one task cycle was obtained. This load–time product (Newton second [N s]) was multiplied by the number of cycles performed on a shift to obtain the cumulative load of the shift, both for compression and shear. By similarly analyzing all other tasks performed by the subjects in the sample total cumulative compression and shear loads in units of Newton second were obtained.
Such mean cumulative compression and shear loads sustained by the no-pain and pain groups for male and female samples in the study are presented in Figures 1.13 and 1.14.
A student t-test of the comparison of the means revealed that the cumulative compression at thoracolumbar and lumbosacral discs was significantly higher in the pain group (p < .05 or better). The cumulative shear in the male pain group was also significantly higher than that of the male no-pain group (p < .02). The mean cumulative daily compression and shear loads and their standard deviations at the thoracolumbar and lumbosacral regions of the male sample respondents with and without pain were not significantly different. The data also revealed that there was no significant difference between pain and no-pain groups in any of the biomechanical spinal load variables on the job when compared for one task cycle. However, the total time spent working by the pain group was significantly
Compression Shear Compression Shear 0
6 12 18 24
Cumulative load (MN s)
Pain No pain
Lumbosacral Male
Thoracolumbar
Figure 1.13 The mean cumulative compression and shear loads in pain and no-pain groups in male sample (Mega Newton seconds [MN s]). (From Kumar, S., Ergonomics, 44, 17–47, 2001.)
higher than that of the no-pain group, thus causing a significant difference in cumulative loads (p< .001). The data for such lifetime cumulative load exposure for pain and no-pain groups are presented in Figure 1.15.
The structural failure of musculoskeletal components can precipitate in either acute or chronic conditions. Most of the studies have concentrated on determination of peak forces in activities with known high stresses (Bartelink 1957; Ayoub 1977; Ayoub et al. 1978, 1980;
Schultz et al. 1982; Kumar and Davis 1983; McGill and Norman 1985; Anderson and Chaffin 1986; Kumar 1991; Kumar and Garand 1992; Waters et al. 1993). Brinckmann et al. (1987, 1988) and Hansson et al. (1987) investigated the fatigue failure of the lumbar spine. In their experimental protocol, loads ranging 20%–30%, 30%–40%, 40%–50%, 50%–60%, 60%–70%, and 75% of the estimated ultimate compressive strength (UCS) of spinal units were applied at a frequency of 0.25 Hz. They found that both the magnitude of the load and
Compression Shear Compression Shear 0
6 12 18 24
Cumulative load (MN s)
Pain No pain
Lumbosacral Female
Thoracolumbar
Figure 1.14 The mean cumulative compression and shear loads in pain and no-pain groups in female sample (MN s). (From Kumar, S., Ergonomics, 44, 17–47, 2001.)
Compression Shear Compression Shear 0
600
300
Mean generic cumulative load (MN)
Pain No pain
Lumbosacral Male
Thoracolumbar
Figure 1.15 The mean generic cumulative compression and shear at thoracolumbar and lumbosacral levels among males (MN s). (From Kumar, S., Ergonomics, 44, 17–47, 2001.)
the number of cycles affect the spinal unit failure. At lower loads high repetition and at higher loads low repetition produced fatigue failures. When their specimens were loaded between 50% and 60% of the UCS, 92% suffered fatigue failures after 5000 cycles. A 91%
fatigue failure rate was reported after 500 cycles when the load was increased by an additional 10%, and at a load of 75% of UCS the fatigue fractures were precipitated only in 10 cycles. Long-range, low-grade loading of the spine will be difficult to control and measure. However, looking at the results of Brinckmann et al. (1987, 1988), it would appear that physiological limitations strongly favor biological safety by preventing cumulative load from rising rapidly as the maximum voluntary contraction level can neither be held for a long time, and nor can it be repeated in quick succession. In addition, the compression generated by the maximum voluntary contraction ranges between 68% and 77% of the UCS (Kumar and Mital 1992). When one considers that the MVC can be sustained only for a few seconds and that it decays exponentially with the duration of the hold (Rohmert 1973), it is obvious that such cumulative compressions cannot rise rapidly. Also, the MVC cannot be repeated without long rest pauses. Rapid repeated trials of force exertion degenerate quickly with a drastic reduction in magnitude, thereby preventing the total exposure (load3 time) from rising. It must be borne in mind that rapid cyclic loading does not allow much needed recovery time to the viscoelastic biological tissues. This in turn pro-gressively accentuates the deformation, rendering the tissues more vulnerable to injuries due to higher stress concentration.