Torso Flexion Loads and the Fatigue Failure of Human

Lumbosacral Motion Segments

Sean Gallagher, PhD, CPE,*

From the *National Institute for Occupational Safety and Health,
 

Pittsburgh, PA; and

William S. Marras, PhD, CPE,†

†Biodynamics Laboratory,

Alan S. Litsky, MD, ScD,‡

‡Orthopaedic BioMaterials Laboratory, and

and Deborah Burr, PhD§¶
 

§Division of Epidemiology and Biometrics,
 

Ohio State University, Columbus, OH.
 

¶Current address: Department of Health Services, Research Management, and Policy, University of Florida, Gainesville, Florida
 

Study Design. Spine loads associated with lifting a

9-kg weight were estimated at three torso flexion angles

(0°, 22.5°, and 45°), and lumbosacral motion segments

were cyclically loaded using these loads until failure or to

a maximum of 10,020 cycles.

Objectives. To simulate the postures and loads experienced by the lumbar spine during repetitive lifting of

moderate weights in different torso flexion postures, and

to analyze the fatigue failure response of lumbosacral

motion segments.

Summary of Background Data. Previous fatigue failure studies of lumbar motion segments have not reproduced the combination of spinal postures, loads, and load

rates anticipated in different torso flexion postures during

lifting tasks characteristic of those in occupational

settings.

Methods. Twelve fresh human lumbosacral spines

were dissected into three motion segments each (L1–L2,

L3–L4, and L5–S1). Motion segments within each spine

were randomly assigned to a simulated torso flexion angle (0°, 22.5°, or 45°) using a partially balanced incomplete block experimental design. Spinal load and load rate

were determined for each torso flexion angle using previously collected data from an EMG-assisted biomechanical model. Motion segments were creep loaded for 15

minutes, then cyclically loaded at 0.33 Hz. Fatigue life was

taken as the number of cycles to failure (10 mm displacement after creep loading). Specimens were inspected to

determine failure mechanisms.

Results. The degree of torso flexion had a dramatic

impact on cycles to failure. Motion segments experiencing the 0° torso flexion condition averaged 8,253 cycles to

failure (±2,895), while the 22.5° torso flexion angle averaged 3,257 (±4,443) cycles to failure, and motion segments at the 45° torso flexion angle lasted only 263 cycles

(±646), on average. The difference was significant at P <

0.0001, and torso flexion accounted for 50% of the total

variance in cycles to failure.

Conclusions. Fatigue failure of spinal tissues can occur

rapidly when the torso is fully flexed during occupational

lifting tasks; however, many thousands of cycles canbe

tolerated in a neutral posture. Future lifting recommendations should be sensitive to rapid development of fatigue failure in torso flexion.

The findings and conclusions in this report are those of the author(s)
 

and do not necessarily represent the views of the National Institute for
 

Occupational Safety and Health.
 

The manuscript submitted does not contain information about medical
 

device(s)/drug(s).
 

Federal funds were received in support of this work. No benefits in any
 

form have been or will be received from a commercial party related
 

directly or indirectly to the subject of this manuscript.
 

Address correspondence and reprint requests to Sean Gallagher, PhD,

National Institute for Occupational Safety and Health, Pittsburgh

Research Laboratory, PO Box 18070, Pittsburgh, PA 15236-0070;
 

E-mail: sgallagher@cdc.gov

Recent reviews of the epidemiology literature have concluded that several work-related physical factors show

consistent and positive associations with the occurrence

of low back disorders.1–3 Among the occupational factors exhibiting positive associations are jobs involving

manual materials handling activities,1,4 – 6 jobs involving

frequent bending,7–9 and jobs possessing a demanding

workload.10 –12 A common theme among these occupational risk factors is that all place the lumbosacral spine

under high levels of stress. Moreover, these stresses may

be experienced several hundred times during a typical

workday,8,13 especially in physically demanding occupations such as construction or mining.

Spinal tissues, like all materials, are subject to fatigue

failure.14 It is apparent that occupational lifting tasks are

capable of providing both loading magnitudes and frequencies sufficient to initiate and propagate fractures of

the vertebral endplates and the underlying trabecular arcades in fatigue testing.13,15,16 However, previous studies have not clearly elucidated the role of torso flexion

during lifting tasks on the fatigue life of lumbosacral

motion segments. Torso flexion influences a number of

factors that impact fatigue life of spinal tissues. One obvious impact is that the magnitude of loading may be

doubled or tripled when lifting in flexion compared with

the neutral posture.17,18 However, several other factors

that may influence fatigue life are also affected by torso

flexion. For instance, flexion of the spine may also alter

the pathway of loading in the spine, with zygapophysial

joints bearing up to 20% of an axial load in the neutral

posture,19 whereas the interbody joints bear a larger

brunt of the load in flexion.20 Biomechanical models also

indicate that shear forces acting on the spine are considerably greater in torso flexion than in the neutral posture.21 The rate of spinal loading (load rate) has been

demonstrated to increase under conditions of increasing

lumbosacral moments and trunk motion, both of which
 
would be greater in lifts from flexed torso postures compared with upright postures.22 Since all spinal tissues

exhibit changes in material characteristics because of

their viscoelastic nature, proper modeling of load rate is

an important consideration in simulating spinal loading

experienced in tasks such as occupational lifting.23,24 To

date, no studies have examined the fatigue life of motion

segments when exposed to the combinations of motion

segment orientation, spinal loads, and load rates associated with varying torso flexion angles experienced in

common occupational lifting tasks. Accordingly, the current study simulates the spinal postures and loading conditions present when lifting a 9-kg load initiated at different angles of torso flexion, and examines the impact in

terms of the fatigue life and failure modes of human

lumbosacral motion segments.

Materials and Methods

Realistic simulation of spinal loading associated with lifting of

a given load at different angles of torso flexion necessarily involves simultaneous adjustment of several factors. In this

study, we simulate lifting a 9-kg load initiated at three torso

flexion angles (0°, 22.5°, and 45°). Important factors that

change with lifting in these flexion angles include the load magnitude, the load rate, and the degree of flexion of the individual

motion segments of the spine. Additionally, different motion

segments of the spine will experience a different combination of

compressive and shear forces within each flexion angle. In this

study, we have attempted to simulate the entire amalgamation

of changes in spinal loading that occur as a given load is lifted

in different torso flexion angles. Thus, lifting this given load in

a neutral torso posture would consist of a lower magnitude of

loading, a lower load rate (important because of the viscoelastic nature of spinal tissues), and motion segment angles characteristic of an erect torso posture. Lifting the load in a flexed

posture would be associated with a higher load magnitude,

load rate, and flexion of motion segments. Thus, when we

describe changes in the fatigue life of motion segments in different torso flexion angles, it must be recognized that these

changes are the result of the changes in the entire ensemble of

factors that would be expected to change with torso flexion in

occupational (or nonoccupational) lifting tasks.

Twelve fresh, frozen human lumbosacral spines (sacrum

through L1) were procured from the anatomic gift program at

Wright State University. Details of the specimens are contained

in Table 1.

Table 1. Specimen Details

Specimen Age Cause of Death Listed on

No. Gender (yr) Death Certificate

1 M 77 Respiratory failure

6 F 82 Provisional

7 M 65 Blunt head trauma

8 M 74 Respiratory arrest

9 F 85 Pneumonia

10 M 80 Gastric carcinoma

11 M 85 Provisional

12 M 84 Congestive heart failure

13 F 91 Congestive heart failure

14 M 73 Cardiopulmonary arrest

16 M 79 Pancreatic carcinoma

17 F 93 Provisional

The sample consisted of 8 male and 4 female lumbosacral spines. The

age of the specimens was 80.7 years (±7.8 years).

As can be seen, the sample consisted of older spines,

a product of the inability of the vendor to procure young specimens during the time-frame of the study. Spines were excised

within 24 hours after death from subjects having no known

history of spinal disease or prolonged bed rest before death.

Intact lumbar spines were thawed at room temperature for

several hours until tissues were sufficiently thawed for dissection into three separate motion segments: L1–L2, L3–L4, and

L5–S1. Excess musculature, adipose tissue, and fascia were removed from the motion segments, and ligaments spanning

multiple levels were sectioned. Ligaments confined to a single

motion segment were preserved.

Anteroposterior and left lateral radiographs of each motion

segment were taken at 50 kV, 5 mA, and a film focus distance

of 38". These initialradiographsserved to detect the presence of

existing defectsin individual motion segments and were used to

determine the relative angles of all four endplates of the motion

segment.

Specimens were mounted in two trays using polymethylmethacrylate (PMMA). Specimens were aligned so that the superior endplate of the inferior vertebral body was placed parallel to the edge of the lower tray (i.e., in a horizontal

orientation), using the angles obtained in the radiographs.

Once inferior vertebrae had been potted, specimens were xrayed again and these radiographs were used to ascertain

whether the superior endplates of the inferior specimens were

indeed in a horizontal orientation, and to determine the Cobb

angle of the motion segment (Figure 1).

Figure 1. Radiograph illustrating measurement of Cobb angle for

potted specimen.

Based on the Cobb angles measured in the second radiograph,

the superior vertebra of each motion segment was potted so that

an angle representative of a neutral posture for each motion segment was achieved when the upper and lowerspecimen trays were

parallel to one another. Neutral posture Cobb anglesforthe three

motion segments studied were operationally defined as: 20° for

L5–S1, 8° for L3–L4, and 0° for L1–L2, based on data provided

by Chen.25 Flexion of the motion segments, necessary to simulate

the 22.5° and 45° torso flexion angles, were calculated relative to

the neutral motion segment posture.

It should be noted that achieving an overall torso flexion

angle isthe result of a summation ofrelatively small amounts of

flexion at the motion segment level.25–27 Indeed, the maximum

amount of flexion possible in a lumbar motion segment in extreme flexion appears to be approximately 10°, with slightly
more flexibility than this in the central lumbar region and

slightly less at either end.26 Data from studies of radiographic

and MRI studies used in this study suggest that lumbar motion

segments are flexed approximately 50% to 75% of their maximum at 45° of torso,25–27 as reflected by the values shown in

Table 2.

Table 2. Loading Details for Motion Segments in the Present Experiment
 

Motion Segment

Trunk Flexion From Resultant Force Load Minimum Maximum Total Cycle Loading Relaxation

Posture Neutral Angle Rate Load Load Time Phase Phase

Level (°) (°) (°) (N/s) (N) (N) (s) (s) (s)

L1–L2 0 0 25 700 500 1300 3 1.14 1.86

22.5 1 26 2100 750 2400 3 0.79 2.21

45 3 24 4800 1050 3150 3 0.44 2.56

L3–L4 0 0 12 700 500 1300 3 1.14 1.86

22.5 3 20 2100 750 2400 3 0.79 2.21

45 6 31 4800 1050 3150 3 0.44 2.56

L5–S1 0 0 -23 700 500 1300 3 1.14 1.86

22.5 3 -6 2100 750 2400 3 0.79 2.21

45 6 7 4800 1050 3150 3 0.44 2.56

Angles of the resultant force are calculated with respect to the perpendicular of the superior endplate of inferior vertebral body for each motion segment. A positive

angle is indicative of a posterior shear.

Once both vertebrae of the specimen were potted, the specimen was placed in a test jig designed to mimic the loading

experienced during lifting tasks, as determined by a dynamic

biomechanical model.21 A loading jig consisting of angled

metal plates (as seen in Figure 2) was used in concert with a

servohydraulic test frame (MTS Bionix 858, Eden Prairie, MN)

to impose a combination of compression and shear forces on

the motion segment for specified levels of flexion.

Figure 2. Illustration of loading jig with specimen attached, inside

environmental chamber.

Compression

and shear loads at the lumbosacral junction were developed

using analyses of lifting tasks starting in each torso flexion

angle using a dynamic EMG-assisted biomechanical model. Estimates of the load rate experienced with lifts initiated in each

posture were also provided by the model. Estimates of compression and shear loads at the upper spinal levels within each

spinal position (e°, 22.5°, and 45° torso flexion) were obtained

by treating the lumbar spine as a rigid object and resolving the

forces experienced at the S1 endplate into normal (compressive) and tangential (shear) forces at the upper levels based on

the angle of the resultant force with respect to the L4 and L2

superior endplates. Thus, while the spinal orientation was different in each torso flexion angle, it was assumed to be rigid

within each torso flexion angle so that upper level loads could

be estimated. Table 2 provides load estimates derived in this

fashion. Figure 3 displays graphically the cyclic loading regimen employed for each torso flexion angle.

Figure 3. Loading profiles for simulated lifting of 9-kg load in 0°,

22.5°, and 45° of torso flexion.

A randomized block partially confounded factorial design

was used in this experiment to evaluate the effects of posture

and lumbar level on fatigue strength (number of cycles to failure) of lumbar motion segments.28 The three motion segments

dissected from each spine (i.e., L1–L2, L3–L4, and L5–S1) were

tested at three levels of simulated torso flexion (0°, 22.5°, and

45°), using load rates and forces characteristic of those experienced during in vivo lifting tasks in these postures, based on

results of the biomechanical model. Blocking on spines helped

control for the large amount of biologic variability typically

observed when testing cadaver materials. In particular, the influences of age and gender are known to have a large influence

on the strength of the lumbar spine motion segments.29 The

current design allowed control of this variability and more

precise estimates of the effects of lumbarlevel and torso flexion.

The design confounded a portion of the level X posture interaction; however, intrablock information was available to recover the within blocks portion of this interaction. As a result,

the interaction between lumbar level and posture remained es-
timable. Two replications of the design were conducted, using

12 complete spines (36 motion segments).

All tests were performed in a humidified environmental

chamber where the temperature was maintained at 37 C. A

preload of 500, 750, or 1,050 N (dependent on the torso flexion angle) was placed on the specimen for a minimum of 15

minutes to precondition the specimen. The loss of height due to

creep experienced by the specimen during this period was measured and recorded. Cyclic loads were placed on the specimens

at a rate of 0.33 Hz (i.e., one cycle every 3 seconds) to simulate

a repetitive lifting task. Figure 3 depicts the loading cycles for

simulated lifting of a 9-kg load in the three torso flexion conditions. Load deformation curves were collected at 100 Hz

using an A/D data acquisition board and were monitored at

regular intervals throughout the test. Fatigue-induced failure of

the motion segments was operationally defined as occurring

when the specimen deformed greater than 10 mm from its

height at the conclusion of the 15-minute pretest creep loading

period. Motion segments lasting the entire 10,020 cycles in the

fatigue failure test (i.e., censored observations) were subjected

to a test of ultimate strength at the same load rate used in the

cyclic loading regimen.

On completion of the mechanical testing, specimens were

removed from the loading jigs and taken to the radiology clinic

to evaluate damage using a lateral radiograph. These radiographs were read to evaluate the post-test status of the endplates, vertebral bodies, trabeculae, intervertebral discs, and

posterior elements. Visual inspection of failed specimens was

performed to evaluate external evidence of failure, particularly

facet disruption and/or evidence of fracture to the vertebral

bodies. The PMMA X specimen interface was scrutinized to

ensure that failure did not occur at this interface. Zygapophysial joint damage was classified into three categories, based on

visual inspection and functional tests involving movement of

the motion segment. The first category was no damage to the

joint capsule. Visual inspection confirmed that the joint was

intact, and functional tests indicated normal joint motion. The

second category of damage was increased joint laxity. Capsular

ligaments were observed to be intact, although increased

stretching of the ligaments was usually apparent, as was abnormal joint motion. The third category was frank disruption of

the zygapophysial joint, where capsular ligaments were completely avulsed and the inferior and superior facets were completely disengaged. Failed specimens were dissected through

the midplane of the intervertebral disc and digitally photographed, and a Galante disc degeneration score was assigned to

each of the discs.30 The endplates were then exposed and examined for damage.

Results

Changes in spinal loading associated with lifting a simulated 9-kg load in different angles of torso flexion had a

highly significant influence on the fatigue life of lumbosacral motion segments (P < 0.0001). Motion segments

loaded in the 0° condition averaged 8,253 cycles to failure (±2,895), specimens at 22.5° lasted 3,257 (±4,443),

while those at 45° lasted an average of only 263 (±646)

cycles (Figure 4).

Figure 4. Cycles to failure by torso flexion angle. Spine loads

associated with lifting 9 kg in 0° flexion resulted in long fatigue life;

however, fatigue failure was rapid when exposed to loads associated with lifting the same weight in 45° torso flexion.

The ratio of maximum to minimum

variance between torso flexion conditions for the untransformed data exceeded 3.0; therefore, a log transformation of cycles to failure was performed and the subsequent analysis variance was found to satisfy the

homogeneity of variance assumption.31 The F statistic

for torso flexion angle in the transformed analysis was

F2,16 = 36.10 (P < 0.0001), and the estimates of the

effectsize forthisfactorindicate that it accounted forjust

over 50% of the variance in cycles to failure (e2 =

0.503). Neither level of the motion segment nor the interaction of flexion X level significantly influenced cycles

to failure (P > 0.05). Figure 5 provides data on the ultimate strength of specimens surviving the maximum

number of loading cycles.

Figure 5. Ultimate strength for specimens lasting maximum number of loading cycles. Lack of a standard deviation bar indicates

only one specimen was tested in this condition.

Although neither motion segment level nor load rate was a significant factor in terms

of ultimate strength (P > 0.05), this is likely because of a

lack of power from the relatively small number of motion segments tested (11 specimens).

Table 3 contains a summary of post test analyses of

the mechanisms and sites of failure for the lumbosacral

motion segments used in this study. This table includes

specimens failing via fatigue as well as those failing in

tests of ultimate strength. Table 4 provides a breakdown

of failure modes by simulated torso flexion angle for

specimens experiencing fatigue failure. As indicated in

this table, only four of 12 specimens failed by fatigue in

the neutral posture, while mild and full flexion had 9 and

12 specimens fail via fatigue, respectively.

As can be seen, failure of the motion segments generally involved endplate fractures, compression/shear

damage to the vertebral bodies, and/or disruption of the

facet joints. Endplate fractures were frequently observed

as a means of failure of the motion segment, often in

combination with (and sometimes as part of) other vertebral body fractures. The classification system devel-
oped by Brinckmann et al13 did not fully characterize the

nature of the endplate fractures observed in this investigation; thus, a modified version of the classification system was developed. This new classification system, illustrated in Figure 6, includes several of the Brinckmann et

al13 classifications but adds some additional classifications of endplate damage based on fracture patterns observed in the present study. The endplate fracture patterns derived from the Brinckmann et al13 classification

scheme include stellate, endplate depression, intrusion,

step, and edge fractures. New endplate damage classifications include the ring fracture, lateral fracture, anteroposterior fracture, and soft (spongy) endplate.

The most frequent endplate damage findings were

endplate depressions (78% of specimens) and stellate

endplate fractures (47% of specimens). Lateral fractures

(9 instances), ring fractures (8 instances), and endplates

with soft, spongy areas (8 instances) were each present in

approximately one fourth of all of the specimens tested.

Step fractures (5 instances), intrusions (2), anteroposterior fractures (2), and edge fractures (2) were seen more

rarely. As can be seen in Table 3, combinations of endplate damage modes were often observed.

Table 3. Summary of Failure Mechanisms for All Specimens
 

Disc Inferior Superior Superior Inferior

Spine Motion Torso Failure Galante Endplate Endplate Vertebral Vertebral Facet

ID Segment Angle Mode Grade (superior VB) (inferior VB) Body Body Disruption Comments

11 L1–L2 22.5 FF 3 1,6 4 Comp/shear Comp/shear L Left pedicle of L2 buckled

L3–L4 45 FF 3 9 Comp/shear Comp/shear

L5–S1 0 US * 6 1,7 Neural arch fracture on L5

8 L1–L2 45 FF 2 6 6 Comp/shear RL L4 facet fractured

L3–L4 0 US 2 6 6 Comp/shear

L5–S1 22.5 US 2 5 9 Comp/shear

1 L1–L2 0 FF 3 2 Comp/shear L

L3–L4 22.5 FF 2 2,1 L (lax)

L5–S1 45 FF 2 2,6 Comp/shear

9 L1–L2 45 FF 3 4,5 5 Transverse

L3–L4 22.5 US * 7 2 Comp/shear Comp/shear RL

L5–S1 0 FF 3 6 6 Comp/shear RL (lax)

7 L1–L2 22.5 FF 3 6.5 6 Comp/shear LTD Blood in posterior central disc

L3–L4 0 US 2 6,3 2,4 Transverse Annular protrusion, left pedicle

of L4 fractured

L5–S1 45 FF 4 6 Comp/shear RL Disc space narrowed

6 L1–L2 0 US 2 3,4 1 Comp/shear Comp/shear

L3–L4 45 FF 2 1 RL (lax)

L5–S1 22.5 US 3 Comp/shear Disc space narrowed

17 L1–L2 22.5 FF 3 1 6

L3–L4 45 FF 2 6 1,6 Comp/shear L

L5–S1 0 FF 2 6 RL

10 L1–L2 45 FF 2 1,4 8 Comp/shear Shear fracture of L1 separated

EP from VB

L3–L4 0 US 2 7 8,4 Transverse R,L (lax)

L5–S1 22.5 FF 3 6,4,5 Comp/shear RL (lax)

13 L1–L2 0 FF 3 6 Comp/shear RL Disc lax in torsional movement

L3–L4 22.5 FF 2 1,6 6 Comp/shear

L5–S1 45 FF 2 1 1 Comp/shear

16 L1–L2 45 FF 2 1 Comp/shear

L3–L4 22.5 FF 1 1,5 Comp/shear

L5–S1 0 US 2 1,6 6,4 Comp/shear RL (lax) Anular protrusion right

posterolateral aspect

12 L1–L2 22.5 FF 3 2,7 Comp/shear RL

L3–L4 0 US 3 2,7 1,6 Comp/shear R,L (lax) L3 spinous process pushed

posterolaterally

L5–S1 45 FF 3 2,5 LTD

14 L1–L2 0 US 3 1,6 2 Comp/shear Comp/shear

L3–L4 45 FF 2 1 6,5

L5–S1 22.5 FF 3 6,5 6,5

See Figure 5 for endplate fracture classifications. Failure mechanisms included endplate fracture, compression and shear fractures of the vertebral bodies, and
 

zygapophysial joint damage.
 

Note: * = Galante grade not determined, FF = fatigue failure, US = ultimate strength, LTD = localized trabecular disruption, L = left facet, R = right facet, (lax) =
 

increased zygapophysial joint laxity.
 

The reader is

cautioned when examining these results that it is difficult

to know the degree to which preexisting endplate damage may have been present and how existing damage

may have influenced the observed results. While a significant portion of the damage would be expected from the

cyclic loading imposed on the motion segments, it cannot

be stated with certainty that the endplate damage observed was solely the product of these loads.

Another recurrent finding in post test examinations

was compression/shear fractures of one or both of the

vertebral bodies of the motion segment. Such fractures

were more common in superior compared with inferior

vertebral bodies. Transverse vertebral body fractures
were observed in only 3 cases, always in the superior

vertebral body of the motion segment. Localized trabecular derangement was observed in radiographs in two

inferior vertebral bodies showing no overt external evidence of fracture.

Examination of the failed specimens also turned up a

number of cases where zygapophysial joints exhibited

significant laxity or were disrupted entirely. Complete

disruption of the zygapophysial joints was more common in the neutral (0°) torso posture, zygapophysial

joint laxity was the more common finding at 22.5° of

torso flexion, and damage to the facets was rarely seen in

full flexion. Intervertebral discs tended not to incur a

great deal of overt damage. However, examination of the

sectioned disc on several occasions led to the observation

that the internal lamellas of the disc appear to have experienced some distortion, as previously observed by

Adams and Hutton,32 which may have been the result of

exposure to the experimental conditions.

Table 4. Failure Modes by Torso Flexion Angle for Specimens Failing by Fatigue
 

0° (4 fatigue failures) 22.5° (9 fatigue failures) 45° (12 fatigue failures)

Inferior EP Superior EP Inferior EP Superior EP Inferior EP Superior EP

Damage Type (Superior VB) (Inferior VB) (Superior VB) (Inferior VB) (Superior VB) (Inferior VB)

Endplate damage None 1 2 1 3 1 5

Stellate 5 5 2

Lateral 1 2 2

Ring 1 2 1

Soft endplate 3 1 2 3

EP depression 2 2 4 4 4 4

Step 1

Intrusion 1

Edge 1

Zygapophysial joint damage Left Right Left Right Left Right

None 1 5 7 9 10

1laxity 1 1 3 2 1 1

Disruption 3 2 1 2 1

Vertebral body fractures Superior Inferior Superior Inferior Superior Inferior

Comp/shear 2 1 5 1 6 2

Transverse 1

Localized trabecular 1 1

disruption

Increasing torso flexion was associated with a greater no. of fatigue failures and increasing severity of damage.

Figure 6. Vertebral endplate fracture classification system. Several classifications of endplate damage are derived from Brinckmann et al.

13 However, additional classifications were included

based on fracture patterns observed in the present study. Numbers in parentheses represent the incidence of observed endplate

fractures in all tests (combined fatigue and ultimate compressive

strength tests), and in specimens failing via fatigue.

Discussion

Changes in spine loading attendant to simulated load

lifting in a range of torso flexion postures had a dramatic

impact on the fatigue life of lumbosacral motion segments. It is remarkable that, even with the elderly cohort

of spines tested in this study, specimens could withstand

several thousand loading cycles when subjected to a load

simulating lifting a 9-kg weight in an upright torso position. Even with mild flexion, specimens lasted over 3,000

cycles, on average. However, the loading conditions associated with full flexion resulted in rapid fatigue failure

of motion segments.

These findings are congruent with evidence from various sources associating torso flexion with low back disorders. For example,severalrecent epidemiologic studies

have indicated that torso flexion is an important risk

factor for low back pain.7–9,33 The retrospective casereferent study by Punnett et al9 disclosed a dose-response

association between the degree of torso flexion (mild

flexion, odds ratio [OR] = 4.9; severe flexion, OR = 5.7

vs. with neutral) and the risk of low back disorders, mirroring the fatigue life results in the current study. Holmstrom et al7 found that time spent in torso flexion was

strongly associated with severe low back disorders

(OR = 2.6), while Marras et al8 found that peak sagittal
flexion angle was a significant predictor of membership

in high-risk versus low-risk group membership for low

back disorders (univariate OR = 1.6). In this study, the

high-risk group had an average maximum sagittal flexion angle of 18°, while the low-risk group averaged 10°

maximum sagittal flexion.8 In addition, interventions designed to limit trunk flexion have been shown to be effective in reducing the incidence of low back pain.34 Results of the current study may have etiologic significance

regarding these relationships.

While the effect of torso flexion on fatigue failure of

the lumbar spine is impressive, there are reasons to believe that the number of cycles to failure for workers in

manual lifting would be greaterthan those reported here.

In the first place, fatigue failure appears to occur more

rapidly during in vitro testing compared with that experienced in vivo.

35 This may be the result of several factors, including the lack of physiologic remodeling capability in nonliving tissue and the possibility that different

strain limits exist in physiologic as opposed to nonphysiologic conditions. Furthermore, the age of the current

cohort is quite high, and specimens undoubtedly possess

reduced bone mineral content compared with those of

working age. Even relatively small reductions in bone

mineral content can significantly affect vertebral ultimate

strength, which will in turn impact fatigue life.39 The

authors are currently attempting to procure a younger

cohort of spines for an additional replication of the current experimental design to evaluate differences in response between older and younger specimens. Nonetheless, despite the age and in vitro factors, spinal loading

associated with lifting in different torso flexion angles

clearly has a major impact on fatigue failure of lumbosacral motion segments.

Lumbar level did not have a significant influence on

fatigue life, either alone or via interaction with torso

flexion. It should be recognized that the rigid body assumption used to estimate loads at the upper levels imposed loads that were somewhat greater than those that

would be experienced by this region in vivo. Although

literature regarding loading at upper lumbar levels is

sparse, estimates have suggested that the differences in

loading between L5–S1 and L3–L4 during lifting may be

relatively small (about 5%), while the difference between

L5–S1 and L1–L2 loading may be between 10% and

20% or more.36,37 Even so, differences between lumbar

levels were not observed for fatigue life.

Evidence of motion segment failure was manifest

most frequently in the vertebral endplates, vertebral bodies, and zygapophysial joints. For specimens failing by

fatigue failure, endplate damage in the neutral posture

tended to be characterized by endplate depression, with

little evidence of fracture. Endplate fracturing was more

evident with increased torso flexion loads. Zygapophysial joint damage was most evident in specimens failing

by fatigue failure in the neutral posture, with progressively less damage as the motion segment flexed. This

pattern of damage corresponds to previous observations

that have shown damage to these joints resulting from

impaction of the superior facets with the lamina below in

neutral postures.24 Compression/shear fracture patterns

observed in the vertebral bodies were fairly frequent but

may have been influenced to a degree by decreased bone

mineral content of these older specimens and the pressures exerted by mounting the specimens in PMMA,

which differs from the manner of loading that would be

experienced in vivo.

Limitations of this study include customary caveats

regarding in vitro testing, which have been well addressed by previous authors.13,35,38 In addition, the

old age of specimens in the current study is a limitation. While the effects oftorso flexion would be expected to

significantly impact the fatigue life of younger specimens in

a similar manner, the absolute fatigue life of younger

specimens would be expected to be greater because of

increased bone mineral content and increased strength of

the vertebrae.15,39 In addition, the likelihood that older

spines may experience spondylosis and other disorders

that may affect the mode of failure must be acknowledged. A final limitation is related to the simplifying assumption that treated the lumbosacral spine as a rigid

body for determination of upper level loads. Although

this represents a common simplifying assumption made

in biomechanics, it is not entirely satisfying since we

know of no object that truly reacts as a rigid body,

the lumbosacral spine included. While some advances

have been made regarding an understanding of lines of

muscle and ligament forces in different postures,24,40,41

our understanding has not yet achieved a level of sophistication to estimate loads on various motion segments in

different torso flexion postures with any high degree of

accuracy.42 Other models of spinal loading (i.e., the

“follower” load hypothesis) have recently been proposed that would suggest an increased ratio of compression to shear force compared with current models.43,44 Results obtained in tests of fatigue failure will

clearly be sensitive to the loading assumptions made

by different models of spine loading.

The findings of this study would appear to have important implications for the current and future manual

lifting guidelines. Based on the data presented in this

study, current lifting guidelines may not pay sufficient

attention to the role that fatigue failure may play in the

development of low back disorders, particularly the

rapid development of fatigue failure under conditions of

full flexion of the lumbar spine. For example, the National Institute for Occupational Safety and Health revised lifting equation45 provides a mild discount in

weight for lifts originating near the floor (i.e., requiring

torso flexion) and a variable discounting factor for frequent lifting when a load originates at or below waist

height (75 cm). However, the present data suggest that

fatigue failure may be strongly influenced by a combination of these factors. That is, fatigue failure is most

likely to occur when frequent lifting is performed in combination with torso flexion. The current recommenda­
tions treat lifting frequency the same no matter what

degree of torso flexion isrequired. As a result, prevention

of fatigue failure of spinal tissues in occupational lifting

tasks may require greater sensitivity to the combined interactive impact of torso flexion and lifting frequency.

Conclusion

Simulation of spine loads when repetitively lifting a 9-kg

weight indicates that torso flexion has a dramatic impact

on the fatigue life of lumbosacral motion segments. The

average number of cycles to failure in 0° trunk flexion

was 8,253, specimens at 22.5° flexion averaged 3,257

cycles to failure, and specimens at 45° torso flexion

lasted an average of only 263 cycles. The level of the

motion segments did not significantly influence the number of cycles to failure, nor did the interaction of torso

flexion X lumbar level. Endplate fractures, vertebral

body fractures, and damage to the zygapophysial joints

were the most common failure modes observed in fatigue

testing. Endplate damage was characterized by endplate

depression in loading conditions corresponding to upright postures, with overt endplate fracturing seen with

the higher loads associated with torso flexion. Endplate

damage patterns differed somewhat from previous studies, perhaps the result of changed loading pathways in

torso flexion, and a revised classification system was proposed. Zygapophysial joint disruption was more frequent in the neutral posture, apparently because of impaction of the descending facet with the lamina of the

inferior vertebral body.

Key Points

● Lifting a load in torso flexion as opposed to an

upright posture will affect the magnitude of loading, load rate, and loading path experienced by

the lumbosacral spine. These may all impact the

development of fatigue failure of motion segments.

● Twelve fresh, frozen human lumbosacral spines

were dissected into three motions each (L5–S1, L3–

L4, L1–L2) and subjected to loads estimated to

occur when lifting 9 kg in three torso flexion postures (0°, 22.5°, and 45°).

● Loads associated with the torso flexion angles

have a dramatic impact on fatigue failure. On

average, motion segments lasted 8,253 cycles in

an upright posture, 3,257 cycles in 22.5° of flexion, and only 263 cycles in 45° torso flexion.

● Future lifting recommendations should be sensitive to the potential for rapid development of

fatigue failure when lifting with a flexed torso.

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