Bone mineral content and fatigue failure of lumbar motion segments in simulated 

flexed lifting: Does specimen age influence the relationship? 

Sean Gallagher1, William S. Marras2, Alan S. Litsky2, Deborah Burr2 

1

NIOSH Pittsburgh Research Laboratory, Pittsburgh, PA 

2

The Ohio State University, Columbus, OH 

This paper describes a comparison of the fatigue failure responses of older versus younger lumbar spine 

cadaver motion segment specimens. These specimens were repeatedly subjected to loads simulating those 

experienced by the spine in lifting a 9 kg load in different torso flexion postures (0, 22.5, and 45 degrees 

of flexion). An older sample of 36 motion segments (average age 81 years + 8 SD) is compared with a 

younger sample of 18 motion segments (average age: 47 years + 9 SD) with respect to the number of 

cycles to failure and results of survival analyses evaluating the effects of flexion and bone mineral 

content with and without the younger data set. Compared to the older sample of spines, the younger 

sample exhibited many more cycles to fatigue failure in all the torso flexion conditions (10020 versus 

8267 average cycles to failure in 0 degrees flexion, 7124 versus 3262 in 22.5 degrees flexion, and 3229 

versus 263 cycles to failure in 45 degrees flexion). The increased cycles to failure in young specimens is 

likely due to the increased bone mineral content (BMC) in younger motion segments (30.7 g + 11.1 g per 

motion segment versus 27.8 + 9.4 g). Cox regression analyses modeling both flexion effects and BMC 

indicate that betas are similar when just older spines are modeled and when younger specimens are 

included in the analysis. Betas for survival analysis models with and without younger specimens 

generally show little change for flexion (2.716 versus 2.756 for 22.5 degrees [0 degrees referent], 3.926 

versus 3.380 for 45 degrees [0 degrees referent]). Bone mineral content shows a similar protective effect 

in both models (Betas: -0.127 versus -0.087). Overall, comparison of the models indicate that risk ratios 

are similar for 22.5 degrees flexion, slightly increased for 45 degrees flexion and slightly more protective 

influence predicted for BMC in the combined sample versus the older sample alone. 

I.	 Introduction There is a decrease in vertebral strength with 

age, particularly beyond the age of 40 years [Bell et 

A vertebral fracture will result when the al. 1967]. This decreased strength is associated 

strength of a vertebra is exceeded by a transitory with a reduction in bone mineral density. 

load or by cyclic, repetitive loads [Brinckmann et Specifically, there [White and Panjabi 1990] 

al. 1988, Hansson et al. 1987]. The strength of a appears to be a significant loss of horizontallyvertebra, and therefore its ability to withstand the oriented trabeculae – particularly in the central 

stresses imposed on it, is primarily determined by portion of the vertebral body [Atkinson 1967]. The 

the amount of mineralized tissue of which the disappearance of the horizontal trabeculae deprives 

vertebra is comprised [Bartley et al. 1966, Hansson the vertical trabeculae of important fortifications, 

and Roos 1981, White and Panjabi 1990]. Several and the load-bearing capability of the center of the 

studies have found that the relationship between the vertebral body becomes degraded as a result 

ultimate compressive strength of a vertebral body [Bogduk 1997]. The decreased strength of the 

and bone mineral content is generally linear cancellous core of the vertebral body as aging takes 

[Bartley et al. 1966, Bell et al. 1967, Galante et al. place is associated with a shift in the load-bearing 

1970, Hansson and Roos 1981] . responsibilities of cancellous and cortical bone. In 

Disclaimer: 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. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

spines less than 40 years of age, the cancellous bone 

bears approximately 55 percent of the compressive 

load, while the cortical shell bears 45 percent of the 

load [Rockoff et al. 1967]. 

The age-related changes detailed above may 

also impact the vulnerability of the endplate to 

failure. Specifically, the weakened trabecular 

network underlying the endplates permits increased 

deformation of the cartilaginous endplates, and may 

increase the propensity to fracture [Bogduk 1997]. 

The finding of increased fracturing in the endplates 

and vertical trabeculae with increasing age tends to 

support this notion [Vernon-Roberts and Pirie, 

1977]. The purpose of the present investigation is 

to compare and contrast the fatigue failure 

responses of older motion segment specimens to a 

younger cohort, and to evaluate the influence of 

bone mineral content on fatigue failure of both 

groups. 

2. Methods 

2.1 Cadaver material 

Three replications of an incomplete block 

experimental design were performed using a total of 

eighteen lumbar spines, each of which was 

dissected into three motion segments (L1-L2, L3­

L4, and L5-S1). The first two reps (12 spines or 36 

motion segments) consisted of an older cohort of 

specimens (averaging 81 years + 8 SD). The third 

rep (6 spines or 18 motion segments) was 

performed using a younger group of specimens 

(mean age: 47 years + 9 SD). 

2.2 Determination of bone density 

The bone density for vertebral bodies L1-L4 

of each spine was measured by means of dual x-ray 

absorptiometry (DXA) using a Lunar DPX machine 

(Lunar, Madison, WI). Specimens were positioned 

on a bed of rice (to simulate body tissue) with the 

anterior aspect of the spine facing up. The bone 

mineral density (BMD) of the specimen was 

expressed in grams divided by the projected area of 

the bone. Bone mineral content (BMC) was derived 

from the BMD and area measurements. Bone 

mineral density, projected area of the vertebral 

bone, and bone mineral content for L1-L2 and L3­

L4 segments were provided in standard reports. 

2.3 Determination of Simulated Lifting 

Loads in Torso Flexion 

A dynamic EMG-assisted biomechanical 

model [Granata and Marras, 1995] was used to 

develop appropriate loads and load rates associated 

with lifting a 9 kg load starting at three trunk 

flexion angles: 0 degrees (neutral), 22.5 degrees 

(partial flexion), and 45 degrees (full flexion). 

These spinal load estimates were obtained from an 

existing large database of lifting tasks. Resultant 

loads were 1300 N, 2400 N, and 3150 N for these 

torso flexion angles, respectively. Since the spine is 

composed of viscoelastic tissues, it was considered 

critical that the rate of loading also be correctly 

modeled in each posture. Average model estimates 

of load rate for the three postures were 700 N/s, 

2100 N/s, and 4800 N/s, with increasing load rates 

in more flexed postures. Motion segments were 

flexed according to endplate angles obtained in a 

radiographic and MRI studies of subjects flexing in 

vivo [Chen 2000]. 

2.4 Testing Procedure 

Motion segments were potted in trays 

containing polymethylmethacrylate (PMMA), the 

flexion angle of each motion segment being 

confirmed by endplate measurements obtained via 

multiple radiographs during the fixation process. 

This flexion angle was maintained throughout the 

cyclic loading regimen for a given specimen. All 

tests were conducted in a humidified environmental 

chamber at a temperature of 37 deg C. Motion 

segments were creep loaded for 15 minutes (at 500, 

750 or 1050 N depending on torso flexion angle) 

and then cyclically loaded at 0.33 Hz until failure or 

the maximum number of cycles (10,020) was 

completed using a hydraulic materials testing 

machine (MTS Systems Corporation, Eden Prairie, 

MN). Failure was defined as a displacement of 10 

mm after termination of the period of creep loading. 

The primary dependent measure was the number of 

cycles to failure. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2.5 Statistical analyses 

Comparisons of the fatigue failure responses 

of older and younger specimens were achieved via 

different survival analysis techniques (KaplanMeier Product Limit Estimation and Cox 

Regression Analysis). Kaplan-Meier Product 

Limit Estimation and mean survival times were 

obtained separately on the younger specimens (18 

motion segments) and older specimens (36 motion 

segments). Plots of survivorship functions for each 

group were obtained. Variables included in the Cox 

regression models included flexion angle, lumbar 

level, bone mineral content, bone mineral density, 

and weight of the motion segment. As Cox 

regression analyses incorporated bone mineral data, 

L5-S1 segments (from which bone mineral content 

data was not obtained) were omitted from the 

analyses. It was not possible to derive Cox models 

for the smaller sample of younger specimens 

(insufficient degrees of freedom), thus comparisons 

were made between Cox regression coefficients of 

models with and without the younger specimens 

included. 

3. Results 

3.1 Mean Survival Times and 

Censored Observations 

Kaplan-Meier Product-Limit analyses 

demonstrated longer survival times and an increased 

number of censored observations for the younger 

cohort of specimens compared to the older group. 

Figures 1A and 1B illustrate the survivorship 

functions s(t) at each angle of torso flexion for older 

and younger spines, respectively. As can be seen 

from these figures, the increased load and load rates 

associated with greater torso flexion are associated 

with decreased survival of specimens in both 

groups. However, the survivorship functions for 

younger spines show a dramatically improved 

survival for the simulated lifting task at all torso 

flexion levels. Table 1 details the difference 

between these groups in terms of censored 

observations (i.e., motion segments surviving the 

entire 10020 cycles of the test) and in terms of mean 

survival time. In all cases, survival was 

significantly enhanced in the younger sample. On 

the average, younger specimens enjoyed an increase 

of approximately 3000 cycles to failure compared to 

older specimens, and an increase of about 35% in 

the number of censored observations. 

3.2 Cox Regression Models 

Table 2 provides the results of Cox 

regression models examining the effects of flexion 

angle and bone mineral content using just the older 

cohort of spines, and using the entire sample of 

spines (old and young). These analyses do not 

include any L5-S1 segments, as bone mineral 

scanning software did not have an algorithm for 

determining the bone mineral content of the sacrum. 

Younger spines were found to have approximately 3 

additional grams of bone mineral per motion 

segment than older spines (30.7 g + 11.1 SD versus 

27.8 + 9.4 SD). 

As can be seen in the first analysis presented 

in Table 2, both flexion angle and bone mineral 

content were significant predictors of the risk of 

failure for older motion segments. Compared to the 

referent posture (0 degrees flexion), simulated 

lifting in 22.5 degrees flexion had a 15 fold increase 

in risk, while the 45 degree condition saw a 29-fold 

increase in risk compared to the referent. As can be 

seen from the negative exponent, increasing BMC 

exerted a protective influence. According to the 

model, every gram of BMC resulted in an 8.3% 

increase in cycles to failure for this older sample. 

When the same analysis was performed on 

all eighteen spines (young and old, 36 motion 

segments), results were quite similar. The estimates 

of risk for the 22.5 degree flexion condition were 

almost identical to the all old sample (15.127 versus 

15.741); however, the risk ratio for the 45 degree 

condition was appreciably higher in the combined 

sample (50.5 as opposed to 29.4). Bone mineral 

content again was again found to be protective, but 

this influence was found to be slightly stronger 

when the younger motion segments were included 

in the model (RR: 0.881 versus 0.917 for the old 

specimens). Thus, each additional gram of bone 

mineral content resulted in an approximate increase 

of 12% in cycles to failure for the combined sample. 

4. Discussion 

Survival analyses of the fatigue failure 

responses of older versus younger spines to 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

simulated lifting loads in three torso angles 

illustrate some significant differences in response as 

well as some similarities. Undoubtedly, the most 

notable difference is the large increase in cycles to 

failure observed at all flexion angles in the young 

cohort. At each flexion angle, it took approximately 

3000 more cycles to achieve failure in younger 

spines than older specimens. The number of 

censored observations also increased significantly 

in each flexion condition for young motion 

segments. Notably, none of the younger specimens 

failed at the 0 degree torso flexion simulated lift. 

Furthermore, while none of the older specimens 

survived the 45 degree torso flexion simulated lift, 

one-third of younger specimens were able to last the 

entire 10020 cycles in this condition. Clearly, 

young spines have a dramatically increased 

tolerance to repeated loading compared to older 

specimens. The increased tolerance exhibited by 

younger spines is undoubtedly due, at least in part, 

to the increased bone mineral content found in 

younger spines. While the absolute difference in 

BMC is small (less than 3 g per motion segment), 

the protective influence appears to be quite 

significant. 

While the absolute difference in cycles to 

failure is significant, Cox models incorporating the 

flexion effect and the influence of BMC are similar 

in terms of the estimates of each effect. The one 

exception is that the risk ratio for the most extreme 

flexion condition was estimated to be significantly 

higher in the combined sample. This may reflect 

the increased disparity between failure experience 

in the 0 degree condition (where no samples failed) 

and the 45 degree condition (where most specimens 

failed) in the young sample. However, other than 

this exception, the models were quite similar, and it 

is notable that the standard errors for each risk ratio 

estimate are reduced in the full sample, indicating 

increased confidence of each estimate. 

Results of this study suggest that exposure 

to spinal loads experienced in flexed lifting of a 

given weight are likely to result in much more rapid 

fatigue failure compared to the lighter loads 

experienced in upright lifting. These findings 

provide strong support for minimizing the amount 

of forward bending performed in lifting tasks. 

These results also suggest that older workers may 

experience fatigue failure much more quickly, 

primarily due to the reduced bone mineral content 

in their spines. This suggests that lifting limits may 

need to consider age-related changes when 

determining appropriate workloads for the older 

worker. 

Certain limitations should be considered 

regarding the data presented here. These include 

the possibility that preexisting medical conditions or 

loading histories might have differentially affected 

the specimens. The sample size of the younger 

spines is also somewhat small, and the average age 

is still middle-aged. 

In conclusion, results of this analysis 

demonstrate much greater tolerance to cyclic 

loading for younger specimens. However, if bone 

mineral content is taken into consideration, Cox 

regression models regression coefficients for the 

flexion effect and bone mineral content are quite 

similar in the combined versus the old sample, with 

the exception of increased risk in the most extreme 

flexion condition. 

References 

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Bartley MH, Arnold JS, Haslam RK, Jee WSS (1966) J 

Gerontol 21:517-21. 

Bell GH, Dunbar O, Beck JS, Gibb A (1967) Calcif Tissue 

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Bogduk N (1997) Clinical anatomy of the lumbar spine and 

sacrum (3rd edition). Churchill Livingstone, New 

York 

Brinckmann P, Biggemann M, Hilweg D (1988) Clin Biomech 

3(Suppl. 1):1-23. 

Chen, Yi-Lang, (2000) Ergonomics, 43(6), 744-751. 

Galante JO, Rostoker W, Ray RD (1970) Calcif Tissue Res 

5:236-46. 

Granata, K.P. and W.S. Marras, (1995) Journal of 

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Table 1. Comparison of censored observations and mean survival times for old and young spine motion segments using spinal loads 

predicted when lifting a 9 kg load in three torso flexion postures. 

 0 degrees 

% Censored Mean Survival 

Observations (cycles) 

22.5 degrees 

% Censored Mean Survival 

Observations (cycles)

 45 degrees 

% Censored Mean Survival 

Observations (cycles) 

Older Specimens (n=36) 67% 8267 25% 3261 0% 263 

Younger Specimens (n=18) 100% 10020 67% 7124 33% 3929 

Table 2. Results of Cox regression models incorporating effects of simulated torso lifting loads at different angles of flexion and bone 

mineral content of the specimens. A model incorporating old spines is compared with a model incorporating both young and old 

specimens. 

Sample df Beta SE P Risk Ratio 95% CI for RR 

Old Spines (24 specimens)

 Flexion 2 0.001 

 22.5 degrees 1 2.716 .872 0.002 15.127 2.741-83.487 

 45 degrees 1 3.380 .941 0.000 29.371 4.641-185.903 

 BMC 1 -0.087 .038 0.022 0.917 0.851-0.988 

Old + Young Spines (36 specimens)

 Flexion 2 0.001 

 22.5 degrees 1 2.756 .862 0.001 15.741 2.905-85.309 

 45 degrees 1 3.923 .921 0.000 50.530 8.310-307.245 

 BMC 1 -0.127 .036 0.001 0.881 0.820-0.946 

a) b) 


Figure 1. a) Survivorship function for old spines by torso flexion angle (n = 36); b) survivorship 

function for young spines by torso flexion angle (n = 18).