We frequently encounter articles on the risks of repeated lifting, but not
too many people question how the deductions are made concerning the dangers
of certain exercises and certain regimes of exercise. The first article
concludes that that errors in estimates of cumulative spinal loading can be
large, depending upon the approach used, which will hinder any progress in
developing a dose-response link between cumulative exposure and an increased
risk of low-back pain or injury.

The remaining articles examine other aspects of spinal loading under
different conditions, with the last article discussing a potential pathway
between psychosocial stress and spine loading that may explain how
psychosocial stress increases risk of low back disorders. One of the
articles shows that, despite the claims of many postural and spinal
conditioning experts about the "best" or "correct" way of stabilising the
trunk, each individual relies on a unique preparatory strategy for managing
spinal loading.

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An evaluation of predictive methods for estimating cumulative spinal loading

Callaghan JP, Salewytsch AJ, Andrews DM
Ergonomics 2001 Jul 15;44(9):825-37

The focus of this study was to assess the amount of error present in several
approaches that have been commonly used to estimate the cumulative spinal
loading during manual materials handling tasks. Three male subjects performed
three sagittal plane lifting tasks of varying loads and postural
requirements. Video recordings of the tasks were digitized and a
biomechanical model was used to calculate the spinal loading (compression,
joint shear, reaction shear, and flexion/extension moment) at L4/L5 for each
frame of data. The 'gold standard' for cumulative loading experienced by the
subjects was obtained by integrating the resultant biomechanical model
outputs for the entire lifting cycle. Five approaches that quantify
cumulative spinal loading, four that use discrete measures and one that
reduces the number of frames used (5 Hz), were used and compared with the
gold standard. The four methods using discrete measures to quantify the
cumulative demands of a task resulted in substantial errors (average error
across task and subjects was 27-69%).

Reducing the number of frames of data processed to 5 frames/s preserved the
time varying information and was the only approach examined that did not
induce significant error into the cumulative loading estimates. This study
indicates that errors in cumulative spinal loading estimates can be large
depending upon the approach used, which will hinder any progress in
developing a dose-response link between cumulative exposure and an increased
risk of low-back pain or injury.

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***And here are some more articles on loading and its effects on the spine.

An assessment of complex spinal loads during dynamic lifting tasks

Fathallah FA, Marras WS, Parnianpour M
Spine 1998 Mar 15;23(6):706-16

STUDY DESIGN: An electromyogram-assisted free-dynamic lifting model was used
to quantify the patterns of complex spinal loads in subjects performing
various lifting tasks. OBJECTIVES: To assess in vivo the three-dimensional
complex spinal loading patterns associated with high and low risk lifting
conditions that matched those observed in industrial settings.

SUMMARY OF BACKGROUND DATA: Combined loading on the spine has been implicated
as a major risk factor in occupational low back disorders. However, there is
a void in the literature regarding the role of these simultaneously occurring
complex spinal loads during manual lifting.

METHODS: Eleven male subjects performed symmetric and asymmetric lifting
tasks with varying speed and weight. Reactive forces and moments at L5-S1
were determined through the use of electrogoniometers and a force plate. An
electromyogram-assisted model provided the continuous patterns of
three-dimensional spinal loads under these complex lifting tasks.

RESULTS: The results showed that complex dynamic motions similar to those
observed in risky industrial tasks generated substantial levels of combined
compressive and shear loads. In addition, higher loading rates were observed
under these conditions. Unlike loading magnitudes, loading rate was a better
indicator of dynamic loading because it incorporated both the duration and
magnitude of net muscle forces contributing to total spinal loading during
the lifting conditions.

CONCLUSIONS: Quantification of spinal combined motions and loading in vivo
has not been undertaken. This study provided a unified assessment of the
effects of combined or coupled motions and moments in the internal loading of
the spine. Dynamic lifting conditions similar to those observed in risky
industrial situations generated unique complex patterns of spinal loading,
which have been implicated to pose a higher risk to the spinal structure. The
higher predicted loading and loading rate during asymmetric lifting
conditions can be avoided by appropriate ergonomic workplace modifications.

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Variation in spinal load and trunk dynamics during repeated lifting exertions

Granata KP, Marras WS, Davis KG
Clin Biomech (Bristol, Avon) 1999 Jul;14(6):367-75

OBJECTIVES: To quantify the variability in lifting motions, trunk moments,
and spinal loads associated with repeated lifting exertions and to identify
workplace factors that influence the biomechanical variability.DESIGN:
Measurement of trunk dynamics, moments and muscle activities were used as
inputs into EMG assisted model of spinal loading.

BACKGROUND: Traditional biomechanical models assume repeated performance of a
lifting task produces little variability in spinal load because the
assessments overlook variability in lifting dynamics and muscle coactivity.
METHODS: Five experienced and seven inexperienced manual materials handlers
performed 10 repeated lifts at each combination of load weight, task
asymmetry and lifting velocity.

RESULTS: Box weight, task asymmetry and job experience influenced the
magnitude and variability of spinal load during repeated lifting exertions.
Surprisingly, experienced subjects demonstrated significantly greater spinal
loads and within-subject variability in spinal load than inexperienced
subjects. Trial-to-trial variability accounted for 14% of the total variation
in compression overall and 32% in lateral shear load. Although the mean
spinal load was safely below the NIOSH recommended limit; due to variability
about the mean, more than 20% of the lifts exceeded the recommended limit.

CONCLUSION: Spinal load changed markedly from one exertion to the next
despite identical task requirements. Trial-to-trial variability in
kinematics, kinetics, and spinal load were influenced by workplace factors,
and may play a role in the risk of low-back pain.

RELEVANCE: Ergonomic assessments considering only the mean value of spinal
load overlook the fact that a large fraction of the lifts may exceed
recommended levels.

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Lifting an unexpectedly heavy object: the effects on low-back loading and
balance loss

van der Burg JC, van Dieen JH, Toussaint HM
Clin Biomech (Bristol, Avon) 2000 Aug;15(7):469-77

OBJECTIVE. This study evaluates the effects of lifting an unexpectedly heavy
object on low-back loading and loss of balance. BACKGROUND. It is often
suggested that lifting an unexpectedly heavy object may be a major risk
factor for low-back pain. This may lead to an increase in muscle activation,
stretch of ligaments and posterior disc, and loss of balance.METHODS. Nine
healthy male subjects were asked to pick up and lift a box as quickly as
possible. The weight of the box was unexpectedly increased by 5 or 10 kg.
Kinematics and force data were recorded throughout the experiment.

RESULTS. Lifting of an unexpectedly heavy box led to a decrease in maximum
torque of the low back compared to lifting the same box mass with correct
expectation. The maximum lumbar angle did not increase compared to the light
box condition. Only the threat to balance appeared to be somewhat increased.

CONCLUSIONS. The lifting of an unexpectedly heavier box appeared not to lead
to an increased balance loss or a clearly increased stress of the structures
of the low back, although a burst of abdominal muscle activity was found in
one condition. These results do not fully clarify the assumed relation
between lifting unexpectedly heavy objects and low-back injury.

RELEVANCE A commonly cited cause of low-back pain is the lifting of an
unexpectedly heavy object. A study of the responses to such perturbation is
important to an understanding of spine mechanics and the etiology of low-back
injury.

----------------------

The following article showed that, despite the claims of many postural and
spinal conditioning experts about the "best" or "correct" way of stabilising
the trunk, each individual relies on a unique preparatory strategy for
managing spinal loading. In particular, the researchers found that the
preparation to cope with the loading always involved the pretensioning of the
erector spinae muscles, although the coactivation of the other trunk muscles
was quite variable across subjects. Those who overemphasize the hypothesised
role of transversus abdominis (TVA) in this task and underemphasise the role
of the erector spinae clearly need to re-examine their claims.

The development of response strategies in preparation for sudden loading to
the torso

Lavender SA, Marras WS, Miller RA
Spine 1993 Oct 15;18(14):2097-105

Sudden and unexpected loading to the torso has been reported in the
literature as a potential cause of low-back disorders. When such loadings
occur, it is hypothesized that the body's response is designed to minimize
the destabilizing postural disturbance, and to minimize the mechanical
loading of the musculoskeletal system. This study tested hypotheses regarding
the role of task experience in the development of preparatory strategies that
potentially minimize the postural disturbance to the body and minimize the
mechanical loading of the spine. These strategies were hypothesized to
consist of muscle pretensioning, postural changes, and intra-abdominal
pressure (IAP). Four subjects participated in five to six experimental
sessions in which a sudden load was applied by dropping a weight once a
minute for 30 minutes. Electromyographic (EMG) data from 10 trunk muscles,
IAP data, and postural data were collected during the initial session and
final sessions for each subject.

The results indicate where each subject developed a unique preparatory
strategy. The preparation always involved the pretensioning of the erector
spinae muscles, although the coactivation of the other trunk muscles was
quite variable across subjects. During the sudden loading the overall
postural disturbance was not consistently reduced; however, the trunk flexion
was significantly reduced in most subjects. Furthermore, the estimated spinal
compression due to muscle loading was significantly reduced in all subjects.

-----------------------

The following article should also be of interest to those who overemphasise
the active role of the abdominal muscles in lifting tasks, because it shows
that trunk extensor muscles generate lifting moments as much as 47% greater
than the applied lifting moment to offset flexor antagonism by muscles such
as the hip flexors and abdominal muscles. This would again emphasise that
the abdominal musculature plays a far more important role in managing the
intraabdominal pressure (IAP) during the Valsalva manoeuvre than actively
playing some anatgonistic role to the spinal erectors.

The influence of trunk muscle coactivity on dynamic spinal loads

Granata KP, Marras WS
Spine 1995 Apr 15;20(8):913-9

STUDY DESIGN. Measured trunk muscle activity was employed in a biomechanical
model to determine the influence of including or neglecting muscle coactivity
on predicted spinal loads. OBJECTIVES. The purpose of this investigation was
to examine the influence of muscle coactivity on spinal load.

SUMMARY OF BACKGROUND DATA. Electromyographic patterns in the trunk
musculature have demonstrated significant levels of cocontraction during
lifting exertions. Biomechanical analyses of musculoskeletal loading are
often mathematically constrained from including muscle coactivity. Models
that attempt to include coactive behavior are complex and difficult to
implement.

METHODS. Electromyographic data were collected from five trunk muscle pairs
while subjects performed dynamic lifting exertions. A validated,
electromyographically assisted biomechanical model was used to compute
relative muscle force, lifting moment, and spinal load. Results were
generated and compared from analyses that included from one to five
simultaneously active muscle pairs.

RESULTS. Trunk extensor muscles generate lifting moments as much as 47%
greater than the applied lifting moment to offset flexor antagonism. Analyses
that neglect muscle coactivity during dynamic lifting exertions may
underestimate spinal compression by as much as 45% and shear forces by as
much as 70%.

CONCLUSIONS. The level of coactive spinal loading is significantly influenced
by the weight of the lifted load as well as trunk extension velocity. Muscle
coactivity significantly influences the modeled load in the lumbar spine
during lifting exertions and should be considered if an accurate measure of
spinal loading of desired.

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The influence of psychosocial stress, gender, and personality on mechanical
loading of the lumbar spine

Marras WS, Davis KG, Heaney CA, Maronitis AB, Allread WG
Spine 2000 Dec 1;25(23):3045-54

STUDY DESIGN: The effects of psychosocial stress on muscle activity and
spinal loading were evaluated in a laboratory setting. OBJECTIVE: To evaluate
the influence of psychosocial stress, gender, and personality traits on the
functioning of the biomechanical system and subsequent spine loading.

SUMMARY OF BACKGROUND DATA: Physical, psychosocial, and individual factors
all have been identified as potential causal factors of low back disorders.
How these factors interact to alter the loading of the spine has not been
investigated. METHODS: Twenty-five subjects performed sagittally symmetric
lifts under stressful and nonstressful conditions. Trunk muscle activity,
kinematics, and kinetics were used to evaluate three-dimensional spine
loading using an electromyographic-assisted biomechanical model. A
personality inventory characterized the subject's personality traits. Anxiety
inventories and blood pressure confirmed reactions to stress.

RESULTS: Psychosocial stress increased spine compression and lateral shear,
but not in all subjects. Differences in muscle coactivation accounted for
these stress reactions. Gender also influenced spine loading; Women's
anterior-posterior shear forces increased in response to stress, whereas
men's decreased. Certain personality traits were associated with increased
spine loading compared with those with an opposing personality trait and
explained loading differences between subjects.

CONCLUSIONS: A potential pathway between psychosocial stress and spine
loading has been identified that may explain how psychosocial stress
increases risk of low back disorders. Psychosocially stressful environments
solicited more of a coactivity response in people with certain personality
traits, making them more susceptible to spine loading increases and suspected
low back disorder risk.

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Dr Mel C Siff
Denver, USA
http://groups.yahoo.com/group/Supertraining/