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Mel Siff
11-17-1999, 09:13 PM
Postural management and lifting technique have long been the focus of
research into back pain and dysfunction. This work seems to recommend that
squat lifting with the legs playing the dominant role is siginificantly
safer and more efficient than stoop lifting with the back far more dominant.

It has also been assumed that a person is able to judge effectively from
personal perception of back effort or strain (RPE) that a lift is too heavy
or being lifted inefficiently. In addition, little distinction has been made
between raising and lowering a load, while the fact that lifting technique is
not invariable, but changes with level of fatigue or RPE.

This selection of papers discusses the above and other assumptions that have
been made regarding lifting etchnique and safety.

Mel Siff

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van Dieen JH et al Stoop or squat: a review of biomechanical studies on
lifting technique. Clin Biomech 1999 Dec; 14 (10):685-96

The squat method of raising loads from the floor has long been recommended as
the safest way of lifting. Yet, intervention studies have failed to show
health effects of this approach and consequently the rationale behind the
advised lifting techniques has been questioned. In this study, biomechanical
comparison of the squat (dominantly leg lifting) and stoop (dominantly back
lifting) does not provide support for advocating the squat technique as a
means of preventing low back pain.

Spinal compression as indicated by intradiscal pressure and spinal shrinkage
appeared not significantly different between both lifting techniques. Net
moments and compression forces based on model estimates were found to be
equal or somewhat higher in squat than in stoop lifting. Only when the load
could be lifted from a position in between the feet did squat lifting cause
lower net moments, although the studies reporting this finding had a marginal
validity. Shear force and bending moments acting on the spine appeared lower
in squat lifting. Net moments and compression forces during lifting reach
magnitudes that can probably cause injury, whereas shear forces and bending
moments remained below injury threshold in both techniques.

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Hagen K & Harms-Ringdahl K Ratings of perceived thigh and back exertion in
forest workers during repetitive lifting using squat and stoop techniques.
Spine 1994 Nov 15; 19(22):2511-7

Local thigh and low back perceived exertion (RPE), sagittal peak load
moments, and leg and trunk muscular activity during repetitive submaximal
lifting, with squat and stoop technique, were investigated. to assess changes
in kinetic variables caused by changes in body movements during the lifting
bouts, and the contribution of the biomechanical and physiologic variables to
the variability in the local RPE responses.

Despite instructions that emphasize the "correct" lifting technique as the
squat technique, the stoop technique is reported as more commonly used in
practice. Few studies have investigated the effect of lifting technique on
differentiated perceptual responses in repetitive lifting.

Low back RPE was higher for stoop than for squat, whereas the opposite was
true for thigh RPE. During the time course in squat lifting at the highest
frequency, the knee load moment decreased and the vertical ground reaction
forces increased.

The study indicates that "movement strategies" are used to reduce the demand
on the knee-extensor muscles during the squat lifting bouts at the highest
frequencies of repetition, which combined with the relatively high assessed
thigh exertions, leads to the hypothesis that quadriceps muscle exertion is
the "weak link" for the squat technique. The study also indicates a
discrepancy between the measured and perceived low back stress in squat
repetitive lifting.

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Dolan P et al Bending and compressive stresses acting on the lumbar spine
during lifting activities. J Biomech 1994 Oct; 27(10):1237-48

Cadaveric studies have shown that intervertebral discs and ligaments are most
vulnerable to injury when loaded simultaneously in bending and compression.
The purpose of the present experiment was to measure bending and compressive
stresses acting on the lumbar spine during forward bending and lifting
activities, and to identify those aspects of lifting which increase the risk
of injury.

Results showed that stoop lifting reduced the peak extensor moment by about
10% compared to squat lifting, but increased the bending torque by about 75%.
Extensor moment and bending torque both increased substantially with
increasing mass, bulk and distance from the feet. Non-sagittal plane lifts
increased the bending torque by about 30%. The fastest lifts increased peak
extensor moment by 60% but did not increase bending torque.

It was concluded that complex spinal loading during lifting tasks depends as
much on the speed of movement, and the size and position of the object
lifted, as on its mass. Analyses of spinal loading which consider only
compressive forces do not give a full indication of the risk of injury to the
intervertebral discs and ligaments.

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Trafimow JH et al The effects of quadriceps fatigue on the technique of
lifting. Spine 1993 Mar 1; 18(3):364-7

A biomechanical analysis was performed of lifting before and after fatiguing
the quadriceps muscles. The hypothesis tested was that when the quadriceps
muscles were fatigued the lifter would change lifting technique from more of
a squat (leg) lift to more of a stoop (back) lift to decrease the demand on
the quadriceps muscles.

The hypothesis was broadly supported, and three variables changed
significantly with fatigue: trunk angular velocity, which increased, and knee
moment integral and hip angles, which both decreased. These changes are all
consistent with a change from more of a squat toward a stoop technique.

The fact that the technique of lifting changes with quadriceps muscle fatigue
underlines the importance of the physiologic condition of these muscles and
suggests that rehabilitation of low-back-injured workers should include the
quadriceps muscles. The amount of work performed should be controlled to
avoid the development of local muscle fatigue and subsequent changes in
performance.

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Hagen KB et al Physiological and subjective responses to maximal
repetitive lifting employing
stoop and squat technique. Eur J Appl Physiol 1993; 67(4):291-7

To establish safe levels for physical strain in occupational repetitive
lifting, it is of interest to know the specific maximal working capacity.
Power output, O2 consumption, heart rate and ventilation were measured in ten
experienced forestry workers during maximal squat and stoop repetitive
lifting.

Perceived low-back exertion was rated significantly lower during squat
lifting than during stoop lifting. The EMG recordings showed a higher
activity for the vastus lateralis muscle and lower activity for the biceps
femoris muscle during squat lifting than during stoop lifting. Related to the
maximal voluntary contraction, the erector spinae muscle showed the highest
activity irrespective of lifting technique.

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Waikar A et al Evaluating lifting tasks using subjective and biomechanical
estimates of stress at
the lower back. Ergonomics 1991 Jan;34(1):33-47

The objective of this study was to evaluate five different lifting tasks
based on subjective and biomechanical estimates of stress at the lower back

The results showed that a lifting task acceptable from the biomechanical
point of view may not be judged as a safe or acceptable task by the worker
based on his subjective perception. This may result in a risk of the worker
not performing the recommended task or not following the recommended method.

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de Looze MP et al Joint moments and muscle activity in the lower
extremities and lower back in
lifting and lowering tasks. J Biomech 1993 Sep;26(9):1067-76

The mechanical loading on the body during the act of lifting has been
estimated frequently. The opposite act of lowering has received much less
attention. The aim of the present study was to compare the mechanical loading
of the musculoskeletal system in lifting and lowering.

Peak moments in lowering were only slightly lower than in lifting (peak
lumbar moments were 5.4% lower). These small differences were related to
different acceleration profiles at the centre of gravity of the body/load
complex. The EMG activity was considerably lower in lowering than in lifting.
The mean EMG in lowering (average for seven muscles) was only about 69% of
the EMG in lifting. This was attributed to the different types of muscle
actions involved in lifting (mainly concentric) and lowering (mainly
eccentric). Furthermore, the EMG results suggest that similar inter-muscular
coordination is involved in lowering and lifting.

The results give rise to the assumption that in lifting and lowering similar
muscle forces are produced to meet the (nearly) equal joint moments, but in
lowering these forces are distributed over a smaller cross-sectional area of
active muscle, which might imply a higher risk of injury.

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Dr Mel C Siff
Denver, USA
mcsiff@aol.com

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