The great majority of pushing, pulling, and lifting activities
undoubtedly occur in everyday tasks. A number of sports, however,
involve the continuous pushing or pulling of external objects. Archery
is an interesting example because it consists of pulling with one
hand while pushing with the other. The same is true of using a slingshot.
Pushing is also used in football, and both pushing and pulling are
used in wrestling. Weight lifting is the prime example of a sport
activity involving lifting.
Rowing and paddling, although classified as forms of aquatic
locomotion, may also be considered activities that involve external
objects. Oars and paddles are both moved by continual pushing and pulling
movements. Pole vaulting, rope climbing (previously classified as
locomotor), and all suspension activities might also be included
in the pushing and pulling category, provided one accepts activities
that involve the moving of the body by means of pushing or pulling
an external object, the object in such cases also serving as the
means of body support.
The magnitude of the force used in pushing, pulling, and lifting
can be increased in two ways. The immediate way is by using the
lower extremities and, in some instances, the body weight to supplement
the force provided by the upper extremities. In many, if not most,
pushing and pulling activities the direction and point of application
of force are interrelated. They both have an important bearing on
the effectiveness of the force exerted, and also on the economy
of effort and avoidance of strain. Economy of effort is ensured
when the force is applied in line with the object’s center
of gravity and in the desired direction of motion. When this application
of force is not feasible, the undesirable component of force should
be as small as possible. For instance, if one desires to push a
heavy box across the floor, it would be difficult to stoop low enough
to push with the arms or even the forearms in a horizontal position.
One should stoop as low as conveniently possible, however, to reduce
the downward component of force that would tend to increase friction.
If it were necessary to move the box down a long corridor, it would
be more efficient to tie a rope to the box and pull it. By using
a long rope, the horizontal component of force would be relatively
great and the vertical or lifting component relatively small. Some
lifting component would be desirable, however, as it would serve
to reduce friction.
When friction is a major obstacle, as when pushing a tall object
such as a filing cabinet across a carpeted floor, the horizontal
push should be applied close to the cabinet’s center of
gravity at a point found by experimentation (Figure 17.4). When
this point is found, it will be possible to push the cabinet without
tipping it. When it does not seem practical to slide a heavy object
along the floor, one may try “walking” it on opposite
corners. This involves tipping the object until it is resting on
one edge of its base and then, by a series of partial rotations,
alternately pivoting it first on one corner and then on the other.
The arms alternate in a lever action, one hand holding the upper
corner that corresponds to the lower one that is serving as the
pivot, and the other hand pushing the diagonally opposite upper
Using the lower extremities and body weight to supplement
the upper extremities in a pushing task.
When attempting to pull an object, the same general directions
apply, but with this exception. As in the case of pulling the box
with a rope, it may be advantageous to pull in a slightly upward direction
because the lifting effect would help reduce friction. Nevertheless,
unless one wishes to rotate the object, the pull should be applied
in line with the object’s line of gravity.
When applying a pull or a push to an object that must move on
a track, such as a window or a sliding garage door or a weight on
a weight machine, it is essential to apply the force in the direction that
the track or runway permits. Force in any other direction is wasted
and friction is increased. As an example, trying to open a heavy
window or one that sticks can be done by standing with the right
side next to it, the arm close to the body, elbow fully flexed,
and the heel of the hand placed beneath a crosspiece of the frame,
and then pushing vertically upward. If more force is needed, the
knees and hips should be flexed and the hands placed against a lower
crosspiece. The extension of the lower extremities then supplements
the force exerted by the arm with little increase in the length
of the resistance arm. If this action is inadequate, both hands
can be used by twisting the trunk to face the window. In pulling
the window down, one should face it, stand as close as possible,
and use both hands, being careful to apply the force vertically
Lifting is a form of pulling; it is pulling a movable object
vertically or obliquely upward. The more nearly vertical the pull
and the more in line with the object’s center of gravity,
the more efficient is the lift. The principle involved here is that
of minimizing the resistance arm of a lever to reduce the amount
of effort needed to lift a given weight. For instance, to give an
extreme example, less effort is needed to lift and hold a heavy
package close to the body than to lift and hold it at arm’s
Of primary concern in lifting are the stresses applied to the
lumbar region of the spine. Conventional wisdom has, for years,
dictated that it is less damaging on the low back to lift from a
squat position than from a stoop, or forward bending, position (Figure
17.5). Recently investigators have called this assumption into question.
The change in resistance arm length between the two postures has
been a major concern, but one must also take into consideration
the nature of the stress produced during a lift. With this in mind,
studies on shear stress (moving the vertebrae across each other),
torque, and compression have found few differences between squat
lifting and stoop lifting. Stoop lifting does produce greater shear
forces, but squat lifting produces greater compressive loads. Torques
produced in the two lift postures are within 5 percent of each other. Lift
posture is often dictated by the requirements of the lift. A compromise
lift technique using a semisquat is proposed as the most efficient
in terms of back stress. In many cases the semisquat is the posture
naturally adopted when no set posture is dictated (Burgess-Limerick
2003; Kingma et al. 2004; Straker 2003).
Picking up a suitcase: (a) semisquat; (b) stoop. Notice
the position of the line of gravity.
In addition to lift posture, a number of other factors must be
considered when examining lifting motions for efficiency and safety.
The velocity of the lift, foot position, load height, symmetry, and
space constraints are among the factors that will affect the lift.
A summary of the principles of lifting as adapted from Burgess-Limerick
(2003) and Ferguson et al. (2002, 2005) follows:
- Greater mass requires greater force for motion, so reduce
load mass as much as possible to reduce the stress on muscles and
passive ligamentous structures.
- The lower the initial load to be lifted, the greater the flexion
at spine, hip, knee, and ankle will be. Avoiding loads at floor
level will reduce the initial extension loading on the joints involved.
- The farther the load is from the joints performing the lift,
the greater the resistance arm. For this reason, loads should be
kept as close to the body as possible. Reducing the resistance arm
of the load will reduce the demands on the musculature and passive
structures. To accomplish this, the feet must be kept as close to
the load as possible. Ideally the feet can be placed on either side
of the load.
- Maintaining the spine in a neutral posture will reduce both
torque and shear stress on the spine. Compressive forces will still
- Avoid trunk rotation while lifting. The lumbar spine is not
structured to accept loading while in rotation. This motion will
place undue stresses on the vertebral facets, discs, and ligaments.
The muscles of the vertebral column are of insufficient size or
strength to counteract these stresses.
- For the reasons stated previously, lateral flexion should
also be avoided while lifting.
- While lifting, both feet should remain firmly on the supporting
surface. The addition of an external load will change the location
of the center of gravity with respect to the base of support; a solid
base is necessary to maintain stability.
- Lift velocity should be held relatively constant. A constant
velocity requires lower accelerations than sudden changes. Lower
accelerations require less muscle contraction force and reduce the force
on passive structures. The lifter should avoid “jerking” the
load to start the motion. Velocity should be maintained throughout
the lift motion if possible. Slowing and restarting the lift to change
hand positions may introduce an unwanted acceleration.
Although holding and carrying are not push-pull patterns, being
static in nature, they are often related to the lifting motion just
described. In holding, effort can be minimized by supporting the object
from underneath, with only enough force applied to counteract the
downward pull of gravity. Again, as in lifting, the closer the held
object is to the line of gravity, the shorter the resistance arm,
thus decreasing the torque produced by the object.
An object held in a pincerlike fashion between the fingers and
thumb (Figure 6.30a) is an inefficient form for holding an object
of any sizable mass, although it is excellent for fine control.
The object held in this grip is supported by the relatively weak
muscle of the thumb and hand supplying sufficient pressure to maintain
the level of friction required. As these muscles fatigue, the pressure
between the object and the fingers is reduced, friction decreases,
and the object falls because of the downward force of gravity. Holding
positions that offer support against gravity or use of the larger
muscle groups of the forearm are more effective and efficient (see
Figure 6.30b, d, e).
The most efficient manner of carrying objects (the translation
of a held object) is that which requires the least accommodation
of the body’s center of gravity. Objects may be carried
in a variety of positions, such as on top of the head, in front
of the body, to one side, or strapped on the back. In each case,
the object becomes part of the moving body and therefore affects
the location of the body’s center of gravity. Therefore,
the larger the mass of the object and the farther away from the
body the object is located, the greater the change in the center
of gravity of the total moving object–body system. The
greater the change in the position of the center of gravity, the
greater the necessity for segmental realignment of posture. As was
discussed in Chapter 15, any postural deviation can increase stress
on the body, often creating weakness or a predisposition to injury. Objects
carried on top of the head raise the center of gravity of the moving
system. The objects are also precariously perched on a relatively
small, rounded base of support. This carrying position, however,
places the object in line with the body’s line of gravity,
causing no relocation of the center of gravity in either the sagittal
or frontal plane and producing little or no external torque on the
vertebral column. In many cultures this is the preferred method
of carrying. In some cultures, women have been found to carry as
much as 60 percent of their body weight on their heads (Bastien
et al. 2005).
Carrying loaded backpacks has become a concern for those working
with schoolchildren. Children carry a variety of bag styles loaded
with school books, school work, and personal belongings every day.
It has been found that backpack carriage alters posture and can
have an effect on the incidence of low back pain. To maintain stability
and equilibrium, the combined center of gravity of the torso and
backpack must be shifted forward over the feet. If this shift occurs
in the spine or at the hips, as is most common, stress will be placed
on the muscles and other structures of the region, usually due to
increased torques. To minimize the effect of carrying a backpack,
it is recommended that pack weight be no more than 10 to 20 percent
of body weight, especially in children. Caution should also be used
when selecting a backpack. Shoulder carriage bags are popular, but
carrying a load on one shoulder tends to produce an asymmetrical
load on the spine, placing an unbalanced stress on one side and
one set of muscles (Brackley and Stevenson 2005; Cottalorda et al.
2003; Murray and Johnson 2005).
A more commonly preferred method of carrying objects is to place
equal loads on either side of the body. This creates a state of
equilibrium within the system. Each separate load offsets the influence
of the other without necessitating a change in posture or any realignment
of the segments.
Whenever objects must be carried on one hip, in back, or in front,
a postural adjustment must be made to maintain stability. To minimize
stress on the joints, including the vertebral column, these adjustments
should be made from the ankles. Thrusting the hips out or adjusting
with the back may predispose one to injury unless compensation occurs
by routinely altering the location of the load.
Lifting weights, whether done for strengthening and conditioning
muscles or competitive lifting contests, involves the use of the
levers of the body to overcome the inertia of relatively large masses.
Depending on the lift being done, either a push or a pull pattern
may be used. The key to safe and successful weight lifting is in
arranging the various levers involved in such a way as to minimize
the torque produced by the external resistance while maximizing
the available muscle torques. To produce an efficient lift, one
must first establish the location of the axis of rotation for the
lift. The moving weight should be kept as close to this axis of
rotation as possible, reducing the resistance moment arm. In the
bench press, for example, it is much safer and more efficient to lift
the bar directly over the shoulders than to allow the bar any horizontal
motion. To further reduce the risk of injury, the axis of rotation
should be well supported. In standing lifts this means that the
axis of rotation should be in line with the approximate center of
the base of support. To allow the axis of rotation to move away
from the base of support requires that postural torques be created
for stability. These torques usually occur in the back and may lead
to injury. The velocity of the lifting motion should be kept low.
A high-velocity lift generates a great deal of momentum, requiring
a large braking force. This braking force usually takes the form
of an eccentric muscle contraction, which must act to decrease momentum.
Punches are simultaneous push-pattern motions. A punch is typically
directed horizontally rather than vertically and usually terminates
with contact to another body that provides the external resistance.
Because the momentum of the punch is to be transferred to the opponent,
it is desirable for punch velocity to be high. The purpose of the
high-velocity punch can be to upset the balance of the opponent,
as in boxing, or to cause injury to the opponent, as in karate and
A boxing punch is typified by a long punch and a long follow-through.
The force of the punch is transferred to the opponent over a broad
area through the boxing glove. Punches to the head produce acceleration
of the head and neck, which may lead to unconsciousness or may produce
angular momentum, causing a loss of balance. A karate punch, on
the other hand, is a shorter punch with little or no follow-through.
The force of a karate blow is delivered to a very small area. Impact
occurs at the peak velocity of the punch, and this impact can break
bone and tear tissue. For this reason, karate is practiced as a
noncontact sport and judged on correctness of form and accuracy.
The simultaneous nature of many karate punches and most boxing
punches allows for maximum force production with a straight-line
motion intended to give the opponent little or no time for defense.
In both sports, quickness of the punch is vital, both for an effective
offense and to allow for a rapid return to a ready position.
Working with implements such as a hoe, rake, mop, or vacuum cleaner
involves a combination of pushing, pulling, and, in some instances,
lifting. The last is usually only for short distances, but it may
occur with considerable frequency. One characteristic of working
with implements such as these is that the body must maintain a more
or less fixed posture for relatively long periods of time, causing
tension and fatigue. Hence the chief problem is that of using the
body in such a way that tension will be minimized and fatigue postponed
for as long as possible. If the implement is used back and forth
in front of the body, the tendency of the worker is to lean forward
(Figure 17.6). This necessitates static contraction of the extensors
of the spine to support the trunk against the downward pull of gravity.
Because implements such as the rake and hoe are lifted at the end
of each stroke and carried to position for the next stroke, the
force of gravity acts on the implement as well as on the worker’s
body. Although the implement may not weigh much in itself, its forward
position means that the lever has a long resistance arm, the effect
of which must be balanced by the muscles. This gives an added burden
to the back muscles and not infrequently causes a backache. A better
method is to stand with the side turned toward the worksite and
the feet separated in a fairly wide stride, and work the implement
from side to side. The reach can then be obtained by bending the
knee of the leg on the same side as the implement and by inclining
the body slightly to the same side. Those who are familiar with
gymnastics will recognize this as a side lunge position. On the
recovery, the knee and the trunk are both straightened. Thus there
is an alternating contraction and relaxation of muscles and there
is no necessity for any of the trunk muscles to remain in static
contraction. Temporary relief can also be obtained by changing sides.
Long-handled implements may force a forward-leaning posture.
The wider stride is beneficial.
The use of a spade or snow shovel primarily involves the act
of lifting. Because the load is taken on the end of a mechanical
lever held in a more or less horizontal position, it is inevitable
that the weight arm of the lever be relatively long. It can be shortened
somewhat, however, by sliding one hand as far down the shaft as
possible, using this hand as a fulcrum, and providing the force
with the other hand by pushing down on the outer end of the handle.
As a variation of this technique, when getting a particularly heavy
load on a shovel, it is possible to bend one knee and brace the shaft
against the thigh, thus using the thigh as a fulcrum. This is only
for the initial lift, however; the hands must then be shifted to
the position previously described to carry or throw the load.
Aside from taking and lifting the load on the spade, there is
the factor of lowering the body to reach the load and of assuming
the erect position for moving it. As in the case of stooping to
lift a heavy object from the floor, the chief problems are economy
of effort, maintenance of stability, and avoidance of strain. These
problems are intensified by the additional factor of taking the
load on a long-handled implement instead of directly in the hand.
As before, separating the feet to widen the base of support, bending
at the knees instead of bending from the waist to lower the body,
and inclining the trunk forward only slightly will, respectively,
increase stability, shorten the anatomical levers involved in the
stooping, and divide the muscular work among the stronger knee,
hip, and back extensors instead of making the weaker back muscles
assume too large a share of the work. Because the lower back is
easily strained by heavy shoveling, it is of great importance to
protect it by observing the principles of good mechanics.