Introduction
In order to reduce the risk of secondary injury and increase the
ability to operate a tractor effectively, farmers/ranchers affected
by Spinal Cord Injuries (SCI) must have the proper seating system
and be properly positioned in it. For many farmers/ranchers with
an SCI, their tractor becomes their primary workstation and command
center. Such tasks as fieldwork, livestock feeding, and operations
oversight often occur while the farmer/rancher is in the tractor
seat, sometimes for hours at a time. Rough terrain, equipment vibration,
long periods of sitting, upper body twisting (in order to see behind
the tractor), and improper choice of seat and positioning can contribute
to pressure ulcers (sidebar), back pain,
and increased spasticity.
Tractors equipped with heat, air conditioning, a cell phone or
FM radio communication system, and an independent suspension seat
are essential for an operator with SCI. The operator’s correct
positioning in the tractor seat is critical in maximizing his/her
ability to control the tractor and prevent pressure ulcers from
occurring. In a study completed at the University of Pittsburgh,
21 farmers with SCI were surveyed. 12 reported having a pressure
ulcer (Willkomm, 1997 p. 38). Preventing the development of pressure
ulcers is crucial, because they can be life threatening and take
months to heal. The average cost of treating a pressure ulcer ranges
from $15,000 to $61,000 (Cook & Hussey, 1995, p. 278).
Postural alignment and pressure management must be considered when
modifying or adjusting existing tractor seats for the operator with
SCI. Although the level of SCI will determine specific seating adaptations
that a farmer/rancher may need, a number of general principles should
be considered when adapting tractors for use by farmers/ranchers
with SCI.
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1. Proper positioning
The operator’s proper postural alignment in the tractor seat
should be addressed first. Improper (asymmetric) pelvic alignment
will increase pressure and shear forces (sidebar)
(Cook & Hussey, 1995, p. 285; Trefler, Hobson, Johnson Taylor,
Monahan, & Shaw., 1993, p. 167). Proper positioning in the tractor
seat can reduce the operator’s potential for pressure ulcers,
increase ability to operate tractor controls, provide needed stability,
and reduce sliding in the tractor seat. Following a spinal cord
injury, a farmer/rancher may have abnormal muscle tones and reflexes
resulting in spasticity in the lower extremities. Leg spasms may
result in the operator bumping and bruising his/her legs by inadvertently
hitting the steering wheel, foot controls, or hand controls. Proper
positioning in the tractor seat can help to reduce injuries from
leg spasms. The following issues and equipment should be considered
when attempting to achieve the optimal seating position to stabilize
the operator’s pelvis and reduce his/her leg spasticity and
injurious pressures on his/her body: seat-to-back angle, seat-incline
angle, armrest position, foot placement, lumbar support, lateral
support, lap belts, and securement straps.
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Figure 1. Seat to back angle: An angle
of 15° to 20° helps stabilize the operator's trunk
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Seat to back angle: A lap belt is frequently
recommended to help an operator maintain a neutral pelvic position
while seated in the tractor. It is also recommended that the seat-to-back
angle be adjusted with the back being reclined approximately 15°
to help stabilize the operator’s trunk and prevent forward
loss of balance (see Figure 1). Reclining the backrest to 20°
is usually recommended for wheelchair users with quadriplegia
(Cook & Hussey, 1995, p. 298). An increased seat-to-back angle
can also reduce the vertical reaction force at the seat base (Wilder,
Magnussen, Fenwick, & Pope , 1994, p. 73).
- Seat incline angle: “A slight incline
of the seat prevents the buttocks from sliding forward and provides
trunk stability” (Trefler et al., 1993, p. 167). Tractor
seats are often equipped with this adjustment. If this option
is not available, several wheelchair seat cushions are designed
with an incline to perform this same function.
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Figure 2. Belts: Pelvic positioning belts
applied at 45° and 90° can help the operator maintain
stability and function while sitting in the tractor seat.
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Belts: Pelvic positioning belts applied at 45°
and 90° (relative to the thighs) can help the operator maintain
stability and function while sitting in the tractor seat (see
Figure 2). Tractor seatbelts can usually be secured at a
45° angle. To secure a lap belt at a 90° angle, however,
would require customizing how the belt is installed. Lap belts
that hold the legs together to prevent spasticity or potential
leg injuries due to spasticity can also provide additional seat
stability. Proper care should be taken to ensure that any positioning
belts used by a tractor operator do not lead to the development
of a pressure ulcer caused by frictional forces that can occur
during tractor operations. The SubASIS bars (sidebar)
or rigid lateral supports used with wheelchairs have not been
seen with tractors (Willkomm, 1997). This equipment has not yet
been tested in tractor operations during which the operator’s
vertical movement can be significant. A rigid lateral support
could contribute to an increase in shear forces to the operator’s
trunk. Thoracic (sidebar) and lateral
supports can also interfere with functional activities and are,
therefore, to be avoided (Trefler et al., 1993, p. 177).
- Armrest adjustments: The position of the armrests
should be slightly above the operator’s elbow height when
seated (Hansson, 1991, p. 14). If this cannot be achieved with
the stock equipment, then the armrests should be modified. Armrests
that are too low promote an asymmetrical posture and armrests
that are too high may lead to shoulder or neck pain (Cook &
Hussey, 1995, p. 299). If a seat cushion is added to the tractor’s
operator seat, it may raise the position of the operator in the
tractor’s operator seat. Therefore, the armrests will need
to be adjusted accordingly.
- Foot placement: Foot placement is critical
in achieving postural stability and good circulation in order
to help prevent a potential pressure ulcer. A footrest may need
to be added in front of the tractor seat so that the operator’s
thighs are horizontal. “Footrests that are too low will
increase pressure under the thigh and encourage sacral
(sidebar) sitting. Footrests that are too high will cause
an undue amount of pressure on the ischial tuberosities”
(sidebar) (Fergusson-Pell, 1990, p. 51). Ischial tuberosities
are the two bony prominences on the bottom of the buttocks. A
footrest can be a piece of wood or rigid insulation (pink board)
stacked, wrapped, and fastened to the floor with non-slip material.
- Lumbar support: A lumbar
(sidebar) pad placed behind the back can help to decrease
six areas of pressure, including pressure on the ischial tuberosities
(Shields & Cook, 1988, p. 168). An existing tractor seatback
rest may have a fixed lumbar support installed in it. If the seatback
does not and it is removed and a customized seatback with lumbar
supports is constructed, remember that it may affect co-workers’
use of the seat. When making any machinery modifications, it is
always important to consider how they will affect all operators.
- Lateral supports: If the operator has difficulty
maintaining a midline position (side to side) of the trunk, then
a lateral support is needed (Cook & Hussey, 1995, p. 266).
How these supports are positioned depends on how much upper body
control the operator has. Operators with high-level SCI may require
more lateral support. Traditional lateral supports used in a wheelchair
may not be appropriate in a tractor. Lateral support in a tractor
seat can be achieved through proper seat selection, adjustments
and modifications to the armrests, and the use of flexible positioning
belts that will “give a little” while the tractor
is moving over rough terrain (Willkomm, 1997a).
- Securement straps: Operators with SCI frequently
report lower extremity spasticity resulting in the need for securement
straps for their legs. These straps can prevent the legs from
inadvertently moving forward underneath the clutch and brake controls.
In addition, securement straps can help prevent the operator from
sliding forward out of the seat during a leg spasm. An additional
(padded) seat belt may be needed to secure the operator’s
pelvis. Operators with high-level SCI have also reported the use
of a chest strap. A chest strap that is too secure and too tight,
however, can cause a potential injury when the tractor hits a
bump. A chest strap could cause friction and rubbing leading to
a potential pressure ulcer. One farmer with a high-level SCI was
observed using a rubber chest strap that was secured to the two
corners of the back of the cab. This solution provided the operator
with some support as he leaned forward to grab the controls and
enabled him to more easily settle back into the seat. Another
operator used a loose chest strap that was padded with sheepskin
material (Willkomm, 1997a).
- Hip, knee, and ankle angles: Each operator’s
positioning needs are different. It may be helpful to start with
measurements taken from the operator’s position in his/her
wheelchair and use them to achieve similar positioning in the
tractor seat. The position needed to operate the tractor controls
effectively, however, differs slightly from the wheelchair positioning
measurements. For example, to achieve optimum pressure distribution
in a wheelchair, the pelvic/hip angle, knee flexion, and ankle
flexion should be approximately 90° (Trefler et al.,
1993, p. 156). In anthropometric (sidebar)
studies of 21 tractor operators with SCI, 90° angles were
not achieved in the tractor’s operator seat for any of the
farmers/ranchers. In fact, a pelvic/hip angle that less than 90°
is often preferred because it helps prevent the operator from
sliding out of the tractor seat (Willkomm, 1997a).
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2. Pressure relief and the tractor seat
An operator should perform a pressure relief every 10 to 15 minutes
for at least 5 seconds (Trefler et al., 1993, p. 175).
It can be performed by leaning forward in the tractor seat with
forearms resting on the thighs or alternately by rocking to one
side. The steering wheel, armrest, or control panel can be used
when doing a pressure relief but this may result in cracked casing
around the control panel, broken tractor seat arm rests, and a broken
tilt mechanism on the tractor steering column. Although some operators
have used an overhead grab bar when performing a pressure relief,
this grab bar could potentially cause a head injury if clearance
is insufficient (Willkomm, 1997a).
In addition to pressure relief activities, operators with SCI
have used a variety of wheelchair cushions to prevent pressure ulcers.
These cushions prevent the development of pressure ulcers through
their unique design properties including: shock absorption, ventilation,
load distribution, product integrity and stability, and ability
to stabilize the pelvis and legs. The following are a few examples
of cushions that have been used in the tractor seat for pressure
management.
- Flotation cushions (e.g., the Roho® cushion) have individual
air cells to allow air circulation. Although a flotation cushion
is excellent in allowing for changes in seat pressure, seating
stability is reduced (Cook & Hussey, 1995, p. 287). This compromise
can be a concern when operating the tractor over rough terrain.
Seat stability is important in order to maintain seat balance
and safe control of the tractor. Therefore, with this type of
cushion, additional support may be needed to maintain stability.
- Combination cushions (“hybrids,” e.g., the Jay®
cushion) are made of a combination of either a) foam and an air-holding
membrane or b) foam and a viscous fluid. They achieve stability
through good envelopment while effectively distributing pressure
(Trefler et al., 1993, p. 268). These specialized seat
cushions also have dampening characteristics that allow for shock
and vibration absorption. Unfortunately, if it is cold in the
tractor cab, it takes some time for the viscous fluid to warm
up and disperse.
- The most common type of seat cushion is made of multi-layered
honeycomb (e.g., the Stimulite® cushion). This type of cushion
provides internal contouring for uniform load distribution, pelvis
stability, proper leg positioning, and excellent shock absorption
and stability over rough terrain. The use of perforated honeycomb
provides excellent ventilation and allows moisture to evaporate
(Willkomm, 1997a).
Regardless of what cushion is selected for pressure management,
the overall height of the operator positioned in the tractor seat
must be considered. To prevent hitting the ceiling of the cab, the
tractor seat should be lowered to allow for sufficient head clearance
but not so low as to cause the operator’s seat to bottom out
when traveling over rough terrain. A clearance of approximately
15 cm (6 in.) between the operator’s head and the cab ceiling
is recommended (Hansson, 1991, p. 10).
The additional cushion will also change the height of the backrest.
Following the recommended back height support for wheelchair users
with the same level of function, the height of the backrest should
be 1.27 cm – 2.54 cm (.5 in. – 1.0 in.) below the scapulae
(sidebar) (Cook & Hussey, 1995, p. 298). For farmers
with high-level SCI, such as C4 and C5 (sidebar),
a higher seat back is needed to support all or part of the scapulae
and to accommodate for less trunk control (Cook & Hussey, 1995,
p. 298).
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Summary
In addition to all of the above suggestions, an evaluation/consultation
by a qualified seating specialist is recommended. These individuals
include physical and occupational therapists with advanced training
in seating and positioning for individuals with spinal cord injuries.
Their input and pressure mapping equipment can be useful in seat
cushion selection as well as recommendations on how the tractor
seat angles should be adjusted to maximize function and reduce potential
secondary injuries.
Proper seating and positioning are critical to enabling farmers
with SCI to operate a tractor effectively and to prevent secondary
injuries. Although this document focuses on operators with SCI,
use of more generalized ergonomic guidelines for tractor seating
can benefit all tractor operators. The level and severity of the
SCI will determine the specific adaptations that will be required.
Modifications made to an existing tractor seat could affect co-workers’
ability to properly and safely operate that tractor. Therefore,
if possible, all custom seat modifications should be easily removed
so as not to interfere with co-workers’ effective tractor
operation.
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Resources:
http://www.agrabilityproject.org/search/index.cfm
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References:
Cook A. M. & S. M. Hussey. 1995. Assistive Technologies:
Principles and Practice. St. Louis, MO: Mosby-Year Book, Inc.
Fergusson-Pell, M. 1990. Seat cushion selection. Journal of Rehabilitation,
Resource Development and Clinical Supplies, No. 2, Choosing a wheelchair
system, 49-72.
Hansson J. 1991:1 Ergonomic Checklist for Tractors and Agricultural
Machinery, The National Swedish Institute of Occupational Health,
NIOH 1991.
Shields, R. K. & T. M. Cook. 1988. Effect of seat angle and
lumbar support on seated buttock pressure. Physical Therapy.
68(11):1682-1686.
Trefler, E., D. Hobson, S. Johnson Taylor, L. Monahan, & G.C.
Shaw. 1993. Seating and Mobility for Persons with Physical Disabilities:
Therapy Skill Builders.
Wilder D., M.L. Magnussen, J. Fenwick, & M. Pope. 1994. The
effect of posture and seat suspension design on discomfort and back
muscle fatigue during simulated truck driving. Applied Ergonomics:
25(2): 66-76.
Willkomm, T. 1997. Risks in using modified tractors by operators
with spinal cord injuries and their co-workers. Ph.D. Dissertation,
University of Pittsburgh.
Willkomm, T. 1997a. Unpublished data collected related to tractor
seating and spinal cord injuries. |
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