Women's Driving Posture: An Overlooked Health Issue
by Dennis Zacharkow, PT.
Driving a car is a continuous vigilance task in a very fixed sitting posture. Unlike sitting in an office chair, safety factors prohibit regular weight shifting when driving. As a result, it is critical to drive in one's most optimal healthy and alert sitting posture.
Many women, however, know that something is not right. They are instinctively aware that by conforming their bodies to car seats, they are gradually distorting their postures and harming their health.
The potential harm begins as soon as a woman leans against the car backrest. Her trunk collapses due to inadequate support for the pelvis and rib cage. Lack of stabilization to these two critical areas can lead to harmful effects on a woman's posture and health.
An open space is necessary at the lower end of an office chair's backrest to provide
adequate space for the posterior protrusion of the buttocks. Without the provision of a large open space, the buttocks will be pushed forward on the seat, inducing a slumped sitting posture.1-3 Car backrests lack such an open space, at best having only a slight recess for the buttocks.
Due to soft tissue differences between the sexes, females in general have a larger gluteal prominence than males.4,63 Women will therefore tend to be pushed further away from the lower backrest when driving, making backrest contact primarily in the gluteal region. This leaves the pelvis unsupported, resulting in pelvic instability when driving.
According to Branton, the freedom of the pelvis to move, which occurs in all sitting
postures when the upper sacrum is not supported by a backrest, results in relatively fast oscillatory movements of the pelvis rocking over the ischial tuberosities.5 The exposure to vertical vibration and road shock on the seat when driving intensifies this rocking motion of the pelvis, thereby increasing the bending stresses to the lower lumbar spine.6,7
Trunk muscle fatigue from vibration impairs the driver's ability to sense a change in pelvic and lumbar spinal position.8-10 Therefore, support to the sacrum is critical to assure accurate positioning of the pelvis at all times when driving. Proper lumbar spinal posture when driving depends directly on the position of the sacrum and pelvis.11 Sacral support may also help relieve posterior pelvic pain, a common site of pain for pregnant women.12
As opposed to advocating sacral support for proper pelvic stabilization when driving,
many health professionals continue to recommend lumbar support instead. This results in three
1. Lumbar support is placed too high to control the oscillatory movements of the
pelvis rocking over the ischial tuberosities.13
2. Lumbar support displaces the upper trunk behind the hips.3,13,14 This moves the driver further away from the steering wheel and windshield. The driver usually adapts to this postural distortion by increasing the thoracic kyphosis, rounding the shoulders, and bringing the head forward.15 Maintaining this posture when driving increases stress to the neck and upper back.
3. Lumbar support results in a relaxation and overstretching of the lower abdominal muscles, critical postural muscles for both sitting and standing posture.3,13
Note: Although not shown here, the "hinge area" for spinal flexion also includes L1, the uppermost lumbar vertebra.
The typical car backrest in the region of a woman's thoracic spine is a combination of
excessive recline, concavity, and softness. When a woman driver leans against such a backrest, the result is a "postural depression," with the front of the rib cage hinging forward and downward towards the pelvis.13,16,17. The greater the gluteal prominence, the greater the postural depression, as a woman must lean further back before contacting and collapsing her trunk into the backrest.
This approximation of the rib cage to the pelvis when driving is due to flexion of the
lower thoracic spine.3,13,18-22 (See Figure 1.) The lower thoracic spine is the region where the
extensor mechanism of the spine is the weakest, and is called the "hinge area" for spinal flexion.12,23,24 Lumbar support does not stabilize this critical area of the spine.
The hinging forward and downward of the rib cage towards the pelvis from the typical car
backrest results in an increased thoracic kyphosis when driving. This increased thoracic
kyphosis should be of much concern to women of all ages for the following reasons:
1. Up to fifty-one percent of healthy women from ages twenty to sixty-four are
kyphotic in their normal posture.25
2. Compared to younger and older age groups, the most dramatic decline in thoracic extension mobility occurs in women in their thirties and forties.26
3. Beyond approximately age forty, the rate of increase in thoracic kyphosis is much higher in women than men.27
4. An increased thoracic kyphosis occurs during pregnancy.28
5. An increased thoracic kyphosis is characteristic of women who habitually wear high-heeled shoes.29
6. An increased thoracic kyphosis is characteristic of women who overdevelop the upper rectus abdominis muscle with sit-ups and crunches.3,13
7. A woman's breast development increases the forward bending moment on the thoracic spine via the rib cage.
8. A habitual postural kyphosis can play a role in bone remodeling among healthy premenopausal women.25
9. T11 and T12, the vertebrae located at the "hinge area" for spinal flexion, are the most frequently deformed vertebrae in postmenopausal women.30
Preventing and correcting the postural depression and the increased thoracic kyphosis when driving require firm support localized to the lower thoracic spine (T10 - T12). This lower thoracic support will elevate and stabilize the rib cage, and elongate the spine. Head, neck, and shoulder posture will all be dramatically improved through firm support to the lower thoracic spine.
The human body can be compared to an open-chain system of links that allows free
movement at the various joints.31 Sitting, however, involves a spontaneous attempt to stabilize the body segments.32 This need for stability demands a closed-chain system of links.
The stability and consistency required for maintaining one's driving posture and operating the controls (steering wheel, foot pedals) necessitate such a closed-chain link system.33 The healthiest and most efficient closed-chain link system for driving involves stabilization of the trunk through proper activation of the diaphragm, transversus abdominis, pelvic floor muscles, and the lower thoracic erector spinae.13,14,17,59
Proper activation of the transversus abdominis is achieved by a "belly-in" position of the lower abdomen.34-37 This is facilitated by maintaining gentle pressure against the sacral support in a backward and upward direction.13,14,34 This slight movement results in a simultaneous reflex activation of the lower thoracic erector spinae and pelvic floor muscles.23,34,59
The firm pressure from the lower thoracic support against the T10 - T12 region of the back when driving immediately facilitates proper diaphragmatic breathing by eliciting the intercostal- to-phrenic reflexes.38,39
(Phrenic motoneurons have been demonstrated to be under the control of reflexes elicited by afferent stimulation of the lower intercostal nerves and the dorsal rami of the lower thoracic spinal nerves. Mechanical stimuli applied to the lower thoracic region, such as pressing on the back muscles or on the lower part of the rib cage, have been shown to have an excitatory effect on the diaphragm from an increase in phrenic motoneuron activity.38
While the intercostal-to-phrenic reflexes can be facilitated with proper lower thoracic support at the T10 - T12 level, afferent stimulation at the mid-thoracic level (T5 - T6) will actually inhibit phrenic motoneuron activity.40,41 This is why the abnormal pressure stimulation against a woman's upper back from typical car backrests impairs diaphragmatic breathing.)
When driving in a position of postural depression, this important closed-chain link
system breaks down, and affects both a woman's internal organs and her musculoskeletal system.
Along with the relaxation of the transversus abdominis muscle in a collapsed trunk posture, there is a simultaneous relaxation of the pelvic floor muscles. The abnormal stress from prolonged driving with relaxed pelvic floor muscles can result in pelvic and rectal pain. Over years, the gradual weakening of the pelvic floor muscles from improper sitting can be a contributing factor in developing stress incontinence.
Due to the relaxation of the lower abdominal muscles and the resulting lowered resting position of the diaphragm, a collapsed trunk posture restricts diaphragmatic breathing. With diaphragmatic breathing restricted, upper chest breathing predominates, leading to shallow inspiration and a chronic state of fatigue.
Upper chest breathing increases the respiratory load placed on the scalene muscles, the neck muscles attached to the upper two ribs. The scalene muscles normally contribute to inspiration through elevation of the upper two ribs. However, when upper chest breathing predominates, the scalene muscles are overworked.
The tendency for the rib cage to move in a downward direction when driving in a position of postural depression also results in a greater load on the scalene muscles to elevate the upper rib cage against gravity.60,61 Overuse of the scalene muscles not only strains the neck, but may also activate "trigger points" in these muscles. The resulting referred pain from these trigger points along the medial border of the scapulae and down the arms and hands resembles the pain pattern of carpal tunnel syndrome.62
Lower thoracic support to the T10 - T12 region when driving is critical for decreasing the
stress placed on the scalene muscles for two reasons:
1. Mechanical support to the lower thoracic spine (T10 - T12) elevates the rib cage and prevents its downward movement. This mechanical support immediately reduces some of the static load on the scalene muscles.
2. The pressure from the lower thoracic support against the lower thoracic region of the back immediately facilitates proper diaphragmatic breathing by eliciting the intercostal-to-phrenic reflexes.38,39
A woman remains in a round back posture for as long as she drives in a postural depression. Eventually, she maintains the same round back posture when standing up.
Unfortunately, a round back posture only receives attention in our society as the
characteristic fixed posture of many elderly postmenopausal women with osteoporosis. However, the constant abnormal stress to the spine from a habitual slumped, round back posture can permanently alter the shape of the spine in healthy premenopausal women!25
It is not surprising that women have a much higher incidence than men of upper back and neck pain, as these pain syndromes are associated with a slumped, round back posture.52
Driving in a postural depression results in an increased thoracic kyphosis, round
shoulders, and a forward head posture.17 In such a driving posture, rotational head movements are decreased, limiting the driver's field of vision.33,47
Backward nodding of the head is not possible when driving with a forward head
posture.47 The loss of head motion in extension with this posture may explain why a whiplash injury can result from a negligible rear-end impact.33
Important Note: For optimal protection against a whiplash injury from a rear-end
collision, the headrest should gently contact the back of the head when driving.48 Unfortunately, this headrest position is difficult to achieve in most automobiles due to a lack of headrest adjustability.
Several factors contribute to the pelvis sliding forward on the seat when driving, leading
to a postural depression and a breakdown of proper trunk stabilization:
1. Driving with extreme extension of the knees to reach the foot pedals increases the tension in the hamstring muscles. This hamstring tension pulls the pelvis forward on the seat in a position of excessive posterior rotation.
2. Movements of the lower extremity with braking and accelerating tend to pull the pelvis forward on the car seat.49
3. Vertical vibration on the seat has a similar effect, causing the pelvis to slide
forward on the seat.50
Suggestions to prevent the pelvis from sliding forward on the seat, in order to maintain
the integrity of the closed-chain link system for driving, include:
a. Moving the car seat slightly forward to increase the knee flexion and reduce the tension on the hamstrings. It is important to note, however, that excessive knee flexion, while being beneficial in relaxing the hamstrings, can be disadvantageous for the application of force to the brake pedal.51
b. A stationary inclined footrest, located to the left side of the brake pedal, enables the left lower extremity to exert effective counter-pressure for preventing the forward migration of the pelvis on the seat.3,33
c. Using a seat cover of woven fabric. As opposed to slippery seat covers such as leather or vinyl, the beneficial friction from a woven fabric keeps the pelvis from sliding forward on the seat.
The least stressful and fatiguing position for the arms, neck, and back when driving is with the upper arms hanging vertically at the hip line.52 However, the commonly recommended ten o'clock - two o'clock hand positions on the steering wheel disturb this balanced vertical arm posture, resulting in a forward flexion of the upper arms.53
This elevated arm posture increases the forward bending moment on the thoracic spine via the rib cage.19 The closed-chain link system breaks down, leading to round shoulders and a round back, along with excessive tension in the neck and upper back.54
The forward flexion of the upper arms can be reduced when driving with lowered hand positions on the steering wheel, such as nine o'clock - three o'clock or eight o'clock - four o'clock.
A slight backrest inclination (approximately 10°) when driving is important to stabilize the body with accelerating, braking, cornering, and other movements of the vehicle. As opposed to a fully vertical driving posture, shifting some body weight to the backrest helps reduce the spinal stress from road shock and vibration.55-57
An excessive backrest inclination when driving distorts the proper upright relationship of the head, neck, and upper back due to the visual requirements of driving. In order to achieve the proper visual angle and distance to the windshield, the driver pulls her head, neck, and upper back forward. In addition, the greater the backrest recline, the more the upper arms are flexed forward to reach the steering wheel, adding further stress to the neck, shoulders, and upper back.13
An excessive recline also distorts the proper upright relationship of the thorax and pelvis,
by displacing the thorax behind the pelvis.3,13 Such a reclined posture relaxes the lower
abdominal wall and depresses the diaphragm,15,58 with the driver having the same trunk muscle
activity as an unconscious person! The end result is a total breakdown of the closed-chain link
system of trunk stabilization that is essential to women's posture and health throughout life.
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