BioMechanics
July 1998

Pain Management:
Gait-related stress leads to chronic low back pain

By Howard Dananberg, DPM

Chronic lower back pain represents a difficult-to-manage problem worldwide. New insight into chronic lower back pain may be gained by examining it not as a problem localized to the spine, but rather as a gait-related process in which stress is applied to the lower back over the millions of step cycles taken each year. The anatomical configuration of the lumbar spine may be normal in appearance and form, yet exhibit pain due to the continued application of gait-related stress. In a previously published retrospective outcome study,¹ 77% of chronic postural pain patients, patients who had found no relief from any of multiple forms of treatment and were considered to be at or near medical endpoint, demonstrated 50% to 100% improvement over a two-year period when gait style was addressed. Michelle Guiliano, DPM, and myself are currently performing a prospective analysis in which the Quebec Back Pain Scale is being used to assess more than 30 patients during a 12 to 24-month period.

A recent issue of the British Medical Journal states that three quarters of the people who go to see their doctor with a new episode of lower back pain are still suffering symptoms a year later.² Other studies have shown that while most patients are no longer disabled from lower back pain after two weeks, 31% report considerable symptoms and 6% remain totally disabled six months later, despite continued care from either their primary-care physician, orthopedic surgeon, or chiropractor.³ Still other studies have shown that the injury that appears to create a disabling event is most often minor and appears out of proportion to the symptoms experienced. Therefore, it would be reasonable to consider that chronic lower back pain, or acute recurrent lower back pain, may be related to an unrecognized repetitive lumbar injury mechanism. Since the studies that examine lower back pain are, by and large, directed at the lower back, an externally applied stress would not have been identified as an etiologic entity.

Mechanics

The initiation of subsequent swing phase then develops. Each lower limb represents about 15% of total body weight. In a 150 lb subject, this equals 22.5 lb. This limb will be lifted into swing phase at least 2500 times per side per day.⁷ How this repetitive activity occurs forms the basis for understanding chronic gait-related stress to the lumbar spine.

The gait cycle, as its name implies, represents a rhythmical series of events. These events occur over a brief time span, with the average step lasting only three quarters of a second. For the sake of this discussion, it can be divided into three sections: initial double support, single support, and terminal double support. The sections make up 20%, 60%, and 20%, respectively, of each gait cycle (Figure 2). During initial double support, the contralateral limb is entering the preswing phase while the ipsilateral limb is experiencing heel strike to foot-flat phase. McGeer⁸ has demonstrated that foot slap resistance by the anterior tibial muscle group can create a very "low cost" swing-phase initiation of the opposite limb. This is, of course, providing that the limb about to enter swing phase is undergoing the proper "preswing" activity as described by Perry.⁹

Preswing motion occurs during the terminal double support phase and involves rapid flexion of both hip and knee. Simultaneously, rapid plantar flexion of the ankle combines with dorsiflexion of the metatarsophalangeal (MTP) joints of the foot, and the limb accelerates forward in preparation for toe-off. (Figure 3)⁹ Since this limb flexion begins precisely as double support occurs, it must be set up by adequate hip joint extension, occurring during the just-completed single support phase. Failure to create adequate hip extension during single support phase would therefore represent a blockage to normal preswing activity. This is analogous to trying to pitch a baseball without a windup or swing at a tennis ball without first "getting the racquet back." Without proper "preload," the creation of each subsequent swing phase would either strain the structures associated with hip flexion or require some type of assistance mechanism for timely and efficient occurrence.

Since the neurology of swing phase is hardwired in the central nervous system, the activity will begin "on cue."¹⁰ If the limb is poorly positioned at the time of toe-off or has failed to develop proper preswing speed, then this disadvantaged toe-off is likely to cause injury to the hip flexors, their vertebral and lower back origin, as well as groin insertion. These injuries result from cumulative trauma, as the process is repeated thousands of times a day. This lack of preswing positioning represents a primary etiologic factor in a repetitive stress injury to the lumbar spine.¹¹

Perry⁹ has shown how the foot provides a series of three "rockers" about which forward advancement of the center of mass occurs. She describes a sequential transfer from the round underside of the heel, to a dorsiflexion of the ankle, and finally to dorsiflexion of the MTP joints. These "rockers" provide a pivotal structure about which the body advances (Figure 4). Should this mechanism fail, then the entire chain of events of lower extremity extension based on the center of mass advancing in front of the stance foot would be adversely affected.

Pivotal changes

When viewing a foot with sagittal plane blockage, several signs are visible. The most common is a late midstance pronation. Rather than a frontal plane accommodation to a flat surface, late midstance pronation can be viewed as a sagittal plane compensation. It occurs when the ankle or the MTP joint motion fails just as the power designed to pull the center of mass forward is reaching full magnitude. The lowering of the medial arch represents only one of the compensations for this pivotal loss. The more proximal joints of the lower extremity will also become limited in their ability to extend during single support phase, or instead will produce an altered forward motion via flexion. This extension limitation is replaced by torso flexion as the center of mass continues to be propelled forward, while the lower extremities' normal sagittal mechanics are blocked. This produces the classic stooped-over, short-strided, flexed-knee gait so typical of chronic lower back pain patients (Figure 5).

Normal pelvic mechanics create stability, as the sacroiliac joints function as a "clutch," close packing via force and form closure and permitting transmission of power from one side of the pelvis to the other.¹³ When flexion of the torso occurs in place of hip extension at midstep, an adverse dynamic situation develops in the sacroiliac joint. A posterior rotation (counternutation) rather than anterior rotation (nutation) takes place, preventing the close-packed orientation from developing. Muscular tension via the piriformis and other groups is then recruited to maintain stability. Loss of the normal integrated support mechanics often results in replacement by high-cost muscular intervention.¹⁴

At toe-off, the iliopsoas muscles have been shown to be primary hip flexors.¹⁵ These muscles originate from the iliac crest as well as the lumbar vertebra, intervertebral septa, and intervertebral disks. When the femur is fixed and the psoas stimulated, it is the lumbar vertebra that move instead, undergoing a flexion/lateral rotation.¹⁶ Since disk degeneration has been shown to be directly related to this type of rotational strain,¹⁷ it would appear logical that the millions of cycles completed daily would—at minimum—create a weakened environment in which the disk functions. Considering the unstable sacroiliac joint base created just before toe-off, the system becomes a veritable house of cards.

When the above situation develops, there exists a near reflexive response on the part of the body to mobilize the "stuck" preswing limb. This motion is the lateral trunk bend. It can be observed at toe-off and occurs away from the side entering swing phase. It is as though the torso is used to drag the swing limb into motion. The two muscular structures most often associated with this trunk bending are the quadratus lumborum and the gluteals, using the iliotibial band, all on the side opposite the limb entering swing phase. Pain develops in the lateral hip and lower back as muscular overuse occurs.

Prior outcome measures and future endeavors

Treatment for these patients consisted of using custom foot orthoses fabricated objectively using a computerized gait analysis system. Patients were examined via in-shoe pressure analysis and dual directional videotaping. Test orthoses were fabricated, adjusted, tested again with in-shoe analysis, and readjusted as necessary until results demonstrated adequate weight transfer and more normal sagittal plane mechanical response. Permanent orthoses were then fabricated, based on the test results, and specifically designed to treat functional hallux limitus. These devices use an area of reduced support specifically under the first metatarsal head area. This permits normal plantar flexion of the metatarsal and prevents development of functional hallux limitus.

For this study, 30 patients who met the criteria were chosen and 20 of these were selected at random for a phone interview. From the interview responses, the following improvements were noted: 42% indicated that they were 75% to 100% better, 35% showed 50% to 75% improvement, and the remaining 23% demonstrated 25% to 50% improvement. No patient reported worse pain. All indicated increased activity levels with decreased pain. A second prospective outcome study is currently under way in which the Quebec Back Pain Scale is used to objectively assess subjective complaints before and during treatment. Results of this study are to be delivered at the 3rd World Congress on Lower Back Pain in Vienna, Austria this fall.

Conclusion

Howard Dananberg, DPM, is president of Podiatry Centers of New Hampshire in Bedford.

References

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