Persons with a history of recurrent LBP position their trunk and spine differently than do controls in response to fatiguing isometric lumbar extension exercise. Overall, isolated lumbar paraspinal fatiguing exercise caused a more forward-flexed trunk and a less lordotic and more laterally bent spine position in healthy participants during jogging gait. Persons with LBP exhibited consistently fewer postural adjustments at the spine in response to isolated paraspinal fatigue, which may illustrate an adaptation to simulated core instability in healthy volunteers. Persons with a history of recurring LBP may be exhibiting a coping mechanism to avoid potentially detrimental spine positioning. However, greater lumbar muscle activation has been reported with more lordotic lumbar positioning versus more kyphotic positioning21
and in more flexed positions of the trunk,22
indicating that these positions may provide better dynamic stability but may increase the risk for fatigue in these muscles during prolonged tasks. Because the force-producing capacity of a muscle is reduced as it fatigues, persons with recurrent LBP may use different positioning strategies while coping with fatigued muscles in an attempt to stabilize the spine. This adaptation may explain why persons with a history of LBP experience excessive fatigue in the muscles that support the lumbar spine, hips, and pelvis.23
Similar to the fatiguing intervention in the current study, isolated lumbar paraspinal fatigue has been used by previous researchers10,13
investigating postural control. Although a variety of methods are available for inducing localized muscular fatigue, these protocols are all similar in that the localized fatigue in essence creates an artificial condition of core instability in healthy volunteers. High levels of fatigability in the lumbar paraspinal muscles have been reported as a risk factor for LBP.6,7,23
In healthy persons without a history of LBP, lumbar paraspinal fatigue resulted in an anteriorly displaced center of pressure and center of mass, indicating a forward-leaning posture13,24
and an altered postural control strategy in response to a balance perturbation after isolated lumbar paraspinal fatigue.24
In addition, lumbar fatigue resulted in a more forward-flexed posture and impaired postural control during quiet standing.13
Our findings are consistent with those of these previous authors in suggesting that a more forward-flexed posture results from localized lumbar paraspinal fatigue in healthy individuals who have never experienced LBP. However, in persons who have experienced recurrent LBP, the opposite occurred. Finally, isolated lumbar fatigue24,25
and a forward-leaning posture26
have been associated with an increase in antagonist muscle activity in the rectus abdominis muscle in healthy persons. We did not record EMG during jogging gait; however, we speculate that this response occurred in the control group, as they experienced forward trunk posture during jogging gait. In persons with recurrent LBP, a lack of abdominal muscle strength and endurance, common in this population, may help to explain why they did not experience a similar adaptive postural mechanism during jogging gait.
Altered trunk and lumbar spine positioning during activity may change compressive loading of the intervertebral facet joints, increasing compressive loading at the intervertebral disc and pressure in the nucleus pulposus.27
This may result in abnormal compression or tension stresses on the anterior and posterior aspects of the intervertebral disc, respectively, potentially increasing the likelihood of LBP.27,28
In addition, a forward-flexed posture of the trunk causes greater intervertebral fluid loss and less nutrient diffusion into the disc.29
In our study, persons with LBP may have been adapting to isolated lumbar fatigue by avoiding potentially injurious trunk and spine positions.
During gait, a more forward-leaning position of the trunk causes a forward excursion of the body's center of mass,30,31
which is characterized by sustained knee flexion during the stance phase in addition to faster ground reaction force loading rates31
and increased metabolic demand.32
These changes may also alter lower extremity joint moments. For example, greater lateral trunk sway has been associated with reduced ipsilateral knee adduction moments.33
Similarly, it is reasonable to expect a change in the sagittal-plane knee moment with a more forward-flexed posture of the spine and trunk. Previously, lumbar paraspinal fatigue resulted in reduced quadriceps activation11,12
and reduction in the external knee flexion moment,34
indicating a coping response that may include quadriceps avoidance during gait.35
In the current study, we observed slightly greater knee flexion angles (however, they were not statistically significant and were of suboptimal statistical power) during the first half of the stance phase of gait. This finding is similar to the finding of compensatory mechanisms for maintaining a more upright posture of the trunk during gait.36
Crouch gait, characterized by excessive knee flexion during terminal swing and the initial phase of stance36
(ie, loading phase), has been observed as an adaptive mechanism to excessive forward lean during gait.31
This lower extremity compensatory mechanism has been observed during gait in persons with postsurgical flatback deformity whose gait patterns resemble those of persons with advanced knee joint osteoarthritis.37
This compensation orients the trunk in a more vertical position but may place abnormal stresses on the lower extremity joints during the stance phase of gait. Based on the current data, we cannot comment on the potential influence of altered trunk posture during gait on lower extremity injury risk.
Persons with chronic LBP typically have poor endurance in the muscles that support the lumbo-pelvic-hip complex; in particular, poor lumbar paraspinal endurance has been linked with risk for developing LBP.7
We used the model of core instability in healthy persons and observed a postural response during jogging gait. Persons in the LBP group were recreationally active and reported recurring episodes of LBP. Through our screening history and physical examination, we attempted to isolate those with muscle-related LBP by excluding volunteers who may have had other conditions contributing to recurring LBP, such as disc or bone injury, tumor, or nerve involvement. Therefore, we hypothesize that the participants in the recurrent LBP group might have been exhibiting a familiar postural coping mechanism that the control group was not able to use. This may represent a shunting response to the core as a global protective mechanism to avoid unnecessary movements of the trunk that may predispose the lower extremity and spine joints to inappropriate or excessive forces. Persons in the LBP group, who were recreationally active, may have experienced altered spine positioning previously and so were using a mechanism to avoid such positions in order to preserve function during jogging gait.
The kinematic changes we observed are small and raise an important distinction between statistical significance and clinical importance. The effect sizes for the control group from prefatigue to postfatigue ranged from small to medium (); however, kinematic movements were recorded during a self-selected jogging task at a comfortable pace and not during a provocative maneuver, such as a drop landing or other simulated perturbation. Tasks that mimic common joint injury mechanisms may magnify subtle changes over time or between groups. However, the effects of very small changes in joint positions observed during jogging gait in the current study may represent changes that would potentially have a greater, cumulative effect on an athlete over the course of an entire game, season, career, or lifetime. Although we did not observe large effect sizes, small fluctuations in spine kinematics may be of considerable clinical importance with regard to the long-term joint health of persons with recurring LBP. Zazulak et al38
measured transverse-plane trunk proprioception in athletes and followed them prospectively for 3 years. A mean, statistically significant difference of 0.7° was seen in transverse-plane trunk active reposition error in females who experienced knee joint injuries during the follow-up period compared with females who did not sustain knee injuries. This small difference resulted in an odds ratio that increased 2.9-fold (in terms of the likelihood of experiencing a knee injury) for every degree in increased transverse-plane active position error.38
In addition, small differences in maximum flexion, extension, and lateral trunk displacement in response to a sudden perturbation were predictors for knee ligament injury, with 91% sensitivity and 68% specificity, indicating that greater trunk displacement predicted knee ligament injury.39
For men and women, a history of LBP predicted risk for knee ligament inury.39
Therefore, small differences in core proprioception and neuromuscular control and LBP history may have profound effects on lower extremity injury risk in active populations.
Effect Sizes for Selected Pre-Exercise to Postexercise Changes in Jogging Gait Kinematics Between Persons with Recurrent Low Back Pain (LBP) and Controls
To conclude, in response to fatiguing isometric lumbar extension exercise, persons with a history of recurrent LBP position their trunk and spine differently than do controls. The observed differences are very small; however, they may represent a necessary adaptation used by persons with recurrent LBP to preserve gait function by stabilizing the spine and preventing inappropriate trunk and lumbar spine positioning. These changes may be of clinical importance in the development of recurrent LBP.