Quadriceps activation was reduced with fatiguing exercise to the lumbar paraspinal muscles, even though the quadriceps were not fatigued (). Overall spinal stability involves contributions from multiple muscles and muscle groups surrounding the lumbar spine.29
Leetun et al30
discussed the role of pelvic and trunk stabilization on control of lower extremity movements. In their study, basketball and track athletes who did not sustain lower extremity injuries had stronger hip abduction and greater external rotation strength.30
Although the authors found no association between trunk extensor endurance and the risk of lower extremity injury, other results16,31–33
suggest a connection between the muscles that stabilize the spine and those that stabilize the lower extremity. First, Hodges and Richardson33
reported that trunk muscles that contribute to spinal stability activate before lower extremity movements. This suggests that the body attempts to stabilize the spine as a foundation for lower extremity movements. In addition, Suter and Lindsay16
reported reduced QA in subjects with a history of low back pain whose lumbar paraspinal muscles were quicker to fatigue during an extended isometric contraction. Finally, conservative treatment for low back dysfunction including sacroiliac joint manipulations caused quadriceps disinhibition in persons with symptomatic anterior knee pain.31,32
Quadriceps activation and quadriceps median frequency (MF) measured simultaneously over time (±SEM)
From our data, we cannot determine whether the reduced QA would be meaningful in an active setting. Quadriceps inhibition (ie, reduced QA) can result in kinematic and kinetic changes during gait.21,34,35
This may compromise the ability of the lower extremity muscles to appropriately respond to joint loading, resulting in changes in gait mechanics. Quadriceps inhibition after anterior cruciate ligament injury36
causes adaptations in gait mechanics.34,35,37
The term “quadriceps-avoidance gait” has been used to describe these detrimental gait adaptations that are commonly associated with quadriceps inhibition, which may have long-term adverse effects.34
Future investigators may help us to understand the meaningfulness of the observed QA reduction after fatiguing lumbar extension exercise during gait.
When a muscle is fatigued, fewer motor units are available to call on during muscle contractions.10
Lumbar muscle fatigue causes biomechanical adaptations during lifting tasks38
and reduces trunk proprioception.8
The fatiguing exercise in our study targeted the lumbar extensors; however, we do not know the rate or extent to which each muscle group in the lower extremity and trunk fatigued during the exercise sets. It is probable that some degree of hamstring and gluteal muscle activation occurred during isometric trunk extension. Hamstring and gluteal fatigue contribute to task failure during isometric trunk extension.39
In the present study, we know that the quadriceps were not fatigued, but we did not measure or control for hamstring or gluteal fatigue during the exercise sets. Future investigators should determine muscular contributors other than the lumbar extensors to QA reductions during isometric trunk extension. In addition, we cannot attribute the change in QA to a peripheral or central mechanism of lumbar paraspinal fatigue. We cannot discount the possibility that changes in QA may have central and peripheral origins that may have neuromuscular or metabolic consequences not explained by the present data. However, review of quadriceps MF and individual force components of the central activation ratio leads us to conclude that peripheral quadriceps fatigue from repeated quadriceps MVIC was probably not a factor in the observed inhibition after lumbar extension exercise ().
Average Force Components of Central Activation Ratio Across Group and Time (Mean ± SD)
Subjects in the HxLBP group exhibited, on average, greater QA than controls. We found no difference in baseline QA between the HxLBP group and the control group. This finding is consistent with that of previous authors,16
who reported no differences in quadriceps inhibition between persons with HxLBP and controls. Our findings were similar despite the fact that the subjects in the previous study were considerably heavier (85.5 ± 17.3 kg) and older (37.4 ± 9.9 years) than those in our study (72.4 ± 12.1 kg and 24.1 ± 3.1 years, respectively). However, subjects in the HxLBP group exhibited greater QA after the fatiguing lumbar extension exercise sets. The small sample size and homogeneity of the subjects in each group may have affected this interaction because the subjects in the HxLBP and control groups did not differ considerably other than in the self-reported history of low back pain. All subjects were otherwise healthy, active individuals who would be expected to have similar QA. The possibility of poor endurance or strength in the spine and pelvis stabilization muscles in the HxLBP group may have resulted in a different strategy to compensate for lumbar paraspinal muscle fatigue. However, we cannot be absolutely certain that we ruled out all bone, disc, nerve, and tumor conditions with our interview and physical examination, nor can we be certain that subjects had poor core muscle strength and endurance compared with the control group, as this was not a part of our physical examination or inclusion criteria.
Although multiple factors may contribute to the different mechanisms by which the 2 groups in this study responded to the fatiguing lumbar extension exercise, we offer 2 possible explanations. The first is based on muscle fatigue. Force production capability most likely was reduced as the lumbar paraspinal muscles fatigued during the trunk extension exercise. Subjects with HxLBP may have compensated for lost lumbar stability due to lumbar fatigue by protecting the quadriceps motor neuron pool in order to maintain anterior-posterior symmetry. This would be the most likely explanation given the fact that the quadriceps did not fatigue but were inhibited. In addition, we examined the individual components of the central activation ratio between groups () and noticed smaller average superimposed burst force in the subjects with HxLBP than in control subjects. This finding suggests that the subjects with HxLBP may have been recruiting quadriceps motor units from a smaller total motor neuron pool. In the absence of quadriceps fatigue, persons with HxLBP may have fewer quadriceps motor units to be inhibited per kilogram of body mass. Therefore, a relationship between the trunk and knee extensors may exist in persons with poor lumbar muscle stability, perhaps as a protective mechanism to preserve knee function during activity.
Another possible explanation is based on the reciprocal relationship between agonist and antagonist muscles. Subjects in the HxLBP group may have had less strength and endurance in the abdominal muscles. As the lumbar paraspinal muscles fatigued and most likely became inhibited,10
the remaining stabilizing muscles should have responded by co-contracting to protect the lumbar spine. In participants with HxLBP, less abdominal strength may place more of a demand on other muscles, such as the quadriceps, to promote stability and proper function at the pelvis and spine.40
Higher QA in subjects with HxLBP after the fatiguing exercise may be a protective response to compensate for reduced lumbar stability. If the lumbar paraspinal muscles were inhibited due to fatigue while the quadriceps muscles were inhibited without fatigue, from a reciprocal inhibition perspective, one would expect simultaneous hamstring and abdominal activation to maintain anterior-posterior muscle balance for trunk and pelvis stability. If persons with HxLBP adapt to lumbar fatiguing exercise by protecting the quadriceps motor neuron pool, possibly because of weak core-stabilizing muscles, it is necessary to determine how the other muscles, such as the hamstrings, respond and/ or contribute to this relationship. Further, we do not know whether this relationship may be more pronounced in subjects with active low back pain.
Quadriceps fatigue is an inherent limitation when using the superimposed burst technique in a repeated-measures design. In our study, subjects performed 12 maximal quadriceps contractions, each lasting 3 to 5 seconds. Reduced force production by a muscle group is a direct result of local muscle fatigue; however, the central activation ratio assumes this reduction in force is due to a central drive failure. Therefore, it is important to include a measure of fatigue when using this technique for estimating muscle activation. In our study, QA calculations were performed using MVIC and MVIC + superimposed burst force data from each level of time when the subsequent ratios indicated a reduction in QA relative to what was electrically recruitable at each postexercise measure. However, the reduction in MVIC + superimposed burst force seen over time (see ) may be interpreted as local quadriceps fatigue, which may be contributing to the decline in QA after lumbar paraspinal exercise. Although this is contradictory to our quadriceps MF data indicating no quadriceps fatigue, we cannot completely rule this out as a contributing factor to QA reduction.
In conclusion, regardless of group, lumbar paraspinal fatiguing exercise reduced QA. The relationship between the lumbar paraspinal and quadriceps muscles may exist to facilitate stability and potentially maintain normal QA during prolonged activities in persons with HxLBP. The mechanism of the association between lumbar paraspinal fatigue and QA requires further research to investigate interactions among muscles in the lower extremity and core during fatiguing exercise. In addition, future investigators should focus on how the observed QA reduction after fatiguing exercises affects gait in patient populations.