In both groups, the quadriceps muscle became more inhibited after fatiguing isometric lumbar paraspinal exercise. These changes in QI occurred without a significant change in quadriceps MedF during MVICs, indicating that subjects were not experiencing quadriceps fatigue due to multiple MVIC efforts. Increased QI after lumbar extension exercise is in agreement with previous findings.
However, we previously found that persons with HxLBP exhibited, on average, greater quadriceps activation (ie, less inhibition). It is important to note that substantially fewer subjects were included in the previous study,
that the inclusion criteria for the HxLBP group were more liberal, and that 3 exercise sets were performed. In the current study, we used considerably more subjects, who were included only if they had previously experienced at least 3 episodes of LBP. These methodologic differences may account for the conflicting results between groups.
In the current study, the quadriceps became about 3% more inhibited after the first exercise set and about 6% more inhibited after the second set. A person who experiences a 6% increase in QI may be considered to have incomplete quadriceps activation, despite possibly complete activation before the fatiguing exercise sets. In this study, 83% (19/23) of the subjects who exhibited complete activation before exercise (ie, <5% QI)
showed more than 5% QI after the fatiguing exercise sets, indicating a change from complete to incomplete quadriceps activation that was not due to knee injury or quadriceps fatigue. This is concerning for persons who have healthy knees and experience QI. Higher amounts of QI are correlated with lower quadriceps strength
and may therefore be detrimental to force attenuation during gait. Excessive QI during prolonged exercise may expose the knee joint surfaces to abnormal stresses during gait, possibly resulting in degenerative processes and injury.
Although the relationship between QI and injury risk remains unclear, we present preliminary data that describe a potential risk to lower extremity joints, a risk that needs to be explored further in a functional setting.
The low back may be exposed to excessive forces that are unabsorbed by weak muscles and transferred through the lower extremities during activity.
Persons with HxLBP commonly suffer from hamstring tightness,
poor spinal flexibility,
and reduced lumbar lordosis,
in addition to weakness and imbalance of the hip musculature.
The results of our current study and previous work
show that the quadriceps seem to become more inhibited as the lumbar paraspinal muscles become more fatigued. However, we do not know from these data whether this relationship exists in a functional setting. Quadriceps inhibition may be a necessary adaptation to preserve function during prolonged exercise in persons who have highly fatigable lumbar paraspinal muscles, and it may cause further lower extremity neuromuscular reorganization during exercise or athletic maneuvers that involve a high demand on the quadriceps.
Quadriceps inhibition may result from muscle or joint damage or a disruption or change in afferent information from the knee joint capsule,
leading to progressive or degenerative conditions of the knee, if mismanaged. The relationship between the knee extensors and the spine is logical to clinicians, as it is prudent to evaluate the hips, pelvis, and spine during a routine physical examination of the knee. However, the neuromuscular relationship between the lumbar paraspinal and knee extensor muscles has not been clearly defined. This relationship was supported by Suter et al,
who reported reduced QI in persons with anterior knee pain after sacroiliac joint manipulations. This relationship may be mediated by a change in afferent information due to activation of the sacroiliac joint mechanoreceptors and proprioceptors through joint manipulation.
Therefore, lower extremity motor neuron excitability changes may result from a change in afferent information from the sacroiliac joint capsule. Similarly, changes in afferent information from the muscle and joint mechanoreceptors and proprioceptors in the lumbar spine due to prolonged, intense fatiguing exercise may affect quadriceps motor neuron excitability. Previously, intramuscular pain induced by hypertonic saline injection into the longissimus muscle at the L4 level caused delayed and reduced transverse abdominis muscle activity during controlled arm movements.
This finding indicates altered motor control of healthy muscles through altered input from lumbar muscle afferents. The observed change in quadriceps motor neuron excitability in the current study makes sense anatomically because afferent nerve fibers from the mid-lumbar vertebrae, muscles, and joints would enter the spinal cord through the same mixed nerve roots that contribute to the femoral nerve from the lumbar plexus. Information from lumbar muscle or joint afferents may synapse with quadriceps efferents or other interneurons in the spinal cord that control the quadriceps motor neuron pool. The mechanism of this relationship may arise from a combination of muscle and joint afferents; however, the origin of altered afferent information is speculative.
It is also possible that an indirect relationship between the lumbar paraspinal and quadriceps muscles explains the results of the current study. Quadriceps inhibition after prone, isometric lumbar extension may be a secondary effect of fatigue in the extensors of the knee, hip, and spine: namely, the lumbar paraspinals, gluteals, and hamstrings. Hamstring and gluteal muscle fatigue contribute to task failure during continuous isometric lumbar extension
; however, if hamstring fatigue had been excessive, the quadriceps motor neuron pool should have become facilitated as a result of the agonist-antagonist relationships among muscles. Because the posterior muscles were likely active during isometric lumbar extension exercise, fatigue experienced in those muscles may have contributed to the observed QI. Another possibility is that the quadriceps become inhibited in response to posterior fatigue-related muscle inhibition, as a mechanism of maintaining the anterior-posterior muscular symmetry necessary to preserve normal function during activity or gait. Regardless of the mechanism, it is important to learn more about how this adaptation affects neuromuscular function in the lower extremity and spine during prolonged functional and athletic activities.
In conclusion, QI increases after fatiguing lumbar extension exercise. These data describe a potential source of QI that may be an adaptive response to maintain symmetry during fatiguing exercise. The observed increase in QI may expose lower extremity joints to injury or degeneration during functional activity, but this possibility requires further research.