This study observed significant association between muscle activities of deep local trunk muscles and pressure measurement of PBU in seated position. The muscle activities of TA and MF during MVIC were lower in the low back pain group than the control group. Muscle activities were significantly higher at each pressure value in the cLBP group than the control group. The slope of the cLBP group was significantly steeper than the control group. The MVIC of MF was negatively correlated with VAS and ODI. TA MVIC was not correlated with VAS or ODI.
MF and TA Muscle Activities in Healthy Controls vs cLBP Patients
Our results indicated that %MVIC of TA and MF were greater and the slope were steeper in participants with cLBP compared with the asymptomatic group. For MF, this was not in line with the study published by Danneels et al. result [36] which suggested that in comparison with the healthy subjects, the chronic low back pain patients displayed significantly lower %MVIC of MF during the coordination exercises. However, findings of our present study were consistent with previous research published by Wang-Price et al. [37] and Ansari et al. [38] which explained the high %MVIC of MF. They suggested that muscle pain could be accompanied by hyperactivity in the back muscles during dynamic conditions, which was called pain adaptation model. In addition, high muscle activity in patients with LBP might be related to muscle spasms [39]. Paraspinal muscles and the connective tissues in the region of the spine contain sensory nerve endings sensitive to changes in position, movement, and tension, which might be initiated by the presence of pain, and some sensory nerve endings related to muscle spasm with increase in activity [40]. Our result that the significant reduction of MVIC of the MF muscle in cLBP group might also explain this phenomenon. The significantly lowered MVIC of MF identified in the present study was consistent with many previous studies [30, 35]. However, the mechanism of MVIC reduction is not totally understood, but may be related to pain inhibition which limits the ability to perform maximum muscle contraction. People with cLBP patients had a greater sensitivity to pain [40], thus, we might speculate that cLBP alters spontaneous neuronal activity resulting in muscle EMG activity changes [30]. Additionally, atrophic changes of MF had been confirmed in around 77–80% of LBP cases, especially at the L5–S1 level [41] (the EMG site of MF in our study) which might contribute to lower MVIC in the cLBP group. TA is an important deep muscle that plays a key role in the dynamic control of the lumbar spine [17, 18]. The present study observed lower TA muscle activity during MVIC in the LBP group than the healthy group. Hodge et al. studied the contraction timing of TA muscle during upper and lower limb movements in patients with LBP and pain-free controls. A delay of TA muscle contraction relative to the agonist muscle that moved limbs was found in patients with LBP [17, 18]. Gildea et al. [42] reported that under contraction status, the thickness of TA was higher in female dancers with cLBP than those without pain. Muscle thickness and contraction ratio were reported to be positively correlated with muscle activity [30]. These data support our finding low muscle activity of the TA during MVIC.
The Difference between TA and MF at Different PBU Pressure in the Two Groups
Our results showed that the TA %MVIC was more active than MF %MVIC at 50 mmHg and 60 mmHg in both groups. These findings are consistent with published studies that investigated the relationship between TA and MF in patients with LBP [43]. The study reported that patients who had adequate contraction of multifidus were of 4.5 times likely to be able to contract TA. At the PBU pressure of 70 mmHg, the cLBP group demonstrated almost equal muscle activity in both TA and MF muscles, whereas in the healthy group the %MVIC of MF had more activity than TA. The potential reason may be related to the fatigue of the multifidus muscle. Deep muscle is mostly affected by inhibition reflex which occurs when sensorial input affects muscle activation [44]. According to published literature [40, 44, 45], sensorial factors influence the recruitment of TA and contribute to MF fatigue. The study by Ramos et al. [46] utilized surface EMG to assess fatigue of MF and PBU to detect activity of TA in patients with LBP. They reported that patients with LBP had difficulties to depress the abdominal wall at the PBU pressure 70 mmHg and higher MF fatigue was observed. Another possible reason is that cLBP patients have reduced flexibility and mobility in the frontal, transverse, and sagittal planes of motion [47]. When TA was contracted at 70 mmHg, there was limited space in the anatomical position [47] that the low back required to complete the motion.
Correlation between EMG activity and VAS, ODI
The present study observed a negative correlation between MVIC of MF and VAS and ODI which was consistent with previous studies [30]. There was no correlation between the MVIC of TA and VAS or ODI. The potential reason was that TA and MF have different roles in maintaining lumbar stability due to their different anatomical structures, different muscle fiber size, different motor unit control properties [16, 40, 41]. Previous studies had suggested that TA was mainly involved in lumbar stability by contractile increase of abdominal pressure [20, 21, 24, 25], and MF might directly maintain lumbar stability through the thoracolumbar fascia [14, 16, 39, 40, 48]. Therefore, compared with TA, MF might be more easily correlated with VAS and ODI. Moreover, maybe TA and MF had different neuromuscular and proprioceptive systems, along with varied changes in biomechanical alignment of spine and developed different models of pain adaptation [49]. However, directly speculating on the connection with neuromuscular control mechanisms and pain was difficult because we know so little about the underlying relationship between brain network and TA and MF muscle activity [43].