In this study, it was found that the amount of muscle activation of the lumbar erector spinae muscles and the abdominal muscles while performing isometric contraction activities could vary after 10-min static deep flexion, and the difference was more pronounced for the antagonist muscles in plank and isometric back extension exercises.
The 10-min static upper body deep flexion task was included in this study as a physically challenging activity that was believed to cause joint laxity and slightly damage spinal stability without pain symptoms. In previous research, prolonged stooping or deep flexion has been found to affect the stress–strain relationship of spinal tissues such as posterior ligaments and passive components of muscle-tendon units [11,17]. Static elongation of the posterior spinal tissues in deep flexion is known to decrease the stiffness of posterior spinal ligaments, and it can cause the low back extensor muscles to generate greater contraction forces to compensate for the reduced tissue stiffness of posterior ligaments .
In the current study, however, the level of activity of the lumbar erector spinae muscles did not increase after deep flexion, and it might be attributable to the straight back posture in the isometric back extension. Different from previous research where participants conducted lifting or weight-holding tasks in forward flexed postures , the participants of the current study made a straight back posture on a roman chair. Since posterior ligaments were less stretched in the stretched back posture compared to when maintaining flexed postures, most of lumbar extension moments during the isometric back extension exercise might have been produced by the active contraction of extensor muscles, minimizing the influence of the reduced stiffness of posterior spinal ligaments on the activation level of the low back extensors.
Significant effects of the 10-min static deep flexion were found on the activation level of antagonist muscles in the isometric back extension. The rectus abdominals and the external oblique muscles were antagonist muscles in the isometric back extension as their activation generated a flexion moment around the lumbar spine while the low back extensors were maintaining the posture by producing an extension moment. Synchronous activation of the agonist and antagonist muscles around a joint helps individuals control movements and/or maintain postures and is known to improve the stability of the joint . The reduced co-activation of the abdominal muscles after the static deep flexion could be indicative of the changes in spinal stability.
Decrease in the antagonist muscle activation level was also observed in the plank as well. The straight upper body of the plank exercise is maintained primarily by the upper body flexion moment from the contraction forces of abdominal muscles. In literature, it has been reported that the lumbar erector spinae muscles and other low back extensor muscles produce contraction forces of less than 10% of their maximum voluntary contraction capacity and contribute to the spinal stability when conducting the plank exercise [19,20]. Similar to the abdominal muscles in the isometric back extension, the decrease in the activation level of the lumbar erector spinae muscles in the plank might be attributable to the 10-min deep flexion and resultant changes in spinal stability.
The co-activation of antagonist muscles is known as the involuntary activation during the voluntary contraction of agonist muscles. The role of antagonist co-activation in maintaining joint stability has been consistently addressed in previous research, and it has been known that the joint stiffness or stability could be improved with greater co-activation of antagonist muscles [21-23]. It has also been found that individuals with damaged joint stability due to muscle fatigue would generate greater co-activation of antagonist muscles to compensate for the reduced stability of the joint .
The results of the current study, however, did not comply with the findings in previous research, and it might be attributable to the difference in the main cause of stability impairment. Different from previous research where damage in the muscle contraction performance caused the instability of the lumbar spine, the current study tested the effects of passive tissue stretching and resultant increase in the joint laxity. The prolonged elongation of posterior ligaments during the 10-min deep flexion might have affected the motor control of adjacent core muscles of the lumbar spine, resulting in the inefficiency of antagonist co-activation . However, it should be noted that additional explanations need further research, specifically on the control mechanism of co-activation of antagonist muscles.
Analysis and interpretation of surface EMG signals in kinesiological EMG and ergonomics areas have been focused mainly on agonist muscles that generate major moments around the joint of interest. Synchronous activation of antagonist muscles has not been considered frequently due to the relatively weak activation level of the antagonist muscles. The results of this study suggest that the responses of antagonist muscles, which have not received much attention before, could be more sensitive than agonist muscles in identifying minor changes in spinal stability.
Compared to previous research [11,17], the 10-min static deep flexion of this study was relatively mild in terms of its physical intensity. It might have caused smaller changes in the spinal stability compared to what have been observed in previous research that tested more physically challenging postures and activities. While major agonist muscles such as the lumbar erector spinae muscles in the isometric back extension and the rectus abdominal muscles in the plank might not be sensitive enough to be influenced by the minor changes in the spinal stability, antagonist muscles that were substantially less activated compared to agonist muscles might be more susceptible to the changes in stability from the static deep flexion.
If the changes in the spinal stability can be better detected or identified from the recruitment patterns of antagonist muscles, the evaluation of antagonist muscles may work better as an efficient and more reliable method for assessing the health status of the low back, specifically when monitoring minor changes in spinal stability in daily activities. The findings of this pioneering study, however, need further evaluation in future research with various test conditions. Specifically, it is of interest whether the results of this study would hold true for other modes of spine health damage such as the development of muscle fatigue.