Twisting of the trunk was shown to be an important risk factor associated with back pain. During axial rotation, electromyographic (EMG) studies have shown that a large number of trunk muscles are recruited and the functional roles of these trunk muscles in axial rotation are multiple. It has been proposed that abnormal patterns of trunk muscle activity could affect the biomechanics of spinal movements and result in back pain. Although it is recognised that fatigue is usually linked to loss of control in spinal stability, current data on fatigue during axial rotation of the trunk is limited. There were no studies examining the fatigue changes in back pain patients. The main aim of this thesis was to examine the EMG activity of the abdominal and back muscles and torque output during isometric axial rotation at different exertion levels and during fatigue in healthy subjects and also in back pain patients.
Significant differences in activity between right and left axial rotation exertions were demonstrated in external oblique, internal oblique, latissimus dorsi and iliocostalis lumborum while no difference was shown in rectus abdominis and multifidus. These results demonstrated that there are different functional roles of the trunk muscles during axial rotation. This is important considering that the abdominal and back muscles not only produce torque, but also maintain spinal posture and stability during axial rotation exertions. The coupled sagittal torque changes from nil to flexion at the maximum exertion of axial rotation. This may indicate the complex nature of the kinetic coupling of the trunk muscles.
During fatiguing axial rotation exertion, decreased coupling torque in flexion and lateral flexion as well as different fatigue rates and activation changes were shown in the trunk muscles. In addition, fatigue was accompanied by an increased variability of torque output and of activation of the trunk muscles. These factors could affect the coordination of the performance and may influence the stability of the spine. Future studies should consider the alternations in the recruitment patterns of trunk muscles in terms of spinal stability and internal loading.
In back pain patients, higher levels of activity for external oblique and lower levels of activity for multifidus were demonstrated during left axial rotation exertion when compared with matched controls. In right axial rotation, back pain patients exhibited lesser activity of rectus abdominis at higher levels of exertion. During the fatigue process, back pain patients demonstrated a smaller fatigue rate for the right external oblique, and a lesser increase in activity for the back muscles. From these findings it was concluded that a decreased activation of one muscle may be compensated for by an overactivity in other muscles. The reduced levels of activity of the multifidus muscle during axial rotation exertion in back pain patients may indicate that spinal stability could be compromised.
As the results indicated that it is the activation pattern of individual trunk muscles which is compromised in the back pain patients, it would be important to address these issues in the rehabilitation process. For this reason activation of back muscles such as multifidus and relaxation strategies to reduce the activity of muscles such as external oblique may be useful in the rehabilitation of back pain patients.