BACKGROUND: The etiology of low back pain (LBP), one of the most prevalent and costly diseases of our time, is accepted to be multi-causal, placing functional factors in the focus of research from early on. Thereby, pain models suggest a centrally controlled strategy of trunk stiffening in the presence of LBP. However, supporting biomechanical evidence is mostly limited to static measurements during maximum voluntary contractions (MVC), probably influenced by psychological factors in LBP patients. Alternatively, repeated findings indicate that the neuromuscular efficiency (NME), characterized by the strength-to-activation relationship (SAR), of lower back muscles is impaired in LBP. Therefore, a dynamic SAR protocol, consisting of relative trunk muscle activity recordings during submaximal loads (SMVC) seems to be a promising alternative. This thesis aimed to investigate the influence of LBP on the NME and activation pattern of trunk muscles during dynamic trunk extensions.
METHODS: The SAR protocol consisted of an initial MVC reference trial (MVC1), followed by SMVCs at 20, 40, 60 and 80% of MVC1 load. An isokinetic trunk dynamometer (Con-Trex TP, ROM: 45° flexion to 10° extension, velocity: 60°/s) and a surface EMG setup (myon, up to 12 leads on main trunk extensors and flexors) was used. Extension torque output [Nm] and muscular activity [V] were assessed in all trials. At the end of the protocol, another MVC trial was performed (MVC2) for reliability analysis. For SAR evaluation the SMVC trial values were normalized [%MVC1] before being compared inter- and intra-individually. The methodological validity of the approach was tested cross-sectionally in an isometric SAR pilot study (S1a: N = 2, female LBP patient vs. asymptomatic male). In addition, the validity of MVC values as reference for SMVCs loads and EMG normalisation was verified by comparing different contraction modes (S1b: N = 17, healthy individuals). In the second stage, the isokinetic protocol was validated in terms of content for its applicability to display known physiological differences between sexes in a cross-sectional study (S2: each n = 25 healthy males/females). Finally, the influence of acute pain on NME was investigated longitudinally by comparing N = 8 acute LBP patients (‘Korff’ and VAS), with the retest after remission of pain (S3). The analysis of the SAR outcome parameter focused on relative agonistic extensor activity and abdominal and synergistic extensor co-activation (t-tests, ANOVA, α = .05) as well as on reliability of MVC1/2 outcomes (indices: ICC, CV, TRV and Bland-Altman).
RESULTS: During the methodological validation of the protocol (S1a), the isometric SAR was found to be descriptively different between individuals, whereby the data of the LBP patient appeared to be rather inconsistent. Whereas torque output was highest during eccentric MVC, no relevant difference in peak EMG activity was found between contraction modes (S1b). The sex comparison by isokinetic SAR protocol (S2), though showing no significant overall effects, revealed higher relative extensor activity at moderate submaximal loads in females (13 ± 4%), primarily caused by pronounced thoracic activity. Similarly, co-activation analysis resulted in significantly higher antagonistic activity at moderate loads compared to males (33 ± 9%). During intra-individual analysis of SAR in LBP patients (S3), a significant effect of pain status on the SAR has been identified, manifesting as an increased relative EMG activity of extensors during acute LBP (11 ± 8%), particularly marked at high load., abdominal co-activation, therewith associated, tended to be elevated (27 ± 11%) just as the thoracic extensor parts seemed to take over proportions of lumbar activity. Taking the data of all studies together, the behaviour of M. erector spinae during the SAR protocol was rather linear with the tendency to rise exponentially during high loads. For the level of relative EMG activity during SMVCs, a clear increasing trend from healthy males to females over to non-acute and acute LBP patients was discovered. This was associated by elevated antagonistic activity and a shift of synergistic towards lumbar extensor activity. The analysis of MVC data revealed overall good reliability, with clearly higher variability during acute LBP.
DISCUSSION: The present thesis demonstrates that the NME of lower back muscles is impaired in LBP patients, especially during an acute pain episode. A new dynamic protocol has been developed that makes it possible to display the underlying SAR using recordings of relative trunk muscle EMG activity during submaximal isokinetic loads. It has been validated by tracking physiological sex differences and revealing a lower NME and an altered recruitment pattern of the trunk in healthy females. Consequently, the protocol shows promise as a biomechanical tool for diagnostic analysis of NME in LBP patients and monitoring of rehabilitation progress. Furthermore, reliability not of maximum strength but rather of peak EMG activity of MVC measurements seems to be decreased in LBP patients. Meanwhile, the findings of this thesis largely substantiate the assumptions made by the recently presented ‘motor adaptation to pain’ model, suggesting a pain-related intra- and intermuscular activity redistribution affecting movement and stiffness of the trunk. Further research is needed to distinguish the grade of NME impairment between LBP subgroups.