Trunk stabilization is achieved differently in patients with low back pain compared to healthy controls. Many methods exist to assess trunk stabilization but not all measure the contributions of intrinsic stiffness and reflexes simultaneously. This may pose a threat to the quality/validity of the study and might lead to misinterpretation of the results. The aim of this study was to provide a critical review of previously published methods for studying trunk stabilization in relation to low back pain (LBP). We primarily aimed to assess their construct validity to which end we defined a theoretical framework operationalized in a set of methodological criteria which would allow to identify the contributions of intrinsic stiffness and reflexes simultaneously. In addition, the clinimetric properties of the methods were evaluated. A total of 133 articles were included from which four main categories of methods were defined; upper limb (un)loading, moving platform, unloading and loading. Fifty of the 133 selected articles complied with all the criteria of the theoretical framework, but only four articles provided information about reliability and/or measurement error of methods to assess trunk stabilization with test-retest reliability ranging from poor (ICC 0) to moderate (ICC 0.72). When aiming to assess trunk stabilization with system identification, we propose a perturbation method where the trunk is studied in isolation, the perturbation is unpredictable, force controlled, directly applied to the upper body, completely known and results in small fluctuations around the working point.

We conclude that there is currently no convincing evidence for the presence of splinting behaviour in LBP patients, because we found no indications for splinting in terms of kinetic and kinematic responses to perturbation and derived mechanical properties of the trunk. Consistent evidence on delayed onsets of muscle activation in response to perturbations was found, but this may have other causes than splinting behaviour.

Measurement of the quality of trunk stabilization is of great interest to identify its role in first occurrence, recurrence or persistence of low-back pain (LBP). Our research group has developed and validated a method to quantify intrinsic and reflex contributions to trunk stabilization from the frequency response function (FRF) of thorax movement and trunk extensor EMG to perturbations applied by a linear actuator. However, the reliability of this method is still unknown. Therefore, the purpose of this study was to investigate the between-day reliability of trunk FRFs in healthy subjects and LBP patients. The test-retest ICC׳s in patients were substantial for both admittance and reflex gains (ICC3,1>0.73 and 0.67). In healthy subjects, the reliability of admittance gain was also substantial (ICC3,1 0.66), but the reliability of the reflexive gain was only moderate (ICC3,1 0.44). Although sample sizes were limited (13 healthy subjects and 18 LBP patients), these results show that trunk stabilization can be measured reliably, and represent a promising step towards using this method in further research in LBP patients.