Leg adjustments as a key: initial insights into quick‐release trials between healthy controls and chronic non‐specific low back pain patients

We state that all authors have made substantial contributions to all of the following: (i) the conception and design of the study, or acquisition of data, or analysis and interpretation of data; (ii) drafting the article or revising it critically for important intellectual content; and (iii) final approval of the version to be submitted. All contributors who do not meet the criteria for authorship will be listed in an acknowledgements section of the manuscript. Back pain has turned out be a diagnostically difficult area because the pain can be classified and distinguished in a wide variety of ways and even today the actual causes are largely unknown. It is generally recognized that patients with chronic non-specific low back pain (CNSLBP) have poor balance and a delayed response to different tasks (Luoto et al. 1998), as well as shorter strides (Khodadadeh et al. 1988) and an increase in contact time during walking (Keefe & Hill 1985). Vogt et al. (2003) observed significant differences in the activation behaviour of the m. biceps femoris. Still, it is nearly impossible to say whether the findings are causative or compensatory mechanisms of back pain. This study therefore examined whether, and how, provoked disturbances during walking lead to different and specific patterns of behaviour in healthy individuals compared to patients with CNSLBP. The classification and matching of the subjects were conducted by the Institute for Sports Medicine (FSU Jena) and were described in Ertelt et al. (2015). The subject’s task was to walk along a 17-m running track with a self-selected walking speed under loading. The loading was a constant traction force equivalent to 13% of the body weight. The traction force acted axially in the direction of walking. The subjects were told that at a specific time (contact with the force plate), an abrupt and total release would occur when the perturbation was applied. The experimental arrangement and the applied measurement systems as well as the disturbance application are described in detail in Ertelt et al. (2015). The study conforms to good publication practice in physiology (Persson 2015). The parameters analysed revealed some significant differences between the two groups studied. Except for differences resulting from specific time-parameters in ground reaction force, the analysis of knee joint acceleration also revealed a significant difference between healthy subjects and CNSLBP patients (Table 1; Fig. 1) when measured in a special way using pre-adjustment processes, measuring the residuals of knee acceleration over 100 ms from before contact up to touchdown (KJResTD). Compared to the healthy group, CNSLBP patients showed a lower adjustment process in the knee joint just before contact (up to 27%; Table 1, Fig. 1). Immediately before and after touchdown, a difference in behaviour was observed between the groups in knee joint acceleration. While in healthy people, knee joint acceleration decreased before touchdown, paused and then accelerated, in patients with CNSLBP their applied strategy was maintained beyond touchdown (Fig. 1, Table 1). The movement strategy plays a key role in the processing of loading and the transfer of load on structures of the human body. With the focus on patients with CNSLBP, these initial findings point to differences in kinetics as well as kinematic parameters. Some of our findings correspond to a known strategy in CNSLBP patients (prolongation of the contact time, lower unloading during contact; Luoto et al. 1998, Khodadadeh et al. 1988). Shortening the swing phase reduces the vertical deflection of the COM (centre of mass), while simultaneously increasing the contact time. Both adjustments lead to a reduction of total loading. The observed behaviour in CNSLBP patients partly refers to a lack of adjustment or maladjustments in the various leg joints, especially the knee joint. In the present case,