Determination of a sagittal plane axis of rotation for a dynamic office chair.

OBJECTIVE: This study investigated the location of the axis of rotation in sagittal plane movement of the spine in a free sitting condition to adjust the kinematics of a mobile seat for a dynamic chair. BACKGROUND: Dynamic office chairs are designed to avoid continuous isometric muscle activity, and to facilitate increased mobility of the back during sitting. However, these chairs incorporate increased upper body movement which could distract office workers from the performance of their tasks. A chair with an axis of rotation above the seat would facilitate a stable upper back during movements of the lower back. The selection of a natural kinematic pattern is of high importance in order to match the properties of the spine. METHOD: Twenty-one participants performed four cycles of flexion and extension of the spine during an upper arm hang on parallel bars. The location of the axis of rotation relative to the seat was estimated using infrared cameras and reflective skin markers. RESULTS: The median axis of rotation across all participants was located 36 cm above the seat for the complete movement and 39 cm for both the flexion and extension phases, each with an interquartile range of 20 cm. CONCLUSION: There was no significant effect of the movement direction on the location of the axis of rotation and only a weak, non-significant correlation between body height and the location of the axis of rotation. Individual movement patterns explained the majority of the variance. APPLICATION: The axis of rotation for a spinal flexion/extension movement is located above the seat. The recommended radius for a guide rail of a mobile seat is between 36 cm and 39 cm.

The effect of muscle fatigue and low back pain on lumbar movement variability and complexity.

INTRODUCTION: Changes in movement variability and complexity may reflect an adaptation strategy to fatigue. One unresolved question is whether this adaptation is hampered by the presence of low back pain (LBP). This study investigated if changes in movement variability and complexity after fatigue are influenced by the presence of LBP. It is hypothesised that pain free people and people suffering from LBP differ in their response to fatigue. METHODS: The effect of an isometric endurance test on lumbar movement was tested in 27 pain free participants and 59 participants suffering from LBP. Movement variability and complexity were quantified with %determinism and sample entropy of lumbar angular displacement and velocity. Generalized linear models were fitted for each outcome. Bayesian estimation of the group-fatigue effect with 95% highest posterior density intervals (95%HPDI) was performed. RESULTS: After fatiguing %determinism decreased and sample entropy increased in the pain free group, compared to the LBP group. The corresponding group-fatigue effects were 3.7 (95%HPDI: 2.3-7.1) and -1.4 (95%HPDI: -2.7 to -0.1). These effects manifested in angular velocity, but not in angular displacement. DISCUSSION: The effects indicate that pain free participants showed more complex and less predictable lumbar movement with a lower degree of structure in its variability following fatigue while participants suffering from LBP did not. This may be physiological responses to avoid overload of fatigued tissue, increase endurance, or a consequence of reduced movement control caused by fatigue.

Pain intensity attenuates movement control of the lumbar spine in low back pain.

INTRODUCTION: Pain intensity attenuates muscular activity, proprioception, and tactile acuity, with consequent changes of joint kinematics. People suffering from low back pain (LBP) frequently show movement control impairments of the lumbar spine in sagittal plane. This cross-sectional, observational study investigated if the intensity of LBP attenuates lumbar movement control. The hypothesis was that lumbar movement control becomes more limited with increased pain intensity. METHODS: The effect of LBP intensity, measured with a numeric rating scale (NRS), on lumbar movement control was tested using three movement control tests. The lumbar range of motion (ROM), the ratio of lumbar and hip ROM as indicators of direction specific movement control, and the recurrence and determinism of repetitive lumbar movement patterns were assessed in ninety-four persons suffering from LBP of different intensity and measured with an inertial measurement unit system. Generalized linear models were fitted for each outcome. RESULTS: Lumbar ROM (+ 0.03°, p = 0.24) and ratio of lumbar and hip ROM (0.01, p = 0.84) were unaffected by LBP intensity. Each one point increase on the NRS resulted in a decrease of recurrence and determinism of lumbar movement patterns (-3.11 to -0.06, p ⩽ 0.05). DISCUSSION: Our results indicate changes in movement control in people suffering from LBP. Whether decreased recurrence and determinism of lumbar movement patterns are intensifiers of LBP intensity or a consequence thereof should be addressed in a future prospective study.

Concurrent validity and reliability of a novel wireless inertial measurement system to assess trunk movement.

INTRODUCTION: Assessment of movement dysfunctions commonly comprises trunk range of motion (ROM), movement or control impairment (MCI), repetitive movements (RM), and reposition error (RE). Inertial measurement unit (IMU)-systems could be used to quantify these movement dysfunctions in clinical settings. The aim of this study was to evaluate a novel IMU-system when assessing movement dysfunctions in terms of concurrent validity and reliability. METHODS: The concurrent validity of the IMU-system was tested against an optoelectronic system with 22 participants. The reliability of 14 movement dysfunction tests were analysed using generalizability theory and coefficient of variation, measuring 24 participants in seven trials on two days. RESULTS: The IMU-system provided valid estimates of trunk movement in the primary movement direction when compared to the optoelectronic system. Reliability varied across tests and variables. On average, ROM and RM were more reliable, compared to MCI and RE tests. DISCUSSION: When compared to the optoelectronic system, the IMU-system is valid for estimates of trunk movement in the primary movement direction. Four ROM, two MCI, one RM, and one RE test were identified as reliable and should be studied further for inter-subject comparisons and monitoring changes after an intervention.