The effect of lumbar posture and pelvis fixation on back extensor torque and paravertebral muscle activation
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Lumbar postures using a high degree of flexion are considered a risk factor associated with manual handling related back injuries. The lumbar posture adopted during lifting is thought to alter the force capabilities of the trunk and the active contribution of the paravertebral muscles. To date, most studies examining back extensor (BE) force and muscle activation have been undertaken in non-functional positions with the pelvis fixated. Furthermore, paravertebral muscle activation has focused on a single spinal level, despite known anatomical and functional differences of upper erector spinae (UES), lower erector spinae (LES) and multifidus. An understanding of how lumbar posture can affect the relationships between trunk extensor torque and paravertebral muscle activity in functional lifting positions may provide important knowledge for vocational manual handling training. Thus, the aim of this study was to determine how lumbar posture and pelvis fixation affect BE torque and muscle activation of 3 divisions of the paravertebral muscles. This study involved 26 healthy participants performing a simulated static lift in 3 lumbar postures (lordotic, flexed and mid-range). A maximal isometric lift was undertaken with and without the pelvis fixated. Participants were provided with real-time feedback of their lumbar posture using a three dimensional (3D) electromagnetic motion monitor. A chest harness connected to a 3D floor mounted force gauge via a metal chain, provided a measure of maximum isometric voluntary BE force. A 9-camera motion analysis system recorded body position, which was used in conjunction with kinetics, to estimate BE torque. Muscle activity of the 3 paravertebral muscles (UES, LES, and multifidus) was recorded using electromyography (EMG). EMG signal for each muscle during peak back extension in each posture were normalised to that produced during maximal exertion in the Biering-Sorensen position. Lumbar posture was found to have a significant effect on BE torque, with torque increasing from a lordotic to flexed posture (P<0.0001). In contrast, a flexed lumbar posture produced significantly less muscle activation for all BE muscle groups than a mid and lordotic posture (P<0.001). As a result, the flexed posture showed a higher neuromuscular efficiency (NME) ratio (torque/activation) when compared to the mid and lordotic posture (P<0.001). Each paravertebral muscle responded to a change in posture in a similar manner. Pelvis fixation was found to have no effect on torque or the intensity of muscle activation. Initiating a lift using a lumbar posture similar to the mid position used in this study may help to avoid loading on the passive structures of the spine, which is thought to occur when using a fully flexed lumbar posture. The mid posture also offers higher levels of NME compared to the lordotic lumbar posture, which may be useful in situations when exerting high BE torque is necessary. Given that pelvis fixation did not influence BE torque and paravertebral muscle activation, it would suggest that functional lifting tasks should not be concerned with restraining the pelvis.