Neuromechanical control of the spine

Abstract

Control of the spine is complex. The spine is inherently unstable and dependent on the contribution of muscles. Yet there is considerable redundancy in the motor system with many muscles that act on the trunk.

This is further complicated by the indirect effects of trunk muscle contraction on the spine, such as increased intraabdominal pressure (IAP), and the multiple functions that must be performed by the trunk muscles such as respiration, in addition to control and movement of the spine. The overall aim of this series of studies was to investigate the control strategies used by the central nervous system (CNS) to control the spine. Studies were conducted to investigate the control of the spine in association with predictable (voluntary arm movements) and unpredictable perturbations (unexpected arm loading and support surface translations) to the trunk. Additional human and porcine studies were conducted to investigate the biomechanical effect on the spine of contraction of the deep trunk muscles that surround the abdominal cavity.

Recordings of electromyographic activity of the trunk muscles were made with intra-muscular electrodes and electrodes placed on the skin or mucosa overlying the muscles. Trunk movement was recorded in three dimensions with markers placed on the skin in human studies or attached to pins inserted into the spinous processes in the porcine studies. IAP was recorded with a pressure transducer inserted via the nose into the stomach. The results provide novel evidence of strategies used by the CNS to control the spine that challenge contemporary views on the mechanisms of control of the spine. A key finding was that the CNS controls the spine using movements rather than simply stiffening the spine. This conflicts with many current models and must be incorporated into biomechanical modelling to understand the mechanisms of control of the spine.

The studies also provided the first in vivo evidence of the effect of IAP on the human spine and provide several arguments for the potential importance of IAP for normal control of the spine. An interesting finding was that tasks may not be coordinated when the demands of multiple functions of the trunk muscles are conflicting. This has potential to compromise normal control of the spine.

In summary, these studies indicate that activity of the deep trunk muscles that surround the abdominal cavity is critical for fine-tuning intervertebral control. Taken together these data have implications not only for the biomechanics and motor control of spinal stability, but also for the management of patients with low back pain.