Biomechanical Models of Spinal Control
Control of upright orientation.
Uprightness and stability are an accurate description of the challenge for humans. How we maintain the optimal curves of the spine over a life-time needs to be understood.
Control of the spine in the regulation of translational and rotational forces at the intervertebral level both in static positions and during movement. This includes standing, sitting, turning, control of weight-shifts and moving.
Intersegmental coordination could be defined as the ability to keep all segments of the spine aware and free to respond. This is not possible without deep system function, as muli-segmental muscles will ‘lock’ many segments of the spine together as a non-preferred stabilising strategy. The most common example of this is an increase in kyphosis, where the external oblique (EO) muscle acts to provide spinal stability. EO is a moving muscle which can also act to brace the trunk, but its role is not informational in the way that the deep system functions.
Control of the pelvis
In upright positions the sacro-iliac joint (SIJ) is subject to considerable shear force as the mass of the upper body must be transferred to the lower limbs via the ilia. When you consider that our big and strong global moving muscles cross the SIJ diagonally via the fascial connection of the lat-glute sling and that we are bound by dominances of hand, foot, eye and vestibular. The difficulty of normalising the forces acting on the pelvis become apparent.
Ground reaction forces are required for moving, including running, jumping, throwing and hitting. All of these forces are registered at and through the pelvis. Awareness of the pelvis which includes the reference of our centre of mass, is essential to move well.
Neural Strategies for Spinal Control
Over a life-time of movement experience, the central nervous system (CNS), builds up an experiential model that includes information of the body and how it is affected by internal and external forces. This is known as the ‘internal model of body dynamics’ or ‘virtual body.’
The CNS has 2 modes of operation;
Feedforward: CNS is able to predict the effects that movements and forces will have on the body to plan strategies in advance of movement to prepare the trunk for predictable perturbations. Another name for this is pre-activation. A good example of this is sprinting. The muscles that need to fire to absorb the next landing, have already begun to fire prior to this happening. This creates a pulse like effect, whereby the muscles function as springs.
Feedback: The response via muscle activity that are part of an overall strategy for recruitment. Feedback is also how we change movement. It is the job of the body to give the brain information regarding quality of movement. Slow movements ramp up the proprioceptive demand, fast movements use momentum. Both are necessary for quality adaptation.
Feedforward Control of Spinal Stability
Feedforward strategies involve muscle activity that counteracts;
- The challenge to postural equilibrium of the entire body
- The challenge to spinal orientation and alignment
- The challenge to intervertebral motion
The deep intrinsic spinal muscles (Deep System) are active in a manner that does not vary based on the directions of limb movement, and therefore the direction of forces acting on the spine. This CNS strategy is to initiate activity of the sleeve of muscles that surrounds the spine to stabilise intervertebral segments. We now know that the diaphragm is the roof of this sleeve and also the pressure regulator responsible for intra-abdominal pressure.
The CNS involves complex strategies to integrate afferent input (body-based information) to initiate trunk muscle activity to optimise spinal control. This postural response involves activity of the deep intrinsic spinal muscles in a manner that is not affected by the direction of forces acting on the spine. Conversely, activity of the superficial muscles is generally controlled in a manner that is specific to the direction of the movement.
The evidence is increasingly clear that our deepest system functions to provide stability and is therefore the only true core stability. Moving muscles do not and cannot function as our stabilising strategy because they span several segments and therefore create ‘lockdown’. Their role is to function in a task-specific manner to brace and move the body, interdependent with deep system stability. Imagine the changes possible in treatment and training disciplines if this understanding was more wide-spread.
When I speak to people about the Deep System, I describe it as informational, in that its job is to be on or aware at all times. We also now know that it is interdependent with breathing because of the dual role of the diaphragm in breathing and stability. The other way that the Deep System is informational is via the ‘push’ generated by intra-abdominal pressure that informs the 3 primary articulations of the body;
- the head/neck
- the shoulder girdle
- the pelvic girdle and hips
Control of Body Equilibrium
The response of the trunk muscles to support surface translations moves the spine to restore balance rather than just maintain spinal orientation and is specifically matched to the direction of the perturbation. This is an example of the strategy of the CNS to use movement for control, rather than simply stiffening. It also highlights the importance of reflexes sitting under purposeful movement. Remember that control of the spine is under brain stem and mid-line cerebellar control, and that this control is reflexive so that it is fast acting. In essence, while the activity of the superficial muscles is closely linked to the direction of the perturbation and the requirement to control equilibrium, the activity of the deep muscles is largely independent of the direction of perturbation.
How active and alive is your Deep System Stability?
In part 3 of this series, I’ll describe how the drive of movement is to liberate degrees of freedom and provide a summary of the important principles regarding spinal stability.