Control of Muscle Stiffness
The modulation of tone in specific muscles provides an underlying degree of stability to the joints. This activity increases the stiffness of muscles that surround the joints.
Muscle stiffness is the property of muscles to act as springs (ratio of length change to force change). Muscle stiffness provides control of forces applied to a joint and contributes to control. This tone or stiffness is itself informational.
The activity related component of muscle stiffness is modulated by feedback from the spindle and ligament afferents (body-based information). It is the stretch reflex and control of the gamma-motoneurons that control the sensitivity of the sensory component of the muscle spindles that control this system.
Muscular Contribution to Spinal Control
Bergmark: local (deep) and global (moving) systems.
Panjabi: considered segmental arrangement as advantageous to CNS in that it is able to control specific motion segment as required.
Examples of local (deep) muscles are the lumbar multifidus and transverse abs via connection to thoracolumbar fascia (TLF).
Global (moving) muscles span the spine from rib-cage to pelvis. These muscles generate torque at the spine but have limited ability to control intervertebral shear and rotation. These muscles are ideal for control of orientation of the spine and act like guy ropes to keep the spine upright. Examples include; obliques, rectus abdominus, and long erector spinae muscles.
Degrees of Freedom
In healthy subjects the CNS uses movement as a stabilising strategy rather than simply stiffening of the spine to overcome the challenges to stability and reduce energy expenditure in many tasks. This is why it is so important to keep moving, even though what you are able to do, may be compromised.
Diaphragm and Pelvic Floor
An important consideration is that for the transverse abs to influence the stability of the spine via an increase in intra-abdominal pressure (IAP) or increased tension in the TLF, it is necessary for the roof and the floor of the abdominal cavity to contract.
What does this mean?
Normal relaxed respiration involves cyclical activity of the diaphragm, parasternal intercostals, and scalene muscles during inspiration with expiration generated passively by the elastic recoil of the lungs and chest wall.
When the the demand for breathing is increased, abdominal muscles are phasically activated during the expiratory phase. This is because of the need to forcefully exhale based on demands of the task.
From a biomechanical perspective, the diaphragm and TA co-contract tonically, yet during inspiration, diaphragm increases its activity and shortens (concentric) and TA decreases its activity and lengthens (eccentric). The converse pattern occurs during expiration. When your intention is to maintain IAP, the diaphragm increases its eccentric activity to concurrently maintain positive pressure during exhalation.
Summary of Key Understandings
- Biomechanical and neurophysiological studies agree that stability of the spine and pelvis is dependent on the control of intersegmental motion, orientation and coordination.
- Different muscle systems and different strategies are used to control each of these elements.
- When people have low back pain there is a general trend for activity of the deep local muscles to be impaired, while activity of the superficial muscles is increased. This depends on each individual context.
- The above changes are considered to be due to changes in motor planning.
- Many methods have been developed to deal with changes in control of the trunk muscles and these techniques are continuing to evolve.
- There is increasing evidence that motor control interventions for the trunk muscles leads to positive clinical outcomes.
The above evidence paints a very different picture to the way that people are currently training from a mainstream conventional perspective. I believe that this understanding is going to change training more than any other intervention over the next 1-5 decades. Stay tuned for updates on training which reflects this understanding.