Anatomy of Stability
To produce a movement, muscles must work in coordinated patterns or chains. Within these patterns some muscles are ideally designed to provide support, and others to produce force. They have specific physiological and neuromuscular characteristics and therefore different roles. A movement needs a firm foundation, and a muscle group called the local stabilisers provides the most fundamental of joint support. The global stabilisers provide control of the moving parts throughout the motion and also produce force. The global mobilisers produce movement (Comerford & Mottram, 2001). This grouping does not exclude mobilisers from having a stabilising role, however based on structural characteristics and behaviour their movement-producing role is most relevant when considering training implications.
The local system provides a foundation for movement in the same way that you build a foundation for a house. When you have the impulse to move, they become active just prior to the movement occurring in order to support the body's structures and provide a stable platform for the movement. This is called a feedforward response. When functioning normally, they activate in this way regardless of the direction in which you want to move (Hodges & Richardson, 1997), and they continue to work throughout the whole movement. This steady continuous behaviour is known as tonic activity.
Local stabilisers do not change length greatly when they contract and are usually positioned to spread closely over a joint, so they are ideally suited to controlling joint position but not for producing a range of motion. Because of these characteristics, the local stabilisers maintain joints in the safest and most suitable position to support muscular forces, and provide a secure axis for movement. Despite their crucial role in efficient movement and joint protection, the local stabilisers can switch off for a variety of reasons.
Pain can inhibit the function of these muscles, and even if the pain resolves, they can remain switched off (Hodges, 2001, Hides & Richardson, 1996). Even fear of pain can alter the function of these muscles (Moseley, Nicholas & Hodges, 2004). Without these local stabilisers functioning as part of the normal neuromuscular pattern for movement, other muscles in the system alter their roles to compensate for them. These larger global muscles normally produce force or movement and are not positioned well to maintain safe and secure joint position, nor are they physiologically suited to the sustained activity required in a stabilising role. The ongoing inhibition of the local stabilisers therefore leads to chronic biomechanical problems, and an athlete can end up in a frustrating injury cycle or performance plateau as a result.
The global system can be divided into the global stabilisers and the global mobilisers. Global stabilisers usually have broad attachments and are suited to controlling joints throughout a movement. Unlike local stabilisers they do change length and therefore can create force. If working in their correct role in the neuromuscular pattern, these muscles can be very powerful. Muscles such as gluteus maximus and the external oblique abdominals are examples of global stabilisers. The global mobilisers have a predominance of fast twitch fibres and are designed to produce movement. The local muscles activate tonically to provide continuous support, but the global mobilisers behave phasically, that is their activity, being task and movement dependent, is an on/off behaviour.
With their relatively long muscle fibres, superficial placement, ability to build tension quickly, and fatigue more readily, the mobilisers are considered to be action muscles. As the global muscles commonly cross more than one joint, an increase in their stabilising activity causes problems differentiating movement from one body part to another. The ability to move body parts independently of one another is called dissociation, and every sport needs this to some degree. The global stabiliser muscles do not always overactivate. Sometimes they become underactive and fail to control a joint through motion. To allow smooth, powerful movement there needs to be a balance between the local and global system. Positioned closely over joints, the local stabilisers have short lines of pull, which are ideal for controlling joints, but produce insufficient force to create and control movement. Global stabilisers have long lines of pull over multiple joints, so they are effective for producing and controlling movement, but when acting without segmental local stabilisers they cause compressive forces, which may lead to joint buckling. When both sets of muscles are working together, stability and mobility are possible.
For further information on the anatomy of stability, read Movement: Functional Movement Systems: Screening, Assessment, Corrective Exercise Solutions for Common Shoulder and Hip Dysfunction, Advances in Functional Training, and Stability, Sport and Performance Movement.