Trunk stability
What is trunk stability?
The terms trunk/central and spinal stability are synonymous and are used interchangeably in clinical practice. Core stability is the equivalent lay term. In essence, trunk stability is a reflection of an individual’s postural control, the ability to orientate and stabilize the body using appropriate balance strategies and responses (Raine et al. 2009). As such, trunk stability is an essential core component of balance, the ability to maintain the centre of mass (COM) within the base of support (BOS) (S3.32) and functional activities, by which it provides proximal stability to allow for the coordinated free movement of the limbs and head (2; 12). Trunk stability is also important in controlling and supporting the spinal segments, so reducing the stresses upon soft tissue structures and helping to avoid injury and pain.
This section will focus upon the core trunk muscles which are commonly divided into two groups according to their functional anatomy (Bergmark 1989):
The local stabilizers
These muscles are anatomically deep and have attachments to each individual spinal vertebra. This allows them to act at a segmental level (between adjacent vertebrae) providing a strong basis for trunk stability. The local stabilizers are composed primarily of Type I (slow oxidative fibres) muscle fibres, which produce low forces but are fatigue resistant. These muscles therefore function as postural muscles. They also have smaller motor units which are easily activated due to their low neural threshold and therefore tend to be recruited first (Henneman principle) during functional activities. In theory, this means that the local stabilizers are activated prior to the surrounding global muscles which have larger motor units. This pre-setting of trunk stability is crucial in providing a stable base upon which limb movement can be based and may be part of an anticipatory response to a perceived COM displacement (MacDonald et al. 2006). The anticipatory activation will be built into a learned movement plan.
The anatomical and physiological features of the local muscle stabilizers make them perfectly adapted to perform their specific trunk stability function, however the nervous system also influences the fine control of these muscles. The force of contraction required to provide segmental stability is estimated to be 3% of maximum contraction (Cholewicki and McGill 1996). As antigravity postural muscles, this background level of activity is controlled by the vestibular and reticular systems (S2.10).
The balance of activity between the abdominal (TAb/IOf) and back extensor (MT) pair is vital in terms of the level and timing of activation. A neutral position of the pelvis and lumbar spine is considered to be highly correlated with the co-contraction of this muscle pair (O’Sullivan 2006). In postures of either anterior pelvic tilt with lumbar spine hyperextension or posterior tilt with lumbar spine flexion the local muscles are switched off and over time become deconditioned (O’Sullivan 2006). This is highly relevant in a population of neurologically impaired patients who often spend many hours sitting in poor postures in inadequate seating.
Why do I need to assess trunk stability?
Trunk instability results in less efficient movement, a limited ability to explore the environment (Cristea et al. 2003) and demands compensatory muscle activity to maintain an upright position. The latter may present as abnormal fixation of the trunk or, in extreme cases, excessive use of the upper limbs, and leads to effortful movement for the individual. Poor trunk stability resulting in deviations in posture may also cause abnormal stresses and potential damage to soft tissue structures leading to pain and further functional loss. Often rehabilitation focuses on improving general trunk stability without specific attention to the interaction between the local and global stabilizers. More focus on segmental stability initially may have the potential to improve the movement sequences and the functional outcome for the individual.
In a neurologically impaired patient, poor trunk stability has been linked with a poor functional outcome in multiple sclerosis (Lanzetta et al. 2004), cerebrovascular accident (CVA) (Hsieh et al. 2002) and Parkinson’s disease (PD) (Dibble and Lange 2006; Nardone and Schieppati 2006).
In CVA, trunk instability has also been linked with poor postural control in standing (Palmer et al. 1996; Slijper et al. 2002) and in sitting (Dickstein et al. 2004) and is viewed as a negative prognostic indicator for recovery. Proprioception (position sense) (Mergner et al. 2003; Peterka 2002) and reduced strength (Karatas et al. 2004) in the trunk have been found to be causal in CVA (Ryerson et al. 2008) and in PD (Vaugoyeau et al. 2007).
