1. to concentrically shorten to produce joint range of motion and accelerate body motion segments, which will be termed ‘mobility function’

2. to isometrically hold position, which will be termed ‘postural control function’

3. to eccentrically lengthen under tension to decelerate motion and control excessive range of motion, which will be termed ‘stability function’

4. to provide afferent proprioceptive feedback to the central nervous system (CNS) for coordination and regulation of muscle stiffness and tension.

Stabiliser and mobiliser function

Rood, in Goff (1972), Janda (1996) and Sahrmann (2002) have described and developed functional muscle testing based on stabiliser and mobiliser muscle roles. Table 2.1 describes stabiliser and mobiliser muscle role characteristics.

Some muscles are more efficient at one of these roles and less efficient in the other role. For example: latissimus dorsi is a powerful multi-joint medial rotator of the shoulder to accelerate the arm in the sagittal plane during throwing actions. Latissimus dorsi is biomechanically suited to large range, high speed, high force movement at the shoulder. This muscle obviously has a power role acting as a mobiliser or sagittal plane movement accelerator. Conversely, subscapularis co-activates synergistically but its co-activation force acts to stabilise the humeral head from excessive translation, while the arm is medially rotating in a throwing action. Subscapularis, with its short lever, small moment arm and capsular attachments, is best suited to non-fatiguing, functional movements in postural control tasks. It is also ideally placed to resist or decelerate excessive lateral rotation of the shoulder. This muscle has a greater stabilisation role and is less suited biomechanically to a power movement role. A muscle’s role must also be customised to its function.

Generating high force may be detrimental in some instances. For example: rectus abdominis is a flexor of the lumbar spine. If it is over-trained and inappropriately strengthened it contributes to pain-related changes in the movement system. It is often over-trained in a misguided belief that it is important to strengthen the abdominals to stabilise and protect the low back or to acquire an abdominal ‘six pack’. However, if this muscle becomes excessively dominant in comparison to the lateral abdominal muscles it increases flexion and compression forces on the lumbar spine and rotation stress and strain is inadequately controlled. This imbalance between rectus abdominis and the lateral abdominal stabilisers has been identified as a common movement-related change in people with low back pain (O’Sullivan et al 1997).

Local and global function

Bergmark (1989) developed a model to describe the muscle control of load transfer across the lumbar spine. He introduced the concept of local and global systems of muscle control. The local and global muscle system characteristics and general features are described in Table 2.2 with examples.

Functional efficiency

The functional efficiency of a muscle is related to its ability to generate tension. A muscle’s tension is not constant throughout a contraction, especially if the muscle is changing length to produce movement. Length and tension properties of a muscle are closely related. The tension or force a muscle produces is the resultant force arising from a combination of both active and passive components of the muscle. The active component of muscle tension is determined by the number of actin–myosin cross-bridges that are linked at any point in time. The passive tension property of muscle is largely due to the elastic titin filaments which anchor the myosin chain to the Z band. Other connective tissue structures within muscle only contribute partially to passive tension. Figure 2.1 illustrates the actin–myosin filament cross-bridges and titin attachments.

The position in range (usually mid-range) where the active length–tension curve is maximal is known as the muscle’s resting length. In this position, the maximum number of actin–myosin cross-bridge links can be established. In a muscle’s shortened or inner range position, the passive elastic components do not contribute to muscle tension. Passive tension only begins to play a role after a muscle starts to lengthen or stretch into the muscle’s outer range, beyond its resting length or mid-range position. Muscles are most efficient and generate optimal force when they function in a mid-range position near resting length. Muscles are less efficient and appear functionally weak when they are required to contract in a shortened or lengthened range relative to their resting length because of physiological or mechanical insufficiency (Figure 2.2).

Physiological insufficiency occurs when a muscle actively shortens into its inner range where the actin filaments overlap each other, thus reducing the number of cross-bridges that can link to the myosin filament. As the muscle progressively shortens, there are fewer cross-bridges able to be linked, and the muscle is unable to generate optimal force. Mechanical insufficiency occurs when a muscle actively contracts in its lengthened or outer range. In this range, the actin filaments do not adequately overlap the myosin filament and again a reduced number of cross-bridges are linked. Consequently the muscle cannot generate optimal force. Mechanical insufficiency during an outer range contraction is offset somewhat by the increase in passive tension from titin filaments.

However, when a muscle habitually functions at an altered length (either lengthened or shortened), its length–tension relationships adapt accordingly. The position in range where it generates optimal force efficiency changes to match the subsequent lengthening or shortening (Goldspink & Williams 1992), as illustrated in Figure 2.2.

When a muscle is persistently elongated or lengthened, it adds sarcomeres in series (the broken line in Figure 2.2). Because the sarcomeres are the force generating units within a muscle, a lengthened or elongated muscle is stronger and is able to generate a higher peak force than normal. This higher peak force, however, is produced in an outer range position and not at its usual resting length, mid-range position. At the muscle test position (inner to middle range), the lengthened muscle is inefficient due to physiological insufficiency, and consequently tests ‘weak’ during muscle testing and fatigues more readily in postural control tasks. A persistently shortened muscle, on the other hand, loses sarcomeres in series and increases in connective tissue (the dotted line in Figure 2.2). Because of the reduced number of sarcomeres, the shortened muscle generates less peak force than normal. Interestingly, a shortened muscle’s resting length may coincide with the muscle test position. Even though the shortened muscle is weaker than its normal control, muscle testing is performed at the point in range where it is optimally efficient. Consequently, shortened muscles frequently demonstrate good strength during muscle testing (Gossman et al 1982). This explains the clinical observation that ‘short muscles test strong and long muscles test weak’.

A muscle’s structure also affects its ability to generate force. Muscles that have long lever arms, such as the multi-joint rectus femoris or hamstrings, can contract through a greater range and are biomechanically advantaged to produce range of movement during concentric shortening. These muscles primarily have a mobility role. These multi-joint mobilisers are not particularly efficient at preventing or controlling excessive movement during eccentric lengthening. The smaller one-joint muscles with short lever arms, such as subscapularis or iliacus, are not biomechanically efficient to produce forceful or high speed movement during concentric shortening. However, they are more efficient during eccentric lengthening to control excessive movement and to decelerate momentum and therefore are more able to protect tissues from overstrain. These muscles primarily have a stability role.

When a muscle has such a short lever arm that it produces minimal length change when contracted, it has a greater potential to control intersegmental translation, for example the single segment fibres of lumbar multifidus.

Functional classification of muscle roles

The concepts of local and global muscle systems and stabiliser and mobiliser muscles provide useful frameworks to classify muscle function. However, alone, they have some clinical deficiencies. By interlinking these two concepts a clinically useful model of classification of muscle functional roles has been developed (Comerford & Mottram 2001).

Table 2.3 summarises this classification in terms of function and characteristics and dysfunction.

Postural adjustments are anticipatory and ongoing and all muscles can have an anticipatory timing to address displacement and perturbations to equilibrium. All muscles provide reflex feedback reactions under both low and high threshold recruitment tasks and demonstrate anticipatory feedforward recruitment when appropriate. However, only muscles with a local stability role exhibit anticipatory timing that is independent of the direction of loading or displacement. Muscles recruited in a global range related role are direction-specific in their anticipatory feedforward responses (Hodges & Richardson 1997; Hodges 2001; Hodges & Moseley 2003). Figure 2.3 illustrates an example of the anatomical inter-relationships between the different muscle roles in the lumbar spine.

Muscle characterisation

Although all muscles can perform all basic abilities, some muscles are ideally suited to some roles to achieve optimal function better than others. An analysis of a muscle’s ideal role should consider the co-relation of the features listed in Table 2.4.

This model of reviewing and analysing muscle function and recruitment provides an opportunity to develop a greater understanding of a muscle’s role in functional activities. By analysing the inter-relationships between a muscle’s anatomy and histology, its biomechanical potential, its recruitment physiology and consistent changes in the muscle related to pain and pathology (see Table 2.4), we can be more critical of some of the oversimplified roles that have previously been ascribed to some muscles.

If an analysis of all four of these features supports a consistent conclusion, we can be reasonably confident that a particular muscle’s primary function or role is understood. Such support is available only for a limited number of the muscles that therapists work with on a regular basis, such as transversus abdominis, external obliquus abdominis, rectus abdominis and hamstrings.

If analysis of these four features provides conflicting conclusions then there may be confusion, misunderstanding or misinterpretation of this muscle’s function. Several possibilities exist to explain this apparent conflict: