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.

Table 2.2

Local and global muscle system characteristics and general features

Implications of local and global characteristics

3. Local muscle ‘system’: the small deep segmental muscles in the local muscle system are responsible for increasing the segmental stiffness across a joint and decreasing excessive intersegmental motion. The relevance of this is that these muscles are ideally situated to control displacement of the path of the instantaneous centre of motion and reduce excessive intersegmental translatatory motion during functional movements. At end range of motion the passive restraints of motion (e.g. ligaments and joint capsules) contribute significantly to controlling translatatory or accessory motion. Local muscles maintain this translatatory control during all functional activities such as postural control tasks, non-fatiguing functional movements, fatiguing high load and high speed activities. Local muscles maintain activity in the background of all functional movements. Their recruitment is independent of the direction of loading or movement and is biased for non-fatiguing low load function, although they maintain the role of controlling intersegmental displacement during fatiguing high load function as well. The local muscles do not significantly change length during normal activation and therefore do not primarily contribute to range of motion. The one-joint (monoarticular) global muscles have a primary stability role, while the multi-joint (biarticular) global muscles have a primary mobility role.

4. Global muscle ‘system’: the muscles that make up the global muscle system are responsible for the production and control of the range and the direction of movement. The global muscles can change length significantly and therefore are the muscles of range of motion. The global muscles participate in both non-fatiguing low load and fatiguing high load activities.

Both the local and global muscle systems must work together for efficient normal function. Neither system in isolation can control the functional stability of body motion segments.

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.

Figure 2.1 Actin–myosin filaments within the sarcomeres

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).

Figure 2.2 Active (contractile) component of a muscle length–tension curve changes when muscles change length: changes in muscle length affect force efficiency in different positions of joint range. Adapted from Goldspink & Williams 1992

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.

Table 2.3

Classification of muscle functional roles in terms of function, 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.

Figure 2.3 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.

Table 2.4

Features of muscle function used for reviewing muscle roles

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:

1. Some discrepancies between biomechanics and neurophysiology need to be explained with some muscles. For example, training latissimus dorsi co-activation with the contralateral gluteal muscles (often referred to as the posterior sacroiliac sling) to stabilise the sacroiliac joint has been proposed by various authors (Vleeming et al 2007). This training would be appropriate to help manage sacroiliac joint pain associated with high load or high speed activities such as running or throwing because these two muscles are automatically recruited in these activities. However, for patients who have sacroiliac joint pain associated with non-fatiguing functional movements (e.g. normal gait) and postural control activities (e.g. static standing), this training is unlikely to be beneficial. There is often an assumption that the muscles used in strength training will be used in all functional activities. However, this is not the case as there is minimal automatic activation of latissimus dorsi in these low load activities.

Another example of measurement discrepancy occurs following the assumption that psoas major is a hip flexor. Biomechanical modelling of psoas major often assumes that it is a fusiform muscle with a straight line of action from the upper lumbar spine to the femur. This is not the case. Psoas major is a pennate muscle with obliquely orientated fibres. A more detailed mechanical evaluation of its pennate orientation (Gibbons 2007) suggests that its maximum shortening potential is approximately 2.25 cm. This is insufficient to produce the flexion range of motion of the hip. The posterior fascia of psoas major is anchored to the anterior rim of the pelvis (Gibbons 2007). This attachment would produce posterior tilt of the pelvis. Posterior tilt of the pelvis is a conjoined movement with hip flexion, and interestingly the range of movement of the pelvis at the psoas attachment point during posterior tilt perfectly matches the predicted range of shortening of psoas major.

2. There may be misinterpretation of research measurement technology; for example, upper trapezius, lower trapezius, psoas major, vastus medialis obliquus. Upper trapezius has been assumed to elevate the shoulder because it demonstrates high levels of EMG activity during scapular elevation tasks. Johnson et al (1994) demonstrate that 90% of the contractile fibres of upper trapezius insert on the ligamentum nuchae below C6 and are horizontally orientated (the vertical fibres are predominantly fascial and connective tissue). They state that the upper trapezius muscle cannot elevate the scapula above C6. It is suggested that the reason for the high levels of EMG activation may be to assist the clavicular rotation (necessary for full shoulder elevation), or an attempt to stabilise the cervical spine during arm load activities.

Similarly, vastus medialis obliquus demonstrates high levels of EMG activity in terminal extension of the knee. Lieb & Perry (1968) demonstrated that vastus medialis obliquus has no biomechanical potential to extend the knee in this last 30°. The high level of vastus medialis obliquus recruitment is best explained by its role of maintaining alignment tracking of the patella during the last 30° of full extension.

3. The muscle is designed to participate in more than one primary functional role; for example, Hodges (2003) suggests that a muscle may have three functional roles:

(i) control of inter-segmental motion

(ii) control of posture and alignment

(iii) to produce and control movement.

Some muscles can effectively perform all three of these functional roles. Gluteus maximus is an example of a muscle that multitasks all three functional roles (Gibbons 2007). Gluteus maximus has deep sacral fibres that run from the inferior lateral corner of the sacrum to the posterior inferior ischial spine. It is believed that these fibres perform a local stability role and have the function of controlling intersegmental translation at the sacroiliac joint. Gluteus maximus also has fibres that run from the medial aspect of the ileum to the gluteal trochanter on the femoral neck. These deep fibres constitute the one-joint part of a muscle that performs a global stability role at the hip joint. The most superficial fibres of gluteus maximus run from the iliac crest and attach into the posterior aspect of the iliotibial band and eventually insert on the anterior aspect of the lateral tibial condyle, below the knee. This multi-joint part of gluteus maximus has a global mobiliser role and produces movement at the hip joint and the knee joint.