Movement actions

The simplest form of classification – muscles with equivalent or similar actions are grouped together, e.g. those muscles which externally rotate the hip are known as the external rotators of the hip. However, it is never so straight forward as the hip external rotators also play an important role in stabilizing and controlling the pelvis. In the torso we talk of the flexor and extensor systems of muscles which contribute to antigravity control as well as flexion extension and so on. Similarly we may talk of the upper limb flexors or the lower limb extensors.

Tonic and phasic muscles

Although a considerable amount of variability exists among muscles in regard to the number, size, arrangement and type of muscle fibres it has however, been common practice by some to classify the muscles themselves into two main groups as either predominantly tonic or phasic muscles.

Tonic muscles are those with a predominance of Type 1 fibres. These muscles work at a low grade of contraction in a sustained manner thus have a lot of endurance. They are often also termed stability or postural muscles as they help to maintain stability of the body,⁴ e.g. the soleus is almost continually active in standing and owing to the high proportion of Type 1 fibres can make small adjustments in muscle tension required to maintain body balance and counteract gravity.

Phasic muscles are those muscles with a predominance of Type 11 fibres. These play a major role in large movements and those requiring more power and speed. They are either called mobility or phasic muscles. However, they fatigue more quickly and following intermittent bouts of high intensity exercise recover more slowly than the tonic muscles,⁴ e.g. the gastrocnemius. Table 5.2 presents the more commonly used distinguishing features⁴ of the tonic and phasic muscles.

Table 5.2 Different characteristics of tonic and phasic muscles (after Norkin and Levangie⁴)

Besides the differences in function and structure outlined above, Kolar⁷ importantly draws attention to the differences in the neural control of the different muscle fibres – it is the type of the motor neuron which determines the type of muscle fibre, creating either tonic or phasic motor units. This difference becomes particularly striking in the light of our individual motor and phylogenetic or evolutionary development. At birth, the infant’s posture is predominantly influenced by the phylogenetically older phasic (as described in Table 5.2) muscle system. (Kolar calls this the tonic system!) The tonic system (Table 5.2) is less evident, but as the central nervous system (CNS) matures these muscles play an increasingly important part in the development of upright posture and its stabilization with movement (Kolar calls this system the phasic system). This system is phylogenetically younger, more vulnerable and tends to become weak. Maturity of this system is not achieved until the child is 4 years old.

It is important to point out at this point that the nomenclature used by the Czech School of Manual Medicine can be confusing and appear contradictory as is evidenced by the paragraph above. This may account for a less wide understanding and acceptance of their valuable work. In addition it may contribute some of the current confusion on the subject. Their use of the terms tonic and phasic is different to that shown in Table 5.2, and further compounded when talking of postural and phasic muscles. This is further elaborated upon when examining Janda’s classification, and the muscle classification debate (see p. 60).

Kolar⁷ does clarify that tonic motor units have a more postural role and phasic motor units have a more kinetic role and that both motor units are present in differing proportions in every muscle. However, he also adopts the nomenclature used by Janda making his work more difficult to interpret.

Importantly, Kolar⁷ questions which position is decisive in opposing gravity and I agree. He sees that in motor control, both types of muscles have dual functions participating in both posture and movement. The decisive difference between them consists of the timing of their development. Postural activity of the tonic system (his phasic) comes into play as central nervous control becomes more highly developed. The functioning between the two systems needs to become integrated and balanced.

Muscle classification according to Vladimir Janda

Professor Janda saw that the muscular system lies at a functional crossroad because it is influenced by stimuli from both the central nervous system and musculoskeletal system.⁸ From the clinical point of view, Janda’s significant contribution has been to show that dysfunction in the muscular system is usually a reflection of dysfunction in the peripheral or central neural system. Impaired central motor regulation results in defective or uneconomical movement patterns. As a consequence, imbalanced action between two structurally and functionally different muscle groups occurs in a systematic, regular and predictable manner.⁹ He proposed a more general classification of muscles throughout the body into two groups based on characteristics of their structure and function and observed actions in the clinical situation. They are the ‘postural muscles’ which are prone to over-activity which in turn tends to create relative underactivity in the antagonistic ‘phasic muscles’. These functional differences are further elaborated:

The ‘postural muscles’ have a tendency to tightness, hypertonia, over-activity and shortening. They tend to be activated early and dominate in a given movement and in states of pain, fatigue, injury, stress and emotional states this tendency is increased. These muscles tend to be relatively ‘strong’.

The ‘phasic muscles’ are prone to inhibition, hypotonia, atrophy and weakening and are less readily activated in most movement patterns, particularly under conditions of injury fatigue and stress. He classified certain muscles into either functional group as shown in Table 5.3.

Table 5.3 Functional division of muscles according to Janda^8,10

You will note that he was undecided about the role of the scalenes and the abdominals, classifying them differently at different times.

Janda defined the muscles which tend to become short and tight as those having an antigravity postural function, particularly those activated when standing on one leg. Janda⁵ considered the postural muscles were approximately one-third stronger than those prone to inhibition. In subjects with altered or poor movement patterns their degree of activation increases. In addition, he notes that in certain structural lesions of the CNS, as seen in cases of cerebral vascular accident or cerebral palsy, the muscles which show evident spasticity are the same as those included in the postural group.

Janda himself said there were a lot of misconceptions and discrepancies about the use of the term ‘postural muscles’.¹¹ However, examining his muscle system groupings it is apparent that they more closely resemble the phasic muscles as described in Table 5.2.

His nomenclature is confusing for our purposes as you will see (p. 60). However, conceptually and functionally his approach has been very helpful.

Local and global muscles acting on the lumbar spine

Bergmark¹² in examining the conditions for mechanical stability in the lumbar spine presented a concept of functional muscle classification into local and global muscles as they acted to control local stability in the lumbar spine including transfer of load between the thorax and pelvis (Fig. 5.1)

Fig 5.1 • The local (A) and global (B) muscles.

The local system includes ‘all those muscles which have their origin or insertion (or both) at the lumbar vertebrae with the exception of psoas’. The muscles included are shown in Table 5.4. This system is involved in the posture of the lumbar spine and used to ‘control the (lumbar) curvature and to give sagittal and lateral stiffness to maintain mechanical stability of the lumbar spine’.

Table 5.4 Local and global muscles of the lumbar spine described by Bergmark¹²

Local muscles	Global muscles
Multifidus Interspinales and intertransversarii Lumbar erector spinae – medial and lateral fibres Medial fibres quadratus lumborum	Thoracic erector spinae which is about of the muscle area Internal and external obliques Rectus abdominus Lateral fibres of quadratus lumborum

The global system ‘consists of the active components i.e. the muscles and IAP which transfer the load directly between the thoracic cage and pelvis’. The muscles have ‘origin on the pelvis and insertions on the thoracic cage’. These include the global muscles shown in Table 5.4. The main role of this system ‘appears to be to balance the outer load so that the resulting force transferred to the lumbar spine can be handled by the local system. Thus large variations of the distribution of the outer load should give rise to only small variations of the resulting load on the lumbar spine. The local system therefore is essentially dependent upon the magnitude (not the distribution) of the outer load and of the posture (curvature) of the lumbar spine’.

He says ‘the global system can be said to respond to changes of the line of action of the outer load whereas the local system responds to changes in the posture of the lumbar spine. Both systems respond to changes in the magnitude of the outer load’. ‘Generally speaking, smaller forces in the global system imply larger forces in the local system as could be expected’. He says intra abdominal pressure (IAP) theoretically has a local and global mechanical role. ‘The global role is to act directly on the thoracic cage or on the curved global muscles. The local action consists of the transverse force in the posterior direction acting directly on the lumbar spine thus inducing a flexion moment’.

This paper has been very influential and is frequently quoted in the literature. This is interesting, as his is a study in mechanical engineering looking at forces and load transfer in the lumbar spine rather than functional control of movement. He admits that how the CNS controls loads is not sufficiently understood to allow detailed modeling; does not include iliopsoas in the local system or accord it any role in lumbar control; and treats the thoracic cage as a rigid body. While he thought psoas should be referred to the global system he excluded both it and latissimus dorsi from his analysis as he felt they do not have a substantial role in maintaining mechanical stability of the back system.

While his muscle classification principle is very useful, it needs to be applied to more than the lumbar spine and we need to appreciate that control of the lumbar spine will be dependent upon control through the entire axial skeleton including the pelvis.

The influence of Bergmark:stabilizers and mobilizers

Richardson et al¹⁴., collectively known as The Queensland Group, have produced some fine research and have been at the forefront of the motor control approach to effective lumbopelvic stabilization in the treatment of low back pain. They have been strongly influenced by Bergmark as seen in Table 5.5, choosing to include transversus abdominus and some of internal oblique into the local group.^14,42

Table 5.5 Categorization of the lumbar and abdominal muscles based on their role in stabilisation according to Richardson et al.¹⁴

In addition, Richardson¹³ also makes distinction between monoarticular, bi-articular and multijoint muscles. Their capacity to provide joint stabilization differs in each category. The monoarticular muscles could also be called local muscles. The multijoint muscles are phylogenetically the oldest and can be called the global muscles.¹³

Comerford and Mottram¹⁵ have interlinked the concepts of local/global and stabilizer/mobilizer into what they see as a more clinically useful classification. This encompasses three different functional muscle roles: local stability muscles; global stability muscles; global mobility muscles. Some muscles stabilize and some mobilize.

However, there are inherent problems for understanding functional control in seeing some muscles as stabilizers and others as mobilizers. As we have seen in Chapter 3 in the process of motor development, movement and stability develop together and are always in constant interaction in mature motor behavior. Kolar⁷ points out that any muscle may be required to work in a stabilizing role one moment and then as a movement producer the next. While some muscles may appear to have a predominantly stabilizing role e.g. the local muscles in the lumbar spine as described by Bergmark, importantly, they also sub serve a postural role and are also producers of fine subtle movements as well as being controllers and discrete adjusters. Danneels et al.¹⁶ found increased multifidus action in concentric lifting which could indicate that it participates in torque production. It is an oversimplification of function and erroneous to consider that they do not produce any appreciable movement and ‘just stabilize’.

Cholewicki and VanVliet¹⁷ refute the classification of muscles into local and global as a means for discriminating between muscles responsible for intersegmental stability and spine motion. All trunk muscles contribute to spine stability and their contribution depends upon many variables including posture and loading conditions. McGill¹⁸ and Kavcic et al.¹⁹ express similar sentiments. The patterns of muscle activation change as the form and magnitude of spine loading patterns change (See ‘spinal stability’ p. 86).