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Active tension (g)

10 8 6 4 2

Description Mobilisers Stabilisers

control

lengthened muscle

80 90 100 110 % Muscle belly length of control

Figure 2: Active length-tension curves for adult animal’s muscles kept in a lengthened position compared to those of a normal length.

motorneurone pool toward the shortened muscle, due to an increased afferent input coming from the stretch receptors (6).

Human movement usually tries to follow the path of least resistance so it is prob- ably reasonable to assume that the way we put a movement sequence together will be the one our individual systems deem the easiest. This is not to say it will be the most efficient, safe and effective method, just the one our system can cope with best.

When we are learning a new skill, there is an immaturity in the way we attempt the task. This reflects a deficit in both motor control patterns (which may result in an increased reliance on more superficial muscles for both movement and stability) and perhaps in our passive subsystem in which muscle length (where a muscle and its connective tissue elements needs to passively lengthen) or joint ROM may be inadequate. With practice and correct instruction this can be improved.

This same principle can be applied to the body after an injury or other insult. Current thoughts in the literature indicate that the return of correct patterns does not coincide with reduction in pain levels and may not necessarily return at all, without correct and adequate instruction (7,8).

Muscles: the stabilisers and the mobilisers Classification of muscles into groups based on functional role has been pro- posed by different authors. In 1983 Janda suggested the existence of postural and phasic muscle groups (4). Bergmark (9) in a study of lumbar stability suggested a global muscle system and a local one. Richardson (10) introduced three groups;

SportEX 11

Superficial Deep

General characteristics Multi joint Single joint

prime stabilisers, local (one joint) muscles and biarticular or multijoint muscles. Future research may well suggest something even more refined.

If

we take all these views we can define two basic categories; the stabilisers, or local muscles, and the mobilisers, or global muscles. These divisions are for general description and the lines are clearly fluid as under altering external requirements muscles necessarily behave differently.

The mobilisers are designed to produce large degrees of torque and ROM over shorter intervals of activity. The main pur- pose of the stabilisers is sustained low level muscle contraction and as they produce little or no movement, they are not as susceptible to the length-tension curve as the prime movers (see Fig. 2).

Muscles important for stability, control and force transfer, in the lumbar spine and pelvis are: ● multifidus (MF) ● transversus abdominis (TA) ● the pelvic floor muscles (PF) ● the muscles of the diaphragm ● the gluteal muscles ● the long head of the biceps femoris (attaching to the sacrotuberous ligament) ● piriformis ● latissimus dorsi

The ‘Neutral Zone’ concept Panjabi (1) described a neutral zone for joints, which is a small range of joint surface displacement in which the osse-

The range of this zone may vary from person to person and can be related to genetics, in that ligamentous composition may vary, for example in people with hypermobile joints. More importantly, it can relate to injury, articular degeneration and/or weakness in the stabilising muscu- lature (11). An increase in the neutral zone, eg. following ligament damage, could lead to an increase in uncontrolled move- ment of the joint, or instability, causing or further exacerbating articular damage.

In this case, ligaments may not return to their original length. The muscular system therefore must step in to work as ‘dynamic ligaments’ to try to control this displacement.

Muscle stiffness Muscle stiffness is defined as the ratio of muscle force change to muscle length change. A muscle is deemed to have a higher degree of stiffness if there is less length change as force increases. Stability muscles, with minimal alterations in length during contraction, can generate increased stiffness. This capability is due to their make up, ie. they are closer to the joint axis, being deep muscles so produce minimal joint movement.

It is thought that this stiffness of the deep musculature helps keeps the

Phasic, fast twitch Tonic, slow twitch

oligamentous system offers little resistance. This joint play is therefore relatively uncontrolled.