What is muscle imbalance?

What is muscle imbalance?

As physiotherapists, we need to be able to differentiate between what is considered a ‘normal’ (or ‘expected’) range of motion (ROM) and that which is deemed ‘abnormal’ – either hypo- or hypermobility – along with which structures may produce or restrict the range of motion.

It must be noted that there may also be an underlying predisposition – or pathology – for hypo-/hypermobility, as in the case of connective tissue conditions, such as ankylosing spondylitis and hypermobility syndrome as distinct from a joint demonstrating hypermobility), or neurological conditions, such as multiple sclerosis. However, it is outwith the scope of this chapter to discuss these specific pathological causes. Muscle imbalance can be either ‘passive’ or ‘active’; passive being identified by muscle length and strength being either less or more than the ideal, and active being

identified when one of a synergistic pair of muscles predominates during the movement (Sahrmann 1987). The resultant functional and structural changes in the muscle appear to be reversible, suggesting that exercise to facilitate muscle length changes will be useful in the management of movement dysfunction (Gossman et al. 1982). Perhaps it is worth reviewing the general characteristics, function and roles of muscles first. Muscles can act as prime movers, or agonists, if they are primarily responsible for producing movement. Their direct opposites are antagonists, which do not normally oppose movement, but have the potential to do so if they are not inhibited; for example, when flexing the elbow, biceps brachii is the agonist whilst the triceps is the antagonist (relaxed, via reciprocal inhibition). Co-contraction of muscles is possible, which can assist movement and/or stabilise. When this occurs to assist the movement produced by the agonist, those muscles are said to be working as synergists (see the example of the scapular force couple in the shoulder section). When muscles contract to stabilise a joint and control the position of the origin of an agonist, they are

known as fixators (or stabilisers, i.e. in the case of elbow flexion as above, the fixators would be those that stabilise, or fixate, the shoulder, such as the rotator cuff). These descriptions are all well and good, but, unfortunately,

prove rather inadequate when it comes to explaining more dynamic, complex movements. Therefore, further classification of the role of muscles has been attempted by various authors according to their anatomy, architecture, fibre type and function, as summarised in. To simplify, some muscles exhibit a tendency to be more stabilising (i.e. multifidius in the spine) or mobilising in their function (i.e. erector spinae), although there are also some that demonstrate characteristics of both and can ‘multitask’ their role depending on the demands placed upon them at the time. Examples of the latter include the vasti muscles of the quadriceps and soleus. Throughout this chapter, muscles will be discussed in relation to their functional role as local and global stabilisers, or global mobilisers. Problems may occur when, for example, a predominantly stabilising muscle is injured or dysfunctional,

leading to instability and altered joint biomechanics, increased strain on structures, such as joint surfaces, and pain. Muscles with a predominantly mobilising role can be used as temporary stabilisers, but as their anatomy and physiology does not lend themselves to this, this may lead to overuse, fatigue and pain. It has been suggested that myofascial trigger points (taut, irritable bands within muscles resulting from overuse) may develop as a result (Simons et al. 1999; Fernández-de-las-Peñas et al. 2006) and which can perpetuate the muscle imbalance through further pain and resultant inhibition. Therefore, in short, if muscle imbalance is present and the muscles are unable to fulfil their role correctly, this can be through either being unable to stabilise or induce/ permit movement because one or more are either too weakened and/or lengthened, or too over-active and/or


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