The muscles which shorten are those which have a primarily postural rather than a phasic (active, moving) role and it is possible to learn to conduct, in a short space of time (ten minutes or so) an assessment sequence in which the majority of these can be identified as being either short or normal.

Janda informs us that postural muscles have a tendency to shorten, not only under pathological conditions but often under normal circumstances. Postural muscles are genetically older; they have different physiological, and probably biochemical, qualities compared with phasic muscles which normally exhibit signs of inhibition in response to stress or pathology.

Postural Muscles
Those muscles which shorten in response to stress comprise the following:
Gastrocnemius, soleus, medial hamstrings, short adductors of the thigh, hamstrings,psoas, piriformis, tensor fascia lata, quadratus lumborum, erector spinae muscles, latissimus dorsi, upper trapezius, sternomastoid, levator scapulae, pectoralis major and the flexors of the arms. The scalenes are a borderline set of muscles - which start life as phasic muscles but which can become through overuse/abuse more postural in their function.

It is now becoming clear that the function of a muscle can be modified - which helps to explain some mysteries - for example why the scalenes are sometimes short and sometimes weak and sometimes both, and yet are classified generally as phasic muscles and sometimes as ‘equivocal’ - maybe postural and maybe phasic’.

Lin, writing in The Lancet examined motor muscle physiology in growing children reviewing current understanding of the postural /phasic muscle interaction and reported that, ‘Buller and Eccles have shown that a committed muscle-fibre type could be transformed from slow-twitch to fast-twitch and vice-versa in their cross innervation experiments, confirming that impulse traffic down the nerve conditions the fibre type’.

The implication of this is that if a group of muscle such as the scalenes are dedicated to movement can, through stress such as occurs i constant upper-chest breathing for example, become ‘postural’ in type, and will therefore develop a tendency to shorten if stressed.

Characteristics of Postural and Phasic Muscles:

All muscles comprise both red and white, fast and slow, fibres which produce both postural and phasic functions, however the classification of a muscle into either a ‘postural’ or ‘phasic’ group is made on the basis of their predominant activity, their major functional tendency.