DPT-218-A&P: Posture & Core Stability

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In this section we cover the principles of good posture and core stability. This includes the roles of muscles and ligaments in spine support, along with other mechanisms such as intra-abdominal pressure and the action of the thoraco-lumbar fascia. We will also include a discussion on the advantages and disadvantages of different methods of stretching.

Postural and Core Stability Defined

The word ‘core’ is used to describe the central area of the body – mainly the lower portion of the vertebral column, or the lower back. The aim of ‘core stability’ training is to improve the function of the muscles surrounding the torso so that they give better protection and support to the joints of the lower back. This in turn leads to better overall postural stability because a stable core promotes better control and alignment of the limbs that attach to it.

Sometimes the definition of the core can be extended to include other regions of the spine and the shoulder and pelvic girdles as well. 

Therefore, a definition of core stability is:

‘The ability to maintain correct spine alignment/shoulder girdle position

pelvic position whilst moving the limbs’

(Central YMCA Qualifications, 2010)

For the purposes of this course we will focus mainly on core stability as it relates to the lower back.

Potential Benefits of Improved Core Stability

Improved core stability should produce a functionally strong torso. This means that, when performing daily tasks, training, or playing sports, our risk of injury to the spine is reduced. 

A strong core should result in:

  • Reduced risk of injury to the lower back
  • Improved posture
  • Increased functional strength – the ability to transfer force between legs and arms is improved.

It is important to note here that core stability training will notdecrease abdominal fat. The exercises will contribute to ‘toned’ abdominal muscles, but unfortunately these muscles cannot be seen unless the fat covering them is lost, and this can only be done with an effective overall programme of training, diet, and lifestyle to create a negative energy balance. Core stability exercises will burn negligible calories when compared to traditional training such as walking, jogging, aerobics, swimming or circuit training. 

Local and Global muscles

Muscles of the core can be roughly divided into two groups: ‘local’ and ‘global’.

As a general rule, surface (superficial) muscles are responsible for large powerful movements of the trunk and for gross stability, but they do not have fine positional control. They are usually referred to as ‘global’ or ‘mobiliser’ muscles. Examples are rectus abdominis and erector spinae. Global muscles are easily visible in someone with good definition, and they are quite simple to train with mobilisation exercises like abdominal crunches and back extensions.

Muscles that lie deeper in the body, closer to the vertebral column, tend to have more subtle stabilizing functions. They are referred to as ‘local’ or ‘stabiliser’ muscles, and they are responsible for making slight adjustments to alignment between adjacent vertebrae. Local stabiliser muscles are recruited prior to gross movement to prevent unwanted movement between vertebrae (known as ‘segmental stability’). Multifidus and transversus abdominis are examples of local muscles. Their role in core stability and lower back pain has been studied extensively (Richardson, Jull, Hodges, & Hides, 1999). 

Local muscles are not visible, nor easily palpated. It is therefore hard, or indeed sometimes impossible, to tell if they are being trained or not. Carefully selected stabilisation exercises, usually performed at low load (i.e. without much resistance) are necessary to ensure local stabiliser muscles are being trained (Comerford & Mottram, 2007).

To understand how the core muscles support the spine we can use the analogy of the vertebral column as a mast on an old sailing ship. The mast must stand up vertically against strong forces from the wind, and to do this it has multiple wires to brace it in all directions to prevent it from buckling. Some wires span from the base to the top of the mast (global). Other wires span just small segments of the mast (local). If any one guy wire is too loose or too tight then the mast becomes unstable, and is pulled out of ideal alignment. 

The core muscles act in the same way as the wires holding up the mast, trying to brace the vertebral column against bending and twisting forces applied in any direction. Thus, if any core muscle is too loose (i.e. a weak/lengthened/underactive muscle) or too tight (i.e. a strong/shortened/overactive muscle) then the spine will be less stable and pulled out of alignment(McGill, 2002).

‘Excessive strength in just one muscle will not promote stability. 

Instead, it is the balance of tension between 

all of the muscles that is essential for overall stability’

The common practice of training just the rectus abdominis, whilst leaving all the other core muscles untouched, simply promotes imbalance and is a mistake. All muscles of the core are important for stability, depending upon the specific activity being performed. 

The aim of core stability training should be to repeat common movement patterns like dead lifts, squats, lunges, push, pull, etc. with good alignment, so that correct neuromuscular recruitment of stabilisers becomes automatic. Once this is achieved then the goal is to progressively increase resistance to develop endurance and strength using a balanced programme.

Ligaments Supporting the Spine

In addition to the muscles there is a comprehensive system of ligaments that gives significant support to the spine. 

You will remember that the function of a ligament is to stabilise a joint by attaching bone to bone. In the spine there are ligaments that attach each vertebra to the next one throughout the whole length of the vertebral column. Some are anterior, some posterior and some positioned laterally to resist extension, flexion and lateral flexion forces respectively. Their location close to the vertebrae gives ligaments limited leverage, and therefore a limited ability to resist forces when compared to the muscles. 

Ligaments give most support at the end of normal range of movement because they cannot be actively contracted like muscles. They simply act as ‘straps’ to prevent too much movement. However, ligaments will gradually adapt and become stronger in the long term if they are subjected to repeated loading. Hence resistance training, when performed safely over weeks and months, will result in tougher, more resilient spine ligaments.

If ligaments are lengthened too far, through use of an excessiverange of movement, they may become stretched or ruptured (sprained) leading to compromised stability and back pain. 

Intra-Abdominal Pressure

When performing exercises like dead lifts and squats, the core muscles may also help to support the spine by forming an abdominal ‘balloon’ and creating intra-abdominal pressure (IAP).

The abdominal balloon is formed by the rectus abdominus, obliques and TVA wrapping around the trunk, with the pelvic floor and diaphragm muscles forming the bottom and top respectively. If these muscles, and their associated connective tissue, are all tensed simultaneously, then pressure inside the abdominal cavity can be increased. The resulting ‘balloon’ may add support to the spine and prevents it from flexing forwards. To what extent this mechanism lends support to the spine is an issue of some debate. 

Note that increasing IAP to any significant level involves a static contraction of the diaphragm, which implies that the breath should be held. Breath holding whilst lifting a heavy weight leads to a rapid increase in blood pressure. Therefore it is not appropriate for any client with existing hypertension or other cardiovascular conditions. With healthy clients, the duration of breath holding should be kept to a minimum. Immediately after the breath is released there is also the possibility of a rebound ‘valsalva’ effect as described in chapter 1. Consider both of these factors before training a client.

Thoraco-Lumbar Fascia

The thoraco-lumbar fascia is located at the base of the spine covering the lower back. It is a flat sheet of tendon that facilitates the attachment for several muscles and transmits forces from those muscles across the lumbar and sacral area from upper to lower body. This connective tissue, when pulled taut, is ideally positioned to give support and stability to the lumbar spine.

Several muscles originate at the thoraco-lumbar fascia and place tension on this sheet of tough connective tissue.

  • Internal obliques, and transverse abdominis pull sideways, like a corset.
  • Latissimus dorsi pull diagonally upwards
  • Gluteus maximus pulls diagonally downwards

The thoraco-lumbar fascia provides additional support to the lumbar spine when it is pulled tight by these muscles. The transverse abdominis and internal obliques create tension to help resist translation movements where one vertebra slips forwards or sideways relative to the adjacent vertebrae. This is one argument for the ubiquitous ‘pull in the abs’ teaching point.

The resultant pull of the latissimus dorsi and gluteus maximus creates tension to help resist forward flexion. This will offer support to the spine during dead lifts, squats, etc. These muscles are also naturally recruited in functional movements like walking and running where the hip extends (gluteus maximus) whilst the opposite arm is drawn back (latissimus dorsi). Not only does the fascia help to stabilise the lower back ad SI joint, it also acts as a spring to store elastic energy when stretched, ready to rebound and give more power to the next step.

Local Muscle Changes due to Poor Posture

Postural alignment is affected daily by any number of factors. If a certain pattern is repeated and held for long enough over weeks, months or even years, then the body becomes ‘set’ in that characteristic position. This is because habitually poor posture leads to changes in the length, tension and tone of the muscles that support the spine. Some muscles become shortened, tight, fibrous and over-worked, whereas others lengthen, weaken and atrophy. Recognising this pattern of imbalance between muscles is important for formulating a plan of treatment and correction to lengthen tight muscles whilst at the same time encouraging movement, activity and strength in the opposing weakened muscles. 

Factors that affect posture include:

A lack of core stability: For example, weakness in the thoracic portion of erector spinae can contribute to a slumped ‘kyphotic’ posture. Corresponding shortness will probably be found in the antagonistic abdominal and pectoral muscles.

Heredity: Genetics supplied by parents, grandparents, and so on decide to a large extent what the shape of the spine will be.

The ageing process: A combination of muscle weakness and osteoporosis cause the vertebrae to degenerate, leading to loss of height and a stooped, kyphotic posture.

Lifestyle factors: Occupations where people remain seated all day inevitably have a detrimental effect on posture. In particular, the constant flexed position of the lumbar spine causes uneven pressure and chronic stress on the inter-vertebral discs. People who have to sit for long periods would be well advised to regularly stand, move about and allow their spine to adopt a more natural lumbar curve.

A poorly designed training programme: If a training programme is poorly designed then imbalances between muscles can develop. A common pattern is to overwork the global muscles [because they are more obvious and visible] whilst under-using the local muscles. The global muscles become dominant and the local muscles weaken, lengthen, lose tone and then atrophy. 

This means that the person appearsstrong because they can perform large trunk movements. However, as they do so, the position of separate vertebrae is not well controlled because of the weakened local muscles. For example, they can dead lift a heavy weight because the erector spinae are strong, But they develop lower back pain because the multifidus is unable to keep the lumbar vertebrae correctly aligned.

Abdominal adiposity: When the distribution of body weight is altered this can have an effect on balance, posture and exercise performance. For example, if a client carries extra fat around their waist then there is a lot of additional weight concentrated in the abdominal region. This shifts the centre of gravity and balance forwards. The spine may then compensate by increasing the lumbar curve to shift the hips and shoulders backwards, resulting in lordosis.

Pregnancy: In the late stages of pregnancy the weight of the developing foetus can also cause a similar shift in centre of gravity and balance, leading to an increased curvature of the lumbar spine.

Clearly, restoring function to core muscles by means of a carefully constructed programme of strengthening and stretching will help to correct poor posture and should, in turn, lead to a reduction in associated pain.

Potential Causes of Back Pain

There are many potential causes of back pain as a result of postural deviations, and the exact source of pain in any particular case can be notoriously difficult to diagnose. Here are some of the more common causes:

Muscle strain: Back pain can be caused simply by over-straining and pulling a muscle. This is common in the lumbar region where the erector spinae are injured whilst lifting, bending or carrying a load. Muscles have a good blood supply, so any tears to fibres and connective tissue repair relatively quickly. The back will usually ache for several days but gradually resolves itself. 

Ligament sprain: Spinal ligaments are most vulnerable to injury when they are stretched beyond their normal range of movement – usually duringexcessive twisting, flexing and hyper extension movements. Minor sprains (small tears in the ligament) will also resolve themselves, but this takes longer than muscle-related problems due to the poorer blood supply to the denser collagen fibres. Major sprains can permanently compromise spine support. Ligaments have limited elasticity and, once stretched, can remain lax.

Damage to the inter-vertebral discs: Ingeneral, movement is good for the spine and for the inter-vertebral discs; it helps to keep discs pliable and healthy. However, sustained positions that repeatedly stress the discs in one direction can lead to injury. 

This can be in the form of a disc bulge or even a prolapsed disc (commonly called a ‘slipped disc’) which impinges on a nerve root, causing intense pain.

Damage to inter-vertebral discs is most common in the lumbar spine. This usually impinges on nerves leading to pain that radiates down to the hip and leg, even reaching as far as the foot. Similarly, a prolapsed disc in the cervical spine can radiate pain to the neck, shoulder, arm and hand. 

Any client with a suspected disc bulge or prolapsed should be referred to their doctor for medical advice before undertaking an exercise programme.

Arthritis: Osteoarthritis (OA) can affect in the facet joints between adjacent vertebrae. This is the result of cumulative wear and tear and will inevitably happen to some extent due to ageing. The resulting inflammation from OA is a common source of back pain. The condition is not reversible, but the effects can usually be managed with a combination of appropriate exercise and medication. In more advanced cases, a surgeon may choose to operate and fuse vertebrae together to prevent the painful rubbing together of the arthritic joints.

Neutral Spine

When performing exercise or everyday tasks it is important to maintain correct spinal alignment. The usual term for this correct alignment is a ‘neutral spine’.

A neutral spine position is one that maintains each of the four natural curves, 

rather than extending, flexing or twisting excessively.

For our purposes here we will focus mainly on the lower back. A neutral lumbar spine can be found somewhere in the middle position between a flat lower back (pelvis tilted fully backwards) and an arched lower back (pelvis tilted fully forwards).

In standing, try rocking your pelvis back and forth between these two extremes and then settle on the mid-point. This is a reasonable indication of your neutral lumbar spine. The challenge then is to maintain this position whilst the lower back is placed under load during lifting, carrying, pushing and pulling movements.

The ability to maintain a neutral spine is important for:

  • Prevention of joint and ligament damage within the vertebral column
  • Performing weight bearing exercises with biomechanical efficiency
  • The transmission of stressors through the pelvis, caused by impact

In the lumbar region there are several nerves that leave the spine to supply the muscles of the hips and lower limbs, particularly the large sciatic nerve and the gluteal nerve. Poor posture, exaggerated pelvic tilt and uncontrolled movements can place the sacroiliac joint and the attached lumbar vertebrae under severe stress. This can result in compressing nerves, overstretching ligaments, tearing of weak muscles and damage to inter-vertebral discs. 

Refer to chapter two for more information about common postural deviations – kyphosis, lordosis and scoliosis.

Core Stabilisation Exercises

Core stabilisation exercises aim to recruit and strengthen the muscles around the trunk in order to improve alignment and reduce the potential for injury.

It is important that the local muscles function properly and can support the vertebrae before heavy loading is applied to the spine. Therefore it makes sense to perform low-load stabilisation exercises first, before using the larger, more powerful mobilisation exercises. Stabilisation exercises usually involve static contractions to hold the spine in neutral alignment, whilst the limbs are moved dynamically to create a challenge. Once these are mastered, then the more familiar mobilisation exercises such as abdominal crunch, back extensions and torso twists can be included. A range of stabilisation and mobilisation exercises for the core is included on your practical personal trainer course.

As previously noted, core stability depends upon the balance of strength and tension between all of the core muscles. If a particular muscle is too short and tight then it is more relevant to lengthen it by appropriate stretching. Therefore we will now move on to a comparison of the different types of stretch available to the exercise teacher.

Different Types of Stretching

The health profession has developed many different methods of stretching muscles with the purpose of improving range of movement (ROM). Here we will briefly look at the advantages and disadvantages of each technique and when it is most appropriate to use.

Static Stretching

A static stretch involves lengthening the muscle slowly until mild tension is experienced, then the stretch is held statically for several seconds. The ACSM recommend holding for 15 – 60 seconds and repeating the stretch at least four times for optimum results (‘developmental’ stretching).

The use of static stretching is common in health clubs because they are low risk and easy to perform. Essentially they are safe for anyone, provided the body position adopted for the stretch is comfortable and stable. Improvements in ROM appear to result from natural adjustments to the stretch reflex, and from lengthening of tightened connective tissue running through the muscle-tendon unit. 

However, a static stretch may not fully prepare a muscle for the rapid and powerful lengthening experienced in many sports. In addition, there is some evidence to suggest that static stretching may slightly decrease force generating capability of muscles during that session. Therefore a static stretch may not always be appropriate, especially where sports performance is your client’s main priority.

A static stretch may be activeor passive. An active stretch makes use of the fact that muscles work in opposing pairs. Stretching (lengthening) of the antagonist is brought about by contraction (shortening) of the agonist. An example is lengthening the hamstrings by contracting the quadriceps. A passive stretch uses another force to bring about the stretch, such as holding the leg with your hand, or using partner assistance.

Active stretching is a natural function of the muscles, and is the type of stretch encountered when performing many activities. Active stretching utilises ‘reciprocal innervation’ between muscles. This means that when the agonist contracts, the nervous system will automatically cause a ‘reflex inhibition’ (relaxation) of the antagonist so that movement can occur. If this did not happen then both muscles would oppose each other and the limb wouldn’t move. Hence, the reflex inhibition of the antagonist, and the resulting relaxation, is thought to help obtain a better stretch. Better muscular control throughout the full range of movement may also result from active stretching when compared to passive stretching, and this may be beneficial to joint stability (Alter, 2004).

The effectiveness of a passive stretch does not rely on the strength of the agonist being sufficient to stretch the target muscle. They can therefore be easy to apply and comfortable to hold, hence their common use in health-related flexibility. However, if used inappropriately, they may promote range of movement beyond that which the muscles are capable of controlling. Hence they may be detrimental to joint stability in the long-term.

Dynamic Stretching

This refers to stretching with movement: The muscle is being continually lengthened then shortened, without being held at full length for any significant duration. Movement should be with control, for example, circling the shoulders through their full range at a moderate speed, or bringing the heels up to touch the backside. 

Dynamic stretching is safe to perform for a majority of clients provided that they have controlled technique. This type of stretching, used as part of a specific warm up, may prepare muscles more fully for rapid lengthening of the type encountered in most sports (Norris, 1995). However, for clients with low skill level, static stretching is still the more sensible choice. Please note that dynamic stretching is not simply a matter of performing any slow and controlled exercise. 

To be effective, a dynamic stretch must be a movement carefully 

selected and performed to lengthen a muscle through its full range.

Ballistic Stretching

If a dynamic stretch is applied with rapid, explosive movements several times in a bouncing fashion then it is referred to as a ‘ballistic’ stretch. Examples would be high-kicking the legs, or bouncing toe-touches. 

Safety of ballistic stretching is considered to be lower because of the greater speed involved, leading to less control of the limbs and possible over-stretch. Therefore it is recommended for use only when the sport demands rapid explosive muscle lengthening.

Proprioceptive Neuromuscular Facilitation

Proprioceptive Neuromuscular Facilitation (PNF) stretching is a term used to describe a number of techniques that originated in physiotherapy, and which are commonly used to increase flexibility. 

The basic principle is to first place the muscle under tension by isometric contraction. When the contraction is released, the muscle appears to become temporarily more relaxed than usual and can be stretched well beyond its usual range. PNF stretching can also be self-appliedquite effectively, but is usually partner-assisted, making it a useful tool for the Personal Trainer.

The increased range of movement is only temporary, but regular use of PNF stretching is effective for long-term increases in range of movement (Alter, 2004). The temporary relaxation of the muscle may due to the action of the GTO’s in response to the isometric contraction – a phenomenon known as ‘autogenic inhibition’. However, it is worth stating not all research agrees on this explanation, and that the exact reason for the greater relaxation in the muscle is not fully understood. However, practical experience shows us that the technique works. 

Partner Assisted PNF Stretch

Safety in PNF stretching is considered to be lower than for normal static stretching because of the degree to which the muscle is being elongated. Also, the Personal Trainer needs sufficient experience to avoid over-stretching the muscle and causing strains. 

Long term, it is not clear whether PNF stretching is superior to regular developmental stretching for increasing range of movement. The technique is popular amongst Personal Trainers because of the impressive immediate results, and because of the inter-dependence developed between client and trainer.

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