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Stability in dance training.

Abstract

Stability training has become a recognized component of dance training in the past decade. Drawing on knowledge applied in sports and spinal rehabilitation, dancers are now able to enhance their performance by applying similar principles in their training. The main issue with stability training is one of being able to recognize the difference between muscle strength training and muscle skill acquisition. Motor control principles underlie stability training as it is the long-term, learned, skill of effective muscle recruitment that is desired rather than short-term strengthening of these muscles. We know that the musculoskeletal system is not an inherently stable structure and ultimately relies on muscle activity to maintain its integrity. This article explores both local and global stability muscle systems, the processes of skill acquisition, and highlight the differences between stability and rigidity.

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It would be fair to say that one of the most discussed topics in both the athletic and the dance worlds over the last decade has been that of stability training. In reality "stability" comes in many forms. Passive stability is provided by the tissue structures that surround joints while dynamic stability is provided by interaction of refined muscular activity to provide smooth and controlled movement.

Many approaches have been proposed but there are some important components that must be considered to provide results that are both beneficial and long lasting. A growing body of research continues to evolve in this area and while many principles of training are being developed it is important to bear in mind that this is a "work in progress' and changes will continue to be made as new evidence comes to light. (1,2) With research on issues relating to cortical motor programming processes affecting muscle function and pathological processes interfering with muscle function now also being included, we realize that there is not one simple answer but that numerous paths may have to be followed in order to develop a range of solutions. (3,4)

Stability or Strength

Traditionally we have though of improvement of muscle function and muscle training in terms of strengthening. We have been lead to believe that by increasing the power and strength of various muscles we should see improvements in performance. (5,6) These beliefs continue to dominate the sporting world but a growing number are seeing alternatives as viable options. One would have to agree that the accent on strength training over the years has had a well publicized downside as well. While it has lead to a myriad of regimens all aimed at improving peak power and torque production, we have also seen the sporting world increase its use of performance enhancing drugs aimed at further improving strength. (7) Interestingly, the significant, and sometimes massive, gains in muscle bulk and torque production in many cases only cause small gains in overall performance. (8)

When it comes to stability training, increased strength and muscle bulk is not necessarily an advantage and can in fact be a disadvantage.

With the increasing literature investigating stability training we now realize that there is a divergence between strength training and stability training. The strength training model does not work well in this arena, particularly if we want the effects to enhance overall efficiency and provide long-term benefits.

The area that has been largely neglected over the years is that of brain function and cortical programming. This is where well-learned, efficient motor programs have the potential to produce longer term benefits than strength training. (2)

So how does stability relate to improved performance? This can be summed up in one simple term--motor learning. (9) The motor learning component relates most directly to stability training.

Why is Stability Training Important for Dancers?

For the dancer, as with any athlete, the importance lies not only in the efficient recruitment of muscle activity but also an understanding of the background structures being protected. Historically, dance has a tendency to self select a more hypermobile population, which is becoming more extreme as choreographic demands change. Passive joint and structural stability begins with the qualities of the collagenous tissue matrix of the individual. This tissue forms passive structures such as ligaments, capsules, fascia, tendons, and the like. Those with a denser, more rigid connective tissue matrix will tend to be classified as hypomobile and, therefore, have a more structurally "stable" joint makeup. On the other hand those with a less stable collagen matrix exhibit greater degrees of joint laxity and tend toward hypermobility and in many cases, structural "instability." As mentioned earlier, current choreographic demands tend to prefer the more hypermobile dancer but there is a mounting concern that these are in fact the more injury prone and likely to have shorter careers due to injury. (10)

Passive joint structures such as ligaments and joint capsules only provide stability at the end of range when they are under tension and load. When a joint is functioning in its mid-range, these structures contribute very little structural support and stability for the structure they surround. This mid-range control is dependant on the active muscular system through its various tendinous and fascial attachments.

The skeletal system devoid of muscle activity is not an inherently stable structure. We know, for example, that the spine will buckle at very low loads over its whole length and is totally dependant on muscle activity to maintain joint congruity and protection. (11)

Local and Global Muscle Systems

In his definitive paper, Bergmark highlighted two muscle systems that related to stability: the local stabilizer and global stabilizer systems (Table 1).12 The local system refers to the deep stabilizers that control inter-segmental position and placement. This includes muscles such as the posterior fascicles of psoas major, (13) transversus abdominus, deep fibers of multifidus, deep cervical flexors, and the myriad of muscles lying close to joint structures. (14)

The function of this system is to control local segmental shearing and translation. They are poor at controlling direction of trunk movement or accepting load against gravity. As primary deep inner units their contraction produces force without muscle length change or range of movement. To stabilize the structures that they surround, activity is continuous at low levels of maximum voluntary contraction (MVC) to provide isometric control of neutral joint position and ultimately increase structural stiffness in mid-range not offered by the passive structures.

The global system, on the other hand, is the more superficial system. Some of the muscles that make up this system include internal and external obliques, superficial erector spinae, rectus abdominus, latissimus dorsi, and those muscles and groups responsible for movement and directional control.

While Bergmark discussed the importance of these systems in their separate roles, he also highlighted that neither system, in isolation, can control functional stability. Both systems do need to integrate for control of movement against gravity.

It was also made clear that joint stiffness increases rapidly and non-linearly with muscle activation and that only modest levels of muscle activity are required to create sufficiently stiff and stable joints. These are the low levels of MVC that are commonly referred to in stability training. While figures of 20% to 30%14 are commonly referred to, these are arbitrary and indicative of submaximal activity with levels as low as 3% MVC also hypothesized as having a direct effect on segmental shear. (15) What is clear is that excessive, strong muscle activation will only result in overly stiff and rigid segments.

Antigravity postures and movement are not controlled by bony skeletal structures, but rather by a highly complex set of controlled muscle actions. The cortical programming necessary to maintain integrity and movement of the human musculoskeletal system is also part of the same process by which we control stability of the system.

Two Main Factors Underlying Stability Training Muscle Fiber Type

Physiologically there are two main types of muscle fiber: those that provide foreground activity and those that provide activity in the background.

The local stabilizer muscles involved in structural stability tend to consist of type I fibers. They function as a background to movement and posture at submaximal levels, generally accepted as being less than 50% MVC. (14)

The global stabilizer muscles, on the other hand, tend to recruit at higher levels of activation and involve the larger type II fibers. These muscles work at higher percentage levels of MVC and are more involved in the movement and movement control process. As a result we often see them as a focus in the strength training process.

So what are the differences between the two muscle fiber systems?

While it is known that strength training, which relates to the higher functioning and higher demand recruitment situation, has a place, we can see there are significant differences in the properties of muscle fiber types and the way that they are used. Rather than type II fibers it is the Type I fibers that we rely on to maintain background stability muscle activity.

The qualities of type I fibers such as slow speed of contraction, low rate of fatigue, early onset with activity, low force generation, and so on are much more suitable for a sustained background muscle pattern that represents structural and functional stability.

During activity, muscular motor units are recruited in an orderly manner. Small (type I) units are recruited first due to their lower thresholds of activation than the larger units. Ideally these small units are firing almost continually in the background of low demand daily activities at a sub-perception level. This represents their bias toward tonic activity. (16,17)

The low percent MVC is a crucial issue to bear in mind as it is not as easy to "feel" the effort of a low level muscle contraction as it is a higher percentage of MVC activity.

Specific exercise training can alter the metabolic characteristics of muscle fiber types. Athletes or individuals involved in endurance activities can appear to have an increase in type I fibers and a reduction in type II fibers as some of the type II fibers acquire the physiological, structural, and biochemical properties of type I fibers. As they alter their ATPase activity, the increase in mitochondria results in increased oxidative capacity, oxidative enzymes, and capillary distribution. (16,18)

Motor Learning

When it comes to muscle stability training it is a case of motor learning versus strength training. As discussed, the current evidence relating to spinal stability training recommends improving activation of central abdominal stabilizers. We are familiar with these muscle groups such as transversus abdominus, multifidus, internal and external oblique, and the like, (2,14) but the problem has been how do we best facilitate activity of these muscles.

Previous investigations have suggested training these muscles in an isolatory fashion, (2) however there has been significant dispute about the role of isolatory muscle activity, which has lead to the realization that these muscles never actually function in isolation but are part of an engram of activity that includes numerous muscles firing in a specific sequence. (18,19) When there is a problem with these muscle groups, timing is generally delayed and insufficient to provide adequate protection for the underlying joint and soft tissue structures.

There are two main reasons why these muscles fail to work efficiently. It comes down to an issue of either recruitment or pathology.

The issue of pathology is a complex one and well beyond the scope of this article since it relates to damaged central and peripheral mechanisms that affect the neural supply to these muscles. We will, though, discuss the relationship between motor control and stability that the cortical planning process plays.

Stability relies on precise muscle activities to protect the predominately "unstable" skeletal system. Training these muscles must be looked at from a motor learning perspective. We are not talking strength in the traditional sense of power and torque production.

The long-term effects of strength training have been well documented. It is seen as a rapid response to demand as a result of overload training. There is local adaptation with hypertrophy of muscle fiber, however this soon regresses when the demand is removed. We can see this with athletes who train toward a specific event. Once that event has passed and they have taken a break for several weeks or even months, they will have to work hard to return to their previous levels of strength. The overall effect is that of foreground activity with minimal motor learning effect in the long term.

Motor learning is a different system. It is more complex and is modulated by the higher central nervous system (CNS). There is central cortical control over complex muscle activation patterns that results in a pattern of learned activity, or motor engram. This skill acquisition process leads to long-term, learned motor patterns that do not regress quickly. Once established, they remain intact and can be improved or maintained with small amounts of practice. Unused they will lie dormant and degrade slowly over time but will return relatively quickly with a small amount of practice. Examples of skill retention such as driving a car, riding a bike, and playing a musical instrument are all motor patterns that are learned as skills. Having learned to ride a bike does not mean you will have to start learning all over again if you have not done it for a year. This learning system though is strongly influenced by and reliant on the afferent proprioceptive input. The ultimate process of learning a skill is dependant on the information supplied to the cortex. It requires afferent load that is appropriate and a task to activate suitable efferent activity. (20,21)

This is where the differences between strength and stability training begin to become apparent. With strength training there is always a perception of effort as the muscle works at higher levels of percentage MVC. It is easy to "feel' this effort.

Stability training is a different experience. If it is being undertaken appropriately, as with any learning process, there will be an initial period of over-recruitment of muscle activity and a perception of effort or difficulty. Low skill levels lead to higher perceived effort due to high levels of recruitment of a few motor units. With repetition this perception of effort will decrease as the cortex sets about establishing an efficient engram of muscle activity activating more motor units at ultimately lower levels of percentage MVC. This is where automatic patterns of learned, preemptive muscle activity are being relegated to the background. As a result, perception of muscle activity, particularly specific or isolatory muscles, will be occurring more at a sub-perceptible level.

A major drawback in current thinking with stability training is the accent on continually trying to perceive (feel) these background muscles working. As these muscles work more efficiently they will activate more preemptively at lower levels of recruitment. There will be a point at which activating them consciously will not only delay their firing but also interfere with the learned motor engram. A simple analogy would be that of driving a car or playing a musical instrument by trying to consciously activate every muscle necessary to the task. The result would not be a smooth well-executed task but a series of jerky poorly performed actions.

For stability training to be successful it must move into smooth task execution with focus drawn away from perception of muscle activity and effort. As with any skill set the ultimate goal is that of smooth and easy movement with minimal effort. The most skilled dancers and athletes do just that; they learn to make it appear easy by exerting minimal effort in execution of the task.

Conclusion

Spinal stability training is a much more complex process than simply strengthening the muscles of the abdominal wall. It is important that we do not confuse muscle stability training with muscle rigidity as the strength and conditioning model does not work well when applied to the motor control and learning based spinal stability training model. To be most effective we must consider that there are a range of higher level neurological processes that are important in achieving stability of movement and control of the musculoskeletal system. Protection of the musculoskeletal system depends on a background of well learned, preemptive motor programs on which we superimpose movement and function.

References

(1.) McGill S: The functional Anatomy of lumbar stability: What are the critical components? In: Proceedings of the 5th Interdisciplinary World Congress on Low Back and Pelvic Pain. Melbourne, 2004, pp. 3-5.

(2.) Richardson C, Jull G, Hodges P, Hides J: Therapeutic Exercise for the Spinal Segmental Stabilisation in Low Back Pain: Scientific Basis & Clinical Approach. Edinburgh: Churchill Livingstone, 1999.

(3.) Moseley L: Psychosocial factors and altered motor control. In: Proceedings of the 5th Interdisciplinary World Congress on Low Back and Pelvic Pain. Melbourne, 2004, p. 137.

(4.) Damiano DL: Reviewing muscle cocontraction: Is it a developmental pathological or motor control issue? Phys Occup Ther Pediatr 12:3-20, 1993.

(5.) Seger JY, Thorstensson A: Effects of eccentric versus concentric training on thigh muscle strength and EMG. Int J Sports Med 26(1):45-52, 2005.

(6.) Rhea MR, Alderman BL: A metaanalysi of periodized versus nonperiodized strength and power training programs. Res Q Exerc Sport 75(4):413-422, 2004.

(7.) Hartgens F, Kuipers H: Effects of androgenic-anabolic steroids in athletes. Sports Med 34(8):513-554, 2004.

(8.) Clarkson PM, Thompson HS: Drugs and sport: Research findings and limitations. Sports Med 24(6):366-384, 1997.

(9.) Higgins S: Motor skill acquisition. Phys Ther 71 (2):123-139, 1991.

(10.) McCormack M, et al: Joint laxity and the benign joint hypermobility syndrome in student and professional ballet dancers. J Rheumatol 31(1):173-178, 2004.

(11.) Panjabi MM: Clinical spinal instability and low back pain. J Electromyogr Kinesiol 13(4):371-379, 2003.

(12.) Bergmark A: Stability of the lumbar spine. Acta Orthop Scand 60:1-54, 1989.

(13.) Gibbons, S: Biomechanics and stability mechanisms of psoas major. In: Proceedings of the 4th Interdisciplinary World Congress on Low Back and Pelvic Pain. Montreal, 2001, p. 246-247.

(14.) Jull G, Richardson C: Rehabilitation of active stabilisation of the lumbar spine. In: Twomey LT, Taylor JR (eds): Clinics in Physical Therapy: Physical Therapy of the Low Back. New York: Churchill Livingstone Inc., 1994, pp. 251-274.

(15.) Cholewicki J, McGill SM: Mechanical stability of the in vivo lumbar spine: Implications for injury and chronic low back pain. Clin Biomech (Bristol, Avon) 11(1):1-15. 1996.

(16.) Enoka R: Neuromechanical Basis of Kinesiology (2nd ed). Champaign, Illinois: Human Kinetics Publishers, Inc., 1994.

(17.) McComas A: Skeletal Muscle: Form & Function. Champaign, Illinois: Human Kinetics Publishers, Inc., 1996.

(18.) Scott W, Stevens J, Binder-Macleod S: Human Skeletal muscle fibre type classifications. Phys Ther 81:1810-1816, 2001.

(19.) Gentile A: Skill Acquisition: Action, movement and neuromotor processes. In: Carr J, Shepard J, Gordon J, Held J. (eds): Movement Science: Foundations for Physical Therapy in Rehabilitation. Rockville, Maryland: Aspen Publishers, 1987, pp. 93-154.

(20.) Nyland J, Brosky T, Currier D, et al: Review of the afferent neural system of the knee and its contribution to motor learning. J Orthop Sports Phys Ther 19(1):2-11, 1994,

(21.) Schmidt RA: Motor Control and Learning (2nd ed). Champaign, Illinois: Human Kinetics Publishers, Inc., 1998.

Craig Phillips, B.App.Sc.(Phty), is the Director of Dance Medicine Australia, Clinical Pilates and Physiotherapy, Melbourne, Australia.

Correspondence: Craig Phillips, B.App.Sc.(Phty), Dance Medicine Australia, 10 Cecil Place, Prahran Victoria 3181, Australia.
Table 1 Local and Global Stabilizer Systems

Local Stabilizer Muscles              Global Stabilizer Muscles

* Primary, deep inner units           * Secondary, movement stabilizer

* Contraction produces force          * Take load but cannot control
  without muscle length change or       segmental movement
  range of movement
                                      * Contraction produces force/
* Activity continuous,                  length change
  independent of direction of
  movement                            * Activity dependent on
                                        direction of movement
* Isometric control of neutral
  joint position, controlling         * Inappropriate use underlies
  shearing and translation              movement dysfunction

* Increases stiffness in mid-range    * Must be recruited against
                                        gravity, postural control
* Unable to counter effects of
  gravity, posture

Table 2 Comparison of Local and Global Fibers (17)

                                       Type 1        Type 2a / 2b / 2x
                                       Local         Global

Contraction speed                      Slow/tonic    Fast/phasic
Myosin ATPase activity                 Slow          Fast
Primary source of ATP synthesis        Aerobic       Anaerobic
Myoglobin content                      High          Low
Glycogen stores                        Low           Intermediate/high
Rate of fatigue                        Low           Intermediate/high
Mitochondria                           Many          Intermediate/low
Capillaries                            Many          Few
Color                                  White         Red
Fiber diameter                         Small         Intermediate/large
Motor unit size                        Small         Intermediate/large
Force generation                       Low           Intermediate/high
Onset with activity                    Early         Late
Maximum voluntary contraction (MVC)    Low           Moderate/high
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Author:Phillips, Craig
Publication:Journal of Dance Medicine & Science
Geographic Code:1USA
Date:Jan 1, 2005
Words:3402
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