Research Review By Demetry Assimakopoulos©

Date Posted:

May 2010

Study Title:

Review: The Use of Instability to Train Core Musculature


Behm DG, Drinkwater EJ, Willardson JM & Cowley PM

Author's Affiliations:

Memorial University of Newfoundland, St. John’s NL; Charles Strut University, Bathurst, Australia; Eastern Illinois University, Charleston, IL; Syracuse University, Syracuse, NY.

Publication Information:

Applied Journal of Physiology Nutrition and Metabolism 2010; 35: 91–108.

Background Information:

For the last decade, the “core” has been a popular topic in research and media. The term “core” has many meanings depending on the literature one reads (30). For the purposes of this literature review, the anatomical core is defined as the axial skeleton – including the skull, ribcage, spine, pelvic girdle and shoulder girdle – and all soft tissues with a proximal attachment to the axial skeleton and a distal attachment to either the axial or appendicular skeletons. The muscles in this group can produce isometric, concentric or eccentric contractions.

Contrary to training for a specific activity that requires dynamic balance (either sport or rehabilitation), core training normally involves a static or isometric task ± movement. Many research studies utilize training protocols for the core that involve unstable and labile surfaces. This large body of research has produced varying results and because of this a synthesis of the literature is required to discover the specificity and effectiveness of the use of instability methods to train the core musculature.

Prior to coming to a conclusion about unstable surfaces, it is necessary to discuss other types of core training. Ground based training – a type of training utilizing body mass or non-traditional equipment (ex. truck tires) – can be very specific to one’s sport. It has been inferred that these methods are adequate to strengthen the core, although they are ballistic and dynamic. Still, this assertion has not been addressed in peer-reviewed literature. Similar benefits can be achieved through the use of Olympic lifts, such as power clean, clean and jerk and snatch.

There is much controversy in the fitness realm as to how to progress one’s fitness program to reach the elite level prior to competition. Do trainers utilize unstable surfaces to train an athlete at the preliminary stages of one’s training regime prior to performing the more ballistic and dynamic resistance lifts? Should instability devices be used at all? These questions and many others will be discussed throughout the paper.

There are many labile training devices on the market, which can be utilized for sport specific training and rehabilitation. “Swiss” or “physio” balls are available in a variety of sizes. The BOSU (both sides up) is assumed by many practitioners to be a great tool for core training. Alternatively, there exist inflatable discs, balance boards, foam tubes, and high/low intensity foam platforms. In addition to these, chains and ropes can be used to decrease one’s stability while performing simple exercises such as pushups and pull ups. Even changing the training surface from concrete to grass to sand can play a role in decreasing stability.

Research directed towards spinal stability and stiffness has been performed extensively in an attempt to prevent injury and perform rehabilitation to the low back (1). Nadler et al (20,21,22) proved that spinal stability is needed to provide a solid base of support for activities of daily living involving the limbs, such as shoveling snow, lifting boxes or even hitting a tennis ball. The preventive nature of spinal stiffness extends to athletic tasks as well.

On the other hand, a decrease in low back muscular endurance is strongly associated with low back pain (18). Research now shows that training programs for the core must be structured differently based on the health status of the client/patient and their goals (i.e. whether that are preparing for athletic competition, or need rehabilitation rehabilitation, etc.).


Physiology and Biomechanics

Anatomical Aspects:

Panjabi (23) described 3 subsystems which contribute to spinal stability: neural, passive and active. The movement of the facet joints in the spinal column is very important in describing spinal stability. Movement that is approximately 1-2° in each plane of movement (sagittal, coronal/frontal or transverse) is possible without passive resistance from the ligaments of the spine (23,24) – this is called the neutral zone. Movement beyond this physiological zone involves recruiting the active, passive and neural subsystems to maintain stability of the vertebral column.

When one or more of these systems are impaired in any way, it places greater stress on the other remaining subsystems. Such compensatory means are said to contribute to chronic low back pain (18).

Biomechanics of spinal stability:

The core musculature is thought to be a link between the lower and upper extremities that aids in transferring torques and momentum during the performance of various physical tasks (sport performance, activities of daily living, etc). Hence, it is reasonable to assume that weakness in the core musculature can cause an interruption in the transfer of torques resulting in a decrease in performance.

Athletes whose training programs lack specific training of the core can suffer from various injuries because their distal joints attempt to make up for lost torque. Such overuse injuries occur in baseball pitchers whose shoulder musculature increases their torque production to increase the speed of a pitch. Had the pitcher been performing the necessary core training, the weakness in the kinetic chain would have been avoided.

Function of all kinetic chains and stability of the vertebral column depends on a proper sequence of firing patterns of the core musculature that change with every movement humans perform. That being said, no single muscle is considered to be the primary stabilizer (13, 23). Amongst the many muscles that participate in stabilization, the muscles that have the greatest moment arms and are thus most capable of stabilizing the spine are: quadratus lumborum and rectus abdominus (4,26,19).

There is a great deal of controversy on the subject of the increase in intra-thoracic/abdominal pressure and its role in spinal stability. Internal and external oblique and transversus abdominus contribute greatly to this (7,8,9). Abdominal hollowing specifically targets these muscles (25). Grenier and McGill (11) concluded that this exercise resulted in 32% less stability than abdominal bracing.

In addition, it was also discovered that hollowing the abdomen decreases the moment arms for the oblique muscles and rectus abdominus, therefore decreasing the capacity to stabilize the spine. Therefore, co-contraction (or “bracing”) of the abdominal musculature is recommended for athletes and the general population alike.

Typically, multi-joint tasks such as Olympic lifts (dead-lift, power clean, snatch, etc) are often suggested to train coordination, motor learning, stability and motor neuron activation. For these reasons, it is suggested to emphasize such lifting techniques to increase sport performance and core muscle activation.

Some researchers and practitioners suggest utilizing various methods of instability combined with multi-joint exercises to train more effectively. However, great care should be taken in creating exercise protocols for athletes, because while instability training increases core activation, it provides the athletes with a less intense limb exercise.

Although, most researchers state that exercising the core musculature is important for performance, it has been shown that performing resistance exercise with a destabilizing component (ie. chest press while supported on a swiss ball, as opposed to a stable surface) may provide a less intense limb exercise while providing higher core activation.

However, there are instances where both muscle groups can be targeted most profoundly. This can be done by correctly modifying the intensity of your exercise. For instance, it has been reported that squats and deadlifts at 80% of 1RM produced greater activation of the core musculature than unstable calisthenics (12). Bearing this in mind, it is important to delineate between training for performance and training for health/rehabilitation: while athletes may want to elicit greater core activation with the use of higher weight free weight exercises, it may be more practical and wise for those that are interested in health and rehabilitation to use an unstable surface to achieve the correct muscle activation to achieve their goals. However, when training an athlete, be certain that their core is properly stabilized and ready to perform the challenging ground-based lifts.

Clinical Application:

Evaluation of core performance:

Performance assessments for the core musculature are important in assessing injury. They are useful in determining pre- and post-surgical rehabilitation progress and offer a prognostic value of injury risk (10, 14, 20, 21). Isometric and isokinetic exercises are typically used to assess strength, whereas endurance tests, such as the Biering-Sørensen back extensor endurance test (3) are performed entirely isometrically to failure.

There are also multiple tests that require an individual to reposition the trunk to a neutral position following a predefined placement. This assesses the patient’s proprioceptive abilities. Other tests are performed using load-release methods, where a person is asked to isometrically contract at a predefined intensity against a load. They are then released and the displacement from neutral is quantified. For load-release tasks, EMG is quantified to determine the on/off activation of muscles post-release.

However, while many of these tests are reliable, they have not been validated experimentally. Because characterizing core stability using one, or even a composite of tests, is unlikely, there is a need for new tests that assess multiple aspects of core function. In addition, these tests must correlate well to physical tasks.

Because of the multifaceted and multidirectional nature of sport, assessing muscles proximal and distal to the core along the kinetic chain is important. There is an inherent synergy between the core musculature and the limb muscles. Because of this, it is very unlikely that only one core muscle is contributing to a decrease in performance and/or an increase in injury risk.

Exercise specificity:

It is recommended that resistance exercise targeted to increase spinal stiffness should be implemented according to the demands of the sport. Many resistance exercises can be modified to increase spinal stabilization demands simply by changing the base of support, the posture in which the exercise is performed (i.e. seated or standing), the type of equipment used (free weights or machine weights – keeping in mind that the body rarely acts in isolation during sport), how the exercise is performed (i.e. unilateral or bilateral) or by adding more weight.

Training for the purposes of athletic performance proves to be challenging. Many times, trainers include core training as an afterthought to training or as a strict, permanent part of a training regimen. However, core training, much like seasonal training, must be periodized. This means that core training should be properly varied throughout the season.

For instance, ground based lifts may be best for improving both stability and specificity to sport. Utilizing an unstable surface may not be best for improving absolute strength and power development because they can lower the intensity of the exercise. Effectively, the athlete may lose out on absolute strength and power. The type of core training depends on what the goal of training is at any given time. On the other hand, training for rehabilitation is another story all together.

For rehabilitation, performing core exercises on an unstable surface has been shown to decrease the incidence of low back pain, and increase the effectiveness of soft tissues in stabilizing the knee and ankle (5, 6, 17). With this being said, it is recommended that unstable devices be incorporated into a periodized program where the intensity used is no greater than 70% of 1 RM. If they are to be utilized over and above this percentage, they should only be used for supplementary training only.

Isolation exercises for the core musculature:

Isolation exercises are defined as isometric or dynamic exercises, designed to emphasize specific core muscles with little activation of the limbs. Examples include trunk flexion to target rectus abdominus, trunk rotations to target internal and external oblique and abdominal hollowing to emphasize the transverse abdominus. Currently, there are only 3 approaches for core training:
  1. closed chain exercises on a stable surface
  2. closed chain exercises on an unstable surface
  3. open chain exercises on either a stable or unstable surface
While it has been shown that isolation exercises are effective in increasing spinal stability (16), there is little evidence to show that they are as or more effective than ground based free weight lifts (12). In fact, it has been debated as to the relevance of utilizing isolation exercises for the purposes of improving sport performance, considering that most sport tasks involve other muscles both proximal and distal to the core along the kinetic chain.

Isolation exercises do not train how to coordinate spinal stability with performing a task (15). Thus, ground based free weight exercises should be used primarily to train the core musculature. Isolation exercises are most useful for developing localized muscular endurance for aesthetic sports (i.e. body building).

Exercise Position:

Isolation exercises are typically performed in a sequence of positions with increasing difficulty. Some authors advocate the hollowing of the abdominal region to activate transverse abdominus, while activating the multifidus at the same time. These exercises are started from a position of full support, then move to the prone position and finally into the quadruped position (2). Once these are mastered, the patient can then move on to perform isolation exercises in less stable positions and dynamic stability movements (28).

While this approach may have some rehabilitative and stabilizing effect, abdominal bracing in various positions (a la McGill’s Big Three Exercises) may be a more effective strategy for lumbar stability that is easier to learn (11,27,29).

Volume, intensity and frequency:

Due to the inborn characteristics of the muscles used to stabilized the spine (greater percentage of type I fibres as opposed to type II), a high volume may be needed to induce fatigue in the lower threshold type I fibres so as to enable recruitment of the higher threshold type II fibres.

Because of this arrangement, the core musculature may respond best to greater than 15 repetitions per set. Intensity can be increased as time goes on. For instance, as opposed to performing a sit up or a crunch to target the rectus abdominus, a more advanced client may perform hip flexions while hanging from a chin up bar. It is a well known fact that volume is inversely proportional to intensity. So as intensity is increased, the number of repetitions and frequency of exercise should be adjusted accordingly. Presently, there is a need for more research to elucidate this topic and to find a specific dose-response relationship.

Study Strengths / Weaknesses:

  • There was no reference as to the safety of instability exercises or core exercises in general. When performed improperly, they can injure a client or athlete. In addition to this, some research has shown the specific, even well and properly executed, exercises may predispose a client to injury. This should have been explored more.
  • There was great diversity in their literature synthesis. They did not focus on any particular camp, but chose to take all available information to form a well justified and broad opinion.

Additional References:

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