A strong foundation of muscular balance and core stability is essential for normal movements and lumbo pelvic integrity. Weakness or lack of sufficient co-ordination in core musculature can lead to less efficient movements, compensatory movement patterns, strain and overuse and injury. The program starts with restoration of normal muscle length and mobility to correct any muscle imbalances. Next, fundamental lumbopelvic stability exercises are introduced, teaching the patient to activate the deeper core musculature. When this has been mastered, advanced lumbo-pelvic stability exercises using the Physioball are added for greater challenge. As the patient makes the transition to the standing position, sensory-motor training is used to stimulate the sub-cortex and provide a basis for more advanced functional movement exercises, which promote balance, co-ordination, precision, and skill acquisition.
The core musculature is composed of 29 pairs of muscles that support the lumbopelvic-hip complex. These muscles help to stabilize the spine, pelvis, and kinetic chain during functional movements. When the system works efficiently, the result is appropriate distribution of forces; optimal control and efficiency of movement; adequate absorption of ground-impact forces; and an absence of excessive compressive, translation, or shearing forces on the joints of the kinetic chain.
The term core has been used to refer to the trunk or more specifically the lumbopelvic region of the body (Bergmark, A, . Mcgill, S.M, Mggill, S.M et al, Panjabi, M.M,). The stability ofthe lumbopelvic region is crucial to provide a foundation for movement of the upper and lower extremities, to support loads, and to protect the spinal cord and nerve roots (Panjabi, M.M). Panjabi defined core stability as "the capacity of the stabilizing system to maintain the intervertebral neutral zones within physiological limits". The stabilizing system has been divided into 3 distinct subsystems: the passive subsystem, the active muscle subsystem, and the neural subsystem (Panjabi, M.M).
The passive subsystem consists of the spinal ligaments and facet articulations between adjacent vertebrae. The passive subsystem allows the lumbar spine to support a limited load (approximately 10 kg) that is far less than body mass. Therefore, the active muscle subsystem is necessary to allow support of body mass plus additional loads associated with resistance exercises and dynamic activities (Mcgill, S.M, Mggill, S.M et al, Panjabi, M.M).
Bergmark divided the active muscle subsystem into "global" and "local" groups, based on their primary roles in stabilizing the core. The global group consists of the large, superficial muscles that transfer force between the thoracic cage and pelvis and act to increase intraabdominal pressure (e.g., rectus abdominis, internal and external oblique abdominis, transversis abdominis, erector spinae, lateral portion quadratus lumborum). Conversely, the local group consists of the small, deep muscles that control intersegmental motion between adjacent vertebrae (e.g., multifidus, rotatores, interspinal, intertransverse). The core muscles can be likened to guy wires, with tension being controlled by the neural subsystem.
As tension increases within these muscles, compressive forces increase between the lumbar vertebrae; this stiffens the lumbar spine to enhance stability (Mcgill, S.M, Panjabi, M.M). The neural subsystem has the complex task of continuously monitoring and adjusting muscle forces based on feedback provided by muscles spindles, Golgi tendon organs, and spinal ligaments. The requirements for stability can change instantaneously, based on postural adjustments or external loads accepted by the body. The neural subsystem must work concomitantly to ensure sufficient stability but also allow for desired joint movements to occur (. Mcgill, S.M, Mggill, S.M et al, Panjabi, M.M).
A key muscle that works with the neural subsystem to ensure sufficient stability is the transversis abdominis. Cresswell and Thorstensson (Cresswell & Thorstensson) demonstrated that this muscle functioned primarily to increase intra-abdominal pressure, which reduced the compressive load on the lumbar spine. Other studies have demonstrated that the transversis abdominis was the first muscle activated during unexpected and self-loading ofthe trunk (Cresswell, Oddsson et al) and during upper and lower extremity movements, regardless of the direction of movement. Hodges and Richardson proposed a feed-forward mechanism associated with function ofthe transversis abdominis.
The neural subsystem utilizes feedback from previous movement patterns to coordinate and preactivate this muscle in preparation for postural adjustments or the acceptance of external loads. In another study, Hodges and Richardson demonstrated delayed activation of the transversis abdominis in subjects with low back pain, suggestive of neural control deficits.
Some practitioners mistakenly believe that the smaller local muscles are involved primarily with core stability, whereas the larger global muscles are involved primarily with force production (Johnson, P, Verstegen, M., & P. Williams). This mistaken belief has prompted ineffective training strategies designed to train the local and global muscle groups separately in nonfunctional positions. For example, the abdominal draw-in maneuver, typically performed in the quadruped or supine body position, has been widely promoted to train the stabilizing function of the transverses abdominis (Johnson, P, Verstegen, M., & P. Williams). Although this muscle is a key stabilizer of the lumber spine, several other core muscles, both local and global, work together to achieve spinal stability during movement tasks (Cresswell & Thorstensson,). For example, local muscles, such as the multifidus and rotators, contain high densities of muscle spindles. Therefore, these muscles function as kinesiological monitors that provide the neural subsystem with proprioceptive feedback to facilitate coactivation of the global muscles to meet stability requirements (Nitz, A.J., & D. Peck).
McGill stated, "The relative contributions of each muscle continually changes throughout a task, such that discussion of the most important stabilizing muscle is restricted to a transient instant in time". Core stability is a dynamic concept that continually changes to meet postural adjustments or external loads accepted by the body. This suggests that to increase core stability, exercises must be performed that simulate the movement patterns of a given activity. The co-contraction of the deeper-layer transverse abdominus and multifidi muscle groups occurs prior to any movement of the limbs, and believe that this neuromuscular pre-activation is critical in stabilising the spine prior to any movement.
The core program
Stability work should be started only after the patient has achieved good mobility, as adequate muscle length and extensibility are crucial to proper joint function and efficiency. Although beyond the scope of this article, a thorough evaluation of the muscular system should include an assessment of the muscles for over-activity, shortening, weakness, inhibition, and quality of motion. This is best accomplished by a skilled physician or therapist using muscle-length tests, strength tests, and tests for the efficiency of basic movement patterns and neuromuscular control. A thorough postural observation and video taping of the patient's gait will help in assessing and identifying any movement imbalances.
Preliminary stretches for shortened muscles should include proprioceptive neuromuscular facilitation (PNF) type or contract-relax stretches that strive for isometric contraction, followed by end-range stretching. These are effective techniques for maintaining muscle length and joint mobility. Myofascial Release Techniques when used in conjunction with stretching techniques, have shown great promise in restoring muscle length and soft-tissue extensibility. Patients can be taught to do their own self-mobilization with use of a foam roll.
Specific exercises for the patient should progress from mobility to stability, to reflexive motor patterning, to acquiring the skills of fundamental movement patterns, and finally, to progressive strengthening. These sequences may not be applicable to all patients; therefore, the key is to analyze the individual in each exercise category and then to tailor an exercise regimen that will best suit that patient's needs. For example, patients with iliotibial band syndrome often have weakness in their hip abductors that predisposes them to increased stress on the iliotibial bands. Thus, a preventative training program for patients with this syndrome must target the hip abductors, particularly the posterior aspect of the gluteus medius that assists external rotation or in decelerating internal rotation of the hip. Other muscles that prove weak or inhibited on evaluation should also be strengthened on a case-by-case basis.
Fundamental lumbo-pelvic stability
The purpose of basic core stabilization exercises is not only to increase stability, but more importantly it is to gain co-ordination and timing of the deep abdominal-wall musculature. It is extremely important to do these basic exercises correctly, as they are the foundation of all other core exercises and movement patterns. These basic exercises emphasize maintaining the lumbar spine in a neutral position.
This first stage of core stability training begins with the patient learning to stabilize the abdominal wall. Proper activation of these muscles is considered crucial in the first stages of a core stability program, before progressing to more dynamic and multi-planar activities.The exercise program should progress sequentially through the fundamental movements as detailed below. The following exercises are to be performed regularly to maximize results, you have to continue the basic pattern even you have mastered the advanced patterns. The patient begins with one to two sets of 15 repetitions and progresses to three sets of 15-20 repetitions. These exercises are taught initially in either a supine, hook-lying position. The patient can progress to the more functional standing exercises, as control is developed. Important concepts taught at this stage include not tilting the pelvis or flattening the spine. We also emphasize normal rhythmic breathing
To instruct your client to perform the Abdominal Hollowing technique follow the following directions:
1. Let your client lay down on the floor or on a treatment table in a supine hook-lying position.
2. Stand next to the client.
3. Ask the client to put both of their hand on their lower abdominal region at he level of their belly button.
4. With permission from the client put your hand on top of their hand as if covering their belly button.
5. Instructions: “Breath in and out normally for a few breaths until relaxed. To accomplish Abdominal Hollowing, take a deep breath then breath out completely, draw your belly button towards your spine at about 30% intensity – hold this position for 10 seconds. breath in, relax and repeat this maneuver.
6. Cues: “Try to suck in your belly button as if you walk on the beach in a bathing suit ”.
7. The pattern will be ‘breath in, breath out, abdominal hollowing (press down your belly button), relax and breath in’.
Supine Bent-Knee Raises
This is a fundamental exercise for recruiting the deep abdominal muscles and for lumbopelvic control. The patient lies on her back, with knees bent and feet flat on the floor. She then braces the abdominal wall, holding the lumbar spine in a neutral position as described above, and slowly raises one foot 15-30cm off the ground with alternate legs. Common errors when performing this exercise include rocking the pelvis, abdominal protrusion, or an inability to maintain the neutral lumbar curve. If this happens, discontinue the exercise for a rest period. Quality more than quantity is stressed.
Progression: The exercise can progress to alternately extending the legs and lowering to the ground. Once the patient can maintain stability with alternate leg lifts. She can add alternate, overhead arm raises for greater challenge. The arm raises should be performed slowly, while maintaining lower abdominal bracing.
Figure 1: Supine Bent-Knee Raises
Quadruped with Alternate Arm/Leg Raises
This exercise prepares the patient for the proprioceptively more challenging, more dynamic exercises of the trunk. It specifically engages the multifidi-the deep transverse spine stabilizer and extensor of the lumbar spine.
The patient should position herself on all fours. She then braces the abdominal wall as described above. While maintaining a midrange/neutral curve of the lumbar spine, the patient should raise the right arm and the left leg (opposite upper and lower limbs) into a line with the trunk, while preventing any rocking of the pelvis or spine (excessive transverse or coronal-plane motion). If it helps to maintain alignment, the patient may use an object, such as a foam roller or wooden dowel, placed along the spine, for added tactile feedback. The leg should be raised only to the height at which patient can control any excessive motion of the jumbo-pelvic region. She then performs the exercise raising the left arm with the right leg.
Progression: A Physioball underneath the trunk can provide significantly more proprioceptive challenge owing to its unstable surface. The goal once again is for the patient to maintain lumbar stability while the opposite arm and leg are raised slowly.
Figure 2: Quadruped with Alternate Arm/Leg Raises
Bridging is a fundamental core-stability and gluteal-strengthening exercise.
The patient begins the exercise on her back, in a hook-lying position, with arms resting at her sides. She activates the abdominals and squeezes the gluteal cheeks prior to initiating the movement. The patient lifts the pelvis and hips off the ground while maintaining neutral lumbar alignment. There should be no rotation of the pelvis. The hips should be aligned with the knees and shoulders in a straight line. The patient should hold the position for 10sec and then slowly lower the pelvis to the floor.
Progression: In the lifted-bridge position, while maintaining neutral lumbar and pelvic alignment. the patient can lift one foot off the ground and extend the leg. By placing her arms across her chest, she can increase the challenge of stabilising the lumbo-pelvic region. To progress further, the patient can raise both arms up to the ceiling and then move one arm out to the side. She should bring the arm back to the centre and repeat with the other side.
Figure 3: Bridging
This is a fundamental, static core-stability exercise.
The patient supports herself with her forearms resting on the mat, elbows bent at 90 degree, and the toes resting on the mat. The patient maintains the spine in a neutral position, recruits the gluteal muscles, and keeps the head level with the floor. She is instructed to breath normally throughout the exercise, while maintaining the abdominal brace. We suggest holding the position for 20sec, working up to one minute for two to three repetitions. No compensatory motion, such as increased lumbar lordosis or sag, should be seen.
Progression: In this position, the patient can add leg lifts for more difficulty: one leg can be lifted off the mat, held for five seconds, and then repeated on the opposite side.
This is a fundamental, static core-stability exercise designed to challenge the patient's body against gravity in the coronal/frontal plane and is an ideal exercise to train the quadratus lumborum.
The patient is lying on her right side with the right arm extended in a straight line up from the shoulder, with the forearm resting on the mat. She then raises the pelvis from the floor and holds it in a straight-line "plank" position. The hips should not be allowed to sag toward the floor. We suggest holding the position for 20sec, working up to one minute holds for two to three repetitions.
Progression: The top foot can be raised to increasingly challenge the core and gluteal musculature.
Figure 5: Side Plank
Advanced lumbo-pelvic stability
Once the patient demonstrates good stability with all static core exercises, they can be replaced with more advanced exercises on the Physioball detailed below. These exercises should be performed at least two times per week to maximize results. The patient progresses to two sets of 10-15 repetitions. Quality is more important than quantity; the patient must maintain lumbar neutral and keep the spine in perfect alignment throughout the exercises.
This exercise is more difficult because the patient positions her body against gravity in a seated position on an unstable surface.
The patient begins by sitting upright on a Physioball, with the lumbar spine in a neutral position. She places her feet hip-width apart While bracing the abdominal muscles, she lifts one leg and foot off the ground. (The limb does not need to be lifted very high, just enough to be off the ground approximately 5cm to start) The patient should focus on controlling the weight shifting to the weight-bearing limb while maintaining lumbo-pelvic stability.
Progression: Once lumbo-pelvic stability can be maintained with alternate leg lifts, the patient can add opposite arm lifts.
Spinal Flexion on Physioball (Advanced versions)
If the patient’s progression is very good and she is mastering all the techniques perfectly; you can transform the program into the next level. The patient pre-activates her abdominal brace in the starting position and maintains this as she rolls back into spinal extension. She then slowly raises the body, focusing the rotation in the thoracic spine. Picture the head and neck as a rigid block on the thoracic spine to prevent flexing the cervical spine. The patient concentrates on attempting to touch the bottom of her ribs to her pelvis (ASIS). The hands can be placed over the ears to eliminate pulling on the neck.
Progression: The patient holds a 2.0 to 4.0kg medicine ball in front of the chest with the arms extended (see Figure 7b).
Alternate Leg Bridge with Shoulders on Ball
The patient starts this exercise sitting on the Physioball and walking forward with his feet on the ground, slowly leaning back until his back rests on the ball. This is called the bridge position. The head, neck and shoulder blades should be supported on the ball. Knees should be bent at a 90° angle, with feet on the ground. While bracing the abdominal muscles, the patient raises the foot and extends the leg off the ground. The weight will be shifted to one side, and the patient should focus on maintaining stability of the lumbo-pelvic region. The patient should strive for stability and balance, while holding this position for 10sec and alternating lower limbs.
Progression: The patient lifts the arms up in the air or out to the sides.
Figure 8: Alternate Leg Bridge with Shoulders on Ball
The patient kneels behind the ball, with both hands on the ball. Keeping the abdominal muscles braced and lower back in a neutral position, she then rolls the ball away from her body a short distance until there is a straight line from the shoulder to hips. While maintaining alignment, she pulls the ball back a short distance, then pushes it away again. The movement should occur only at the shoulders, not the back
Progression: The patient can gradually straighten the body until she is up on her toes. There should be a straight line from the back of the head to the knees. Now she can again move the ball away and back toward the body a short distance with the arms.
Figure 9: Abdominal Rollout
Squat Ball Thrust
Keeping the abdominal muscles braced and lower back and shoulder blades in a neutral position, he patient uses her abdominal contraction to move the ball forward and back. Keep the spine in neutral alignment throughout the movement. If the exercise shown is too challenging, start with the shins instead of the toes on the ball.
Progression: The patient can perform the exercise with only one foot on the ball (see Figure 10b).
Development of balance and motor control
The following movements require reflexive control. The patient can establish this control using an unstable surface and taking advantage of the numerous proprioceptors in the soles of the feet, and by activating the neck muscles, which contribute greatly to postural regulation. This sensory-motor training is an attempt to provide the sub-cortex with a basis for movement that is progressively more challenging. It involves exercises that stimulate balance, coordination, precision and skill acquisition.
Various devices are useful to progressively challenge balance, including a balance board with a whole sphere underneath the board (which creates multi-planar instability) or a rocker-board with a curved surface underneath the board (which allows single-plane motion). Dynamic foam rollers are an inexpensive alternative to the boards that also can be used to challenge balance, proprioception, and stability. These include half-rollers and full-sized rollers. Two other items that are invaluable to challenge balance and core stability and aid proprioceptive training in the standing position are the Bosu Balance Trainer and the Dyna Disk (these can be used interchangeably.) The Bosu has two functional surfaces that integrate dynamic balance with sports-specific or functional training: the domed surface is convex, the other side is flat and can be used for less challenge. The Dyna Disk is an air-filled plastic disc that can be firmly inflated. It has a smaller diameter than the Bosu and can be used like the Bosu Trainer, as it creates an increased proprioceptive challenge to the patient while standing on it. The Dyna Disk is unstable and does not have a base like the Bosu trainer.
In this exercise, a rocker-board is used to challenge balance in the frontal plane of motion. Standing on the rocker-board with both feet in perfect postural alignment, the patient gently rocks forward and backward. (To maintain ideal posture, the patient can create an imaginary line through the joints of the ankle, knee, hip, and shoulder. The ear should align in a straight line with these joints, with no excessive extension [swayback] of the lumbar spine or anterior pelvic rotation.) While rocking, there should be no excess body movement in the coronal or transverse planes. This exercise should be performed for several minutes. The goal is to optimally align the spinal curves and lower extremities.
Progression: The patient can progress to a slight flexed-knee position, with fast and slow movements to stimulate the righting reflexes and balance reactions. She also can progress the stepping motion to the three axes of motion.
Figure 11: Forward/Backward Rocking
Single-Leg Balance-3 Planes
This next exercise progresses the patient to a single-leg stance. The rocker-board is used in the three planes of motion. This exercise also can be performed with a balance board, which is more demanding as it incorporates all planes of motion simultaneously. The patient takes one step forward while maintaining alignment and balance, controlling aberrant motion, mimicking a forward running motion. The goal is to maintain lumbo-pelvic alignment. The patient controls movement in the three planes of motions by placing her feet in various positions on the board. The patient then alternately steps forward and backward onto the rocker-board.
Progression: Once the patient achieves static stability and can remain stable while standing on the rocker board, she can add an accessory motion. The patient can swing the arm and the non-weight-bearing opposite leg (as though mimicking running). No excessive motion in the pelvis or lumbar spine should occur during the swing phase.
Figure 12: Single-leg Balance-3 Planes
Weight Transfers with Proper Alignment
The preceding exercise progresses to "falling" onto an unstable surface. Figure 13 shows a rocker-board and "falling" onto a circular balance board. Again, the emphasis is on spinal alignment from the head to the sacrum. The patient steps forward quickly and catches herself from falling over with a quick forward movement of the leg onto the board.
Functional movements require acceleration, deceleration, and dynamic stabilization. A functional exercise regimen includes single-leg drills, three-dimensional lunges, resistive diagonal patterns of the upper and lower extremities, and tri-planar movement sequences. Patients can progress through the three planes of motion by performing similar exercises on balance boards, the Dyna Disk or Bosu type trainers, as static trunk and core stability have been mastered. Once these exercises are performed at a high level, the therapist can be assured the patient has the necessary core stability to start plyometric drills.
This exercise provides a functional movement pattern that is similar to running. The exercise seeks to increase stability of the lower abdominal muscles while using a forward motion at the hip. The exercise is designed to develop sagittal-plane control. While balancing on one leg, the patient imitates a running motion. As the upper thigh is lifted forward in a running motion, she concentrates on maintaining the abdominal brace and lumbo-pelvic stability while avoiding excessive anterior or posterior pelvic rotation. The patient raises the opposite arm simultaneously into flexion, while maintaining postural alignment with an erect spine, allowing only the extremities to move.
Progression: Once the patient can maintain lumbar spine stability without effort, she can attach a pulley or resistive cord to the ankle to increase the challenge to the hip flexors.
The patient begins this exercise with a forward lunge. Again, the emphasis is on maintaining a neutral spine position and abdominal brace throughout the entire movement. As the patient steps forward, knee flexion of the forward leg is limited to 90°. The knee joint should be over the ankle joint and the patella aligned with the second toe. The lower part of the leg should be perpendicular to the ground, as shown in Figure 15.
Progression: Once strength and stability in the forward (sagittal) plane have been achieved, the patient can begin stepping out at oblique angles, creating a narrower lunge or a wider lunge in the coronal or transverse planes. The patient can also step out onto an unstable surface such as a Bosu Trainer or Dyna Disk, which vastly increase the proprioceptive and dynamic core-stability challenge.
This exercise is a continued progression of multi-directional lunges and must not be started until strength and stability in that exercise has been achieved.
This exercise utilises a sports cord to resist shoulder and hip flexion while doing Step-ups. The movement pattern is similar to the running gait. The patient's opposite arm and leg are resisted simultaneously to increase the strength and co-ordination of this movement pattern.
Figure 16: Resisted Alternate Arm/Leg Step-ups
Multi-Directional Resisted Alternate Arm/Leg Step-Ups
This is a continued progression of the previous exercise. Once strength and stability is achieved in the frontal plane of motion, the patient can begin stepping up at a 45°.
Figure 17: Multi-directional Resisted Alternate Arm/Leg Step-ups
Standing Pulley or Medicine Ball Rotation
This resistive, dynamic trunk pattern challenges the core with a rotational movement pattern while the patient maintains stability in the hips and pelvis. It requires strict bracing of the abdominal muscles and locking the rib cage and pelvis together to avoid unnecessary stress from torsion on the spine.
The patient stands with feet about shoulder-width apart and knees slightly bent. She activates the abdominal brace prior to the movement. It is important to emphasize postural alignment, with the scapulae retracted and depressed. The patient should maintain neutral spinal angles throughout the movement. Holding a straight-arm position (elbows extended) while grasping the pulley handle or medicine ball with both hands, the patient rotates the trunk by activating the abdominal obliques and spinal rotators. She concentrates on keeping the arms extended in front of the chest. It is important that the pelvis remains stable in the movement. Resistance is perpendicular to the body.
This exercise can be done in the same manner using a 2.0 to 4.0kg medicine ball. Progression: The patient can add diagonal motions with the pulley or medicine ball.
Figure 18: Standing Pulley of Medicine Ball Rotation
Forward Lunge with a Medicine Ball with Trunk Rotation
The purpose of this exercise is to challenge the trunk muscles with appropriate weight shift, balance, and control on one leg. It uses a resistive movement of the trunk with a lunge that demands a high level of lumbo- pelvic and lower extremity stability as the patient moves the ball in a diagonal pattern across the body.
The patient will need approximately 30m to complete this exercise. She stands upright, holding onto a 2.0 to 4.0kg medicine ball, with arms outstretched, perpendicular to the body. She steps forward with the medicine ball in front of her chest with the arms extended. Once the lunge portion is completed, she rotates the trunk by bringing the ball across her body towards the same side as the front leg and then returns the ball to midline as the next step is made. It is important that the knee joint on the step- ping limb does not come forward past the vertical angle relative to the ankle joint. The second toe is aligned perpendicular with the patella.
Figure 19: Forward Lunge with a Medicine Ball with Trunk Rotation
Standing Reverse Wood-Chop with a Medicine Ball
This exercise is a resistive diagonal pat- tern of the trunk that demands a high level of lumbo-pelvic stability and combines upper- and lower-chain integration as the ball is moved in a diagonal pattern across the body.
The patient stands, holding onto a 2.0 to 4.0kg medicine ball with both hands, with the feet approximately shoulder-width apart. While holding the arms in front of the body with elbows extended, the patient moves the ball from a lower position at the hip, raising it across the body to the opposite shoulder, simulating a wood-chopping motion. The motion is then reversed by starting at the lower knee position and bringing the ball diagonally across the body, ending overhead at the opposite shoulder. This exercise also can be performed with resistive cords or a pulley system simulating the same motions.
Progression: The patient can progress to standing on one leg, using the opposite arm to complete the motion.
This article is intended to provide an understanding of the importance of core musculature to low back pain patients and to offer exercises that will help them achieve desired stability, balance, and neuro- muscular control. It is highly recommended, however, that patients consult a skilled physiotherapist to address individual needs and maximize results from a program of this nature.
Bergmark, A. Stability of the lumbar spine: A study in mechanical engineering. Aeta Orthop. Scand. 230(Suppl.):20-24. 1989.
Cresswell, A.G., L. Oddsson, and A. Thorstensson. The influence of sudden perturbations on trunk muscle activity and intra-abdominal pressure while standing. Exp. Brain Res. 98:336-341. 1994.
Cresswell, A.G., and A. Thorstensson. Changes in intra-abdominal pressure, trunk muscle activation, and force during isokinetic lifting and lowering. Eur. J. Appl. Physiol. 68:315-321. 1994.
Johnson, P. Training the trunk in the athlete. Strength Cond. J. 24(1): 52-59. 2002.
Mcgill, S.M. Low back stability: From formal description to issues for performance and rehabilitation. Exere. Sport Sci. Rev. 29(1):26-31. 2001.
Mggill, S.M., S. Grenier, N. Kavcic, and J. Cholewicki. Coordination of muscle activity to assure stability of the lumbar spine. J. Electromyogr. Kinesiol. 13:353-359. 2003.
Panjabi, M.M. The stabilizing system of the spine. Part I. Function, dysfunction, adaptation, and enhancement. J. Spinal Disord. 5:383-389. 1992.
Panjabi, M.M. The stabilizing system of the spine. Part II. Neutral zone and instability hypothesis. J. Spinal Disord. 5:390-397. 1992.
San Francisco Spine Institute. Dynamic Lumbar Stabilization Program.
San Francisco: San Francisco Spine Institute, 1989.
Nitz, A.J., and D. Peck. Comparison of muscle spindle concentrations in large and small human epaxial muscles acting in parallel combinations. Am. Surg. 52:274. 1986.
Verstegen, M., & P. Williams. Core Performance. New York: Rodale, Inc., 2004.
Hodges, P.W., & C.A. Richardson. Inefficient muscular stabilization of the lumbar spine associated with low back pain: A motor control evaluation of transversus abdominis. Spine. 21:2640-2650. 1996.
Allison, G.T., P. Godfrey, and G. Robinson. EMG signal amplitude assessment during abdominal bracing and hollowing. J. Electromyogr. Kinesiol. 8:51–57. 1998.
Axler, C.T., and S.M. Mcgill. Low back loads over a variety of abdominal exercises: Searching for the safest abdominal challenge. Med. Sci. Sports Exerc. 29:804–811. 1997.
Barnes, D. What type of strength training do distance runner do or need? Modern Patient Coach 40:27–37. 2002.
Bartonietz, K., and D. Strange. The use of Swiss balls in athletic training—An effective combination of load and fun. New Stud. Athletics 13:35–41. 1998.
Behn, D.A., K. Anderson, and R.S. Curnew. Muscle force and activation under stable and unstable conditions. J. Strength Cond. Res. 16:416–422. 2002.
Carriere, B. The Swiss Ball. Theory, Basic Exercises and Clinical Applications. Berlin: Springer, 1998.
Check, P. Swiss ball exercises for swimming, soccer and basketball. Sports Coach 21:12–13. 1999.
Cram, J.R., and G.S. Kasman. Introduction to Surface Electromyography. Gaithersburg, MD: Aspen Publishers, Inc., 1998.
Cusi, M.F., C.J. Juska-Butel, D. Garlick, and G. Argyrous. Lumbopelvic stability and injury profile in rugby union players. N. Z. J. Sports Med. 29:14–18. 2001.
Franklin, B.A. ACSM Guidelines for Exercise Testing and Prescription. Philadelphia: Lippincott, Williams, and Wilkins, 2000.
National Institute of Occupational Safety and Health (NIOSH): National Occupation Research Agenda. Washington, D.C.: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, NIOSH; 1996.
Morse L, Owen D, Becker CE: Firefighter's health and safety. In Environmental and Occupational Medicine Edited by: Rom WN. Boston: Little Brown; 1992:197-1204.
Rivera F, Thompson D: Systematic reviews of injury- prevention strategies for occupational injuries. Am J Prev Med 2000, 18:1-3.
Firefighter Injuries for 2003. NFPA Journal 2004, 1:56-78.
U.S. Fire Administration. Topical fire research series 2001, 2:2-22.