INJURY PREVENTION & PERFORMANCE ENHANCEMENT

Monique Mokha, PhD, ATC, Report Editor

Core Stability, Part 1: Overview of the Concept Marisa A. Colston, PhD, ATC • The University of Tennessee at Chattanooga

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ow back injuries represent 10 to 15% of all athletic injuries.1 With over 30 million participants in organized and recreational sports in the United States, 2 a thorough understanding of normal core function, as well as core dysfunction and its remediation, will advance a clinician’s ability to promote optimal performance. The lumbo-pelvic-hip complex comprises the core.3 Think of this complex as a three-dimensional box that is formed by muscles and connective tissues Key Points (i.e., deep investing fascia, tendons, and Changes occur in specific core muscles folligaments) that attach lowing low back injury. to the underlying skelAll muscles of the abdomen and back are etal framework. The important contributors to core stability. diaphragm forms the ceiling of the box, Facet joint mechanoreceptors may play and the pelvic floor an important role in maintenance of spine muscles and proximal stability. attachments of the hip/thigh musculature form the base. The anterior and lateral sides of the box are formed by the abdominal musculature, and the posterior side is formed by the paraspinal muscles.4,5 These active and passive structures function to provide static and dynamic stability and provide a solid foundation for extremity movement.6 The purpose of this report is to provide an overview of research evidence pertaining to core function, core stability, and risk for injury to the lumbo-pelvic-hip complex.

Fundamental Concepts of Core Stability Pioneering research in the area of core was performed by Panjabi 7 and Bergmark. 8 Panjabi7 described three subsystems for stabilization of the spine: (a) passive osseoligamentous, (b) active muscular, and (c) neural control (Figure 1). The passive system does not generate spine motions, but it is dynamically active in monitoring spine position and produces reactive forces at end ranges of motion that resist spine motion.7 Some of the structures in the passive system incorporate abundant mechanoreceptors that relay sensory information to the central nervous system (CNS) concerning spine position and movement (i.e., facet joint capsules and ligaments of the vertebral column).9 The muscles and tendons of the active subsystem are the means by which forces

Figure 1  The three subsystems of Panjabi’s stabilization system.7

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are generated to maintain spine stability.7 The inherently unstable osseo-ligamentous system has been shown to buckle in vitro at loads less than 100 N (~ 20 lbs).10 The magnitude of muscle force generation is monitored by proprioceptors in the tendons (i.e., Golgi tendon organs) and muscles (i.e., muscle spindles), thereby linking the muscular system to the neural control subsystem.7 The neural control subsystem receives information from the specialized mechanoreceptors located in both the passive and active subsystems. The CNS controls movement and stability through both feedback and feed-forward motor control mechanisms.9 Feedback to the CNS that is generated by elongation of a ligament or joint capsule, and from muscle tension development, mediates adjustment of muscle activation patterns to meet the requirements for maintenance of spine stability. The CNS also activates muscles in a feed-forward manner (i.e., anticipatory) in preparation for impending spine movement or external load application (e.g., activation of transverse abdominis prior to initiation of movement).7 The stabilizing subsystems function in an integrated manner to meet the stability demands of the spine in response to changes in posture. In a healthy state, the spine segments are maintained within physiologic limits referred to as the neutral zone (i.e., positioning that requires minimal tension within pas-

sive structures to resist displacement).7,9 The overall stability of the spinal system will be affected when any one of the three subsystems fails to function in an optimal manner. Bergmark8 classified muscles as either global (i.e., producing trunk motion) or local (i.e., providing segmental stability) on the basis of muscle location in relation to the vertebral column (Table 1). The global muscles have greater mass and longer moment arms, which makes them capable of generating large forces. This group includes the large, superficial muscles (i.e., rectus abdominis, external oblique abdominis, erector spinae), which act as prime movers during dynamic activities and provide multisegmental stiffness over a wide range of motion.8 The local muscles are smaller and deeper and have shorter moment arms, which makes them better suited for generation of intersegmental stiffness that stabilizes adjacent vertebrae (i.e., multifidus, rotatores, interspinalis, intertransversalis).8 This classification system has been criticized for oversimplification of the function of specific muscles, which may lead some clinicians to assume that the local muscle group is more important than the global muscle group for maintenance of core stability. To the contrary, both muscle groups provide important contributions to core stability. A third category that could be added is the axial-appendicular force transfer group of muscles (Table 1). These muscles connect the

Table 1. Modification of Bergmark’s Muscle Classification System8

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upper extremities to the pectoral girdle and the lower extremities to the pelvic girdle, thereby transferring forces through kinetic chain during the performance of functional activities.9 Core muscles function in a manner that differs from that of extremity muscles. They stabilize spinal segments when stopping motion to a greater extent than initiation of motion. Core stability depends on both muscle capacity (i.e., endurance) and neuromuscular control.11 Like guy wires supporting a tall antenna, core muscles support the spinal column.12 Whereas the stiffness of the guy wires is constant, muscle stiffness can be continuously adjusted during activity. Muscle coactivation in the upper and lower extremities is integrated through the fascial system, which has been referred to as a “serape effect” (Figure 2).4 The anterior and middle layers of the thoraco-lumbar fascia surround the quadratus lumborum and the erector spinae muscle group.13 The posterior layer is ideally positioned to transfer tension through extensive attachments to both local and global muscles. This layer forms an intricate criss-crossing pattern of fibers that connect the latissimus dorsi on one side to the contralateral

gluteus maximus.13 This anatomic linkage creates a functional relationship between the core and the lower extremity through the portion of the gluteus maximus that attaches to the fascial system of the iliotibial tract.

Core Stability Defined There is no universally-accepted definition of core stability.5 Kibler et al3 defined core stability as “The ability to control the position and motion of the trunk over the pelvis and legs to allow optimum production, transfer, control of force and motion to the terminal segment in integrated kinetic chain activities.”3(p. 190) Core stability is greatly enhanced by cocontraction of antagonist muscle groups,3,14 with only relatively low levels of activation needed to adequately stabilize the spine.15,16 Adequate response to loading requires proper muscle recruitment, sufficient muscle force generation, and correct timing of the sequence of activation of various muscles. Because sudden loading is a common low back injury mechanism, conditioning for proper response to sudden imposition of spine loads is an important consideration for injury prevention.17 Muscle integrity may be normal, while endurance and neuromuscular coordination of movement are suboptimal.18 Muscle endurance has been shown to be a better predictor of low back problems than muscle strength.19-21 Following first-time occurrence of low back pain, transverse abdominis and multifidus muscle function is altered (Table 2).

Muscle Dysfunction Following Low Back Injury — Transverse Abdominis The transverse abdominis (TrA) muscle is an important contributor to core stability. When contracted, its horizontal fiber orientation decreases abdominal circumference and increases tension within the

Table 2. Muscle Dysfunction Following Low Back Injury

Figure 2  The ‘serape’ effect created by the superficial layer of the thoracolumbar fascia with the latissimus dorsi and contralateral gluteus maximus muscles.

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thoraco-lumbar fascia. Cocontraction of the TrA with the diaphragm and pelvic floor muscles contributes to the development of intraabdominal pressure (IAP),22,23 which increases trunk stiffness (i.e., resistance to displacement) and may decrease compressive load on the spine during exertion.11 Function of the TrA is altered when low back dysfunction exists, regardless of pathology (e.g., disc herniation, facet sprain, extensor muscle strain). In a healthy state, the TrA is activated before other muscles of the core or a moving extremity, regardless of the direction of extremity movement.24-26 This protective feed-forward response that normally occurs with all extremity movement is lost when low back dysfunction exists. Delayed and direction-specific activation of the TrA makes the spine vulnerable to excessive displacement of its segments during extremity movements.25,26 Normally, the TrA exhibits a low level of tonic activation during standing,26,27 which may contribute to prolonged maintenance of core stability.23 During high-velocity limb movement, the TrA responds with a burst of tension development at the initiation of the movement, which is followed by maintained activation at a lower tension level.23,28 When low back dysfunction exists, the TrA exhibits a low activation threshold during slow-velocity limb movement, tension is developed in phasic bursts, and its function is not well-coordinated with that of other trunk muscles.26,29,30 During high-velocity limb movements, post-injury TrA function is characterized by a higher activation threshold, which is similar to the responses of other abdominal muscles.31

of the superficial and deep muscle fascicles of the multifidus and their segmental innervation (i.e., dorsal ramus of the respective level spinal nerve)33 enhances the specificity of segment motion control in response to loading.36 Changes in multifidus function of low back pain patients include differing activation patterns37,38 and greater fatigability.39,40 Morphologic changes include selective atrophy of type II muscle fibers and change in the internal structure of type I fibers41 that have been shown to occur in as little as three weeks following injury.42,43 Furthermore, neural inhibition of the multifidus leads to atrophy,44 and fatty infiltration decreases the density of the muscle tissue.45

Muscle Inhibition Research involving porcine models has provided evidence that the facet joint has a regulatory function in maintenance of vertebral segment stability. Saline injection into the facet joint,46 and transection of the

Muscle Dysfunction Following Low Back Injury — Multifidus Similar postinjury changes occur in the multifidus muscle, which is most well-developed in the lumbar region of the spine (Figure 3). The multifidus provides a minor contribution to the total extensor moment of the lumbar spine (i.e., 20% versus 80% from the erector spinae muscle group),32 which is primarily due to its short moment arm. The multifidus has extensive direct attachments to the vertebrae, virtually smothering their posterior elements with its multisegmental fascicles.33 This anatomic relationship to adjacent vertebrae facilitates maintenance of segmental stability, particularly through control of small-amplitude displacements between vertebral segments that generate shear loads on spine structures.34,35 The segmental arrangement international journal of Athletic Therapy & training

Figure 3  The multifidus muscle in the lumbar region january 2012  11

medial branch of the dorsal ramus or disruption of the intervertebral disc,47 have been shown to produce a reflex inhibition of the lumbar multifidus muscle at the same segment level. Neural drive to the multifidus may be reduced by afferent input to the CNS that originates from the mechanoreceptors in the intervertebral disc or facet joint capsule, which results in a reduction of alpha motoneuron excitability. Greater understanding of CNS processing of afferent signals derived from mechanoreceptors in the facet joint, intervertebral disc, and multifidus muscle may be a key to advancing management of low back dysfunction. Despite clinical recovery from low back injury (i.e., complete resolution of symptoms), pathologic changes in the multifidus muscle may persist.17,48,49 Therefore, pain level and general performance capabilities should not be the only factors considered in making a decision about return to participation in high-demand activities. Performance capabilities of the core musculature should be evaluated through static or dynamic tests (e.g., back extension, trunk flexion, side planks, etc.). Ideally, such tests should be administered prior to the beginning of a sport season to identify athletes that may possess elevated risk for injury. Individuals identified as having high-risk status can be provided with a customized program for remediation of performance deficiencies. A critical fact to remember is that a single episode of low back pain can result in long-lasting changes in trunk motor control, even when the individual is capable of returning to a prior level of competition. Because most acute low back pain episodes are self-limiting, inadequate management of altered neuromuscular control capabilities may leave the individual highly vulnerable to future low back injury. This phenomenon may explain the high recurrence rate for low back pain following a first-time episode.

Muscle Responsibility for Stability Despite the emphasis on the TrA and multifidus muscles, the evidence clearly indicates that no single muscle, or group of muscles, is the dominant contributor to spinal stability.16,50,51 The changes that have been documented to occur in the abdominal and paraspinal muscles following injury, however, must be addressed by a rehabilitation program, and failure to do so would not be consistent with evidence-based practice. Fortunately, proper rehabilitation can reverse the pathologic changes. Spine stability requires rapid 12  january 2012

muscle response to continually changing external forces. The predominant muscle that provides stability during the performance of one task may contribute less during the performance of a different task. Thus, stability depends on contributions from numerous core muscles, all of which must be addressed by a conditioning or rehabilitation program.50-52 Conditioning should involve multi-planar movement patterns that incorporate many muscles of the lumbo-pelvic-hip complex.52 Pelvic girdle muscles that attach distally in the thigh should not be neglected, due to the functional relationship between the core and lower extremity that has been linked to injury risk.53

Summary Core stability is a complex, multifactorial concept that relies on the integration of osseo-ligamentous, muscular, and neural control subsystems. Deficient function of any one of these subsystems can lead to reliance on compensatory mechanisms that may not adequately preserve spine stability. The changes that have been documented to occur in core muscles following low back injury must be addressed to minimize susceptibility to injury recurrence. Part 2 of this series on core stability will review research evidence that has linked core muscle function to low back and extremity injury risk, and strategies for injury prevention will also be presented. Simple core muscle endurance tests can identify athletes who possess elevated injury risk, and such tests can also be used to guide return-to-play decisions. 

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52. Zazulak BT, Hewett TE, Reeves NP, Goldberg B, Cholewicki J. Deficits in neuromuscular control of the trunk predict knee injury risk: a prospective biomechanical-epidemiologic study. Am J Sports Med. 2007;35:1123-1130.

30. Hodges PW, Richardson CA Altered trunk muscle recruitment in people with low back pain with limb movement at different speeds. Arch Phys Med Rehabil. 1999;80:1005-1012. 31. Hodges PW, Richardson CA. Delayed postural contraction of transversus abdominis in low back pain associated with movement of the lower limb. J Spinal Disord. 1998;11:46-56. 32. Bogduk N, Macintosh JE, Pearcy MJ. A universal model of the lumbar back muscles in upright position. Spine. 1992;17:897-913.

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53. Hewett TE, Myer GD, Ford KR, et al. Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study. Am J Sports Med. 2005;33:492-501.

Marisa Colston is with the Health and Human Performance Department at the University of Tennessee at Chattanooga.

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