INTRODUCTION AND DEFINITION

This chapter defines the CORE, identifies the anatomical structures within the CORE, and discusses the pathophysiology of muscle imbalances. The evaluation of the CORE and strength training concepts are discussed in Chapter 12.

For the purposes of this chapter the CORE is defined as a clinical manifestation in which a delicate balance of movement and stability occurs simultaneously. The author of this chapter describes the upper quadrant CORE to include the glenohumeral joint and the scapulothoracic joint and the lower quadrant CORE to include the hip and trunk. This chapter discusses the function and special considerations of the muscles of the CORE. In addition, muscle imbalances and dysfunction of the CORE are discussed.

Speed is allegedly an innate talent and cannot be changed with training. However, muscle imbalances, joint restrictions, and pain may prevent the athlete from achieving his or her maximum innate abilities. The job of the sports-specific rehabilitation specialist is to identify deficits within the CORE and design a rehabilitation program to promote an increase in strength, power, and endurance specific to the muscles and joints that are in a state of dysfunction. Specificity of the rehabilitation program can help the athlete overcome musculoskeletal system deficits and achieve maximum potentials of his or her talents. A combination of power, strength, and endurance is critical for the muscles of the CORE to allow the athlete to perform at his or her maximum capabilities.

The lower quadrant CORE is identified by the muscles, ligaments, and fascia that produce a synchronous motion and stability of the trunk, hip, and lower extremities. The initiation of movement in the lower limb is a result of activation of certain muscles that hold onto bone, referred to as stabilizers, and other muscles that move bone, referred to as mobilizers. The muscle action within the CORE depends on a balanced activity of the stabilizers and mobilizers. If the stabilizers do not hold onto the bone, the mobilizing muscles will function at a disadvantage. The lack of harmony between the stabilizers and the mobilizers can result in muscle imbalances and injury.

Thirty-five muscles attach directly to the sacrum or innominate, or both. This chapter is not intended to describe the detailed anatomy of each of these muscles but rather to highlight the muscle groups and their specific application to movement and stabilization. In addition, the chapter presents evidence of how the muscle groups within the trunk and hip CORE are important to the athletes’ performance and prevention of injuries. The CORE trunk muscles include the abdominals, thoracolumbar, lumbar, and lateral thoracolumbar muscles. The CORE hip muscles include the hip flexors, extensors, adductors, abductors, and internal and external rotators.

CORE MUSCLES OF THE TRUNK AND HIP

The CORE muscles of the trunk include the thoracolumbar muscles (longissimus thoracic pars thoracis, and the iliocostalis lumborum pars thoracis), the lumbar muscles (lumbar multifidus, iliocostalis lumborum pars lumborum, longissimus thoracic pars lumborum, intertransversarii, interspinalis and rotatores), the lateral thoracolumbar muscle, the quadratus lumborum, and the abdominal muscles (the transverses abdominis, rectus abdominis, internal and external oblique abdominals). Although the thoracodorsal fascia is not a contractile tissue, it does enhance CORE trunk stability as a result of the contraction of several trunk muscles attached to it.

The CORE hip muscles include psoas, iliacus, gluteus maximus, gluteus medius (anterior and posterior fibers), rectus femoris, and the hamstring muscle group. The external and internal rotators of the hip include a large group of muscles. These muscles are important to the hip and trunk for movement and stability. Lack of strength and power of the muscles noted as follows can contribute to injury of the lower extremity or reduced performance, or both.

FUNCTIONAL ANATOMY OF THE MUSCLES OF THE TRUNK AND HIP

This section discusses the specific function of individual muscle groups within the CORE and their importance to performance in the athlete. Muscle is the body’s best force attenuator. Eccentric control of rapid movement is critical for performance enhancement and prevention of injury. In addition, muscles create forces that play a role in the production of movement and in stabilizing of joints for safety and performance.

The physiological cross-sectional area (PCSA) of muscle determines the force-producing potential, while the line of pull and moment arm determine the effect of the force on movement and stabilization.¹ The small muscle of the thoracic and lumbar spine includes the intertransversarii, interspinales, and rotatores. These muscles have small cross-sectional areas and work through a small moment arm.¹ Their total contribution to rotational axial twisting and bending torque is minimal. Bogduk² and McGill¹ hypothesized that these small muscles may not predominate as mechanical stabilizers but instead have a proprioceptive role. The rotatores and intertransversarii muscles are highly rich in muscle spindles, 4.5 to 7.3 times more than the multifidus.³ Muscle spindles are the proprioceptors of muscle. These receptors are stimulated by stretch.

The major extensors of the thoracolumbar spine are the longissimus, iliocostalis, and multifidus groups. According to Bogduk⁴ and McGill and Norman,⁵ the longissimus and iliocostalis are divided into lumbar and thoracic portions, longissimus thoracis pars lumborum and pars thoracic, and iliocostalis lumborum pars lumborum and pars thoracis (Figure 9-1, A). The pars thoracis component of these two muscles attaches to the ribs and vertebral components and has relatively short contractile fibers with long tendons that run parallel to the spine attaching to the sacrum and iliac crest. These muscles have the greatest extensor moment with a minimum of compression to the spine.¹ The lumbar components of these muscle groups have a line of pull that is not parallel to the spine but rather have a posterior and caudal direction that causes them to generate posterior shear and an extensor moment to the spine.¹ The multifidus muscles have a low density of muscle spindles and are involved in producing extensor torque with small amounts of twisting and side-bending torque.¹ The lumbar multifidus muscles span two to three spinal segments. Therefore their forces affect only local areas of the spine. The multifidus is a good example of a muscle that stiffens the spine, acting as a stabilizer (Figure 9-1, B) (Table 9-1).

Figure 9-1 A, Longissimus and Iliocostalis muscles. B, Multifidus muscle.

(A From Willard FH: The muscular, ligamentous and neural structure of the low back and its relation to back pain. In Vleeming A, Mooney V, Dorman T, et al, editors: Movement, stability and low back pain, Edinburgh, 1997, Churchill Livingstone. B From Lee D: The pelvic girdle: an approach to the examination and treatment of the lumbopelvic-hip region, ed 3, Edinburgh, 2004, Churchill Livingstone. Courtesy Gracovetsky personal library.)

Table 9-1 CORE Muscle Tables

* Lumbar multifidus and transversus abdominis are local trunk stabilizers.