The shoulder complex is composed of three bones: (a) the clavicle (collarbone), (b) the scapula (shoulder blade), and (c) the humerus (arm bone). These bones, together with the scapular and shoulder muscles and connective tissue, perform the complicated task of allowing dynamic stability. The shoulder complex has a larger range of motion than any other part of the body. At the same time, it has to provide a stable base for the rest of the upper extremity so the hand can perform fine motor skills.
The clavicles and the scapulae form the shoulder or pectoral girdle. The anterior portion of the pectoral girdle is formed by the clavicles while the posterior portion is comprised of the scapulae. The proximal end of a clavicle and the sternum (breast bone) form the sternoclavicular joint. This is the only bony attachment for the upper extremity to the axial skeleton. Muscles secure the shoulder girdle to the rest of the body.
The lateral portion of the scapula forms a small cup-like projection that is called the glenoid fossa (Gr. glene, a socket; L. fossa, trench or ditch). The head of the humerus (arm bone) articulates with the glenoid fossa and forms the glenohumeral joint. As a spheroidal or ball-and-socket joint, the glenohumeral joint provides a great deal of mobility to the upper extremity. However, the scapula can also move to further increase the available range of motion for the upper limb.
While the scapula moves along the thorax (rib cage), it is not a true anatomic joint because it has none of the characteristics of a joint. The movements of the scapula, however, are commonly described as motions of the scapulothoracic joint. The motions of the scapula include elevation/depression, abduction/adduction, and upward rotation/downward rotation (Figures 7-9A, B, C,D).
The dynamic stability of the shoulder complex can be demonstrated in the motions necessary to change a light bulb in the ceiling. In order to raise the hand over the head, the glenohumeral joint has to flex while the scapula rotates in an upward direction. Once the hand is in position, the muscles of the pectoral girdle have to stabilize the entire shoulder complex so the hand can unscrew the light bulb.
The spinal column.
Because most motions of the shoulder complex involve both the scapulothoracic joint and the glenohumeral joint, a complete exercise program should include exercises for muscles that move the scapula and muscles that move the humerus. Table 7-4 shows the major muscles of the pectoral girdle and the glenohumeral joint.
The Vertebral Column
Figure 7-10 shows that the vertebral column is typically composed of 26 vertebrae: (a) 7 cervical (neck), (b) 12 thoracic (where the ribs attach), (c) 5 lumbar (lower back), (d) 5 sacral (fused to form the sacrum), and (e) 4 coccygeal (fused to form the coccyx or tailbone). The majority of the vertebrae have a body, a spinous process in the back, and transverse processes on either side. Behind the body is an opening or foramen through which the spinal cord passes. The spinal cord conducts afferent, or sensory, messages from the body up to the brain and efferent, or motor signals, down from the brain to various parts of the body. Certain afferent signals trigger specific motor responses in the spinal cord without going to the brain. These automatic responses that do not require our conscious thought are known as spinal reflexes.
Located between adjacent vertebral bodies to form the cartilagenous joints are the intervertebral discs. The discs have a soft, gel-like center called the nucleus pulposus surrounded by the annulus fibrosus (L. annulus, ring; fibrosus, composed of fibers). While the nucleus pulposus is usually found in the center of a disc, it is located more posteriorly in the cervical and lumbar regions. While each joint has limited movement, the intervertebral joints together contribute to the motions of the entire spinal column.
As a major portion of the axial skeleton, it is important that the vertebral column be properly aligned to protect the spinal cord, to bear the weight of the body and to provide, with the ribs, a framework for the attachment of our internal organs. There are four distinct curves in the normal vertebral column. Two of them, the cervical curve and the lumbar curve, are concave posteriorly (open to the back of the body). The thoracic and sacral curves are concave anteriorly. The purpose of these curves is to provide flexibility and shock-absorbing capacity to the spinal column. The spinal curves allow the vertebral column to handle axial compressive loads up to ten times greater than what could be expected from a straight spine. That is why proper alignment through the trunk is important during all of our regular activities including exercising.
Several structures work together to help maintain proper alignment of the spine. The shape of the vertebral bodies and the intervertebral discs is primarily responsible for the four spinal curves. The muscles that can affect the curves of the spine should be balanced in strength and flexibility. Many ligaments also provide stability throughout the length of the vertebral column (Figure 7-11).
Along the front of the vertebral bodies is the anterior longitudinal ligament, which is considered by some researchers to be the strongest ligament in the body. Along with the spinal muscles, the anterior longitudinal ligament limits spinal hyperextension and provides strength to the anterior portion of the intervertebral disc during lifting activities. The posterior longitudinal ligament reinforces the posterior annulus fibrosus and runs along the back side of the vertebral bodies. However, it is much narrower than the anterior longitudinal ligament and only attaches to the margins of the vertebral bodies to allow the blood vessels and lymph vessels to enter and exit the vertebrae. In the lumbar region, it is often narrowed to a cord-shaped filament and gives reduced support to the intervertebral discs.
In the lumbar region, another posterior stabilizing ligament, the interspinous ligament, has been found to be weak or ruptured in 90% of the subjects studied who were over 40 years of age. The posterior longitudinal ligament, along with other ligaments and spinal muscles, such as the erector spinae, limit the degree of forward flexion.
In the United States, some studies have found that 80% of the general population will suffer from back pain. Back pain is currently the most expensive ailment among people in the 30-60-year-old category. While many disease processes can produce low-back pain, the primary causes of low-back pain appear to be poor posture, faulty body mechanics, stress, decreased flexibility, and poor physical fitness. Examples of poor posture include sitting in a slumped posture, leaning forward while standing, and standing with an excessive lumbar curve. Faulty body mechanics include one of the most common mechanisms for back injuries: bending forward and lifting while twisting.
All of these factors do not allow the vertebral column to maintain its natural spinal curves. Therefore, the flexibility and strength of the spine is compromised in these situations. While muscle contractions or spasms are commonly associated with low-back pain, some professionals do not believe that muscle spasms are the primary cause of the pain. These people believe that the back muscles tighten to prevent further movements that may aggravate the pain. Poor posture and faulty body mechanics can lead to back pain by causing intervertebral disc protrusions or herniations. Because the nucleus pulposus is a gel-like substance, it can protrude through the annulus fibrosus and irritate the spinal nerves as they exit the vertebral column.
The higher incidence of intervertebral disc problems in the lumbar area may be due to the posterior position of the nucleus pulposus and the decreased strength of some of the posterior ligaments in this area. Both of these factors increase the opportunity for an intervertebral disc to bulge or herniate posteriorly. When a disc protrudes posteriorly, it can cause many different signs and symptoms, including low-back pain, if it puts pressure on the spinal cord or spinal nerves in the area.
There is considerable controversy among health professionals about how much muscle weakness contributes to low-back pain. Some people feel that the erector spinae, as well as abdominal muscles and other spinal muscles, should be strengthened; others feel that muscle endurance is more important than strength; and still another group believes that there is not a clear cut relationship between muscle strength and endurance and low-back pain. It is also important to look at the other muscles that can affect increased lordosis the lumbar curve. Near the bottom of the vertebral column is the sacrum, which is wedged between the two ileum (the posterior portions of the pelvic girdle). It is at this junction, the sacroiliac (SI) joint, that the weight of the head, upper body, and trunk is transferred to the pelvis and then the lower extremities.
Muscles that can directly change the position of the lumbar spine or indirectly affect the spine by changing the position of the pelvic girdle include the rectus abdominus, the erector spinae, the iliopsoas, the rectus femoris (part of the quadriceps), and the hamstrings. These muscles need adequate strength and flexibility to maintain the pelvis in neutral alignment. Too much of an anterior tilt of the pelvis will lead to an increased lordotic curve (Figure 7- 12A), while a posterior tilt (Figure 7-12B) can eliminate the normal curve of the lumbar spine (Figure 7-12C).
Either extreme can be a contributing factor to loback pain. Weak abdominals and hamstrings together with tight erector spinae and iliopsoas can cause an increased lordosis. Tight hamstrings with weak hip flexors and erector spinae can lead to a posterior tilt and lack of a normal lumbar curve. For muscle balancing, the tight muscles should be stretched and the weak muscles should be strengthened. AFAA’s Basic Exercise Standards and Guidelines provides several recommendations for strengthening the erector spinae and stabilizing the torso safely and effectively.
Abnormal curvatures of the spine.
The recommendations are designed for participants with normal, healthy backs. Instructors may wish to refer people with a predisposition for back problems to a licensed health care provider for more specific exercise prescriptions.
Abnormal curvatures of the spine, shown in Figure 7-13, include scoliosis, kyphosis, and lordosis. Scoliosis (Gr. skolios, twisted) is the most common of the three conditions and is a lateral bending of the spine. Kyphosis (Gr. kyphos, a hump) refers to an exaggerated curve in the thoracic area. Lordosis (Gr. lordos, bent backward) is an increased concave curve in the lumbar portion of the spine. This condition is often accompanied by an increased anterior tilt of the pelvis.
Core strengthening exercises are normally designed to increase the stability of the axial skeleton. Specialty programs like this require a deeper understanding of additional core muscles known as the pelvic floor muscles or the pelvic rotator cuff. These muscles work with the abdominal and spinal muscles to hold or move the trunk. Exercises that push the pelvic floor muscles downward or create a muscular imbalance among the core muscles can lead to pelvic or low-back pain, incontinence, and problems with balance. While considered a women’s issue, both men and women can be affected by these conditions. If your participants have complaints of this type, refer them to a licensed professional with training in treating pelvic floor conditions.
The hip is a spheroidal, or ball and socket, joint like the glenohumeral joint. The joint is formed where the pelvic girdle meets the femur (thigh bone) (Figure 7-14). The pelvic girdle is composed of the two coxa or innominate (L. innominatus, nameless) bones. The innominate bones are united anteriorly at a fibrocartilagenous joint, the symphysis pubis. Posteriorly, the coxa bones meet the sacrum to form the sacroiliac joint. Each coxa bone is constructed from three fused bones: (a) the ilium, (b) the ischium, and (c) the pubis. The three bones meet and form a socket known as the acetabulum. This is where the femur articulates with the pelvis. While they are both ball and socket joints, the hip joint is more stable than the glenohumeral joint. The acetabulum forms a deeper socket than the glenoid cavity and the head of the femur forms a more complete sphere than the humeral head. Several strong ligaments add to further stability at the hip joint. The hips are the transition area where the weight of the body is transferred to the legs.
Movements of the hip include flexion/extension, abduction/adduction, and medial/lateral rotation. The primary muscle that flexes the hip joint is the iliopsoas assisted by the rectus femoris, a portion of the quadriceps that also crosses the hip. The hip extensors include the hamstrings and the gluteus maximus. The primary mover for hip abduction is the gluteus medius assisted by the tensor fasciae latae. The hip adductors include several different muscles, only one of which crosses the knee, the gracilis. As previously discussed, muscles that cross the hip joint may also lead to changes in the lumbar spine due to their attachments on the spine or their ability to affect changes through the pelvis.
The knee has two distinct purposes: (a) to provide stability in activities such as standing, and (b) to allow for mobility in movements such as sitting or squatting. The knee is actually two separate joints-the tibiofemoral joint (where the thigh bone meets the shin bone) and the patellofemoral joint (where the kneecap meets the thigh bone). The fibula, which is the bone at the lateral aspect of the lower leg, is not considered a part of the knee since it is not included in the joint capsule of the knee. Because the femur (thigh bone) and the tibia (shin bone) do not fit together particularly well, they are said to be somewhat incongruent. To increase the area of contact between the two bones, there are the two fibrocartilagenous menisci on the superior surface of the tibia. Their purpose is to (a) serve as shock absorbers, (b) help lubricate and give nutrition to the knee, (c) decrease the friction, and (d) increase the area of contact between the femur and the tibia.
When looking at the leg anteriorly, you can see that the femur and the tibia do not form a straight line. The angle formed by these two bones is known as the Q-angle (Figure 7-15). To measure the angle, two lines should be drawn: (a) from the anterior superior iliac spine of the pelvis to the midpoint of the patella, and (b) from the tibial tuberosity to the midpoint of the patella. Figure 7-15 shows a normal Q-angle, which is approximately 15°.
Several ligaments and muscles that cross the knee joint also help provide stability. On either side of the knee are the medial collateral ligament and the lateral collateral ligament. Within the joint are the anterior cruciate ligament and the posterior cruciate ligament. The main muscles crossing the knee joint are the quadriceps (anteriorly) and the hamstrings (posteriorly). The quadriceps muscle is composed of four parts: (a) the rectus femoris, (b) vastus lateralis, (c) vastus medialis, and (d) vastus intermedius. The rectus femoris (L. rectus, straight) runs down the front of the thigh and crosses both the hip and knee joints. As a biarticular muscle, the rectus femoris can flex the hip or extend the knee. The three vasti (L. vastus, large) muscles attach to the femur (thigh bone) and form the common quadriceps tendon that attaches at the tibial tuberosity (on the shin bone). Embedded within the quadriceps tendon is the patella (L. patera, little plate), the body’s largest sesamoid bone. Sesamoid bones are found in tendons, and they serve as a pulley and protect the tendon from excessive wear and tear. The main purpose of the patella is to increase the effective strength of the quadriceps by increasing its leverage or mechanical advantage.
The patella fits into the femur along the patellofemoral groove. The vastus lateralis, along with other structures, tends to cause the patella to track laterally during knee extension. A small, oblique portion of the vastus medialis appears to offset the lateral tracking to keep the patella centered in the patellofemoral groove. A weak vastus medialis oblique can lead to abnormal tracking of the patella in the patellofemoral groove and result in irritation of the lateral aspect of the patella. The entire quadriceps muscle is worked in exercise programs that include standing, squatting, and stepping. It works concentrically to extend the knee from a squat position or to step up on a bench.
The quadriceps muscle works eccentrically when lowering into a squat or stepping off of a bench. In both cases, all aspects of the quadriceps are working in a closed kinetic chain to support the weight of the body against gravity. It is important to balance the strength of the quadriceps with its opposing muscle group, the hamstrings. In an exercise program without additional equipment, it is difficult to strengthen the hamstrings as well as the quadriceps.
All three of the hamstring muscles cross the hip joint and help support the weight of the body against gravity in squats and stepping by concentrically extending the hips in a step up and eccentrically allowing the hips to flex in a squat. However, the short head of the biceps femoris does not cross the hip joint. Because it only crosses the knee joint, it is necessary to perform knee flexion against the pull of gravity to work this portion of the hamstring. Because this will be an open kinetic chain, you will only be lifting the weight of the lower leg against gravity rather than the trunk and upper extremities.
Research indicates that the trunk and upper extremities are about 60% of total body weight while the lower leg is only about 6%. Assuming that in a squat both legs work symmetrically, each quadricep supports about 45 pounds; in a hamstring curl, the weight lifted is only about 9 pounds in an individual weighing 150 pounds. Because of the difference in resistance provided by body weight alone, knee flexion with additional external resistance would help balance the strength of the quadriceps and hamstrings.
Stability and Mobility
Moving the body is one of the most basic components of exercise. Movement requires that some joints remain stable while others are mobile. As a fitness professional, ensure that your participants understand the difference between stability and mobility. When they perform their exercises accurately, they decrease their risk of injury.
When using equipment, verify that the equipment fits the participant properly and is used correctly. For example, many weight training machines have specific instructions about how to align the participant’s moving joint with the pivot point of the machine. By following the manufacturer’s guidelines, you help ensure that non-moving joints are stabilized while mobile joints have the range of motion needed to perform effectively.
Various exercise programs prioritize joint stability and mobility differently. In performance sports, the actual performance is given more weight than the individual’s musculoskeletal system. In core strengthening programs, the focus is on the stabilizing joints rather than on the moving joints. In aerobic programming, the priority is the individual’s cardiac response. Giving different priorities to the body’s musculoskeletal system is part of the diversity that the body needs to respond well to different situations.
By having a general understanding of anatomy and kinesiology, fitness professionals can plan effective exercise programs for their participants. It is easy to see why it is important that opposing muscle groups be balanced in strength and flexibility. Being able to understand how muscles work and the factors that affect their efficiency allows fitness professionals to design safe and effective muscle strengthening and endurance activities.