Reaction Forces Inside the Joints

The bones of the skeleton are essentially levers and each joint serves as a fulcrum (see Fig. 4-1).

Levator scapulae Semispinalis

Occipitoclavicular section of trapezius \

Figure 4-3 The cervical joint serves as a class I lever. The center of gravity of the head lies anterior to the atlanto-occipital joint. In normal adult cervical posture, the momentum arm of the extensor muscles (l ) = 4 cm, and the momentum

arm of the head's weight (lwejgh) = 2 cm. Thus the extensor muscles use approximately half the weight of the head to support the normal head posture.

Occipitoclavicular section of trapezius \

0.04m

W = weight of the head

MF = extensor muscle force

Figure 4-3 The cervical joint serves as a class I lever. The center of gravity of the head lies anterior to the atlanto-occipital joint. In normal adult cervical posture, the momentum arm of the extensor muscles (l ) = 4 cm, and the momentum

arm of the head's weight (lwejgh) = 2 cm. Thus the extensor muscles use approximately half the weight of the head to support the normal head posture.

Upper posterior

Upper posterior

Upper anterior

Upper anterior

0.05m n

0.15m

W = weight of the body (upper)

MF = lumbar muscle force = 3 W

Figure 4-4 The traction force of the back muscles counterbalances the weight of the trunk.

Most of the skeletal muscles of the human body operate in first- or third-class lever systems with mechanical advantages less than 1 as the tendons of the muscles that operate the joints usually are inserted close to the joints. As Figure 4-1 shows, the fulcrum of the lever system sustains the physical force equal to the sum of muscle force and loading force. For example, in Figure 4-4, if the upper body weighs W = 50 kg (110 lb), the lower back muscles have to exert F = 150 kg (330 lb) to support the standing in normal posture, and the reaction force Rj to support the normal posture in the lumbar joint Rj = 50 kg + 150 kg = 200 kg (440 lb). If low back pain occurs, the lower back muscles have to use much more force to support the posture, and the reaction force inside the lumbar joint is much higher. It is not surprising that an overloaded joint is more likely to be subsequently subject to wear and tear, resulting in osteoarthritis or other joint and muscle injuries. Therefore frequent de-stress treatment with dry needling technique introduced in this book is of great importance to maintain optimal physical performance and a career in sports.

Lever Shoulder Joint
Figure 4-5 During abduction, the shoulder joint function as a typical class III lever.

Support of Body Weight

The skeleton can maintain a posture that supports body function while accommodating large external forces, as in sports such as weight lifting or jumping. The bones of the lower vertebrae, the pelvic bones, and the bones of the lower extremities are larger than their counterparts in the upper body (upper vertebrae, bones of the shoulder, and bones of the upper extremities), in proportion to the amount of body weight that they bear. The muscles that move these bigger bones are correspondingly bigger and stronger than their upper body counterparts.

Upper Limb

The upper limb consists of the shoulder girdle, the humerus, the radius, the ulna, and the bones of the wrist and hand. The design and structure of the upper limbs enable people to grasp and manipulate objects and to perform many other actions.

Muscle

Muscles are the major powerhouse of motion in both daily life and sports. Techniques in molecular biology were among the first methods to be used for understanding the mechanisms of muscle contraction. The physiologic and pathologic mechanisms of muscle are a focus of both sports science and pain medicine. Many textbooks deal with these topics, but this book focuses on the physiologic mechanisms that are related to sports performance, injury prevention, and rehabilitation after injury.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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