Joints of Class I Levers

The mechanical advantage of this type of joint can be less or more than 1. If the joints are positioned properly, mechanical advantage is better; otherwise, the muscles must deal with a lower mechanical advantage, more physical work, and higher energy consumption. These factors cause muscle fatigue to build up more rapidly, with more physical stress in the joints and a higher likelihood of injury such as disc herniation or bone fracture.

Elbow

The elbow joint functions as both a class I and a class III lever, according to how it is used. It is a class I lever when the external force F acts on one side and the internal force F acts on the other (Fig. 4-2). The arm will be still if F X l = F X l, where l is

Figure 4-2 The elbow joint functions as both a class I and a class III lever, according to the function. The joint serves as a class I lever when the external force F acts on one side and the internal (muscle) force Fm acts on the other. The arm is held still if F x l = F x l, where l is the length of m m ' m c the lever arm generating internal force and l is the length of the lever arm being acted upon by the external force.

Figure 4-2 The elbow joint functions as both a class I and a class III lever, according to the function. The joint serves as a class I lever when the external force F acts on one side and the internal (muscle) force Fm acts on the other. The arm is held still if F x l = F x l, where l is the length of m m ' m c the lever arm generating internal force and l is the length of the lever arm being acted upon by the external force.

m the length of the lever arm generating internal force and l is the length of the lever arm being acted upon by the external force.

The ball will accelerate if F increases, or F X

Neck

The center of gravity of the head lies anterior to the atlanto-occipital joint (Fig. 4-3). In normal adult cervical posture, the momentum arm of the extensor muscles (l ) = 4 cm, and the momentum v extensor'

extensor muscles use approximately half the weight of the head to support normal head posture. If the head is positioned forward, l may reduce and extensor l ., will increase. In this head-forward posture, the weight extensor muscles have to use much more force to balance the head.

Lower Back

As Figure 4-4 shows, the traction force of the back muscles counterbalance the weight of the trunk. If the momentum arm of the body weight is three times longer than the momentum arm of the lower back muscles, these muscles have to use a force of three times the body weight to achieve this balance. In the act of bending forward, the lower back muscles must sustain a physical stress equal to several times body weight to maintain the posture.

Arthritis Joint Pain

Arthritis Joint Pain

Arthritis is a general term which is commonly associated with a number of painful conditions affecting the joints and bones. The term arthritis literally translates to joint inflammation.

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