It is important to define what is or is not a repetitive motion, because such activities are the primary cause of repetitive stress injuries (RSIs). In general, motions that are produced constantly and over a long period of time are considered repetitive. However, less constant motions produced over a shorter period of time yet with greater applied force also are considered repetitive motions. These motions can occur in almost any joint, but the limbs are most susceptible.
Joint Mechanics
The function of every joint in the body is to provide movement. Some movements are more pronounced than others, providing more amplitude or movement distance. Compare, for example, movement of the shoulder versus the jaw. Usually, the more amplitude a joint can produce, the greater the chances of pathology. While in theory repetitive motion disorders can occur with any joint, the condition occurs most often in particular joints, specifically those in the arms and legs.
Hand and Wrist
The hand by far is the body part most susceptible to repetitive motion injuries, since we use our hands in almost all daily activities. Repetitive motions can occur by wrist flexion and extension movements, such as in typing or using hand tools. The hand also can pronate and supinate, or flip over and back. Rapidly producing these motions as occurs in assembly line work and using manual screwdrivers also can produce repetitive stress injuries. Wrist abduction and adduction, or side-to-side hand motions as produced when hammering, also contribute to such injuries.
Fingers
Rapid finger flexion and extension motions, such as from typing and playing the piano, are considered a primary contributor to repetitive stress injuries. The muscles controlling these movements originate in the forearm. Their tendons, passing through the wrist and into the hand and fingers, can become injured anywhere along their length. Carpal tunnel syndrome and tendinitis are just two conditions associated with such finger movements. Text massaging has been implicated in the rapid rise in deQuervain's syndrome.
Elbow and Shoulder
The elbow can undergo rapid flexion and extension motions. Such activities often are attributed to sports, like tennis and golf, and are common contributors to repetitive stress injury. The elbow also can flex and extend slower but with much greater force, as might occur with weightlifting, which is another contributor to these injuries. The shoulder is more complex in its moments, because it is capable of three-dimensional movements. Any one of these movements performed on a repetitive basis can produce injury. This is why repetitive stress injuries often occur with golfers, baseball pitchers, weightlifters, and rock climbers.
Knee, Ankle and Foot
The joints of the leg and foot also undergo motions, which make them highly susceptible to repetitive stress injuries. Normally, legs and feet are used daily, and they often are stressed beyond normal limits. Movement of the ankle joint commonly produces repetitive stress injuries due to its high degree of mobility and the weight bearing forces it maintains. Conditions such as Achilles tendinitis are frequent and painful consequences of repetitively stressing the Achilles tendon by running, jogging or just excessive walking. Some believe plantar fasciitis also is a repetitive stress disorder.
