Certain voluntary movements have a tendency to be accompanied by other involuntary responses called the associated, or synkinetic, movements. These are defined as automatic modifications of the attitude of certain parts of the body as a reflex response to the volitional motion of some other portion. Associated movements may be either physiological or pathological phenomenon. Pathologic associated movements are usually expressions of activity in paretic groups of muscles that are stimulated by active innervation of other groups.
There may be generalized (global) associated movements, symmetric (imitative, mirror) associated movements and coordinated associated movements. Most often they can be seen in patients with hemiparesis or hemiplegia of cerebral origin. Generalized associated movements tend to produce the characteristic position of the extremities in patient with hemipareis: the upper limb is held in a position of flexion of the fingers and wrist, and the elbow, and flexion and adduction at the shoulder (the paralysis of the extensors is more marked than that of the flexors). The lower extremity is held with extension at the hip and knee and plantar flexion at the ankle and toes, and with more marked paralysis of the flexors. These characteristics are increased with exertion. Straining and attempts to grip with the paretic hand may cause an increase in the spasticity, with increased flexion of the wrist, elbow, and shoulder. Involuntary movements such as yawning, coughing, and stretching may also increase the tonus and cause the affected arm to extend at the elbow, wrist, and fingers.
Symmetric associated movements are usually seen in the paretic limb when the opposite healthy one is forcibly moved. Thus, in squeezing the examiner's hand with the healthy hand the paretic hand is seen to flex.
Coordinated associated movements are characterized by a spread of response from one muscle or group of muscles to others. They alter the position of the part and lead to the adoption of new postures.
Figure 2.2. Somatotopical distribution of the central motoneurons (homunculus)
Figure 2.3. Babinski sign (D.Purves, D. Fitzpatrick et al., 2001)
