Fixation chains between the upper limbs

Aim: to explore the features of the fixation chains between the upper limbs during abduction-adduction of one arm on the horizontal plane. Methods: 6 experimental subjects seated on a stool with the upper limbs in two different configurations: i) both arms flexed to horizontal and directed forward, elbows fully extended; ii) arms along the body, elbows at 90°, forearms pointing forward. Hand palms faced the midsagittal plane. Subjects performed series of impulsive adduction or abduction of the right arm or forearm in the horizontal plane, while the left hand touched a firm vertical support with its palmar or dorsal face. Movement of the right limb and ElectroMyoGram (EMG) from left and right Infraspinatus and Pectoralis Maior, and from left Extensor and Flexor Carpi Radialis were recorded. Results: In both arm configurations, synchronously with the activation of the right arm muscles, EMG changes were recorded in the muscles of the left limb. According to the contact of the left hand with the support (palmar or dorsal) and to the direction of the right (fore)arm movement (adduction or abduction) those muscles were excited that would produce a movement of the left limb which is mirror-symmetrical to the actual right limb movement, while muscles acting anti-symmetrically were inhibited. These EMG changes, featured as Anticipatory Postural Adjustments (APAs) realise a fixation chain which contrasts the trunk rotation reactive to the primary arm movement. Conclusion: the pattern of these postural activities would favour mirror-symmetrical oscillations of the (fore)arms in the horizontal plane and hinder. anti-symmetrical oscillations. It is therefore proposed that such APAs may justify the preference, extensively described in the literature, for mirror-symmetrical movements during coupled oscillations of the upper limbs.