Immobilization of the hand affects arm and shoulder postural control

Objectives: Neuromuscular consequences of limb immobilization are widely reported in literature, however, most papers describe changes in the motor pathways deserving muscles of the immobilized joint. Conversely, the present study investigates the effect of a short-term immobilization on the activation of both the prime mover and the associated postural muscles. It has been recently observed that when rapidly flexing the index finger, the forearm equilibrium is preserved thanks to the postural adjustments occurring in arm and shoulder muscles prior to the movement onset (APAs). These postural adjustments are excitatory in Triceps Brachii (TB) and inhibitory in Biceps Brachii (BB) and Anterior Deltoid (AD). In this study we tested if and how a 12 hours immobilization affects the APAs development in these muscles. Methods: Subjects (n=10) were sitting on a chair with the right arm along the trunk, the elbow flexed at 90° and the prone hand in axis with the forearm. Starting with the index finger extended, subjects performed a rapid flexion (about 5-7 cm), repeated every 4s for 120 times. The metacarpo-phalangeal and elbow joints angles were recorded, as well as the EMG activity from the prime mover Flexor Digitorum Superficialis as well as from BB, TB and AD. At the end of the session, the EMG electrodes were left in place and fingers and wrist joints immobilized by a cast which was removed 12 hours later. The whole experimental protocol was then repeated. Results: Short-term immobilization significantly reduced the excitatory APA in TB (-36.0%) and increased the inhibitory APAs in BB and AD (+19.4% and +30.3% respectively). The movement amplitude and duration, as well as the magnitude of the prime mover activation were instead unchanged. Conclusions: Following immobilization of a joint, even of brief duration, the overall motor impairment may be partly due to APAs modifications in muscles acting on other non-immobilized joints of the same limb.