Changes in “intra-limb” anticipatory postural adjustments after a short-term immobilization of both wrist and fingers

Neuromuscular effects of limb immobilization are widely reported in the literature, however, most papers describe changes in the motor pathways deserving the prime movers 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 postural adjustments that occur 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 12h immobilization affects the APAs development. Subjects (n=7) were sitting on a chair with the right arm along the trunk, the elbow 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 and from the postural muscles (BB, TB and AD). At the end of the session, the EMG electrodes were left in place and the fingers and wrist joints immobilized by a cast which was removed 12 hours later. We then repeated the whole protocol. Short-term immobilization effectively reduced the excitatory APA in TB and increased the inhibitory APA in BB and AD. The movement amplitude and duration, as well as the magnitude of the prime mover activation were unchanged. We conclude that the overall motor impairment following immobilization of a joint may be partly due to APAs modifications in muscles acting on other non-immobilized joints of the same limb.