Ischemic block of the forearm abolishes finger movements but not their associated anticipatory postural adjustments

Abstract

Voluntary movement is known to induce postural perturbations that are counteracted by unconscious anticipatory postural adjustments (APAs). Thus, for every movement, two motor commands are dispatched: a voluntary command recruiting the prime mover and a postural command driving the APAs. These commands are classically thought to be separated; this study investigates whether they could be instead considered as two elements within the same motor program. We analyzed the APAs in biceps brachii, triceps brachii and anterior deltoid that stabilize the arm when briskly flexing the index finger (prime mover flexor digitorum superficialis). APAs and prime mover activation were recorded before, under and after ischemic block of the forearm. Ischemia paralyzed the prime mover, thus suppressing the finger movement and the ensuing postural perturbation. If the two commands had been separated, it would have been expected that after a few failed attempts to flex the index finger, the APAs were suppressed too, being purposeless without postural perturbation. APAs were still present under ischemia even after 60 movement trials. No significant changes were found in APA amplitude in biceps and triceps among different conditions, or in the average APA latency. Inhibitory APA in anterior deltoid was reduced but still present under ischemia. In addition, the pharmacologic block of the sole median nerve produced similar effects. APAs were instead almost abolished when applying a fixation point to the wrist. The observation that APAs remained tailored to the expected perturbation even when that perturbation did not occur supports the idea of a functionally unique motor command driving both the prime mover and the muscles of the APA chain.