Latency of antIcIpatory postural adjustments depends on the intended, not on the actual movement velocIty

Literature reports that the Anticipatory Postural Adjustments (APAs) preceding voluntary movements are programmed according to several task parameters, including movement velocity (for a review: Bouisset & Do, Neurophysiol Clin 2008). However, the linkage between APAs and velocity has been highlighted within single subjects, who were asked to perform several movements at different velocities; therefore, till now, it has been impossible to discern whether the key factor determining the APA latency was the intended movement velocity or the actual one. Aim of this study was to distinguish between the above two factors. We analyzed the APA chain that stabilizes the arm during a brisk flexion of the index finger (Caronni & Cavallari, Exp Brain Res 2009): such movement, driven by the prime mover Flexor Digitorum Superfcialis (FDS), is preceded by an APA chain in the upper limb, consisting of an excitatory burst in Triceps Brachii (TB) and in an almost contemporary inhibition in Biceps Brachii (BB) and Anterior Deltoid (AD). These APAs allow to counteract the elbow and shoulder flexion induced by the upward perturbation that the index finger movement causes on the metacarpophalangeal joint. Experiments were carried out in two groups of subjects: 1) 29 who composed our database from previous experiments and were asked to flex the finger “as fast as possible” (go-fast), but actually performed the movement with different speeds (from 238 to 1180°/s), and 2) ten new subjects who performed the go-fast movement at more than 500°/s and were then asked to go-slow at about 50% of their initial velocity, thus moving at 300 to 800°/s. Subjects sat on a chair with both arms along the body, elbows flexed at 90°, dominant hand prone, in axis with the forearm, and index-finger extended. Subjects produced a sequence of 30 finger flexions for each required velocity. Recorded variables were the index-finger movement and the rectified EMG from FDS, TB, BB and AD. For each sequence, the 30 traces were aligned on the FDS onset (taken as time zero) and averaged; then the latency of APAs in arm postural muscles was measured. No correlation between the APA latency and the actual movement speed was observed when all subjects had to go-fast (for all muscles: r² < 0.015, p > 0.50), while APAs delayed of about 20-25 ms were found in the ten new subjects when they had to go-slow (p < 0.001). Moreover, in the speed range between 300 and 800°/s, the APA latency depended only on movement instruction: subjects going fast showed APAs that were about 15-30 ms earlier than those of subjects going slow (p < 0.001). These data suggest a stronger role of the intended movement velocity versus the actual one in setting the timing of postural muscles recruitment with respect to the prime mover. Such linkage between the APA programming and the intended velocity of voluntary movement also strengthens the idea that the postural and prime mover muscles are driven by a “shared” motor command, as proposed by Bruttini et al. (Exp Brain Res 2014).