Intended rather than actual movement velocity determines the latency of anticipatory postural adjustments

Abstract

The literature reports that anticipatory postural adjustments (APAs) are programmed according to movement velocity. However, the linkage between APAs and velocity has been highlighted within single subjects who were asked to voluntarily change movement velocity; therefore, till now, it has been impossible to discern whether the key factor determining APA latency was the intended movement velocity or the actual one. Aim of this study was to distinguish between these two factors. We analyzed the APA chain that stabilizes the arm during a brisk index finger flexion in two groups of subjects: (1) 29 who composed our database from previous experiments and were asked to "go-as-fast-as-possible" (go-fast), but actually performed the movement with different speeds (238-1, 180°/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 300-800°/s. No correlation between APA latency and actual movement speed was observed when all subjects had to go-fast (p > 0.50), while delayed APAs 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 earlier APAs than those going slow (p < 0.001). These data suggest a stronger role of the intended movement velocity versus the actual one in modifying the timing of postural muscles recruitment with respect to the prime mover. These results also strengthen the idea of a shared postural and voluntary command within the same motor act.

The Organization and Control of Intra-Limb Anticipatory Postural Adjustments and Their Role in Movement Performance

“…Another proof that intra-limb APAs adapt to changes in the postural context within few trials comes from Bruttini et al ( 2014 ) who illustrated that when tapping with the right index-finger, intra-limb APAs were deeply reduced (Figures 2A,B ) if the right forearm was fixated at the wrist, i.e., closer to the voluntary moving segment. Moreover, Esposti et al ( 2015 ) confirmed that also the intra-limb APAs adapt to changes in movement speed (Figure 2C ).…”

“…Finally, Berrigan et al ( 2006 ) reported that when pointing is performed towards small targets (i.e., under high accuracy constraints) from an unstable position (i.e., standing vs. sitting), slowing movement speed represents a strategy to reduce the equilibrium disturbance and the associated APAs. However, the above results might simply be an outcome of the relationship between APAs and intended movement speed (Shiratori and Aruin, 2007 ; Esposti et al, 2015 ), given the well known inverse relation (Fitts, 1954 law) between target width and movement speed. In order to get rid of such possible bias, Caronni et al ( 2013 ) studied inter-limb APAs during an upper limb pointing movement toward a target of fixed dimension, while wearing (or not) prismatic lenses, which shift the binocular eye field and make the subject to miss the target (Redding et al, 2005 ).…”

“…The oneness of postural and voluntary command is also supported by the finding that the APA latency depends on the movement instruction, not on its actual velocity (Esposti et al, 2015 ). This conclusion was based on two observations.…”

“…Time 0 (vertical dashed line) refers to prime mover EMG onset. Reproduced from Esposti et al ( 2015 ), © Springer-Verlag Berlin Heidelberg 2014, with permission of Springer .…”

Transcranial direct current stimulation of SMA modulates anticipatory postural adjustments without affecting the primary movement

“…Our recent works provided evidence that APAs are programmed with the prime mover recruitment as a shared motor command (Caronni et al, 2013;Bruttini et al, 2014;Esposti et al, 2015). However the ability of the CNS to selectively adjust the APA to changes in the postural context, like adding or removing a fixation point, without affecting the prime mover recruitment (e.g.…”