Preparation of Anticipatory Postural Adjustments Prior to Stepping

Abstract

.Step initiation involves anticipatory postural adjustments (APAs) that propel the body mass forward and laterally before the first step. This study used a startle-like acoustic stimulus (SAS) and transcranial magnetic stimulation (TMS) to examine the preparation of APAs before forward stepping. After an instructed delay period, subjects initiated forward steps in reaction to a visual "go" cue. TMS or SAS was delivered before (Ϫ1,400 or Ϫ100 ms), on (0 ms), or after (ϩ100 ms for TMS, ϩ200 ms for SAS) the imperative "go" cue. Ground reaction forces and electromyographic activity were recorded. In control trials, the mean reaction time was 217 Ϯ 38 ms. In contrast, the SAS evoked APAs that had an average onset of 110 Ϯ 54 ms, whereas the incidence, magnitude, and duration of the APA increased as the stimulus timing approached the "go" cue. A facilitation of motorevoked potentials in the initial agonist muscle was observed only when TMS was applied at ϩ100 ms. These findings indicate that there was an initial phase of movement preparation during which the APA-stepping sequence was progressively assembled, and that this early preparation did not involve the corticomotor pathways activated by TMS. The subsequent increase in corticomotor excitability between the imperative stimulus and onset of the APA suggests that corticospinal pathways contribute to the voluntary initiation of the prepared APA-stepping sequence. These findings are consistent with a feedforward mode of neural control whereby the motor sequence, including the associated postural adjustments, is prepared before voluntary movement.

Differential Effects of Startle on Reaction Time for Finger and Arm Movements

“…Thus they suggested that sufficient details of a prepared movement may have been stored subcortically, possibly in the brain stem and spinal centers that were accessible to the startle volley, so that in some cases, it could be triggered early. Several later experiments replicated and extended these findings (Carlsen et al 2003b(Carlsen et al , 2004b(Carlsen et al , 2007Castellote et al 2007;Cressman et al 2006;MacKinnon et al 2007;Siegmund et al 2001). For example, it was found that response kinematics and EMG patterns are unchanged between control and startle elicited movements (Carlsen et al 2004b) and that the paradigm can be extended to different effectors and movement types such as saccades (Castellote et al 2007) and anticipatory postural adjustments (MacKinnon et al 2007).…”

Postural adjustments for online corrections of arm movements in standing humans

“…It is known that there is a significant cortical involvement in predictive postural behavior. Mackinnon et al (2007) demonstrated a facilitation of the muscles involved in APAs, whereas anticipatory adjustments of arm muscles were absent in patients with damage to their motor cortex (Viallet et al 1992). Moreover, animal studies have suggested that the cortex contributes to motor planning of reaching during stance (Martin and Ghez 1985;Perfiliev 2005;Perfiliev 1998;Vicario et al 1983) and the feedforward adjustments accompanying the reach (Yakovenko and Drew 2009).…”

Motor preparation of spatially and temporally defined movements: evidence from startle

“…Studies employing a startling stimulus have consistently shown that the movement triggered during startle trials is similar in movement kinematics and EMG configurations to that of control trials. This has been shown for such diverse tasks as upper arm and wrist movements (e.g., Carlsen et al 2004b, Maslovat et al 2008Valls-Solé et al 1999), stepping and gait initiation (MacKinnon et al 2007;Queralt et al 2010;Reynolds and Day 2007), head rotations (Oude Nijhuis et al 2007;Siegmund et al 2001), sit to stand (Queralt et al 2008), and rise to tiptoes (Valls-Solé et al 1999). However, most of these experiments have used a spatially defined movement whereby participants move to a predetermined target as fast as possible.…”