Observation of others' actions during limb immobilization prevents the subsequent decay of motor performance

Abstract

There is rich clinical evidence that observing normally executed actions promotes the recovery of the corresponding action execution in patients with motor deficits. In this study, we assessed the ability of action observation to prevent the decay of healthy individuals' motor abilities following upper-limb immobilization. To this end, upper-limb kinematics was recorded in healthy participants while they performed three reach-to-grasp movements before immobilization and the same movements after 16 h of immobilization. The participants were subdivided into two groups; the experimental group observed, during the immobilization, the same reach-to-grasp movements they had performed before immobilization, whereas the control group observed natural scenarios. After bandage removal, motor impairment in performing reach-to-grasp movements was milder in the experimental group. These findings support the hypothesis that action observation, via the mirror mechanism, plays a protective role against the decline of motor performance induced by limb nonuse. From this perspective, action observation therapy is a promising tool for anticipating rehabilitation onset in clinical conditions involving limb nonuse, thus reducing the burden of further rehabilitation.

Keywords: action observation; early treatment; mirror mechanism; motor rehabilitation.

Fig. 1.

Experimental protocol. The figure must be read left to right, following the time line. (A) The actions executed by participants during the motor task (Top, A-Low; Middle, A-High; and Bottom, L-Low). (B) Example of wrist trajectories (solid lines) acquired during the execution of the three movements in their anatomical reference planes during the preimmobilization phase. The dashed lines connect shoulder (s), elbow (e), and wrist (w) joint centers captured at the beginning of the reaching motion, at maximum elbow flexion, and at the end of the reaching motion. (C) The immobilization period (orange bar) and VR sessions (blue diamonds) based on actions or natural scenarios for the AOT and CTRL groups, respectively. (D) Postimmobilization kinematics (wrist trajectories and joint center positions) for the same subjects shown in B depicted alongside the preimmobilization trajectory (gray lines). Note the lower postimmobilization elbow flexion, leading to a less ballistic wrist trajectory.

Fig. 2.

(Left and Center) Time courses of elbow angles averaged across preimmobilization trials (light colors) for the AOT and CTRL groups (green and red, respectively) for the three movements (A-Low, A-High, and L-Low); time courses of mean elbow angles acquired during the first trial of the postimmobilization phase (T1 post) are superimposed in corresponding dark colors. (Right) Means and SEs of AM evaluated at T1, T4, and T9 for the AOT and CTRL groups (green and red, respectively).

Fig. 3.

Using AOT as a tool to prevent the motor impairment caused by limb nonuse. The continuous gray line indicates a hypothetical time course of limb motor capabilities before the onset of immobilization. Motor abilities diminish at the time of injury (i.e., immobilization onset) and are represented by dashed lines. The solid red and green lines that begin after immobilization offset indicate the motor recovery of CTRL and AOT groups, respectively. “AOT benefit” indicates the advantage provided by AOT during immobilization in terms of residual motor abilities.