Functional effect of short-term immobilization: Kinematic changes and recovery on reaching-to-grasp

Abstract

Although previous investigations agree in showing significant cortical modifications related to short-term limb immobilization, little is known about the functional changes induced by non-use. To address this issue, we studied the kinematic effect of 10 h of hand immobilization. In order to prevent any movement, right handed healthy participants wore on their dominant hand a soft bandage. They were requested to perform the same reaching-to-grasping task immediately after immobilization, 1 day before (baseline 1) and in other two following days without non-use (baseline 2 and baseline 3). While no differences were found among baseline conditions, an increase of the total duration of reaching movement together with an anticipation of the time to peak velocity were observed in the first trial after immobilization. Interestingly, these initial effects decreased quickly trial-by-trial, following an exponential function till reaching values equal to those observed in the control conditions. The present findings show firstly that the transport phase of the reaching-to-grasp task was affected by a temporary reduction of sensory and motor information. Secondly, a trial-by-trial recovery of the immobilization-related changes, likely driven by the sensory inputs and motor outputs associated to the repetition of the movement has been observed. All together these results confirm a fundamental role of a continuous stream of sensorimotor signals in maintaining motor efficiency and in driving recovery process.

Introduction

Plasticity is an intrinsic property of the human brain that exhibits itself during the whole lifespan (Sanes and Donoghue, 2000, Jones and Lederman, 2006). It has been documented by using two main approaches: the first refers to brain changes related to an increase of sensory inputs and motor outputs (see for a review Pascual-Leone et al., 2005), while the second concerns the deprivation of these signals (e.g. Kaas, 2000). In this theoretical framework an important contribution is given by the recent model of short-term limb immobilization in healthy participants. Authors typically asked young subjects not to use their upper-limb for a limited period of time in order to verify the central effect induced by the non-use. For instance, Facchini et al. (2002) through Transcranial Magnetic Stimulation (TMS) recorded the amplitude of motor-evoked responses (MEP) after 4 days of ring and little fingers’ immobilization. They found an important decrease of motor cortex excitability contralateral to the immobilized limb. A similar effect was subsequently confirmed by Huber et al. (2006) following 12 h of upper-limb non-use and by our group in a recent study aimed to verify the role of the left free arm’s use during right hand immobilization (Avanzino et al., 2011). Beside a general agreement about the cortical changes induced by immobilization in healthy participants, little is known on the functional effects on motor behavior (Moisello et al., 2008).

As far as we know, alterations in the kinematic of a motor task after short-term immobilization were reported only in planar out-and-back movements executed with a digitizing tablet (Huber et al., 2006, Moisello et al., 2008). Here, we would like to extend these previous findings investigating the effect of 10 h of right hand immobilization in a different motor task: the reaching-to-grasp. It represents a daily life common motor behavior, fundamental for interaction with the surrounding environment. Moreover, reaching-to-grasp requires several visuo-motor transformations (see for a review Castiello, 2005). In fact, in order to plan the trajectory to bring the hand to the object and grasp it properly, visual and proprioceptive sources are essential to compute the current hand configuration and the arm posture. Previous studies suggest that this information is continuously updated through motion (for a review Wolpert and Ghahramani, 2000). Because movements are completely prevented during immobilization, one could suppose that the planning of reaching-to-grasp task is not supported by these inputs after non-use, leading to kinematic changes. Conversely, no effect after immobilization could be observed at least for two main reasons. First, our sensorimotor system seems to be robust to a brief period of immobilization, as during everyday life after comparable common experience of hypo-activity (like after long travels in airplane). Second, because the reaching-to-grasp is a very common activity, subjects might be so widely trained in the task that behavioral alterations would not be measurable. Moreover, it is worth to note that the reaching-to-grasp task involves a continuum of feedforward and feedback processes (Seidler et al., 2004). Predictions about the state of the motor system and environment in addition to sensory feedback can be used to update parameters of the next motion (e.g. Gentilucci et al., 1997). Thus, repeating the task trial-by-trial (i.e. 15 times) could give the subjects some important inputs to “recalibrate” the system after immobilization. To verify this hypothesis, possible changes in motor performance during trial repetitions were investigated.