Malouin, Francine PhD; Richards, Carol L. PT, PhD; Durand, Anne PT, PhD; Doyon, Julien PhD
The beneficial effects of mental practice on motor function after stroke have been reported by several groups. Although most of the clinical studies have focused on the recovery of upper limb function,1 positive effects on gait2–4 and locomotor-related tasks have also been described.5–7 However, the contribution of mental practice to the extent of the gains reported is difficult to determine either because of the absence of a control group or because the amount of physical and mental practices is not controlled.
The large variability in the training procedures described from one study to the next is a limitation in determining the real effect of mental practice. Indeed, the amount, the specificity, and the timing of the physical training when combined to mental practice are quite variable across studies.2–15 For instance, in a study investigating the effects of a home-based program of mental practice on gait, there was no physical practice and no monitoring of real walking activities over the six-week study.4 In other studies, physical and mental practices are carried out independently with or without supervision.9–12 Moreover, physical practice may consist of nonspecific physical or occupational therapy for one to two hours per day, two to three days per week over four to six weeks, and followed later by mental practice targeting specific activities of daily living (ADL).9–11 In other studies, physical and mental repetitions of a specific motor task were intermingled in variable proportions within the same training session5–7,13,14 and finally in another study, physical practice was carried out only on the less affected side.15 More important, except for a few studies,5–7 the amount of physical and mental practices is generally not controlled or monitored, and study designs do not allow for the teasing out of the specific effects of mental practice or its additional effects when combined with a small amount of physical practice. Because mental practice can promote the physical use of trained limbs,8 it is important to control for the amount of physical repetitions as it could account for some of the gains observed in controlled studies.9–13
Another important factor when investigating the effects of mental practice on motor recovery is the specificity and sensitivity of the outcome measures. For example, although sometimes the effects of training are assessed with outcome measures that are directly related to the aims of training, such as gait speed,2–4 weight distribution,5,6 movement time,7,12,13 and movement accuracy,14 others have focused on motor impairment and functional ordinal scales9–11 that included items unrelated to the motor tasks trained, making the outcomes less specific to the training and less sensitive, given the ordinal scales.
Mental practice is an adjunct to habitual therapy, and the mental rehearsal of a task does not replace physical practice of the same task.16–18 Moreover, although motor gains can be obtained with mental practice alone, better results are obtained when mental rehearsals are combined with a small amount of physical rehearsals.16–18 In addition, the afferent information provided by the physical execution of the target task has also been found to be helpful for consistent reproduction of the next imagined movement19 and for promoting the internal representation of the motor task. Therefore, training protocols including series of mental repetitions separated by a physical repetition likely offer optimal training conditions.
In a previous feasibility study combining mental with physical repetitions (five mental repetitions separated by one physical repetition over one training session), substantial gains in limb loading were observed on the affected lower limb during rising up from a chair and sitting down (R-S).5,6 In the present pilot trial, a similar approach was used to examine the additional effects of combining mental with physical repetitions in individuals with chronic stroke over a four-week training program. To determine the additional effects of mental practice when combined with physical practice, three groups were compared: one group had no training (NOT group) and served as control; the other two groups were trained physically, but in one group, training was combined with mental practice (MP group), whereas in the other group physical training was combined with cognitive training (Cog group).
It was hypothesized that if motor imagery training had enhancing effects on motor performance, gains would be greater in the MP group than the Cog group that received a similar amount of physical practice. In addition, it was predicted that because the NOT group did not train physically and that the Cog group had a small number of physical repetitions, the gains in limb loading of both the Cog and NOT groups would not be significant. The main objective of the study was to examine the added effects of mental practice when combined with a small amount of physical practice on relearning the R-S tasks.
Persons who had sustained a stroke were recruited after reviewing charts from the Institut de Réadaptation en Déficience Physique de Québec (IRDPQ), in Quebec City. Then they were screened by a research therapist to determine whether they met the following inclusion criteria: (1) men or women between the age of 30 and 80 years who (2) had had a first stroke (ischemic or hemorrhagic) more than three months earlier, (3) had residual limb loading asymmetry during R-S after the completion of a rehabilitation program, (4) were able to stand up and sit down from a chair without using their hands, (5) demonstrated a good understanding of verbal instructions sufficient to enroll in a motor retraining program, and (6) had the ability to engage in motor imagery [pass the Time-Dependent Motor Imagery (TDMI) screening test and the Kinesthetic and Visual Imagery Questionnaire (KVIQ), see later].20–22 They were excluded if they had (1) lesions in the cerebellum or midbrain (confirmed by magnetic resonance imaging or computed tomography scan), (2) associated conditions causing pain in the lower limbs (eg, osteoarthritis, arthritis, fracture), (3) knee or ankle contracture, (4) joint replacement, (5) severe aphasia (based on the clinical evaluation of the speech therapist), (6) severe perceptual problems (based on clinical tests of the treating occupational therapist), and (7) cognitive impairments (based on the neuropsychologist’s evaluation) or other neurological conditions (eg, Parkinson’s disease and dementia).
Before screening, individuals who had accepted to participate in the study signed an informed, written consent that had been approved by the ethics committee of the Institut de Réadaptation en Déficience Physique de Québec (IRDPQ), in Quebec City, where the study took place. The screening was carried out over two visits and after the second visit, the participants were randomly assigned to one of three groups using a random numbers table. The MP group (n = 5) practiced the task mentally in addition to physical training; the Cog group (n = 3) in addition to physical training received cognitive training; and the NOT group (n = 4) did not train.
Main Outcome Measure
The main outcome was the loading of the affected leg during R-S calculated in percent of body weight. Limb loading was assessed at baseline, post-training, and at follow-up three weeks after training. During testing, participants sat on a chair with a seat height standardized to 100% of lower leg length, the hips were flexed 90 degrees and the knees 100 degrees (0 degree = full extension), with the seat supporting two thirds of the thighs and the feet positioned in a natural and comfortable position (slight external rotation). Tape was used to mark the position of the feet on the force plates, and a tape marker on the lateral face of the thighs guided the subject’s return to the initial seated position for each trial. Anthropometric measurements (hip, knee, and ankle) were taken, and footprints were traced on a paper for proper replication of the setup at subsequent assessment. On hearing an auditory cue, the participants were required to stand without using their hands and to sit down on a second auditory cue. They were instructed to hold their paretic hand with their sound hand and to keep their elbows flexed in front of them. Five trials were recorded. The ground reaction forces were recorded using three AMTI force plates (Advanced Mechanical Technology, Inc., Watertown, MA) embedded in the floor, one placed under the chair and one under each foot. The position of the center of mass (COM) was tracked with noncollinear, active infrared markers on the lower back (Optotrak system, Model 3020; Northern Digital, Waterloo, ON, Canada). The recording of the body kinematic data started 500 msec before the auditory cue (tone) to begin the task and stopped after the subjects had returned to sitting. The kinematic and kinetic data were collected synchronously at a sampling rate of 100 and 1000 Hz, respectively, and filtered later using a fourth order Butterworth, 0 lag filter, with a cutoff frequency of 6 and 50 Hz, respectively.
A clinical assessment was conducted at baseline. It included two measures of motor imagery ability: the TDMI screening test20,22 and the KVIQ.21 Hand and leg dominance was assessed using the modified version of the reliable measure of hand and foot preference questionnaire.23 Three domains of working memory (visuospatial, verbal, and kinesthetic) were also assessed using testing procedures similar to those described in detail elsewhere.5,24,25 Finally, motor impairment of the foot and leg was assessed with the Chedoke-McMaster Stroke Assessment Scale (1 = low; 7 = high level).26 The level of locomotor disability was assessed by different tests: Timed Up and Go,27 gait speed (5-m walk test),28 and dynamic balance.29 These tests were applied according to their respective standardized procedures. Testing was carried out by a research physiotherapist trained in the use of these tests and blind to group assignment.
Training was provided three times per week for four weeks individually in a quiet room by one of two physical therapists. Each therapist was specifically involved in the training of participants of either the MP group or Cog group.
Participants in the MP and Cog groups received similar physical training, which consisted of practicing R-S. The participants were taught to focus on the loading of the affected limb using the guidelines described by Carr and Shepherd30 wherein biomechanical constraints (seat height, sitting position, feet position, and speed) were manipulated to increase the difficulty of the task. Both therapists had been trained to apply similar procedures for the physical practice of the R-S tasks. Written training guidelines and procedures were prepared to standardize the training; in addition, therapists met regularly to make sure that they followed similar procedures over the 12 training sessions. Each physical repetition of the R-S tasks was separated by a series of mental repetitions of the same task (MP group) or by mental activities unrelated to the R-S tasks (Cog group). Participants in both the MP and Cog groups were repeatedly instructed to apply the training strategies when they perform R-S during their ADL but to refrain from practicing physically R-S outside their regular ADL.
For participants in the MP group, the physical repetition was followed by a series of mental repetitions of the R-S tasks. First, participants were briefed about first-person motor imagery (internal imagery), and the focus was placed on the sensations (kinesthetic imagery) associated with the ? R-S tasks.5,6 Mental practice training was organized into a series of blocks, each consisting of one physical repetition followed by a series of mental repetitions. The number of mental repetitions for each physical repetition was increased over the training sessions according to individual ability. During mental rehearsal, patients were required to close their eyes and to imagine R-S and were asked to verbally signal the beginning and end of each repetition. To control for time spent on mental practice, the therapist recorded the duration of the physical and mental repetitions. In the first session, to help the participants develop an inner image of their R-S performance, they were provided visual feedback of the force exerted under each foot5,6 by means of two force plates and a monitor showing net vertical force traces that illustrated the amount of loading on the affected (green trace) and less affected limb (red trace). Participants were instructed to modify their motor strategies to increase the loading on the affected limb, to relate their movements to the outcome shown on the screen, and to remember the feeling and the movement sequences associated with success or error to develop an inner representation of their performance. They were also asked to verbally describe (explicit knowledge) what they did to improve their performance (eg, “shift my body to the right and then move forward and up”), so that they could mentally reactivate these cues later during mental repetitions.5,6 Feedback was removed when the participants were able to provide a good autoestimation of their performance as judged by the physical therapist who continued to monitor the limb loading on the visual display. The number of repetitions and their duration were recorded by the training therapist. The training session including preparation, instructions, mental repetitions, autoestimation of motor imagery vividness, physical repetitions, and rest periods took approximately one hour.
Instead of practicing mentally the R-S tasks between each physical repetition, participants in the Cog group were involved in one of several mental activities for a time equivalent to the time dedicated to mental repetitions. The level of difficulty and the nature of these activities were adapted to each individual’s interests and ability to maintain a continuous challenge. Examples of the cognitive mental tasks include mental calculation, recall of numbers, questions from Mc Wiz 2000 about sports, sciences, geography, sentence completion, answering questions about a text read by the therapist, word recognition from mixed letters (scrabble). The duration of the mental activities period was recorded by the training therapist. The total duration of a session including preparation, instructions, mental activities, physical repetitions, and rest periods was approximately one hour.
The participants in the NOT group did not receive any intervention but were tested at the same time intervals as the participants in the MP and Cog groups.
Data Reduction and Statistical Analyses
The R-S tasks were divided into a partial loading phase (1 and 4) and a full loading phase (2 and 3) using the signals from the force plate under the chair and the COM signals (Fig. 1). Partial loading is defined as the period when body weight is supported by the chair and both feet, whereas the full loading phase represented the period without contact with the chair. Rising started with the partial loading phase 1 and continued with full loading phase 2; sitting started with full loading phase 3 followed by partial loading phase 4. The onset of rising was determined by a change (≥10 N) in the vertical force signal recorded from the force plate under the chair and ended when the COM was stable (full elevation of the body). The sitting task started with the auditory cue and ended when the signals from the force plate under the chair were stable. For each phase, the vertical ground reaction force (vertical impulse) was calculated (area under the curve) and averaged over the five trials. Only the full loading phases for rising (phase 2) and sitting (phase 3) were examined in this study. The analyses were carried out by a research assistant unaware of the group assignment.
The number of movements imagined (TDMI screening test) over each time period (15, 25, and 45 seconds) was averaged for each group. The scores from each subscale (visual and kinesthetic) of the motor imagery questionnaire (KVIQ) were averaged for each group. For gait speed and Timed Up and Go, the mean of two trials was computed for each group. For the working memory tests, a Z score was calculated for the number of sequences and the number of items correctly recalled.5 Between-group comparisons at baseline were made using the nonparametric Kruskal-Wallis one-way analysis of variance, followed by the Mann-Whitney U test. Within-group comparison were carried out between baseline and post-test to determine the effects of training on limb loading using the Wilcoxon rank sum test. Between-group comparisons were made with the change scores computed between baseline and post-training [(post - baseline)/baseline × 100] for the three groups using the Kruskal-Wallis one-way analysis of variance, followed by the Mann-Whitney U test. To determine whether the effects on limb loading were retained at follow-up, change scores at follow-up [(follow-up-baseline)/baseline × 100] were compared with those post-training within each group using the Wilcoxon rank sum test. Because of missing data in the NOT group, retention was examined only in the two groups who received training (MP and Cog groups). Statistical tests were performed with SPSS 11.0 for Windows.
The subject characteristics are reported in Table 1. The comparisons between the three experimental groups did not reveal significant differences (P > 0.05). The results from the TDMI screening test indicated that each group demonstrated a significant increase in the number of movements imagined with increasing time periods, suggesting that all were able to engage in motor imagery. There was no difference between groups in the limb loading deficit at baseline (P > 0.05).
Amount of Training
Participants in the MP and Cog groups attended all 12 sessions. Details about the amount of training received in the MP and Cog groups are given in Table 2 and Figure 2. The mean time in minutes (Table 2) dedicated to physical practice did not differ between the MP and Cog groups (P > 0.05) nor did the time dedicated by each group to mental activities (P > 0.05). The number of physical repetitions at each training session ranged from 9 to 11 (Fig. 2A) and the mean (standard deviation) total number was 119.3 ± 1.2 and 124.6 ± 5 in the MP and Cog groups, respectively; there was no difference between groups (P > 0.05), indicating that both groups performed a similar total amount of physical practice. Note that the mean number of mental repetitions increased gradually (Fig. 2B) over the first three sessions to reach a plateau by the fourth session; the mean number of repetitions for the 12 sessions was 1094 (±33.8) repetitions. Likewise, the number of mental repetitions for each physical repetition increased over time (Fig. 2C) from three to 10 indicating a better capacity for mental practice with time.
Effects of Training on Limb Loading
Within-group comparisons indicated that only the MP group had a significant increase in limb loading during rising (P = 0.04) and sitting (P = 0.04). As shown in Figure 3, limb loading increased in all participants of the MP group, and these increases contrast with those observed in the other two groups. Medians and ranges of change scores [(post-training − baseline)/baseline × 100] are illustrated in Figure 4. Between-group comparisons of post - training change scores showed significant differences for rising (P < 0.02) and sitting (P < 0.04), and further analyses showed that the MP group had larger (P < 0.03-0.007) change scores compared with the Cog and NOT groups. There were no significant differences between change scores of the Cog and NOT groups. The comparisons between post-training and follow-up change scores in the MP group revealed no significant differences. The gains at follow-up in the MP group corresponded to >50% of those measured post-training thus reflecting retention of the improved performance.
In this pilot study, the addition of mental practice to physical practice on the loading of the affected leg during R-S was examined in persons with chronic stroke. The results indicated that only individuals in the MP group who combined physical practice with mental practice showed a statistically significant increase in the loading of their affected leg after 12 training sessions. More specifically, all participants in the MP group (Fig. 3) showed some gains irrespective of their initial motor deficit at baseline and displayed similar post-test and follow-up patterns, a finding that contrasts with small changes and inconsistent patterns found in the other two groups. The lack of significant increase in the Cog group implies that the amount of physical training received over the 12 sessions was minimal and is unlikely to account for the substantial gains observed in the MP group.
This is the first study in which the amount of physical practice of a specific motor task was strictly monitored and controlled in each group, and, thus, the differences in outcomes between the MP and Cog groups cannot be attributed to a difference in the amount of physical practice. In addition, in the Cog group, unspecific mental activities between each physical repetition for a time equivalent to the time dedicated to mental repetitions, controlled for the time between each physical repetition as well as the total training time and time in contact with a physical therapist. The main difference between these two groups was therefore the specificity of the mental task. Therefore, it is proposed that the gains found in the MP group reflect priming effects of mental practice on physical execution.31–33 Priming effects of mental rehearsal on subsequent physical performance observed in healthy individuals are in line with the notion that mental practice has a preparatory effect and enhances the efficacy of subsequent physical training.31–33 Such a finding supports the idea that part of the behavioral improvement observed with mental practice may be latent, waiting to be expressed after minimal physical practice, and underlines the advantage of combining mental and physical practices.31–33 The latter phenomenon offers attractive clinical applications especially for tasks that are physically demanding (eg, walking, rising up, and sitting down) for individuals with severe motor impairment or who are in the early stage of rehabilitation.
In this study, the number of mental repetitions was gradually increased to reach, by the fourth session, a maximal ratio of 10 mental repetitions for each physical repetition. Such a progression is necessary and should be adapted to individual needs because mental rehearsal requires much attention and concentration especially for complex motor task involving displacement of the whole body in space.5–7
The improvement of motor performance in the MP group further extends evidence from other studies20,35–37 that the ability to engage in motor imagery is retained after stroke and can be successfully used to obtain the benefits of mental practice.4,6,7,9 Clinicians should be aware, however, that the ability to use imagery varies greatly among individuals, and “good” and “bad” imagers coexist after stroke.20 Thus, it is imperative to evaluate motor imagery ability before introducing mental practice. On the basis of recent findings that mental representation of actions is highly modulated by imagery practice,38 individuals who initially demonstrate difficulty in generating mental representation of movements may eventually improve their motor imagery ability with repeated exposures to mental practice.
This pilot study that was designed to test a novel intervention is in line with stage 2 of development of concept trial.39 This stage gives the opportunity to test enriched strategies that lead to the best possible experimental treatment for future randomized controlled trials.39 Although this pilot trial involved a relatively small number of participants, the absence of substantial gains in the group that received equal time of physical training, the stability of loading deficits over time in the NOT group, and the fact that all participants in the MP group showed improvement argue for a possible treatment effect. Nonetheless, the small sample size is a limitation and suggests caution before drawing definite conclusions from the actual findings until they can be replicated with larger groups of participants.
This pilot study provides promising results on the added value of combining mental practice with a small amount of physical practice of the same task within the same training session for relearning motor strategies post-stroke. However, these results are preliminary and need to be confirmed and extended in studies with larger samples.
The authors thank the subjects who participated in this study. The authors also extend their gratitude to Daniel Tardif and Guy St-Vincent for their technical assistance.
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