Full length articleInterlimb transfer of motor skill learning during walking: No evidence for asymmetric transfer
Introduction
The issue of “motor transfer” − i.e., the ability to learn a new motor skill and transfer it to another task or situation − is fundamental to learning and development, and is considered to be a key factor for motor recovery after neurologic or orthopedic injuries [1], [2]. One specific type of transfer is interlimb transfer where the neuromotor system retrieves information relevant to learning from the trained limb, and transfers it to the opposite, untrained limb [3], [4], [5], [6], [7]. This phenomenon of interlimb transfer is not only interesting from a theoretical standpoint of how movements are represented, but also to rehabilitation specialists, as it has therapeutic implications for individuals with unilateral deficits like stroke.
The concept of interlimb transfer has been studied extensively in the upper-limb, and various factors (e.g., type of task, conception of the task, spatial reference frame, duration of training, motor variability during training, ageing, sleep) have been identified to affect the extent and stability of transfer [8], [9], [10], [11], [12], [13], [14]. One specific finding is that the amount of interlimb transfer of motor learning appears to be asymmetric or side-specific, with one limb showing greater ability to learn from practice on the other limb [3], [5], [15], [16], [17]. Whether it is the dominant or non-dominant limb that shows greater transfer seems to be a function of the task, and this asymmetry has been linked to the idea of hand/hemispheric dominance [16], [18], [19], [20]. However, while there is strong evidence for this asymmetry in the upper limb, the evidence for inter-limb transfer in the lower-limb has been mixed. van Hedel et al. used a novel obstacle avoidance task to study the extent and asymmetry of transfer of motor skill learning from one leg to the other [21]. They observed a significant transfer of motor skills between limbs, but reported no asymmetry in the amount of transfer between limbs. In contrast, Houldin et al. evaluated the extent of interlimb transfer using a unipedal walking task and reported limited transfer of locomotor adaptations from the right leg to the left leg [22]. Finally, a recent study by Stockel and Wang [11] showed that the asymmetry in the lower limb was mediated by the task context, depending on whether the feedback was spatial (kinematic) or dynamic (force).
Given the paradigms used to investigate interlimb transfer in the lower limb have been varied, we investigated the amount of interlimb transfer in a paradigm that we have specifically used for gait rehabilitation [23], [24]. The paradigm involves a foot trajectory-tracking task that necessitates participants to produce greater hip and knee flexion by coordinating their muscle activation patterns during the swing phase of the gait, and has been used to promote motor recovery after stroke [23], [24], [25], [26], [27], [28]. Here, we used this functional paradigm to study the extent of interlimb transfer and test whether there is an asymmetry in the amount of transfer between the dominant and the non-dominant legs. We hypothesized that significant improvement in performance would be observed in the transfer leg after training with the other leg, and that this transfer would be asymmetric between the dominant and the nondominant leg.
Section snippets
Participants
Twenty right-legged (as determined by their preferred leg to kick a ball) healthy adults (Age: 22.8 ± 5.8 years, Height: 1.7 ± 0.1 m, Weight: 66 ± 15.9 kg) participated in this study. All participants signed an informed consent document that was approved by the University of Michigan Institutional Review Board.
Experimental protocol
The schematic of the experimental protocol is shown in Fig. 1. Prior to the experiment, participants were randomized into two experimental groups: (1) right-to-left (RL) and (2) left-to-right (LR)
Baseline performance
Exemplar data showing tracking performance of a subject in the RL group during key time points of the experiment are shown in Fig. 3. There were no significant differences in baseline performance (i.e., tracking error) between the training and the transfer leg for both the target-matching with visual feedback (F [1], [18] = 0.047, p = 0.831) and no visual feedback conditions (F [1], [18] = 0.5, p = 0.489). There were also no significant group (TM: F [1], [18] = 1.098, p = 0.308; NVF: F [1], [18] = 0.387, p =
Discussion
In this study, we tested the amount of interlimb transfer during walking and evaluated whether this transfer was asymmetric—i.e., transfer occurred to a greater extent from one leg to the other than vice versa. Two participant groups (RL and LR) practiced a motor learning task that has been previously used for gait rehabilitation. The RL group trained with their right leg and tested for transfer in their left leg, whereas the LR group trained with their left leg and tested for transfer in their
Conflict of interest
The authors declare no conflicts of interest.
Acknowledgements
This work was supported in part by Grants R03-HD069806 and R01-EB019834 from the National Institutes of Health and grants from the University of Michigan Office of Research and Undergraduate Research Opportunities Program. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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