Design and Evaluation of a Novel Experimental Setup for Upper Limb Intermuscular Coordination Studies

Developing new intermuscular coordination patterns through an electromyographic signal-guided training in the upper extremity

BACKGROUND

Muscle synergies, computationally identified intermuscular coordination patterns, have been utilized to characterize neuromuscular control and learning in humans. However, it is unclear whether it is possible to alter the existing muscle synergies or develop new ones in an intended way through a relatively short-term motor exercise in adulthood. This study aimed to test the feasibility of expanding the repertoire of intermuscular coordination patterns through an isometric, electromyographic (EMG) signal-guided exercise in the upper extremity (UE) of neurologically intact individuals.

METHODS

10 participants were trained for six weeks to induce independent control of activating a pair of elbow flexor muscles that tended to be naturally co-activated in force generation. An untrained isometric force generation task was performed to assess the effect of the training on the intermuscular coordination of the trained UE. We applied a non-negative matrix factorization on the EMG signals recorded from 12 major UE muscles during the assessment to identify the muscle synergies. In addition, the performance of training tasks and the characteristics of individual muscles' activity in both time and frequency domains were quantified as the training outcomes.

RESULTS

Typically, in two weeks of the training, participants could use newly developed muscle synergies when requested to perform new, untrained motor tasks by activating their UE muscles in the trained way. Meanwhile, their habitually expressed muscle synergies, the synergistic muscle activation groups that were used before the training, were conserved throughout the entire training period. The number of muscle synergies activated for the task performance remained the same. As the new muscle synergies were developed, the neuromotor control of the trained muscles reflected in the metrics, such as the ratio between the targeted muscles, number of matched targets, and task completion time, was improved.

CONCLUSION

These findings suggest that our protocol can increase the repertoire of readily available muscle synergies and improve motor control by developing the activation of new muscle coordination patterns in healthy adults within a relatively short period. Furthermore, the study shows the potential of the isometric EMG-guided protocol as a neurorehabilitation tool for aiming motor deficits induced by abnormal intermuscular coordination after neurological disorders.

TRIAL REGISTRATION

This study was registered at the Clinical Research Information Service (CRiS) of the Korea National Institute of Health (KCT0005803) on 1/22/2021.

Expanding the repertoire of intermuscular coordination patterns and modulating intermuscular connectivity in stroke-affected upper extremity through electromyogram-guided training: a pilot study

Abnormal intermuscular coordination is a major stroke-induced functional motor impairment in the upper extremity (UE). Previous studies have computationally identified the abnormalities in the intermuscular coordination in the stroke-affected UE and their negative impacts on motor outputs. Therefore, targeting the aberrant muscle synergies has the potential as an effective approach for stroke rehabilitation. Recently, we verified the modifiability of the naturally expressed muscle synergies of young able-bodied adults in UE through an electromyographic (EMG) signal-guided exercise protocol. This study tested if an EMG-guided exercise will induce new muscle synergies, alter the associated intermuscular connectivity, and improve UE motor outcome in stroke-affected UE with moderate-to-severe motor impairment. The study used the six-week isometric EMG signal-guided exercise protocol that focused on independently activating two specific muscles, the biceps and brachioradialis, to develop new muscle activation groups. The study found that both the stroke and age-matched, able-bodied groups were able to develop new muscle coordination patterns through the exercise while habitual muscle activation was still available, which led to improvements in the motor control of the trained arm. In addition, the results provided preliminary evidence of increased intermuscular connectivity between targeted muscles in the beta-band frequencies for stroke patients after training, suggesting a modulation of the common neural drive. These findings suggest that our isometric exercise protocol has the potential to improve stroke survivors' performance of UE in their activities in daily lives (ADLs) and, ultimately, their quality of life through expanding their repertoire of intermuscular coordination.Clinical Relevance- This study shows the feasibility of expanding the intermuscular coordination pattern in stroke-affected UE through an isometric EMG-guided exercise which positively affects task performance and intermuscular connectivity.

Feasibility of Isokinetic Training to Modify Coupling of Upper Limb Muscle Synergy Activation in Stroke-affected Upper Limb

Abnormal intermuscular coordination in stroke-affected upper limbs contributes to motor deficits after stroke. In particular, abnormalities in the activation of upper limb muscle synergies after stroke were demonstrated for endpoint force control during isokinetic exercises. This study aimed to investigate the feasibility of isokinetic training to alter these abnormal synergy activations and improve motor control. Muscle synergies and Wolf Motor Function Test Functional Ability Scale (WMFT-FAS) score were compared before and after three weeks of electromyography-based training. The proposed training changed the synergy activation and improved the WMFT-FAS score in a chronic stroke survivor while preserving the muscle weights of the synergies.Clinical Relevance- This study presents the feasibility of neuromuscular training to modify the activation of upper limb muscle synergies against stroke-specific patterns of intermuscular coordination and improve WMFT-FAS score.

Feasibility of inducing new intermuscular coordination patterns through an electromyographic signal-guided training in the upper extremity: a pilot study

Abnormal intermuscular coordination has been highlighted in the field of post-stroke upper extremity (UE) rehabilitation. Relatively recent studies have quantified the altered "muscle synergies", distinctive co-activation patterns of a group of muscles, which characterize the stroke-induced abnormal intermuscular coordination. Nonetheless, whether targeting the altered muscle synergy(ies) would ameliorate the stroke-induced motor impairment and improve motor function remains unknown. Our ultimate aim is to design an exercise protocol that modifies abnormal muscle synergies and improves motor function in UE after stroke. In this study, the feasibility of an electromyographic (EMG) signal-guided exercise protocol, which targeted the alteration of an elbow flexor synergy, was tested in healthy subjects. Four neurologically intact adults participated in a six-week isometric exercise to activate two major elbow flexor muscles, biceps and brachioradialis, in isolation. Participants performed an isometric reaching in a virtual three-dimensional (3D) force space to assess any potential changes in muscle synergies during the assessment at week zero, two, four, and six of the training. EMGs of 12 UE muscles and 3D forces were collected simultaneously. A non-negative matrix factorization (NMF) was applied to the EMGs to identify synergies. From the third-to-fourth week of the training, when the participants intended to use the newly learned motor skill, they were able to activate the targeted muscle pair in isolation and induce the formation of newly emerging synergistic muscle groups. As the participants practiced to expand their repertoire of intermuscular coordination patterns, their motor control of the trained UE was improved. These findings suggest that our isometric exercise protocol can potentially modulate impaired muscle coordination in a way that benefits stroke survivor's performance in activities of daily living (ADLs) and, eventually, their quality of life.

Alterations in intermuscular coordination underlying isokinetic exercise after a stroke and their implications on neurorehabilitation

Background

Abnormal intermuscular coordination limits the motor capability of stroke-affected upper limbs. By evaluating the intermuscular coordination in the affected limb under various biomechanical task constraints, the impact of a stroke on motor control can be analyzed and intermuscular coordination-based rehabilitation strategies can be developed. In this study, we investigated upper limb intermuscular coordination after a stroke during isokinetic movements.

Methods

Sixteen chronic stroke survivors and eight neurologically intact individuals were recruited. End-point forces and electromyographic activities of the shoulder and elbow muscles were measured while the participants performed isokinetic upper limb movements in a three-dimensional space. Intermuscular coordination of the stroke survivors and the control participants was quantified in the form of muscle synergies. Then, we compared the number, composition, and activation coefficients of muscle synergies and the end-point force between the groups. The correlation between the alteration of muscle synergies and the level of motor impairment was investigated.

Results

Four and five muscle synergies in the stroke and control groups were observed, respectively. The composition of muscle synergies was comparable between the groups, except that the three heads of the deltoid muscle were co-activated and formed one synergy in the stroke group, whereas those muscles formed two synergies in the control group. When the number of muscle synergies between the groups matched, the comparable composition of muscle synergies was observed in both groups. Alternatively, the modulation of synergy activation coefficients was altered after a stroke. The severity of motor impairments was negatively correlated with the similarity of the post-stroke synergies with respect to the mean control synergies.

Conclusions

Stroke-affected upper limbs seemed to modularize the activation of the shoulder and elbow muscles in a fairly similar way to that of neurologically intact individuals during isokinetic movements. Compared with free (i.e., unconstrained) movement, exercise under biomechanical constraints including the isokinetic constraint might promote the activation of muscle synergies independently in stroke survivors. We postulated the effect of biomechanical constraints on the intermuscular coordination and suggested a possible intermuscular coordination-based rehabilitation protocol that provides the biomechanical constraint appropriate to a trainee throughout the progress of rehabilitation.