Task directionality impacts the ability of individuals with chronic hemiparetic stroke to match torques between arms: Preliminary findings

Beta-Band Cortico-Muscular Phase Coherence in Hemiparetic Stroke

Following a stroke, compensation for the loss of ipsilesional corticospinal and corticobulbar projections, results in increased reliance on contralesional motor pathways during paretic arm movement. Better understanding outcomes of post-stroke contralesional cortical adaptation outcomes may benefit more targeted post-stroke motor rehabilitation interventions. This proof-of-concept study involves eight healthy controls and ten post-stroke participants. Electroencephalographic (EEG) and deltoid electromyographic (EMG) data were collected during an upper-limb task. Phase coupling between beta-band motor cortex EEG and deltoid EMG was assessed using the Multi-Phase Locking Value (M-PLV) method. Different from classic cortico-muscular coherence, M-PLV allows for the calculation of dynamic phase coherence and delays, and is not affected by the non-stationary nature of EEG/EMG signals. Nerve conduction delay from the contralateral motor cortex to the deltoid muscle of the paretic arm was estimated. Our results show the ipsilateral (contralesional) motor cortex beta-band phase coherence behavior is altered in stroke participants, with significant differences in ipsilateral EEG-EMG coherence values, ipsilateral time course percentage above the significance threshold, and ipsilateral time course area above the significance threshold. M-PLV phase coherence analysis provides evidence for post-stroke contralesional motor adaptation, highlighting its increased role in the paretic shoulder abduction task. Nerve conduction delay between the motor cortices and deltoid muscle is significantly higher in stroke participants. Beta-band M-PLV phase coherence analysis shows greater phase-coherence distribution convergence between the ipsilateral (contralesional) and contralateral (ipsilesional) motor cortices in stroke participants, which is interpretable as evidence of maladaptive neural adaptation resulting from a greater reliance on the contralesional motor cortices.

Assessing Neural Connectivity and Associated Time Delays of Muscle Responses to Continuous Position Perturbations

Both linear and nonlinear electromyographic (EMG) connectivity has been reported during the expression of stretch reflexes, though it is not clear whether they are generated by the same neural pathways. To answer this question, we aim to distinguish linear and nonlinear connectivity, as well as their delays in muscle responses, resulting from continuous elbow joint perturbations. We recorded EMG from Biceps Brachii muscle when eight able-bodied participants were performing a steady elbow flexion torque while simultaneously receiving a continuous position perturbation. Using a recently developed phase coupling metric, we estimated linear and nonlinear connectivity as well as their associated delays between Biceps EMG responses and perturbations. We found that the time delay for linear connectivity (24.5 ± 5.4 ms) is in the range of short-latency stretch reflex period (< 35 ms), while that for nonlinear connectivity (53.8 ± 3.2 ms) is in the range of long-latency stretch reflex period (40-70 ms). These results suggest that the estimated linear connectivity between EMG and perturbations is very likely generated by the mono-synaptic spinal stretch reflex loop, while the nonlinear connectivity may be associated with multi-synaptic supraspinal stretch reflex loops. As such, this study provides new evidence of the nature of neural connectivity related to the stretch reflex.

Individuals With Hemiparetic Stroke Accurately Match Torques They Generate About Each Elbow Joint

Background: Successful execution of a task as simple as drinking from a cup and as complicated as cutting food with a fork and knife requires accurate perception of the torques that one generates in each arm. Prior studies have shown that individuals with hemiparetic stroke inaccurately judge their self-generated torques during bimanual tasks; yet, it remains unclear whether these individuals inaccurately judge their self-generated torques during unimanual tasks. Objective: The goal of this work was to determine whether stroke affected how accurately individuals with stroke perceive their self-generated torques during a single-arm task. Methods: Fifteen individuals with hemiparetic stroke and fifteen individuals without neurological impairments partook in this study. Participants generated a target torque about their testing elbow while receiving visual feedback, relaxed, and then matched the target torque about the same elbow without receiving feedback. This task was performed for two target torques (5 Nm, 25% of maximum voluntary torque), two movement directions (flexion, extension), and two arms (left, right). Results: Clinical assessments indicate that eleven participants with stroke had kinaesthetic deficits and two had altered pressure sense; their motor impairments spanned from mild to severe. These participants matched torques at each elbow, for each target torque and movement direction, with a similar accuracy and precision to controls, regardless of the arm tested (p > 0.050). Conclusions: These results indicate that an individual with sensorimotor deficits post-hemiparetic stroke may accurately judge the torques that they generate within each arm. Therefore, while survivors of a hemiparetic stroke may have deficits in accurately judging the torques they generate during bimanual tasks, such deficits do not appear to occur during unimanual tasks.

Accuracy of Individuals Post-hemiparetic Stroke in Matching Torques Between Arms Depends on the Arm Referenced

Background: Prior work indicates that 50–75% of individuals post-hemiparetic stroke have upper-extremity weakness and, in turn, inaccurately judge the relative torques that their arms generate during a bimanual task. Recent findings also reveal that these individuals judge the relative torques their arms generate differently depending on whether they reference their paretic vs. non-paretic arm.

Objective: Our goal was to determine whether individuals with hemiparetic stroke inaccurately matched torques between arms, regardless of the arm that they referenced.

Methods: Fifteen participants with hemiparetic stroke and 10 right-hand dominant controls matched torques between arms. Participants performed this task with their right arm referencing their left arm, and vice versa. Participants generated (1) 5 Nm and (2) 25% of their reference elbow's maximum voluntary torque (MVT) in flexion and extension using their reference arm while receiving audiovisual feedback. Then, participants matched the reference torque using their opposite arm without receiving feedback on their matching performance.

Results: Participants with stroke had greater magnitudes of error in matching torques than controls when referencing their paretic arm (p < 0.050), yet not when referencing their non-paretic arm (p > 0.050). The mean magnitude of error when participants with stroke referenced their paretic and non-paretic arm and controls referenced their dominant and non-dominant arm to generate 5 Nm in flexion was 9.4, 2.6, 4.2, and 2.5 Nm, respectively, and in extension was 5.3, 2.8, 2.5, and 2.3 Nm, respectively. However, when the torques generated at each arm were normalized by the corresponding MVT, no differences were found in matching errors regardless of the arm participants referenced (p > 0.050).

Conclusions: Results demonstrate the importance of the arm referenced, i.e., paretic vs. non-paretic, on how accurately individuals post-hemiparetic stroke judge their torques during a bimanual task. Results also indicate that individuals with hemiparetic stroke judge torques primarily based on their perceived effort. Finally, findings support the notion that training individuals post-hemiparetic stroke to accurately perceive their self-generated torques, with a focus of their non-paretic arm in relation to their paretic arm, may lead to an improved ability to perform bimanual activities of daily living.

Development of a Training Game to Coordinate Torques Produced Between Arms

The ability of individuals to accurately judge the forces that they generate is integral to seamlessly controlling their movements during everyday life. Individuals with chronic hemiparetic stroke have been shown to be impaired when matching forces between arms; this impairment may make activities as simple as carrying a tray challenging. Our goal was to develop a training protocol that individuals with stroke could use to improve their accuracy in judging the torques that they generate between arms. We designed a torque coordination game for this goal and tested its feasibility in six individuals without neurological impairments. Participants interacted with an instrumented isometric device at each arm and received automated audiovisual cues in response to the torques that they generated about each elbow joint. During the game, the participant's task was to keep a launched ball on its planned course. The participant achieved this task by sequentially applying required elbow torques at the correct times to close a left flap using the left arm and a right flap using the right arm. Participants performed this task 20 times when initiating with their left arm and 20 times when initiating with their right arm. Results indicate that all participants had a success rate in the range of 60% to 80% regardless of the arm dominance of the leading arm. Additionally, all participants anecdotally reported the game to be intuitive, and they provided an average difficulty rating that indicated the task was relatively easy to learn (i.e., 3 out of 10). Based on these findings, we conclude that this game may be suitable, enjoyable, and motivational for training coordination of torques between arms in individuals with stroke.