They are even telling you the task to do to improve your corrective response. So go to a crowded bar and walk around with a drink in your impaired hand. In my case I could never even get the drink glass into my left hand without spilling it right there. Damned spasticity.
Impaired corrective responses to postural perturbations of the arm in individuals with subacute stroke
Journal of NeuroEngineering and Rehabilitation volume 12, Article number: 7 (2015)
Abstract
Background
Stroke is known to alter muscle stretch responses following a perturbation, but little is known about the behavioural consequences of these altered feedback responses. Characterizing impairments in people with stroke in their interactions with the external environment may lead to better long term outcomes. This information can inform therapists about rehabilitation targets and help subjects with stroke avoid injury when moving in the world.
Methods
In this study, we developed a postural perturbation task to quantity upper limb function of subjects with subacute stroke (n = 38) and non-disabled controls (n = 74) to make rapid corrective responses with the arm. Subjects were instructed to maintain their hand at a target before and after a mechanical load was applied to the limb. Visual feedback of the hand was removed for half of the trials at perturbation onset. A number of parameters quantified subject performance, and impairment in performance was defined as outside the 95th percentile performance of control subjects.
Results
Individual subjects with stroke showed increased postural instability (44%), delayed motor responses (79%), delayed returns towards the spatial target (79%), and greater endpoint errors (74%). Several subjects also showed impairments in the temporal coordination of the elbow and shoulder joints when responding to the perturbation (47%). Interestingly, impairments in task parameters were often found for both arms of individual subjects with stroke (up to 58% for return time). Visual feedback did not improve performance on task parameters except for decreasing endpoint error for all subjects. Significant correlations between task performance and clinical measures were dependent on the arm assessed.
Conclusions
This study used a simple postural perturbation task to highlight that subjects with stroke commonly have difficulties responding to mechanical disturbances that may have important implications for their ability to perform daily activities.
Background
When holding a drink in a crowded room and someone bumps your arm, you must rapidly respond to keep the drink from spilling. Recent studies highlight that the motor system is capable of generating intelligent corrective responses to unexpected forces applied to the body [1, 2]. For example, perturbation responses have been shown to account for subject intent [3–7], urgency to respond [8], properties of the limb [9] and properties of the goal and environment [10, 11]. These task-related responses can be observed in the long-latency time period, ~50 ms after perturbation onset, implying these responses are generated through a transcortical feedback pathway [12–14]. Historically, the transcortical feedback pathway is considered to involve primary motor and somatosensory cortices [15–17], but many other cortical and subcortical areas may contribute to these corrective responses [2].
Stroke can damage cortical and subcortical regions of the brain or spinal cord, leading to a wide range of sensory and/or motor impairments [18]. This damage often leads to patients having slow, jerky and uncoordinated movements post-stroke [19]. The impact of stroke on corrective responses in the upper limb has principally been explored by quantifying long latency stretch responses in muscles. In stroke, long-latency responses have been found to be delayed and/or absent in wrist muscles [20] and the biceps brachii [21]. Subjects with stroke do not modulate their long latency responses when their arms counter stiff versus compliant environments [10] and also display inappropriate coupling between different muscle groups [22].
Only a few of the aforementioned studies that quantified changes in long-latency stretch responses in the upper limb also examined behavioural responses to perturbations [14, 19]. Many perturbation studies were designed to assess spasticity and instructed subjects to relax and not intervene or react to the perturbation [14, 19, 23–25]. Other studies have instructed subjects to actively assist [26] or actively resist [27] the perturbation. In these paradigms the applied perturbation directly controlled limb motion so that the subject could not actively achieve the behavioural goal. Therefore, it is difficult to interpret the subjects’ behavioural impairments without an attainable goal [2].
Previous studies also highlight potential deficits in the ipsilesional arm post-stroke. For example, subjects with stroke have shown bilateral impairments in modulating long latency responses between stiff and compliant environments [10]. Marsden and colleagues also found that 5 out of 12 subjects with stroke had reduced long latency responses in the both arms [15]. Again, it is not clear how altered long-latency responses in the ispilesional limb are linked to behavioural impairments for these individuals.
Finally, vision plays an important role in voluntary motor control and could provide an important alternate source for correcting limb disturbances when somatosensory feedback is impaired. Bonan and colleagues found that visual feedback was critical for maintaining whole-body posture [28]. Piovesan and colleagues found arm stiffness was reduced during reaching when post-stroke subjects used visual feedback [29]. However, little is known on the relative contribution of visual and somatosensory feedback to counter limb perturbations [30]. Visual feedback is slower than limb somatosensory feedback. Thus it is predicted that impairments in somatosensory feedback can be compensated for by visual feedback except for a slight delay in the corrective response.
Our goal was to create a behavioural task to quantify the ability of subjects with stroke to actively correct for unexpected disturbances of the arm during a goal-directed motor action. Subjects had to maintain their hand at a spatial goal and a constant load was applied to the limb so that subjects must respond to the disturbance to achieve the behavioural goal [2]. Corrective responses were assessed in both the contra- and ipsilesional arms. As well, we examined corrective responses with and without vision to quantify whether impairments in the use of somatosensory feedback could be compensated for with vision. Subjects with stroke were often slower than controls in decelerating the arm in response to the imposed load, took longer to return to the goal or undershot the target. Endpoint error was the only parameter that showed improvement when visual feedback was provided to subjects. About half of subjects with stroke who showed task impairments with their more affected arm also showed impairments with their other arm. Thus, this robotic postural perturbation task quantifies post-stroke impairments in the use of limb afferent feedback to generate motor corrections.
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