The aim of this preliminary study was to assess changes in the control of grasping forces in patients with subacute stroke who participated in an intensive 2-week CIT program. Results indicated that CIT, in general, led to an increase in maximum precision grip force and improved consistency and modulation of grasping forces. Results from the clinical motor assessment measure (WMFT) indicated the CIT led to an overall improvement in UE function as time to perform these tasks decreased.
The WMFT data are consistent with previous investigations showing that CIT is effective in improving UE motor function as assessed by clinical rating scales,13-15
whereas the FMA data are not as clear. The overall reduction in time to perform the WMFT, in particular, the key-turning, was dramatic for 4 of the 5 CIT patients. These data suggest that CIT leads to an improvement in distal UE function of the hemiparetic limb. Improved hand function of the more affected limb of stroke patients is encouraging and may lead to CIT being embraced as a technique for improving the motor performance of stroke patients. To our knowledge, these are the 1st data that provide evidence for improvement in the dexterous abilities of patients with stroke following CIT.
Despite promising reports of CIT improving UE motor function,7,13
little is known regarding the mechanism(s) underlying improvements in motor function. Our data suggest that CIT led to a reduction in force variability for this type of dexterous task. The impulse-variability theory assumes that movements are programmed and that the variability in the forces produced and in the durations over which these forces are applied are the major determinants of the variability of limb trajectory.16,17
Based on our pretest data, patients in both groups produce highly variable and irregular grasping forces during key-turning. However, after CIT, 4 of the 5 patients were able to decrease the variability of their digit forces and 2 of these patients exhibited modulation of grasping forces. The consistency of the force-rate profiles across trials for those patients who modulated their grasping forces suggests that they were able to reduce the overall variability of digit forces and the duration necessary to apply these forces. A possible mechanism responsible for improved dexterous ability after CIT is improved control of muscle force. CIT may lead to a generalized improvement in the control of forces, as patients in the CIT group improved performance on the key-turning task even though this task was not part of the CIT training activities. Greater consistency and predictability in the control of forces could result in patients utilizing more of a preprogramming or feed-forward movement control strategy. Our data suggest CIT may allow patients to utilize more of a preprogramming movement control strategy, because the rate of force production was greater and more consistent, and movement time to perform UE tasks tended to decrease after the CIT intervention.
In a review of the strength training literature related to the rehabilitation of stroke patients, Ng and Shepherd18
suggested that strength directly relates to functional improvements in stroke patients. Daily tasks do require a minimum amount of strength for their performance; if individuals lack adequate strength levels, then performance will certainly be impaired. The current data suggest that absolute strength is not a predictor of dexterous ability. One patient (pt. 8) in the immediate group showed nearly a 128% gain in maximum force production (15.4 N to 35.1 N) from pre- to posttesting sessions. This patient had the greatest maximum precision grip of patients from either group. Even with these impressive strength gains, this patient could not perform the key-turning task in either the pre- or posttest sessions. Furthermore, despite a large overlap in maximum strength levels between groups, the delayed group produced irregular and inconsistent forces and torques during key-turning. Successful performance of daily activities does require a minimum force. However, these data indicate the ability to control muscle forces with precision is of greater importance for the performance of fine motor activities involving the distal musculature, such as turning a key in a lock, than maximum strength levels, especially if one has sufficient strength to perform a given task.
A recent study investigating the effects of stroke on the control of grasping forces highlighted the need to use precise objective and quantitative measures when attempting to understand the effects of stroke on hand function.11
Although patients with stroke were able to transport and cycle objects throughout space, they produced excessive grip forces, relative to healthy controls, during the performance of these actions.11
Excessive grip force or “safety margin” has been suggested to contribute to diminished hand function of older adults19,20
; it is reasonable to suggest that this lack of precision in the control of grasping forces in older adults also contributes to their decline in dexterous function. In the current study, the modifications and fabrication procedures for implementing the force transducer into the key required a modest amount of effort. Data collection and analyses programs were easily created with LabView and MatLab software. The force and torque data gathered during this well-practiced functional activity allows for an objective and quantitative assessment of how stroke affects hand function. Accurate measurement of grasping forces provides a more complete picture of specific movement parameters that may be impacted by a therapeutic intervention, such as CIT. A clearer understanding of what movement parameters are changing and the nature of their change can provide insights into the mechanisms responsible for improved motor performance as a result of a given intervention. Future studies interested in the effects of stroke and rehabilitation interventions on hand and UE function should consider employing objective biomechanical measures that capture the dynamic control properties of the movement in addition to maximum strength measures.
The absence of kinematic data in the current study is a limitation, as estimates of key position had to be made. The grasping force and torque data collected during the pretest sessions were so irregular that estimating key position was impossible. As grasping force production improved for the immediate group, identifying the extreme positions of the key was possible based on changes in force magnitude. Collecting position data from the entire UE, trunk, and key would allow for a more complete understanding of how these key-turning movements are controlled. Kinematic measures would provide insight into how stroke affects the control of multiple degrees of freedom of the limb and trunk and how CIT affects the control of the proximal and distal degrees of freedom. Larger scale follow-up studies are planned to examine these issues systematically. Nevertheless, the change in rate of grip force production and modulation of grip forces in 3 of the 5 patients undergoing CIT were evident and suggest that CIT is improving digit force production. To further understand the effects of stroke and CIT on structure-function relationships with respect to the control of grasping forces, efforts should be made to collect data from a more homogeneous patient group in terms of lesion location. Future studies are planned to investigate the persistence of changes in force control in subacute stroke patients undergoing CIT and how the group of chronic patients respond to CIT in term of force control.