Purpose-Outline the biomechanics of reaching both in healthy individuals and in individuals with acquired brain injury (ABI), and to discuss the clinical implications for using valid biomechanical models to assess reaching.Methods-A review of current literature, including a MEDLINE search using keywords of reaching, acquired brain injury, stroke, biomechanics and motor control.Results-Current assessments of the upper extremity in acquired brain injury (ABI) are focused on single joint characteristics of range of motion, strength, and spasticity. However, reaching is a functional multijoint task requiring interjoint coordination in addition to feedback and feedforward control to optimally position the hand at a desired location so that it may interact with the environment. From the literature, biomechanical measures of reaching such as movement time, movement distance and interjoint coordination have been shown to discriminate changes to hand path quality following brain injury. These measures also have been shown to correlate with measures of sensorimotor function (e.g., Fugl-Meyer) in the upper extremity.Conclusions-Further development of reliable and valid multijoint biomechanical evaluations is required, particularly for natural and goal-oriented reaching movements. The biomechanical assessment of reaching in ABI can provide an understanding of the specific deficits in physiological structures or motor planning underlying altered reaching ability, assist in the evaluation of new therapies, and characterize the recovery process following ABI.

. The control and execution of movement could potentially be altered by the presence of stroke-induced weakness if muscles are incapable of generating sufficient power. The purpose of this study was to identify compensatory strategies during a forward (sagittal) reaching task for 20 persons with chronic stroke and 10 healthy age-matched controls. We hypothesized that the paretic anterior deltoid would be maximally activated (i.e., saturated) during a reaching task and that task completion would require activation of additional muscles, resulting in compensatory movements out of the sagittal plane. For reaching movements by control subjects, joint motion remained largely in the sagittal plane and hand trajectories were smooth and direct. Movement characteristics of the nonparetic arm of stroke subjects were similar to control subjects except for small increases in the abduction angle and the percentage that anterior deltoid was activated. In contrast, reaching movements of the paretic arm of stroke subjects were characterized by increased activation of all muscles, especially the lateral deltoid, in addition to the anterior deltoid, with resulting shoulder abduction power and segmented and indirect hand motion. For the paretic arm of stroke subjects, muscle and kinetic compensations increased with impairment severity and weaker muscles were used at a higher percentage of their available muscle activity. These results suggest that the inability to generate sufficient force with the typical agonists involved during a forward reaching task may necessitate compensatory muscle recruitment strategies to complete the task.

Understanding stroke-induced changes to the motor control of the more affected arm of people with stroke may lead to more effective rehabilitation interventions that improve function. Reaching movements of the more affected arm in persons with stroke are slow, segmented and indirect. Such changes may be related to a reduced capacity to transmit motor commands in the presence of neuromotor noise. In tasks requiring both speed and accuracy, transmission capacity can be characterized by the linear relationship between movement time and task difficulty (Fitts' law). This study quantified Fitts' slope and intercept coefficients in stroke during reaching tasks and their relationship to kinematic measures of path accuracy (directness), trajectory corrections (segmentation), and planning strategy (skewness). We compared Fitts' slope and intercept and kinematics among the more and less affected arm of twenty persons with stroke and the nondominant arm of ten healthy persons. Slope and intercept were significantly increased in the more affected arm of the group with stroke and related to clinical measurements of motor impairment and tone. For both the more and less affected arm of the group with stroke, increased slopes and intercepts were correlated to more indirect, segmented, and positively skewed movement. Our findings suggest that stroke results in greater neuromotor noise which has consequences on both motor execution and planning. Individuals with stroke demonstrate substantially more deviation from straight-line paths than controls, despite using more conservative strategies (i.e., leftward shift of velocity profile) and extensive feedback control (i.e., segmentation).