Modulation of finger muscle activation patterns across postures is coordinated across all muscle groups.

Successful grasp requires that grip forces be properly directed between the fingertips and the held object. Changes in digit posture significantly affect the mapping between muscle force and fingertip force. Joint torques must subsequently be altered to maintain the desired force direction at the fingertips. Our current understanding of the roles of hand muscles in force production remains incomplete, as past studies focused on a limited set of postures or force directions. To thoroughly examine how hand muscles adapt to changing external (force direction) and internal (posture) conditions, activation patterns of six index finger muscles were examined with intramuscular electrodes in 10 healthy subjects. Participants produced submaximal isometric forces in each of six orthogonal directions at nine different finger postures. Across force directions, participants significantly altered activation patterns to accommodate postural changes in the interphalangeal joint angles but not changes in the metacarpophalangeal joint angles. Modulation of activation levels of the extrinsic hand muscles, particularly the extensors, were as great as those of intrinsic muscles, suggesting that both extrinsic and intrinsic muscles were involved in creating the desired forces. Despite considerable between-subject variation in the absolute activation patterns, principal component analysis revealed that participants used similar strategies to accommodate the postural changes. The changes in muscle coordination also helped increase joint impedance in order to stabilize the end-point force direction. This effect counteracts the increased signal-dependent motor noise that arises with greater magnitude of muscle activation as interphalangeal joint flexion is increased. These results highlight the role of the extrinsic muscles in controlling fingertip force direction across finger postures.NEW & NOTEWORTHY We examined how hand muscles adapt to changing external (force direction) and internal (posture) conditions. Muscle activations, particularly of the extrinsic extensors, were significantly affected by postural changes of the interphalangeal, but not metacarpophalangeal, joints. Joint impedance was modulated so that the effects of the signal-dependent motor noise on the force output were reduced. Comparisons with theoretical solutions showed that the chosen activation patterns occupied a small portion of the possible solution space, minimizing the maximum activation of any one muscle.

Finger-thumb coupling contributes to exaggerated thumb flexion in stroke survivors.

The purpose of this study was to investigate altered finger-thumb coupling in individuals with chronic hemiparesis poststroke. First, an external device stretched finger flexor muscles by passively rotating the metacarpophalangeal (MCP) joints. Subjects then performed isometric finger or thumb force generation. Forces/torques and electromyographic signals were recorded for both the thumb and finger muscles. Stroke survivors with moderate (n = 9) and severe (n = 9) chronic hand impairment participated, along with neurologically intact individuals (n = 9). Stroke survivors exhibited strong interactions between finger and thumb flexors. The stretch reflex evoked by stretch of the finger flexors of stroke survivors led to heteronymous reflex activity in the thumb, while attempts to produce isolated voluntary finger MCP flexion torque/thumb flexion force led to increased and undesired thumb force/finger MCP torque production poststroke with a striking asymmetry between voluntary flexion and extension. Coherence between the long finger and thumb flexors estimated using intermuscular electromyographic correlations, however, was small. Coactivation of thumb and finger flexor muscles was common in stroke survivors, whether activation was evoked by passive stretch or voluntary activation. The coupling appears to arise from subcortical or spinal sources. Flexor coupling between the thumb and fingers seems to contribute to undesired thumb flexor activity after stroke and may impact rehabilitation outcomes.

Effects of wrist tendon vibration on arm tracking in people poststroke.

The goal of this study was to evaluate the influence of wrist tendon vibration on a multijoint elbow/shoulder tracking task. We hypothesized that tendon vibration applied at the wrist musculature would improve upper arm tracking performance in chronic stroke survivors through increased, Ia-afferent feedback to the central nervous system (CNS). To test this hypothesis, 10 chronic stroke and 5 neurologically intact subjects grasped the handle of a planar robot as they tracked a target through a horizontal figure-8 pattern. A total of 36 trials were completed by each subject. During the middle trials, 70-Hz tendon vibration was applied at the wrist flexor tendons. Position, velocity, and electromyography data were evaluated to compare the quality of arm movements before, during, and after trials with concurrent vibration. Despite tracking a target that moved at a constant velocity, hand trajectories appeared to be segmented, displaying alternating intervals of acceleration and deceleration. Segments were identifiable in tangential velocity data as single-peaked, bell-shaped speed pulses. When tendon vibration was applied at the wrist musculature, stroke subjects experienced improved tracking performance in that hand path lengths and peak speed variability decreased, whereas movement smoothness increased. These performance improvements were accompanied by decreases in the muscle activity during movement. Possible mechanisms behind improved movement control in response to tendon vibration may include improved sensorimotor integration or improved cortical modulation of spinal reflex activity.

Analysis of muscle fiber conduction velocity during finger flexion and extension after stroke.

BACKGROUND: Stroke survivors experience greater strength deficits during finger extension than finger flexion. Prior research indicates relatively little observed weakness is directly attributable to muscle atrophy. Changes in other muscle properties, however, may contribute to strength deficits. OBJECTIVES: This study measured muscle fiber conduction velocity (MFCV) in a finger flexor and extensor muscle to infer changes in muscle fiber-type after stroke. METHODS: Conduction velocity was measured using a linear EMG surface electrode array for both extensor digitorum communis and flexor digitorum superficialis in 12 stroke survivors with chronic hand hemiparesis and five control subjects. Measurements were made in both hands for all subjects. Stroke survivors had either severe (n = 5) or moderate (n = 7) hand impairment. RESULTS: Absolute MFCV was significantly lower in the paretic hand of severely impaired stroke patients compared to moderately impaired patients and healthy control subjects. The relative MFCV between the two hands, however, was quite similar for flexor muscles across all subjects and for extensor muscles for the neurologically intact control subjects. However, MFCV for finger extensors was smaller in the paretic as compared to the nonparetic hand for both groups of stroke survivors. CONCLUSIONS: One explanation for reduced MFCV may be a type-II to type-I muscle fiber, especially in extrinsic extensors. Clinically, therapists may use this information to develop therapeutic exercises targeting loss of type-II fiber in extensor muscles.

Contributions of voluntary activation deficits to hand weakness after stroke.

BACKGROUND: Hemiparetic stroke survivors often exhibit profound weakness in the digits of the paretic hand, but the relative contribution of potential biomechanical and neurological impairment mechanisms is not known. Establishing sources of impairment would help in guiding treatment. OBJECTIVE: The present study sought to quantify the role of diminished capacity to voluntarily active finger flexor and extensor muscles as one possible neurological mechanism. METHODS: Two groups of stroke survivors with "severe" (N = 9) or "moderate" (N = 9) hand impairment and one group of neurologically intact individuals (N = 9) participated. Subjects were asked to create isometric flexion force and extension force, respectively, with the tip of the middle finger. The maximum voluntary force (MVF) and the maximum stimulated force (MSF) produced by an applied train of electrical current pulses (MSF) were recorded for flexion and extension. Percent voluntary activation (PVA) was computed from MVF and MSF. RESULTS: Significant deficits in both MVF and PVA were observed for stroke subjects compared to control subjects. For example, activation deficits were >80% for extensor digitorum communis (EDC) for the "severe" group. Maximum voluntary force and PVA deficits were greater for EDC than for flexor digitorum superficialis (FDS) for stroke subjects with severe impairment. Maximum voluntary force and PVA correlated significantly for stroke subjects but not for control subjects. CONCLUSIONS: Although extrinsic finger muscles could be successfully recruited electrically, voluntary excitation of these muscles was substantially limited in stroke survivors. Thus, finger weakness after stroke results predominantly from the inability to fully activate the muscle voluntarily.

Effect of Tendon Vibration on Hemiparetic Arm Stability in Unstable Workspaces.

Sensory stimulation of wrist musculature can enhance stability in the proximal arm and may be a useful therapy aimed at improving arm control post-stroke. Specifically, our prior research indicates tendon vibration can enhance stability during point-to-point arm movements and in tracking tasks. The goal of the present study was to investigate the influence of forearm tendon vibration on endpoint stability, measured at the hand, immediately following forward arm movements in an unstable environment. Both proximal and distal workspaces were tested. Ten hemiparetic stroke subjects and 5 healthy controls made forward arm movements while grasping the handle of a two-joint robotic arm. At the end of each movement, the robot applied destabilizing forces. During some trials, 70 Hz vibration was applied to the forearm flexor muscle tendons. 70 Hz was used as the stimulus frequency as it lies within the range of optimal frequencies that activate the muscle spindles at the highest response rate. Endpoint position, velocity, muscle activity and grip force data were compared before, during and after vibration. Stability at the endpoint was quantified as the magnitude of oscillation about the target position, calculated from the power of the tangential velocity data. Prior to vibration, subjects produced unstable, oscillating hand movements about the target location due to the applied force field. Stability increased during vibration, as evidenced by decreased oscillation in hand tangential velocity.

Isokinetic strength and power deficits in the hand following stroke.

OBJECTIVE: The purpose of this study was to compare the torque production in the paretic and non-paretic hands during isokinetic tasks following stroke. METHODS: We compared torque and power production at the MCP joint of chronic stroke survivors during isometric and isokinetic tasks. We also recorded surface electromyography (EMG) activity in the forearm muscles. RESULTS: Isokinetic torque production at all velocities was less than that produced during isometric trials. Both torque and power produced by the paretic hand was substantially impaired. Deficits were substantially greater in extension than flexion trials. EMG data suggests that excessive co-contraction of agonist and antagonist muscles does not greatly affect isokinetic torque production at the MCP joint. CONCLUSIONS: The fact that movement velocity produces larger deficits in torque and power production may explain why patients who have limited strength impairments still experience functional deficits. SIGNIFICANCE: This study demonstrates that strength testing of stroke survivors under isometric conditions may underestimate the overall level of impairment.

Effects of wrist tendon vibration on targeted upper-arm movements in poststroke hemiparesis.

BACKGROUND: Impaired motor control of the upper extremity after stroke may be related to lost sensory, motor, and integrative functions of the brain. Artificial activation of sensory afferents might improve control of movement by adding excitatory drive to sensorimotor control structures. The authors evaluated the effect of wrist tendon vibration (TV) on paretic upper-arm stability during point-to-point planar movements. METHODS: TV (70 Hz) was applied to the forearm wrist musculature of 10 hemiparetic stroke patients as they made center-out planar arm movements. End-point stability, muscle activity, and grip pressure were compared as patients stabilized at the target position for trials completed before, during, and after the application of the vibratory stimulus. RESULTS: Prior to vibration, hand position fluctuated as participants attempted to maintain the hand at the target after movement termination. TV improved arm stability, as evidenced by decreased magnitude of hand tangential velocity at the target. Improved stability was accompanied by a decrease in muscle activity throughout the arm as well as a mean decrease in grip pressure. CONCLUSIONS: These results suggest that vibratory stimulation of the distal wrist musculature enhances stability of the proximal arm and can be studied further as a mode for improving end-point stability during reaching in hemiparetic patients.

Strategies for arm stability in post-stroke hemiparesis - biomed 2009.

This aim of this study was to evaluate the ability of hemiparetic stroke subjects to adapt to reaching in an unstable force field compared to the adaptation experienced by healthy subjects reaching in the same environment. Endpoint stability, muscle activity and grip force were measured while subjects made planar reaches while holding a two-joint robotic arm. Neurologically intact (NI) individuals adapted to the divergent force field while reaching by increasing muscle co-contraction in the arm and tightening hand grip. Chronic stroke (CS) survivors began with higher levels of muscle activation and grip pressure, but did not show signs of adaption to the destabilizing force field. Weakness and saturated muscle activity could contribute to the CS subject's inability to adapt. Knowledge of proper and improper sensorimotor adaptation to unstable environments could help determine the most appropriate therapy for increasing strength and muscle activation during reaching.