Effect of Tendon Vibration on Hemiparetic Arm Stability in Unstable Workspaces

Abstract

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.

Fig 1. Experimental Set-Up.

A) Planar arm movements were made while each subject grasped the handle of a two-joint planar robot. Optical encoders provided measurements of position data, which was converted to a global x (medial/lateral) and y (forward/backward) coordinate system. Tasks were projected onto a horizontal screen, which obstructed the subject’s view of their hand and arm. The subject’s arm was supported by an armrest and the base of the pronated wrist was attached to the robot’s manipulandum with a wrist brace. Grip pressure was measured by a pressure bladder, which was placed in the subject’s palm. Subject depicted in figure is a computer generated image and is in no way representative of any actual subject. B) Twenty trials were conducted in each experimental condition totaling 200 trials. Initially, two blocks of trials were conducted to quantify baseline movement in proximal and distal workspace. Then in one workspace (proximal or distal) a block of trials was conducted measuring the response to a divergent force field (F_d). Tendon vibration was applied in the following block, in addition to F_d, to determine the effect of vibration on the ability to stabilize the hand at the end of movement. Aftereffects of vibration were evaluated during a block of trials with only F_d, which was followed by another block of baseline trials. The treatment and washout blocks were then repeated in the other (proximal or distal) workspace.

Fig 2. Pilot Study Results: Adaptation to Divergent Force Field.

Plots of mean stability error (averaged every 5 trials) observed across stroke and control subjects during a pilot experiment aimed at selecting force parameters. Each subject made 90 forward movements from the home to the target location in the proximal workspace. In the initial 30 trials, no force field was present in order to assess baseline performance. The following 30 trials included the force field (Control G = 40Ns/m; Stroke G = 20 Ns/m). The final 30 trials again were without the force field to investigate kinematic after-effects.

Fig 3. Characteristic Movement.

Plots of typical position and velocity profiles representing arm movements made by a chronic stroke and an age-matched neurologically intact control subject while making arm movements at baseline, into a divergent force field (F_d), and into F_d with vibration (V). Compared to baseline trials, F_d increased instability at the target position. Tendon vibration appeared to improve stability at the target for both subjects as evidenced by decreased movement at the target position and decreased oscillation amplitudes in the tangential velocity data during the stabilization period. EMG data indicated that control subjects co-contract antagonist muscles and increase their grip in response to F_d. Stroke subjects, whose EMG and grip activity was already heightened during baseline trials were unable to invoke the same compensation.

Fig 4. Effect of Vibration on Arm Movements.

A) Stability error (mean ± SD) reported at proximal and distal targets for stroke (n = 10) and control (n = 5) subjects. Stability error increased for both subject groups while stabilizing in F_d. Tendon vibration decreased stability error. B) The frequency of hand oscillation (Hz, mean ± SD) at the target was not significantly affected by the F_d or tendon vibration. However, control subjects exhibited significantly higher error frequencies than the stroke group. C) Changes in grip pressure (psig, mean ± SD) across blocks of trials as subjects made movements at proximal and distal target locations. Control subjects increased their grip pressure while attempting to stabilize their hand during movements made in F_d. Stroke subjects did not demonstrate significant increases in grip pressure while stabilizing in F_d.

Fig 5. EMG Data.

Normalized EMG (mean ± SD) area are shown for stroke and control subjects’ wrist flexors (WF), wrist extensors (WE), brachioradialis, (BRD), biceps (BI), triceps (TRI), anterior deltoid (AD), posterior deltoid (PD), and pectoralis major (PEC) muscles. Stroke subjects always exhibited higher muscle activation levels at distal targets. Control subjects used relatively similar levels of muscle activation at each target for all but the AD (greater in distal workspace) and PD (greater in proximal workspace) muscles. Control subjects also increased muscle activity in response to F_d.