Effects of wrist tendon vibration on arm tracking in people poststroke

Abstract

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.

Fig. 1.

Experimental setup. A: subjects were seated at a planar robot. Tendon vibrators were affixed adjacent to the wrist flexor tendons and secured with a wrist brace that eliminated wrist movement. The tracking task and cursor representing hand position were projected onto a horizontal screen. B: a single repetition consisted of 1 trace of the figure-8 pattern. A sample set of trials consisted of 6 counterclockwise (CCW) repetitions followed by 6 clockwise (CW) repetitions. The set of trials was repeated with the application of tendon vibration (TV). The final set of trials was completed without TV. The order of CCW and CW trials was randomized between subjects to eliminate an order effect. C: segmentation in movement was identified as multiple peaks in velocity data. A single segment was defined as the movement occurring between 2 adjacent local minima in velocity data.

Fig. 2.

Tracking response to TV. A: plots of single trial trajectories made by a chronic stroke (S) subject before TV, the same S subject during TV, and a neurologically intact control (C) subject. Both S and C subjects segmented movement, slowing or stopping at several points throughout the trial. B: segmentation was more pronounced for the S subjects who spent more time at the beginning/end of each segment. C: histograms depicting the distribution of velocity (V) during each trial indicate that TV caused S movement to become normally distributed around the target velocity similar to the tracking behavior of C subjects. ave, Average; sd, standard deviation.

Fig. 3.

Tracking performance. Performance (means ± SD) measures for S (n = 10), C (n = 5) and S-sham trials Pre-TV, during TV, and Post-TV. S subject's tracking performance significantly improved (ANOVA; *P < 0.05) in response to TV as evidenced by decreased total path length, improved movement smoothness, and decreased standard deviation of velocity. The mean number of segments, absolute error, and velocity did not significantly change in response to TV for S or C subjects. No effect on tracking performance occurred with the sham vibration. IMS, index of movement smoothness.

Fig. 4.

Plotting the median IMS value across stroke subject data for each trial indicates improvements in IMS occur immediately with the application of TV to the forearm muscles.

Fig. 5.

Muscle activity while tracking. A: modulation of electromyography (EMG) activity in the posterior deltoid (Post Delt; PD) muscles during vibration for a single chronic S subject. The magnitude of the root-mean square (RMS) signal fluctuated at a frequency corresponding to the frequency of hand movement. The magnitude of fluctuation decreased during TV. Med/Lat, medial/lateral; MVC, maximum voluntary contraction. B: changes in the power of the RMS EMG activity (means ± SD) recorded at the median frequency (Freq) of hand movement for all muscles. Significant decreases in biceps brachii long head (BI), PD, and pectoralis major (PEC) activity was recorded between Pre-TV and Post-TV trials.

Fig. 6.

Correlations between tracking performance measures and Fugl-Meyer scores. A negative correlation existed between tracking performance and functional ability level for stroke subjects. UE, upper extremity.