Reduction in voluntary activation of elbow and wrist muscles in individuals with chronic hemiparetic stroke

Abstract

After a stroke, descending drive is impaired due to the loss of corticospinal and corticobulbar projections which causes a reduction in voluntary activation or an inability of the nervous system to activate muscles to their full capacity, which in turn contributes to weakness of the upper extremity. Voluntary activation has not been quantified at specific joints in the upper extremity, in part because directly assessing changes descending drive is difficult. In this study, voluntary activation of elbow and wrist flexors and extensors was assessed in participants with chronic hemiparetic stroke using twitch interpolation. Twitch interpolation uses electrical stimulation to estimate voluntary activation and relies on the principle that there is an inverse relationship between the amplitude of a twitch evoked by a stimulus and the voluntary force output during stimulation (Taylor, 2009). We measured voluntary activation using twitch interpolation as well as maximum voluntary torque (MVT) of the elbow and wrist flexors and extensors in the paretic and non-paretic limb of ten participants post stroke and the dominant and non-dominant limb of 2 control participants. Results show, MVT interlimb differences were significantly greater for stroke participants compared to control, across muscle groups (p≤0.005).For stroke participants, MVT interlimb differences were significantly greater at the wrist compared to the elbow (P=0.003). Voluntary activation was significantly less in the paretic limb compared to the non-paretic, dominant and non-dominant limbs, across participants and muscle groups (p<0.005 for all four muscle groups). For the stroke participants, the voluntary activation interlimb difference was significantly greater for the wrist muscles compared to the elbow muscles (p<0.005). There was a significant positive correlation (r = 0.39, P = .022) between each participant's impairment level, as measured by a hand specific subscore of the Fugl-Meyer Assessment, and the wrist extensor voluntary activation in the paretic limb but the relationship was not significant for the other muscle groups.

A novel time-based surface EMG measure for quantifying hypertonia in paretic arm muscles during daily activities after hemiparetic stroke

“…Because our method is targeted to quantify the sEMG active duration in paretic muscles, where low SNR (i.e., due to paresis and deficit in voluntary activation [41][42][43]) and presence of abnormal neural drive (e.g., involuntary, sporadic firing [56]) are expected, we emulated such conditions by injecting simulated noise to the sEMG recorded from healthy individuals. In particular, we tested robustness of resulting sEMG active duration against two types of noise.…”

“…In the case of RMS, in contrast, much of the signal was buried with low SNR (Fig. 4A, middle), which would be problematic especially when applied to muscles with substantial paresis [41][42][43].…”

“…Especially, the near-zero% value at the Baseline and near-100% value at MVC were very consistent, which was the case for all muscles and individuals. This result indicates that the measure (e.g., with default parameters selected in this study) provides highly reliable and consistent lower and upper bounds that is comparable across muscles and individuals, in stark contrast to magnitude-based sEMG measures that suffer from normalization issues [41][42][43].…”

“…Participants performed the TUG test, as described before. While MVC were collected for reference, data from these trials were not included in the analysis (see below) because individuals have difficulty in voluntarily activating the muscles, especially with more severe motor impairments [41][42][43].…”

“…For example, measurements via sEMG inherently suffers from nonstationarity of the signal, where magnitude of the signal varies with measurement condition such as electrode-skin impedance and electrode location. Further, deficits in stroke survivors to voluntarily activate their muscle raises issues in defining a reference for comparison [41][42][43].…”