Acosta, A. M. | bioRxiv Science

bioRxiv2019

Volume and intramuscular fat content of upper extremity muscles in individuals with chronic hemiparetic stroke

Stroke survivors often experience upper extremity deficits that make activities of daily living (ADLs) like dressing, cooking and bathing difficult or impossible. Survivors experience paresis, the inability to efficiently and fully activate muscles, which combined with decreased use of the upper extremity, will lead to muscle atrophy and potentially an increase in intramuscular fat. Muscle atrophy has been linked to weakness post stroke and is an important contributor to upper extremity deficits. However, the extent of upper extremity atrophy post hemiparetic stroke is unknown and a better understanding of these changes is needed to inform the direction of intervention-based research. In this study, the volume of contractile tissue and intramuscular fat in the elbow and wrist flexors and extensors were quantified in the paretic and non-paretic upper limb using MRI and the Dixon technique for the first time. Total muscle volume (p[≤]0.0005) and contractile element volume (p[≤]0.0005) were significantly smaller in the paretic upper extremity, for all muscle groups studied. The average percent difference between limbs and across participants was 21.3% for muscle volume and 22.9% for contractile element volume. We also found that while the percent intramuscular fat was greater in the paretic limb compared to the non-paretic (p[≤]0.0005), however, the volume of intramuscular fat was not significantly different between upper limbs (p=0.231). The average volumes of intramuscular fat for the elbow flexors/extensors and wrist flexors/extensors were 28.1, 28.8 and 19.9, 8.8 cm3 in the paretic limb and 29.6, 27.7 and 19.7, 8.8 cm3 in the non-paretic limb. In short, these findings indicate a decrease in muscle volume and not an increase in intramuscular fat, which will contribute to the reduction in strength in the paretic upper limb.

bioengineering↗

bioRxiv2019

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

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 participants 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.

bioengineering↗