Objectives: To determine the effects of load type (elastic, viscous, mass) on muscle activation and co-activity during research in individuals with spastic hemiparesis, post-stroke. Design: single session, mixed repeated measures. Setting: Marquette University’s Neuromotor Control lab – Participants: Cross-sectional sample of 10 right-handed individuals with chronic hemiplegia and 10 neurologically intact matched controls. Interventions: Upper extremity reaching against elastic, viscous, mass loads. Outcome measures: Agonist muscle activation (triceps, anterior deltoid) and co-activation at the shoulder and elbow from normalized EMG impulses during reaching. Results: For each load, muscle activation and co-activation were generally greater in both arms post-stroke. Whereas load type did not affect activation or co-activation levels in the paretic arm, shoulder muscles did respond differentially across loads in the non-paretic arms. In comparison, healthy adults demonstrated stronger shoulder muscle activity with elastic and viscous loads, and little evidence of co-contraction regardless of load type. Conclusions: Despite the fact that elastic and viscous loads require only agonist activity, and mass loads required approximately timed agonist and antagonist, hemiparetic arms demontrated high levels of muscle activation and co-contraction regardless of the load type. The non-paretic arm also demonstrated increased muscle activation and co-contraction, albeit with some load differentiation at the shoulder. Clinical significance: For resisted reaching with the non-paretic limb, viscous loads may be a more effective form of resistance than mass or elastic loads since it elicited greater muscle activation wihout increased co-contraction at the shoulder. For the paretic limb, all loads were equally effective in recruiting muscle activity, although co-activity was substantial for each of them. Further research is needed to determine whether trainig with one or more of the different loads may promote normalization of muscle activation patterns post stroke.
