Impaired motor preparation and execution during standing reach in people with chronic stroke

Abstract

Movement preparation of both anticipatory postural adjustments (APAs) and goal directed movement during a standing reaching task in adults with chronic hemiparesis and healthy controls was investigated. Using a simple reaction time paradigm, while standing on two separate force platforms, subjects received a warning light cue to “get ready to reach” followed 2.5s later by an imperative light cue to “reach as quickly as possible” with the paretic arm (matched arm for controls) to touch a target in front of them for a total of 90 trials. In 30 of the reaching trials a loud acoustic stimulus (LAS) of 123 dB was randomly − −200, or 0ms relative to the “go” cue. APA (postural) responses were characterized by the onset and maximal posterior displacement of center of pressure (CoP) and onset/offset of electromyography (EMG) from tibialis anterior (TA), soleus (SOL), while reach was characterized by onset and maximal forward displacement of the reach hand and onset of the anterior (AD), biceps brachii (BB) and middle deltoid (MD). Subjects with stroke, demonstrated a marked reduction in the occurrence of the StartReact responses for both APA and forward reach at all LAS time points indicating movement preparation dysfunction. Movement execution during a cued reach showed significant delays in APA and reach onsets, significant reduction in the magnitude of APA (posterior CoP displacement) and reach excursion, and an increased latency between the APA and reach compared to controls. EMG activation patterns for the TA and SOL demonstrated co contraction compared to the temporally sequenced pattern of control subjects. When LAS was provided at the “go” there were earlier but not significant differences in APA onset latency compared to reaching without LAS and significant delays in reach onset latency when compared to control subjects with or without LAS. An early burst of EMG in biceps brachii muscles with a further delay of the reach onset compared to reaching without LAS may be indicative of interference of a classical startle reflex activating elbow flexors. Results indicated impairments in movement preparation of both APA’s and goal directed UE movement in individuals with stroke which impact the functional performance of reaching in the standing position.