title: Impaired force steadiness is associated with changes in force frequency composition in subacute stroke.
abstract: We tested the hypothesis that impaired force steadiness early after stroke is associated with changes in frequency composition of the force signal during constant-force task. The power spectra and the relationship between power spectra and force variability during isometric knee extension (10%, 20%, 30%, and 50% of peak torque for 10s) were studied in the paretic and non-paretic legs of 34 stroke patients (64±14years, 8-25days post-injury) and the dominant leg of 20 controls (62±10years). Power spectrum analysis of the force signal included the median frequency, peak power frequency, relative peak power, and relative power in 0-3, 4-6, and 8-12Hz bands. Force variability, quantified by coefficient of variation (CV), was increased in patients at 3 of the 4 contraction levels (P⩽0.001). Median frequency across all force levels was decreased and the relative peak power was increased in the paretic and non-paretic legs compared to controls (P⩽0.001). The relative power was increased in 0-3Hz band and decreased in both 4-6 and 8-12Hz bands in the paretic leg only (P⩽0.001). Progressively stronger contractions brought about a significant decrease in relative power in the 0-3Hz band and increase in 8-12Hz band in controls but not in stroke subjects. The hypothesis was confirmed by significant non-linear correlations between CV and each relative spectral power found in the paretic leg at most contraction levels (0.22⩽R(2)⩽0.72, P⩽0.0004) and in the non-paretic leg at 10% only (0.35⩽R(2)⩽0.52, P⩽0.0002), but not in controls. Fugl-Meyer lower extremity motor and sensory scores were not related to the frequency measures in stroke subjects (P>0.05). Limited modulation of frequency spectra and the emergence of non-linear relation between power spectra and force variability suggest that less broadband force output may account in part for impaired force steadiness in paretic and non-paretic legs early after stroke.
authors.affiliation: Center for Neuroscience and Neurological Recovery, Methodist Rehabilitation Center, Jackson, MS 39216, USA. jchow@mmrcrehab.org
authors.forename: J W
authors.initials: JW
authors.lastname: Chow
authors.forename: D S
authors.initials: DS
authors.lastname: Stokic
mesh_terms: D000368:Aged; D016022:Case-Control Studies; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D007717:Knee / Q000503:physiopathology; D008297:Male; D008875:Middle Aged; D010291:Paresis / Q000150:complications / Q000503:physiopathology*; D020521:Stroke / Q000150:complications / Q000473:pathology* / Q000503:physiopathology*; D019415:Torque