title: The effect of signal acquisition and processing choices on ApEn values: towards a "gold standard" for distinguishing effort levels from isometric force records.
abstract: Approximate entropy (ApEn) is frequently used to identify changes in the complexity of isometric force records with ageing and disease. Different signal acquisition and processing parameters have been used, making comparison or confirmation of results difficult. This study determined the effect of sampling and parameter choices by examining changes in ApEn values across a range of submaximal isometric contractions of the first dorsal interosseus. Reducing the sample rate by decimation changed both the value and pattern of ApEn values dramatically. The pattern of ApEn values across the range of effort levels was not sensitive to the filter cut-off frequency, or the criterion used to extract the section of data for analysis. The complexity increased with increasing effort levels using a fixed 'r' value (which accounts for measurement noise) but decreased with increasing effort level when 'r' was set to 0.1 of the standard deviation of force. It is recommended isometric force records are sampled at frequencies >200Hz, template length ('m') is set to 2, and 'r' set to measurement system noise or 0.1SD depending on physiological process to be distinguished. It is demonstrated that changes in ApEn across effort levels are related to changes in force gradation strategy.
authors.affiliation: Department of Sport and Exercise Science, Aberystwyth University, United Kingdom.
authors.forename: Sarah M
authors.initials: SM
authors.lastname: Forrest
authors.affiliation: Biomechanics Laboratory, Department of Kinesiology, The Pennsylvania State University, USA.
authors.forename: John H
authors.initials: JH
authors.lastname: Challis
authors.affiliation: Department of Sport and Exercise Science, Aberystwyth University, United Kingdom; School of Sport and Exercise Sciences, University of Kent, United Kingdom. Electronic address: S.L.Winter@kent.ac.uk.
authors.forename: Samantha L
authors.initials: SL
authors.lastname: Winter
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D019277:Entropy; D005260:Female; D005385:Fingers / Q000502:physiology; D006801:Humans; D007537:Isometric Contraction; D008297:Male; D008875:Middle Aged; D053581:Muscle Strength Dynamometer; D005082:Physical Exertion; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted; D055815:Young Adult
keywords: Complexity; First dorsal interosseus; Isometric muscle force