Approximate entropy based on attempted steady isometric contractions with the ankle dorsal- and plantarflexors: reliability and optimal sampling frequency.

The aim of this study was to (1) examine the test-retest reliability of approximate entropy (ApEn) calculated for torque time-series from attempted steady isometric contractions performed at two different days, and (2) examine the significance of the sampling frequency for the ApEn values. Eighteen healthy young subjects (13+/-3 years, mean+/-1 S.D.) performed attempted steady isometric submaximal contractions with the ankle dorsal- and plantarflexors at two different days. Relative (ICC(3.1)) and absolute (standard error of measurement [S.E.M.], and S.E.M.%) test-retest reliability was assessed for the ApEn values calculated for torque time-series down-sampled to 30 and 100Hz, respectively. The relative reliability was generally moderate (0.360< or =ICC(3.1)< or =0.897), with an absolute reliability (S.E.M.%) of 6-14%. The mean ApEn values varied considerably depending on the applied down-sampling frequency (5-200Hz). When ApEn was used to quantify structure in the torque time-series, the relative and absolute reliability of steady isometric contractions with the ankle proved to be good in healthy young subjects. We propose that an optimal time-series down-sampling frequency exists for ApEn calculations, which will increase the sensitivity for biological system-changes, reduce adverse effects of random noise, and ensure that biological information in the signal is preserved. We recommend estimating this frequency using a variable high-pass filter-method for frequency analysis. Based on this method, the optimized time-series down-sampling frequency was around 30Hz for the isometric contractions performed with the ankle in the present study.