The multiplicative noise components of isometric force fluctuations

The experiment tested the relative contribution of additive and multiplicative noise components in isometric force fluctuations. We implemented the Hilbert-Huang transform to analyze variability in the steady state region of isometric force time series generated by participants in three adult age categories: (1) young (20-24 yrs); (2) old (60-69 yrs); and older-old (75-90 yrs). Each participant generated isometric force output in response to constant target force levels (5-40% maximum voluntary contraction). The empirical mode decomposition component of the Hilbert-Huang transform was applied to each isometric force time series resulting in a set of intrinsic mode functions (IMFS). The amplitude modulation envelopes of the IMFS obtained from the modulus of the Hilbert transform were random sequences. These random sequences were the source of the multiplicative noise. Analyses showed significant age group differences in the multiplicative noise structure of the force output. The results provide preliminary evidence that multiplicative noise dominates additive noise in isometric force output and can discriminate among the force fluctuations of healthy adult participants of different ages.