Required test duration for group comparisons in ligament viscoelasticity: a statistical approach.

The goal of this study was to determine the duration of time that ligaments from a study group need to be loaded in order to adequately determine their collective viscoelastic behavior. Rat ligaments were subjected either to creep or stress relaxation for 1,000 s or stress relaxation for 10,000 s to compare estimates of viscoelastic behavior for different test durations. Stresses versus time (relaxation) or strains versus time (creep) were fit with power law models (tbeta where beta is the rate of creep or relaxation on a log-log scale). Time intervals were separated by logarithmic decade and analyzed using a Random Coefficients approach to compute residual specimen error as a function of the number of decades of data analyzed. Standard Regression was also used for comparison. Results show that by testing for </=100 s (i.e. two logarithmic decades of time) offers 1% less accuracy than testing for 1,000 seconds (i.e. three decades) when estimating the viscoelastic behavior of a specimen. These 100 s power law estimates are far more accurate than the between specimen dispersion of viscoelastic properties. Hence, a better way to compare viscoelastic behavior between study groups is to test more specimens for shorter durations. This reduces experimental time per sample and therefore increases efficiency.