Measurement of changes in muscle viscoelasticity during static stretching using stress-relaxation data

This study investigates how muscle viscoelasticity changes during static stretching by measuring the state of the muscle during stretching using continuous time-series data. We used a device that applied a force to the muscle during stretching and measured the reaction force. The device was attached to each participant, and time-series data of the reaction force (stress-relaxation data) during stretching were obtained. The spring-pot model, which uses fractional calculus, was selected as the viscoelastic model for the muscle, to which the stress relaxation data were fitted on a straight line on a double-logarithmic plot. The stress-relaxation data formed a broken line comprising two segments on the double-logarithmic plot, showing that viscoelasticity changed abruptly at a particular time during static stretching. Considering two viscoelastic states, before and after the change, the stress-relaxation curve was fitted through segmented regression to the double-logarithmic data with high accuracy (R2 = 0.99 and NRSME = 0.0018). We compared the parameters of the spring-pot model before and after the change in muscle viscoelasticity. By examining these continuous time-series data, we also investigated the time taken for the effects of stretching to become apparent. Furthermore, we measured the changes in muscle viscoelasticity during static stretching before and after a short-term exercise load of running on a treadmill.