Passive stiffness of Drosophila IFM myofibrils: a novel, high accuracy measurement method.

As the smallest muscle-cell substructure that retains the intact contractile apparatus, the single myofibril is considered the optimal specimen for muscle mechanics, although its small size also poses some technical difficulties. Myofibrils from Drosophila indirect flight muscle (IFM) are particularly difficult to study because their high passive stiffness makes them hard to handle, and too resistant to stretch to produce enough elongation for the accurate measurement of sarcomere length change. In this study, we devised a novel method for accurate stiffness measurement of single relaxed myofibrils using microfabricated cantilevers and phase contrast microscopy. A special experimental protocol was developed to minimize errors, and some data analysis strategies were used to identify and exclude spurious data. Remarkably consistent results were obtained from Drosophila IFM myofibrils. This novel, high accuracy method is potentially an effective tool for detecting small passive stiffness change in muscle mutants.