IONOptix BV 1 article published in JoVE Biology Isolating Myofibrils from Skeletal Muscle Biopsies and Determining Contractile Function with a Nano-Newton Resolution Force Transducer Martijn van de Locht1, Josine M. de Winter1, Dilson E. Rassier2, Michiel H.B. Helmes1,3, Coen A.C. Ottenheijm1 1Department of Physiology, Amsterdam UMC, 2Department of Kinesiology and Physical Education, Faculty of Education, McGill University, 3IONOptix BV Presented here is a protocol to assess the contractile properties of striated muscle myofibrils with nano-Newton resolution. The protocol employs a setup with an interferometry-based, optical force probe. This setup generates data with a high signal-to-noise ratio and enables the assessment of the contractile kinetics of myofibrils.