Learning about impedance in our bodies is challenging. There’s no definitive text or study guide on the topic. Most of the best material is from published papers or book chapters. The Impedance Bibliography will list some of the best materials I have found. Some of these will be entirely abstract; some will have equations and graphs. Don’t worry about understanding all the math. At the same time, feel free to stretch yourself and flex your math muscle. In my experience, mathematics is much more fun when you explore territory you’re already familiar with.
The first entry is a book chapter from a 1997 book: “Computational Biomechanics”. Dr. Ito’s book chapter is “Dynamic Control of the Musculoskeletal System“. He introduces the idea that co-activation of agonist/antagonist muscle groups is used to achieve stiffness.