Here is another presentation, taking you through muscle contraction step by step:
A 5m TED-Ed intro to How the Muscular System works and from Crash Course, Muscles, part 1 – Muscle Cells: Crash Course A&P #21 in 10m and Muscles, part 2 – Organismal Level: Crash Course A&P #22 in 10m 40s.
Paul Andersen explains The Muscular System in 5m 59s.
In Crash Course Biology #30 – The Skeletal System: It’s ALIVE!, Hank introduces us to the framework of our bodies, our skeleton and in #31 – Big Guns: The Muscular System, he tells us the story of the complicated chemical dance that allows our skeletal muscles to contract and relax.
This is an amazing gif from tabletop whale – you have to check it out: