When you observe glycerinated muscle shorten what is happening microscopically?

When glycerinated muscle shortens, the individual actin and myosin filaments within the muscle fibers move past one another (a process called filament sliding), causing the entire fiber to shorten. This process is driven by the release of calcium ions, which bind to the regulatory protein troponin, causing a change in the shape of the troponin molecule and releasing the inhibitory action of troponin on the actin-binding site of the myosin head.

This allows the myosin head to form crossbridges with actin, pulling the actin filament toward the center of the sarcomere. The energy for this process is derived from the hydrolysis of ATP by the myosin head.