Proko is a great resource to understand anatomy, but it is a premium resource.
If you have a budget, Proko is great, You can also buy some anatomy books such as Anatomy 4 sculptors, Facial Expression anatomy, etc.
But if you don't have budge, you use some free [anatomy reference websites such as Line of Action, Sketch Daily Reference, Pinterest, etc.
With these websites, you can find high-resolution posed models that are great to understand anatomy.