Expressive charcoal

I'm doing an expressive charcoal drawing class at the moment. I didn't intend to sign-up until next year, but the recent intake happened to coincide with my experimentation with charcoal in mixed media.

Here are some examples from a recent exercise...

 Stretching with umbrella

 Yoga pose

Twirling

Dog running with Frisbee

Skipping-walking

The idea was to work quickly, with time restrictions on how long you could observe the subject. Which is why I drew such basic figures. Yet clearly, they do possess movement.

While some of the exercises feel uncomfortable, as I'm not used to working within some of the limitations, I'm still enjoying the class. Some pennies are dropping for me, in areas I've always wondered - how do I incorporate energy, without making it look unnatural?