I like to think about what is 'natural' from a training and movement perspective. I spent 5 years working primarily in the outdoors doing fairly vigorous work and the initial season smoked me. This is both a literal and figurative statement as I was working as a Wildland Firefighter. During the previous 5 years, I was a college athlete and basically trained like it was my job. On paper, I was in the best shape of my life, but when I went to work, the constant variety, uneven terrain, temperature fluctuations, altitude, and odd-objects raised the difficulty level significantly. The variety of movement and range-of-motion was exponential from what I experienced in the college weight room let alone what one would expect to experience in the 'real world'. However, this type of work, movement, and exercise is exactly what the human body expects in nature. It is the modern world that is unnatural, especially when viewed from a movement perspective.
If I had to choose one word to describe natural movement it would be this: variety.
That is what I love about pocket monkii, monkii bars 2, and the Isocore. They all allow you to move in an infinite number of ways while also engaging the entire body. This is how we evolved and our training should honor these wild roots.