Like most of us, I spent my formative years traipsing from one dimly lit dojo to another to discover the arts we are all so passionate about now. Church halls, YMCAs, garages, boxing gyms, we congregated wherever some teacher had rented, begged, or borrowed to figure out how to beat, bloody, or force a tap from a willing opponent in relative privacy.
And as much as I have come to love the smell of stale sweat that hangs in the air of any reputable training hall, there is nothing to me now like the smell of fresh air. Of course, there is also nothing like the taste of dirt grinding into my teeth and face as an added level of reality as some dude tries to drive my head into the soil while positioning me for an arm-bar.
Outdoors sounds like (and is) an alien environment to most martial artist practitioners. But in truth it is probably the most effective place to hone your skills as a Systema practitioner.
I currently conduct the majority of my training and teaching outside, and for good reason, Vladimir once stated something which articulated precisely why I feel this way, he said never to train where you are comfortable.
This could mean (as many things in a principle based system can) many things. It means never do a push-up you can easily do, do one you cannot so easily do. It means never fall into a comfortable training regime, test the limits of your abilities, even if it means losing in a training environment to discover something new. It means step outside the place where you know you can look good and out into an environment you have no knowledge of.
Practicing Systema on a purely physical level is no mean feat by any standards, we go through conditioning routines that many gymnasts can stand back from and appreciate. But still, trust me, just on a physical level anything you can do in the school is made exponentially harder outside. Even a simple push-up or squat changes as the body is forced to adjust constantly to angles and positions that are impossible to duplicate in a school.
Uneven terrain can change from slippery to rocky to grassy soft in a matter of meters. It is something your feet must to learn to contend with on their own terms while the rest of your body is fighting for survival. Independent movement is not a new concept to any Systema practitioner, but your impression it will change radically on open ground. Just learning how to slip effectively on loose soil, snow or mud is a great tool and something that simply cannot be learned in even the dingiest of schools. Other physical factors that should be considered are

1. Light conditions. Which generally remain constant in a school can literally change by the minute outside, to your advantage or disadvantage. Personally, I often position myself, if possible, so the sun is in my training partners eyes when knife fighting or sparring outside, to see how they react. Sometimes whilst taking advantage of higher ground when available. Conversely, I will actively place myself in the counter position to see how I deal with these conditions and work to gain the advantage, much as a boxer will willingly place themselves on the ropes to learn how to get out, or how make use of it. As the light drops, depth and speed perception changes. In total darkness many of the rules completely go out the window as sound and intuition become more of a factor. These are not things you want to discover for the first time on the street, or in the field depending on your profession.

2. Temperature and traction. Your overall sensitivity changes as the temperature changes. As your body fights to maintain your core temperature it may push blood to the surface or pull it away. This can affect your sensitivity and fine motor skills, especially when handling weapons. And of course just physical factors such as your hands being wet or slippery with mud, or how dust affects breathing and irritates eyes if you are too close to the ground on a dry day. In any given moment, you might be surprised to discover when it is cold and wet just how unwilling your unconscious self is to getting dirty and clammy, even when faced with an armed opponent and going to the ground is the only option you have left to you.

3. Striking. Punching a body skin-to-skin is one thing. Striking through a T-shirt which gives you a good amount of friction to place a solid shot in a school or even in the street may also be ideal. But punching through a jacket or multiple layers of clothes takes skillful practice or even just the knowledge that it is impractical to do so in certain situations and necessitates different effective target selection. Also understanding the use of clothing to bind someone or the optimum grips needed for throws is instruction not often covered in most school environments.

These are just a few examples of the factors I rarely considered until I took my training outdoors. And, of course, there are many more. Along with many psychological factors to contend with too. One of which I never could quite grasp but always seem to manage to experience along with everyone else that I train with outside. It is the phenomena of well-being that we all seem to feel after training. Until recently I thought maybe it was psychosomatic, maybe our proximity to the earth possibly triggers a deep-seated feeling that we all experience when we remove ourselves from our urban lives and get down and dirty with nature again. And maybe it is just that, but recently I read a study by a leading university that stated, apparently, when we play in the dirt there are microbes in the soil that enter our system and act exactly like anti-depressants. So we feel great.

Who knew?