system
A chat on 4chan
human
Yes, there is. However, it's not so simple and usually as effective, especially if you'll understand how things work, what's the point of a ritual and so on.
Most of the magic you get from some grimoire doesn't carry actual power. It's just a set of symbols practitioner who may or may not already have some power can understand and employ in the context of the tradition the text stems from. A complete random can try supposedly powerful spells and they'll get nothing or a string of some very random, potentially harmful yet dismissable a coincidencies events.

So, most of it depends on the practitioner. Now, it may be a bit easier if you are about contacting other entities (said demons) to work on your behalf rather than trying to bring the effect yourself, but you still need either some skill and practice or at least quite some dedication and luck to reach them.

Lastly, both with beings and with one's own working, the effect doesn't have to come the way you want it. Many beings who won't care about morality of the example MMA fighter deeds yet also care about doing something for him despite him being just some shmuck of little value to them may not offer too strong a boon or their boon may be realized in ways that aren't that great. For example, if it's possible, most of the magic will work through coincidencies and work with possibilities of events already happening there - and the MMA fighter may get his 'demonic strength' but it may turn out it was through influencing his trainer to slip him some stimulants of questionable legality.

People thorough history keep thinking that magic is basically cheat codes for reality. It is a way to achieve certain things through mystical venues when there's no alternative, but like any science and skill, it has its own requirements and issues one has to overcome. Said MMA may achieve possibly more using occult knowledge if he'll supplement his training with meditation and energy work, for example.
gpt
What kind of energy work?
human
Plenty of traditions that can improve one's health and mesh well with physical endeavors, from stuff like tai chi to yoga or qi gong