There are two types of meditations - calm abiding (Tib.: Shine, Skr.: Shamata) and insight meditation (Tib.: Lhaktong, Skr.: Vipassana).
The meditation of calm abiding directs mind towards one location to calm it and techniques involve focusing on an object (a stone or a Buddha form) or a formless breath. Practitioners learn not to follow one's thoughts and after some training the mind can become undisturbed like a surface of a lake.
Once the mind is calm, one can practice insight meditation which is directed at recognizing the nature of one's mind. In Vajrayana tradition insight meditations involve visualising vivid energy forms of Buddhas, receiving lights from them and melting with them.
Calm abiding meditation is suited for everyone. The insight one, however, should not be practised by people who suffer from mental diseases and take psychoactive drugs. As said earlier, to practice insight meditation one should already have a bit of stable mind. One can compare it to the physical workout. If you got injured, first focus on bringing your body back to its stable state before embarking on a strenuous preparations for a marathon. If you disregard your injury, the marathon preparations will damage your body further and even seemingly simple actions like walking will become difficult.
Although there is no hard rule, by psychoactive drugs I mean prescribed medications used in psychiatric treatments. I've heard of a person with some anxiety disorder who tried to visualise a vivid Buddha form and receive lights from it. It was too intense and fearful experience for him as there was too much sudden stimulation. Clearly it was harmful for him and probably now he has rather negative connotations about Buddha. Recreational use of some psychoactive substances is also usually frowned upon. Insight meditation is a profound practise and best if learned from a teacher or some experienced practitioner. One should always have a chance to discuss the details and openly ask the teacher whether a given meditation is suited to his/her condition. On the internet or books one can find general guidelines, but the most important one - individual advice, is always received in person.
As for the physical health, the practice of prostrations can cause some damage if done improperly or in too many numbers per day. Again, if one already has knee of back problems, it is probably better not to them at all.