I had a concussion a year ago and i've read i've to abstain from strenuous physical activities for some time. is it true? when can i start exercising?

Wrong opinion: Appropriate exercise is good even right after a concussion. The major risk is further brain injury. Head trauma with loss of awareness is a concussion. The brain needs to rest and recover. However, good health for the rest of the body is necessary. Good sleep, good nutrition and good exercise should start immediately. Sports that jolt or shock the brain should be avoided. Be positive.

When symptom-free: In the first few days after a concussion, physical activity can worsen and even prolong symptoms of concussion. You need total physical and mental rest until symptom free, then you can start activity. However, after a certain period of time, typically around 3 weeks, even if you're still symptomatic, light aerobic physical activity can be started as tolerated. So you can start now; just start slow.

Concusion: After a year of concussion, you should be okay to start exercising as tolerated.

Not problem: You are likely fully healed and there should be no prohibition, but you might want to use a personal trainer as you have not exercised for about a year. Agree that future head injuries could be problematic, so no true contact sports or activities at heights.