Hi, after psyhical activity 15 min my heartbeat goes to 140 after 4 min it decreases to 100. but ut did not easily decrease below 90. is it normal.

Could be, yes.: If you are well-conditioned, and if you do strenuous activity, it would be expected that your heart rate would go up, and a rate of 140 is not unusually high (depending of course on exactly what you were doing). The return to 100 after 4 minutes is also normal. Decrease to baseline will depend upon your level of physical conditioning; the more you get in shape, the faster it will occur.