Do I have to stretch everytime I start and finish with my training session. For example I run in the morning, so I stretch before and after the running session. Then I have boxing session in the evening and strenth training at night. So in these 3 sessions do I have to stretch everytime before and after the training session ?
When I was running a good bit many years ago, I'd stretch before running to limber the muscles up but after the run I'd do a "cool down" walk of a quarter mile or so as opposed to more stretching. Since your evening session is likely quite a few hours later, it can't hurt to stretch before getting into the hard work just in case you have a few kinks from the morning session. I really never was one to stretch after a workout and still don't.
I've actually stopped streching before my workouts. I warm up well though in order to avoid injuries. Now I'm warming up in a cardio rowing machine for three minutes, After that I take two small dumb bells and do reps of different exercises to target more specific muscles and warm them well before I actually start working out. I do bicep curls, shoulder press, squats, some short streching movements, etc. I do all this this in high reps, without sets, and without rest since the movements just seem to get more blood pumping through the muscles but don't really fatigue the body. After feeling well warmed up I start my training, and I have to say I feel pretty good through out my workouts. After I'm done I do stretch all of my body. I am by no means an expert. But if I were you I'd probably stretch my legs and torso after running. I would stretch after boxing, and I would definitely stretch after strenght training. I would focus on warming up well before working out rather than actually stretching cold muscles.