I want to gain muscle and lose belly fat, so i'll really like to combine these 2 ways of working out but i don't know if it's good and how could i make a routine.
Swimming is intense cardio, while weight lifting is intense resistance training. It is good to do both but I wouldn't advise to do them on the same days, unless you're prepared to compromise on the intensity of either.
Swimming is a sport that uses most of the bodies muscles. If you swim directly before/after weight lifting, you may overtrain some muscles (Quads, Pecks and Shoulders in particular), because you will be working them twice on the same day.
If your schedule allows, I would advise you to do your cardio on separate days to your weight training.
Your muscles recover and grow when they are at rest, not when you are training them.
It is also important to note that cardio isn't everything when its comes to losing belly fat. The key to losing belly fat is to operate in a calorie deficit.
Here is an example of a simple 5 day split which you could implement. Note that there is 2 upper body workouts, but only 1 lower body workout. You can switch this up week to week depending on your goals. Also you could remove one of the upper body workouts to turn it into a 4 day split.
I'm a competitive swimmer and also do a lot of weight training. I can certainly recommend doing cardio alongside a resistance routine - not only will it help burn additional calories to reduce body fat (assuming a sensible diet), but the improved cardiovascular fitness will enable you to push yourself harder in the gym, thus benefitting your weight sessions.
As for when to swim, avoid doing it before your weight sessions - all this will do is consume your muscle glycogen stores, meaning you will be tired in the gym and unable to maximize your weight routines.
It's far better to do weights before cardio.
In fact, the benefits of swimming after the gym are:
- You will already be warmed up, so less chance of injury.
- Your glycogen stores will be depleted so you are more likely to start tapping into your fat stores for energy in the pool.
The only issue with point 2 is that you will be more fatigued in the pool, so yes it will feel more difficult to swim, but unless you are swimming to achieve specific times/distances, this isn't really a negative. Just push through the fatigue and know it's helping burn calories and improve your fitness.
Don't think too much about this, just do it. That is how you learn. When I was younger, in the prime of life, I would run to the gym, lift weights, swim, and run home. Oh, those were the days.
ps. My opinion would be weights before the swimming. When you are handling weights, you want the ligaments of your shoulder joint capsule to be a little stiff, in order to protect the joint. When you swim, these ligaments will relax a bit because of the wide range of motion. So weights first, in my experience.