I'm a 14-year old guy, and I'm also skinny. My current routine is: 1) 20 mins treadmill (3-4 mp/h, incline 1.0)
2) 35 push-ups (sets of five with 3 min breaks in between)
3) 25 reps on each arm of 4KG dumbells.
Is there any way I can improve this for more/better muscle mass/gain? I'm trying to focus on my biceps, but forarm and core suggestions will also help.
Thanks!
Tweaking what you currently have would be pointless. You need to follow an actual plan if you want good results. Under the best of circumstances a man can only realistically hope to build a single kg of muscle per month. That's utilizing a quality plan, getting proper nutrition, and enough rest. You are making the common mistake of believing yourself to be qualified to make your own plan when you aren't. Most lifters aren't, not even advanced lifters are as inclined towards this as you might imagine. When determining what plan is most suitable for you, consider your fitness goals and the quality of the program itself. This can be judged at a glimpse by a combination of reputation and testimony, but digging deeper into the program's creator and holding any available free content to scientific scrutiny can also be revealing.
Be careful as both a new lifter and teenager to avoid ego lifting. Make sure that you always exercise with proper form and only lift weights that you can control. Having a challenge is good, ideal even, but make sure you are controlling that challenge. Injuries are generally the result of doing things you shouldn't be doing.
Nutrition is important too. As a teenager you should be slowly gaining weight regardless of whether you are lifting or not, but since you are lifting you should definitely be gaining weight. Weight is added by maintaining a caloric surplus. You don't have to count calories if you don't want to, but you do need to be in a caloric surplus regardless. You also need plenty of protein, 1.75 grams per kg of bodyweight is about as much as your body can utilize for muscle growth, doing more won't do anything more. A TDEE Calculator can help you to determine how much to eat (add 300-500 to your result), and the free app "MyFitnessPal" makes tracking super easy. But again, you don't have to count for it to count. You could try a more intuitive approach, just utilize your weight scale to make sure you are on track. Gaining a total of 1 kg per month is going to be plenty, try to stick as closely to that as possible.
Finally, your muscles don't grow in the gym, they grow the other 23 hours of the day - especially when you sleep. Your training provides the stimulus while your nutrition and rest basically give your muscles permission to grow. You don't have to spend hours in the gym to build muscle, nor do you have to train multiple times a day. Spending 30-60 mins a day training is plenty. Your activities for the rest of the day can be whatever you're feeling up to, if you want to play then play, if you want to relax then relax. But give your all during your training sessions so that the rest you get later is earned.
The following infographic is in imperial units, so just convert them to metric if that is your preference. It's an overview at what you can hope to expect in building muscle.
