What Erics44 says is true. If you really want to change, what he says is true. I know because I am suggesting it you will probably dismiss it but if that is so then listen to what he said
"honestly tho man you wont put on muscle unless you do it with a bit of direction"
I am guessing he said that because he found it to be true.I am not an advanced Bodybuilder, Boxer or athlete and I have health issues but I manged to put on a couple of stone of muscle over a couple of years. I started with the Mark Rippetoe starting strength basic Barbell training program and did it all at home, safely with no spotting partner.
Like Eric said if you want to put on muscle then you need to up your carbs and protein to gain mass. You dont need ridiculous amounts but you need to eat a little more to get a little bigger. Also because of your reduced thyroid problem you will no doubt realise that your below average testosterone levels may well be largely responsible for you slow mass gains. To supplement anything your healthcare professionals are doing to remedy this you really need to incorporate squats into your training routine.
You have already recognised the benefit of full body exercises that recruit many muscles at once. This is important because it means with reduced lung function you can do more work, more efficiently, in less time. The Squat is the king of the full range of motion exercises and will help you build, Strength, power and size. it will help increase the production of testosterone and make you bigger. Indian wrestlers do it hundreds of times a day with no weights and are incredibly strong.
If you can already do pull-ups, which are fuckers and chin ups add the squat, deadlift, and press and you can make real noticeable gains. For example you can do as many chins as you like but however big your biceps and well defined your back gets you will look weird without a well developed chest and shoulders and having the strength in your legs to lift heavier and increase your mass. This is true even if you just want to put on a half stone of useful nice looking muscle of 6 stones of Arnie type beefcake.
Whatever you do I have found the best way to learn is to listen to the experiences of someone who has achieved what you wish to. If Eric managed to get himself into good enough shape to fight in the ring then there must be something to his advice.
Bookmarks