I am 6 feet 2 inches tall. I have been eating and exercising in the “evolutionary” or “primal fitness” way for about 18 months, and I was in good physical condition prior to that. I have been lifting weights for years. I am fit and active with a low percentage body fat. My stomach is flat. You can tell that I have abdominal muscles. But here is my hang up: I can’t seem to pack on any extra muscle. I weigh in at 150 pounds. I am the ultimate hardgainer, as they say in the iron game. I’m not looking to become huge. I have a lanky, Jimmy Stewart kind of frame, and no amount of training will turn me into Arnold. But what the heck does a guy have to do to gain a lousy 5-10 pounds of muscle? — Ed
First off, keep doing what you are doing. You’re building the most important base, namely that of health and strength. Eating and training like you are, your body is able to find and develop its perfect, natural design. Ultimately, if you have low body fat, good strong muscles and lots of energy, the most important ratio is power-to-weight.
Your goal of adding a few more pounds of muscle seems doable with intensive effort. Most hardgainers can add 10-20 pounds of muscle with some work. Beyond that it gets increasingly difficult to gain and/or maintain.
Since I don’t know the specifics of your diet and lifting routine, I’ll offer some general pointers here. In terms of diet, extra protein is critical. I’d recommend at least 1 gram of protein per pound of lean mass per day. If you’re older than 50, check out the Mature Muscle  post from last month. In it, we mentioned research suggesting that a high protein post-workout snack was especially important for the more seasoned crowd and that high protein food might be a more effective source for the snack than a protein supplement.
I would also suggest including more healthy fats in your diet. We mentioned in the same post that fish oil, which I always recommend for various reasons, can enhance the conversion process of food protein to muscle protein. That suggestion holds for anyone.
In terms of your workout, you can always see a good trainer to ensure you’ve perfected your form. Otherwise, I don’t see any way around the necessity of lifting harder to get the results you want. I’d specifically recommend “CrossFit ” style work for you. That means compound movements using larger muscles, which generates human growth hormone  immediately post-workout. Again, a trainer who is experienced in CrossFit can offer tips and help you take things to the next level.
On that same note of human growth hormone, it’s essential to get enough sleep. HGH is secreted during sleep, and building takes place during good old shut eye as well.
Good luck, Ed, and let me know how it goes.
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