An ectomorph is somebody with a naturally thin frame and a metabolism that helps keep the weight off easily. If you're only worried about weight loss, this body type is an absolute blessing. For those who want to build muscle mass, it's exactly the opposite. The same physiology that keeps fat off also makes it a challenge to put muscle on. The basic method for building muscle on skinny arms is the same for putting it on burly arms -- you just have to work harder and more faithfully to see the results you want.
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Eat more. Extra calories provide your body with the basic building blocks of muscle. According to Dr. Melina Jampolis at CNN Health, your body can put on at most 1/2 pound of muscle per week. This equates to eating about 250 to 500 extra calories per day in addition to the extra calories from your workouts.
Comprise your extra calories of lean protein sources such as lean meats, legumes and nuts. The amino acids in proteins are the raw materials your body uses to construct muscle.
Eat foods rich in B and C vitamins. B vitamins are vital to the process of building muscle, while C vitamins accelerate healing between workout sessions. It doesn't hurt to take a multivitamin to round out your nutrition, especially when you're already making alterations to your diet.
Consider dietary weight gain supplements, available in any sports nutrition store. The landscape of sports supplements changes so rapidly that your best plan is to ask the staff at a local outlet about the best available products for your needs.
Schedule two sessions per week of dedicated arm work. Working out less often can slow your progress, and working out more risks overtraining and injury.
Do three to four exercises for the biceps and triceps during each session. Most biceps exercises are some kind of curl using equipment like barbells, dumb bells, cable machines or kettlebells. Some examples of triceps exercises include lying triceps extentions, behind the head presses and cable extensions.
For each exercise, perform three sets of three to five repetitions each. Set the weights so that you struggle with the last half of each set, and need help with the final repetition more often than you don't. This kind of high weight, low repetition weight lifting is best for developing large muscles.