Since I am soon heading into my own 'mass phase' where I will be looking to build a bit of muscle, I figured now would be as good of time as any to discuss the facts on muscular growth. There may be some myths or misconceptions that some people have so hopefully I can clear the air the best I can.
This form of muscular growth is one of the most debated amongst researchers because many are not even sure if the process is possible. Hyperplasia is the action of growing a muscle by increasing the number of muscle cells/fibers. This seems to be one of the biggest misconceptions amongst individuals who look to grow their muscles. The shape and size of your muscles is largely determined by the number of muscle fibers contained in each muscle which is set by the time you reach puberty. Many people think that by working out, your increase the number of muscle fibers or cells therefore increasing size, which is not true.
There have apparently been studies that have shown cases of hyperplasia via longitudinal fiber splitting as a response to high-intensity resistance training (actually sounds painful!), but these studies have been largely ruled-out. Other such cases have been reported when muscles are treated to optimal conditions. Some theorize that muscle fibers only experience hyperplasia once they reach a theoretical size limit. If they can then be pushed beyond this point - oftentimes with anabolic steroids - they may potentially split to form new fibers.
Muscular hypertrophy refers to muscular enlargement resulting from training. Muscle fiber disruption and damage during intense resistance exercise stimulates muscle growth. Once the damage has occurred, the repair process on the muscle fibers involves many different mechanisms such as hormonal variations, immune system functions, and metabolic demands. The various bodily systems can only assist in repair if adequate amounts of specific building blocks are present (i.e. protein). Protein synthesis involves 3 main steps. First, there needs to be adequate water intake. Secondly, the body will restructure noncontractile protein structures. Lastly, contractile protein synthesis will take place.
One common misconception amongst individuals starting a new resistance training program is that they may initially experience strength gains from resistance training, but this is generally caused by increased neuromuscular adaptations. What this basically means is that the brain becomes much more efficient and develops new and stronger pathways in order to contract muscular fibers. To simplify this even further, understand that the muscle fibers simply become more coordinated in their contractions. When more fibers contract at once, you can produce more force. For hypertrophy to occur however, research suggests that an individual would need to exercise beyond 16 intense workouts or so. Mostly all strength gained within those first 16 workouts are attained from neural adaptations. Beyond that, the muscle fibers will then begin to grow.
Magnitude of Hypertrophy
So how does one maximize muscular hypertrophy? First of all, a well-structured program is key to reaching a goal of muscular growth. There are two main factors at play in order to increase muscle size. Mechanical and metabolic systems must be stressed in a pretty specific way. Mechanical factors include lifting heavy loads with progression. Also known as progressive overload, muscles must be continually pushed and challenged in order for continued growth. An example of this would be increasing either repetitions or resistance during every single workout. If an individual can perform 40lbs bicep curls for 8 reps for a set, they should try and push themselves to 10 reps the next workout, then potentially increasing resistance to 45lbs the workout following that.
Short rest periods are also encouraged during resistance training to increase stress on the glycolytic energy system. This increases concentrations of metabolites that may be involved in muscular growth. This is the second component of hypertrophy mentioned above - the metabolic system.
Muscle Fiber Types
Muscle fibers can be categorized into two different types - type 1 and type 2. Type 1 muscle fibers are referred to as 'slow-twitch', have lower force production, and rely heavily on oxidative energy systems. Type 2 on the other hand is referred to as 'fast-twitch' muscle fibers, have higher force production, and rely heavily on non-oxidative energy systems. When a muscle is resistance trained, both types of fibers are recruited for force production, therefore they both possess the affinity for growth. However, type 2 muscle fibers have a much higher affinity for growth so muscle size is largely dependent on an individual's ratio of type 1 to type 2 muscle fibers.
Load and Repetition Goals
Hypertrophy training should aim for 67-85% of 1RM with a repetition goal in the 6-12 rep range. I personally feel that individuals should try to not use resistance that would force them to drop too far below an 8-rep goal for assistance exercises. Assistance exercises are those that are single-joint and usually single-muscle (i.e. bicep curl, tricep pushdown). By using too heavy of resistance for assistance exercises, individuals can place far too much stress on those single joints.
In order to build mass, your body will require more calories than what you are used to. Having said that, if you wish to try and build as lean of mass as possible, your intake of calories should be clean. What I mean by this is that you still do not want to ingest total junk calories such as processed foods or refined carbs. Also, your macronutrient ratios should be spot on. Calorie counts will vary based on the individual and their mass building goals, but remember that building about a pound of muscle a week requires pretty serious dedication so do not set goals too much higher than that!
I have seen different opinions on this, but for the most part, ideal macronutrient ratios for lean mass building should be somewhere in the ballpark of: 40-50% carbohydrates, 30-40% protein, 20-30% fat. You will need carbohydrates (clean; whole grains, fruits, vegetables) to fuel your intense workouts and protein to build tissue.
Baechle, Thomas R. and Earle, Roger W. Essentials of Strength Training and Conditioning Third Edition