Getting stronger and more muscular are goals that most weight lifters strive for. However, they are rarely mutually exclusive. In order to get stronger, it is often beneficial to also attain more muscle mass. And vice-versa; If progressive overload is required to build muscle consistently, an increase in strength is instrumental. Lets look at the two types of hypertrophy to further elaborate this concept.
Sarcoplasmic Hypertrophy
Sarcoplasm is the material within muscle fibers that contain a very large amount of glycosomes. In order for muscle contractions to take place, an electrical signal must past down the motor nerve to a muscle, triggering a depolarization of the the sarcolemma (transparant sheath that envelops the fibers of muscles). This creates a reaction that triggers the sarcoplasmic reticulum to release calcium ions into the muscle, resulting in a muscle contraction. When the muscle is relaxed, calcium is pumped back to the reticulum. 1 In other words, calcium ions are critical to muscle contractions, and sarcoplasm in particular, store a very high concentration of calcium ions unlike cytoplasm found in other types of cells.
Lifting weights in higher rep ranges (8-15 reps) typically results in the hypertrophy (the increase in size of cells) of sarcoplasm. The volume of the sarcoplasmic fluid in the muscle cell increases, with little effect on strength. This is often desired by body builders that want to emphasize adding volume and size to their bodies in the form of lean muscle mass. The volume of training within the 8-15 rep range puts a lot of fatigue on the muscle cells, which forces them to adapt and increase the amount of fluid within the sarcoplasm, thus increasing in size.
Myofibrillar Hypertrophy
Myofibrils are long (several centimeters) filaments that run parallel to eachother and are contained within the sarcolemma. Their structure consists of thin filaments (actin and nebulin), and thick filaments (myosin). Myosin is primarily responsible for force generation. These filaments, thin and thick, run down the length of the myofibrils and give the muscle its striated appearance. A group of myofibrils make up what is known as a muscle fiber.
When a nerve impulse arrives to a muscle fiber, the myosin binding sites are exposed, allowing myosin to bind with actin. In the event of a muscle contraction, actin is pulled along myosin until they are overlapped and the muscle is shortened.
Strength training, or Olympic lifting, utilize a smaller rep range of 1-5 or so reps. It is in this rep range, that actin and myosin contractile proteins increase in number (myofibril hypertrophy), allowing the lifter to generate more force, and therefore, get stronger. Although muscle size can increase as a result of strength training, it is not necessary in order to see gains in strength.
It is important to note that both types of hypertrophy do not occur independently, even when isolation is attempted. The ratio between the two however, does change based on how you train. The important take away from this is that BOTH should be utilized to further promote one over the other.
1. http://www.els.net/WileyCDA/ElsArticle/refId-a0000674.html