In the bodybuilding and fitness community and even in some academic books skeletal muscle hypertrophy is described as being in one of two types: Sarcoplasmic or myofibrillar. According to this theory, during sarcoplasmic hypertrophy, the volume of sarcoplasmic fluid in the muscle cell increases with no accompanying increase in muscular strength, whereas during myofibrillar hypertrophy, actin and myosin contractile proteins increase in number and add to muscular strength as well as a small increase in the size of the muscle. Sarcoplasmic hypertrophy is characteristic of the muscles of certain bodybuilders while myofibrillar hypertrophy is characteristic of Olympic weightlifters.[15] These two forms of adaptations rarely occur completely independently of one another, one can experience a large increase in fluid with a slight increase in proteins, a large increase in proteins with a small increase in fluid, or a relatively balanced combination of the two. In contrast to this theory it should be noted that when viewed in microscope, muscles are filled entirely by myofibrils, whether or not the muscles from bodybuilders or powerlifters are used. Also, very little actual evidence actually supports that the non-myofibrillar part of the sarcoplasm ever expands. Antagonists to this theory suggest that the cause of this popular notion is twofold: First, it is derived from fractioning of muscle used when measuring protein synthesis. This is a technique in which muscle proteins are separated biochemically into myofibrillar, sarcoplasmic, membrane and mitochondrial fractions for protein synthesis. This separation is poorly validated and also, the results of this fractionation and the usual following stable isotope protein synthesis measurement does not tell anything about the relative abundance of these protein fractions (as changes in protein synthesis are by definition relative (i.e. a change of 50% in a substance that constitutes 1% of the muscle is still insignificant in a physiological context). Secondly, the sarcoplasmic/myofibrillar proponents use their theory to explain why bodybuilders have less relative strength than strength athletes. But this theory is not necessary to explain these differences. The physiological changes associated with training with very high volume and degrees of muscle fatigue produce neuromuscular adaptations that are different from those experienced by strength training with very high mechanical loads and less muscle fatigue.