sarcoplasmic hypertrophy (common in bodybuilding) involves the growth of the sarcoplasm (fluid like substance) and non-contractile proteins that do not directly contribute to muscular force production. Filament area density decreases while cross-sectional area increases, without a significant increase in strength. Myofibrillar hypertrophy occurs due to an increase in myosin-acting filaments. Contractile proteins are synthesized and filament density increases (Zatsiorsky 1995). This type of hypertrophy leads to increased strength production. Sarcoplasmic Hypertrophy Muscle fibers adapt to high volume training by increasing the number of mitochondria (organelles in the cell that are involved in ATP production) in the cell. This type of training also leads to the elevation of enzymes that are involved in glycolytic and oxidative pathways. The volume of sarcoplasmic fluid inside the cell and between the cells is increased with high volume training. This type of training contributes little to maximal strength while it does increase strength endurance due to mitochondria hypertrophy. Growth of connective tissue is also present with sarcoplasmic hypertrophy.
Myofibrillar hypertrophy occurs due to increases in the number of myosin/actin filaments (sarcomeres) inside the cell. This leads to increased strength and size of the contractile unit of muscle. Ultimately this means greater force production. This is often referred to as functional muscle, while sarcoplasmic hypertrophy is referred to as non-functional muscle. ATP and Muscular Growth as we said earlier, increasing the number of mitochondria in the cell means increased ATP production. ATP is required for protein synthesis to occur. Low levels of ATP will halt muscular growth as well as inhibit other metabolic functions that take place inside the muscle cell.
Size vs. Strength
In general, bodybuilders are more muscular than powerlifters, but powerlifters are stronger. How does training with weights that are 90% of 1RM develop strength and power, but do very little for hypertrophy? Studies have shown an intense set of 5 reps involves more fibers than an intense set of 1rep. Research has shown that using loads in the 90% range causes failure to occur before a growth stimulus has been sent to the cells. Therefore other factors besides muscle fiber fatigue result in termination of the set. The muscle simply does not have sufficient time under tension to stimulate the growth process. High rep training produces high levels of phosphate and hydrogen ions, which enhance the growth process. Research has shown heavy lifting enhances neural efficiency (improved motor recruitment, and firing rates), which enhances strength, but does not necessarily result in muscular growth.
With this information you can see why the strength, and size levels are different between bodybuilders and powerlifters. There are powerlifters that possess muscularity comparable to bodybuilders. There are also bodybuilders who have equal or greater strength than powerlifters. Do not misinterpret this article to mean there is no relationship between strength and size.
If you gain 30lbs. of lean tissue you will probably become stronger. The basic idea presented in this article is there is a relationship between size and strength, but strength increases can occur due to other reasons. Just as size increase can occur with a non-linear strength increase.
References
The weight trainer (2001) Muscle Growth part 1811: Why, And How Does A Muscle Grow and Get stronger? http://weightrainer.virtualave.net/training/growth/.html Zatsiorsky, V. (1995) Science and Practice of Strength Training. Human Kinetics. Copyright 2001 Jamie Hale
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