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Increasing Strength and Mass of Muscles
Part 3. Science of Muscles on Cellular Level
Skeletal Muscle: Macroscopic
Skeletal muscle attaches to bones for the movement of the skeleton, to other muscles and to the base membrane of the skin. There are approximately 700 skeletal muscles in the body that vary greatly in size and shape.
Each muscle is composed of parallel bundles of individual muscle cells. Bundles of these cells, called fascicles are wrapped in a layer of connective tissue, which holds the bundle together. Skeletal muscle tissue constitutes ~40 to 45% of total body weight in humans and is among the body’s most metabolically active tissues.
Skeletal Muscle: Microscopic
Skeletal muscle cells are made up of approximately 75 percent water, with myofibrillar proteins (actin and myosin) and non-myofibrillar (sarcoplasmic) proteins comprising the balance. The fluid surrounding these proteins is the sarcoplasm, and it is critical for maintaining proper muscular function. It includes ATP, glycogen, creatine phosphate, and water. The myofibrillar proteins are the ones responsible for carrying out muscle contraction.
Skeletal muscle cells are produced during development of a fetus by fusion of many mononucleated myoblasts. Muscle fibers do not divide after initial tissue formation. Because the muscle cells are made up of multiple cells, each one has more than one nucleus.
The myotubes start to develop myofibrils. Myofibrils are added around the nuclei and the nuclei remain in the long axis.
Aerobic Endurance Exercises (EE) and Resistance Exercises (RE) activate satellite cells. In the adult skeletal muscles after Progressive Overload (PO) during exercise satellite cells get activated and undergo extensive proliferation. Most of them fuse with myofibers and contribute to the increase in myonuclei. It is assumed that activated satellite cells donate cell nuclei to a muscle cell and thus increase the cell’s capacity to produce new proteins.
The skeletal muscle cells can be very long, ranging from 1mm to 30 cm in length with a diameter of 1-10µm. The muscle cells are tubular in shape, and appear striated or striped. The striations come from bands of protein which act to cause a shortening of the cell, and contraction of the muscle as a whole. The cells also have large numbers of mitochondria to supply the cell with energy in a form of ATP molecules. EE and RE increase skeletal muscle mitochondrial volume & density by an increase in numbers of existing mitochondria.
When a muscle contracts, the actin and myosin filaments do not change length, but instead slide past each other until the filaments are completely overlapped. The H zone becomes smaller and smaller and the muscle shortens. Thus when the muscle is fully contracted, the H zone is no longer visible.
There are three mechanisms of hypertrophy: mechanical tension, metabolic stress, and muscle damage. The more time spent under load, the more mechanical tension provided. Muscle Soreness is a very common sensation experienced by individuals after undertaking exercise, this is a result of micro tears in the muscle as a result of damage. This leads to a large build-up of metabolites like lactate, hydrogen ions etc. The resulting metabolic stress placed on the muscles has an anabolic effect leading to molecular signalling and an increasing hormonal response by the body.
And there are two types of muscular hypertrophy: myofibrillar, which is an increase in number of myofibrils, and sarcoplasmic, which is an increase in muscle glycogen storage and volume of sarcoplasm in a muscle fiber. Myofibrillar hypertrophy, the multiplication of myofibrils, increases muscle fiber strength because more contractile units are pulling on your bones. This will lead to higher maximum load in one repetition is increasing. During myofibrillar hypertrophy, there is also an expansion of the sarcoplasm, but the ratio between myofibrillar protein and water remains relatively constant.
Sarcoplasmic hypertrophy will increase the muscle’s work capacity. This means more sets and repetitions can be performed till failure. This occurs much in the same way as myofibrillar hypertrophy, through your body overcompensating during the recovery phase after your energy sources have been depleted during a workout. It, therefore, increases energy stores such as ATP and glycogen to prevent depletion during training. The tricky part is that these two types of muscle growth are impossible to separate. .