The muscular systems specialize in movement; of food and air into and out of our body, of circulation of the blood within our body, of different parts of the body relative to one another as when we change position, and of our body through space – what we normally think of as “movement”.
The muscle tissue cells are crowded with mitochondria, thousands of small factories constantly turning out molecules of ATP, refined fuel. The muscle cells use the fuel to manufacture strong and flexible proteins, with which the muscles build and repair themselves and do work.
Muscle activity accounts for most of the body’s energy consumption.
Muscle and Tissue Types
The skeletal muscles move the bones. These muscles make up a substantial part of the body mass and most of what we eat goes to fuel the metabolism of the skeletal muscles. Muscles are attached to bones on the inside of our body and skin on the outside, with various types of connective tissue. Together with our skin and skeleton, the muscles shield our internal organs from injury due to impact or penetration.
Contracting and releasing a muscle moves the bone it is attached to relative to the rest of the body. The movement of the bone moves all the tissue attached to it through space.
The heart has its own very special type of muscle tissue, called cardiac muscle. Cardiac muscle tissue is on the job, day and night, from birth to the moment of death. The cardiac muscle cells contract regularly and simultaneously more than 2.5 billion times throughout a lifetime.
Smooth muscle tissue lines the organs and structures of many organ systems, including the digestive system, the urinary system, the respiratory system, the circulatory system and the reproductive system. Smooth muscle contraction is typically slow, strong and enduring. Smooth muscle can hold a contraction longer than skeletal muscle.
Muscle and Homeostasis
Muscle contributes to homeostasis by generating heat to balance the loss of heat from the body surface. Muscle contraction uses energy from the breakdown of ATP and generates heat as a byproduct. The heat generated by the muscles interacts with other physiological processes that release heat from the body, e.g. sweating, to maintain thermoregulation.
When we undergo physical exercise there is trauma to the muscle fibers that is called muscle injury or damage. This disruption to muscle cell organelles activates satellite cells to proliferate to the injury site. A biological effort to repair or replace damaged muscle fibers begins with the satellite cells fusing together. The satellite cells have only one nucleus and replicate by dividing. As the satellite cells multiply, some remain as organelles on the muscle fiber, whereas the majority differentiate and fuse to muscle fibers to form new muscle protein stands, myofibrils, or repair damaged fibers. The muscle cells’ myofibrils increase in thickness and number. Muscle growth occurs when the rate of muscle protein synthesis is greater than the rate of muscle protein breakdown. 
The muscles adapt, so in order to make them grow a progressively higher load of stress has to be applied. This means that when doing resistance training, the weights have to increase over time.
Rest 24 hours between working on a specific body part and doing it again. When doing resistance training, damage is done to the muscle. It takes time for our body to repair it, while also making it slightly stronger. In other words, resting is when the muscles actually grow.
Aging and Muscles
As we grow older, our skeletal muscles tend to wither and weaken, we lose muscle mass. This phenomenon is known as sarcopenia. Sarcopenia starts around the age of 40 and accelerates after 75.
The harmful effects of aging on muscles can be restrained or even reversed with regular resistance exercise. A study of 40 competitive runners, cyclists, and swimmers, ranging in age from 40 to 81, found no evidence of muscle loss – the athletes in their 70’s and 80’s had almost as much thigh muscle mass as the athletes in their 40’s. Also, resistance exercise improves the connective tissue harness surrounding muscle, thus being most beneficial for injury prevention and in physical rehabilitation therapy.
There is a fairly widespread misconception about muscle turning into fat as we grow old. This is not possible as muscle and fat are different kinds of tissue. One cannot morph into the other. What often happen is that sarcopenia together with lower physical activity and maintained caloric intake lead to less muscle and increased fat. Without being aware of it, people end up eating more than they move, lean muscle depleting and fat stores expand. In essence, muscle being replaced by fat.
A study done by researchers at Columbia University Medical Center have discovered the biological mechanism behind age-related loss of muscle strength and identified a drug that may help reverse this process. Check article relating to the study.
1. How do muscles grow?, Young sub Kwon, M.S. and Len Kravitz, Ph.D.
Exercise, protein metabolism, and muscle growth, Int J Sport Nutr Exerc Metab. 2001 Mar;11(1):109-32.