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A characteristic of all living systems is homeostasis, or the maintenance of stable, internal conditions within specific limits. In many cases, stable conditions are maintained by negative feedback.

In negative feedback, a sensing mechanism (a receptor) detects a change in conditions beyond specific limits. A control center, or integrator (often the brain), evaluates the change and activates a second mechanism (an effector) to correct the condition; for example, cells that either remove or add glucose to the blood in an effort to maintain homeostasis are effectors. Conditions are constantly monitored by receptors and evaluated by the control center. When the control center determines that conditions have returned to normal, corrective action is discontinued. Thus, in negative feedback, the variant condition is canceled, or negated, so that conditions are returned to normal.

Anatomic terminology

In order to accurately identify areas of the body, clearly defined anatomical terms are used. These terms refer to the body in the anatomical position—standing erect, facing forward, arms down at the side, with the palms turned forward. Terms often used are summarized:

Term Definition Example
Superior Above another structure. The heart is superior to the stomach.
Inferior Below another structure. The stomach is inferior to the heart.
Anterior/ventral Toward the front of the body. The navel is anterior to the spine.
Posterior/dorsal Toward the back of the body. The spine is posterior to the navel.
Medial Toward the midline of the body. (The midline divides the body into equal right and left sides.) The nose is medial to the eyes.
Lateral Away from the midline of the body (or toward the side of the body). The ears are lateral to the nose.
Ipsilateral On the same side of the body. The spleen and descending colon are ipsilateral.
Contralateral On opposite sides of the body. The ascending and descending portions of the colon are contralateral.
Intermediate Between two structures. The knee is intermediate between the upper leg and lower leg.
Proximal Closer to the point of attachment of a limb. The elbow is proximal to the wrist.
Distal Farther from the point of attachment of a limb. The foot is distal to the knee.
Superficial Toward the surface of the body. The skin is superficial to the muscle.
Deep Away from the surface of the body. The skeleton is deep to the skin.

Human Kinetics

Human movement is accomplished through the functional integration of three systems within the human body, the nervous, skeletal, and muscular systems. The nerves, muscles, and joints must work together, or be linked (chain) to produce motion (kinetic) or human movement. The three systems responsible for human movement are also referred to as the kinetic chain. All components of the human movement system must work together to produce movement. If one component of the human movement system is not working properly, it will affect the other systems and ultimately affect movement. Therefore, it is important that personal trainers understand the systems involved in human movement and how they work together, forming a kinetic chain to produce efficient movement.

The Nervous System

The nervous system is one of the main organ systems of the body and consists of a network of specialized cells called neurons that transmit and coordinate signals, providing a communication network within the human body. It is the fast-acting control system of the body and responds to stimuli by activating muscles and glands. The nervous system is divided into two parts, the central and peripheral nervous systems. The central nervous system (CNS) is composed of the brain and spinal cord. The peripheral nervous system (PNS) contains only nerves and connects the brain and spinal cord (CNS) to the rest of the body.

The Skeletal system

The skeletal system serves many important functions; it provides the shape and form for our bodies in addition to supporting, protecting, allowing bodily movement, producing blood for the body, and storing minerals. It is important to note that the growth, maturation, and functionality of the skeletal system are greatly affected by posture, physical activity, and nutrition status. For example, poor nutrition and physical inactivity contribute to osteoporosis, which has a negative effect on skeletal health and human movement. It is composed of bone, cartilage, and ligaments and protects and supports body organs. It also provides the framework for muscles. Muscles are connected to bones by tendons. Bones form junctions that are connected by muscles and connective tissue. These junctions are known as joints. Joints are the sites where movement occurs as a result of muscle contraction.

Bone Growth

Throughout life, bone is constantly renewed through a process called remodeling. This process consists of resorption and formation. During resorption, old bone tissue is broken down and removed by special cells called osteoclasts. During bone formation, new bone tissue is laid down to replace the old. This task is performed by special cells called osteoblasts.

During childhood through adolescence, new bone is added to the skeleton faster than old bone is removed. As a result, bones become larger, heavier, and denser. For most people, bone formation continues at a faster pace than removal until bone mass peaks usually by the time individuals reach their thirties. It is also worth noting that remodeling tends to follow the lines of stress placed on the bone. Exercise and habitual posture, therefore, have a fundamental influence on the health of the skeletal system. Incorrect exercise technique, coupled with a generally poor alignment, will lead to a remodeling process that may reinforce the predominating bad posture.

The Muscular System

The body uses the muscular system so that the nervous system can command to move the skeletal system. Muscles generate internal tension that, under the control of the nervous system, manipulates the bones of our body to produce movements. Muscles are the movers and stabilizers of our bodies. It is composed of muscles and tendons to allow manipulation of the environment, locomotion, and facial expression. It also helps to maintain posture and produces heat.

Skeletal muscle is one of three major muscle types in the body; the others are cardiac and smooth muscle. Skeletal muscle is made up of individual muscle fibers, and the term muscle refers to multiple bundles of muscle fibers held together by connective tissue. Bundles of muscle fibers can be further broken down into layers from the outer surface to the innermost layer.

Muscle Types

Muscles provide the human body with a variety of functions that allow for the manipulation of forces placed on the body and to produce and slow down movement. These muscle functions categorize the muscle as an agonist, synergist, stabilizer, or antagonist.

  • Agonist muscles are muscles that act as prime movers, or, in other words, they are the muscles most responsible for a particular movement. For example, the gluteus
  • maximus is an agonist for hip extension.
  • Synergist muscles assist prime movers during movement. For example, the hamstring complex and the erector spinae are synergistic with the gluteus maximus during
  • hip extension.
  • Stabilizer muscles support or stabilize the body, whereas the prime movers and the synergists perform the movement patterns. For example, the transversus abdominis,
  • internal oblique, and multifidus (deep muscles in the low back) stabilize the low back, pelvis, and hips (lumbo-pelvic-hip complex) during hip extension.
  • Antagonist muscles perform the opposite action of the prime mover. For example, the psoas (a deep hip flexor) is antagonistic to the gluteus maximus during hip extension.
Muscle Type Muscle Function Exercise
Agonist Prime mover Chest press, Overhead press, Row, Squat
Synergist Assist prime mover Chest press, Overhead press, Row, Squat
Stabilizer Stabilize during workout Chest press, Overhead press, Row, Squat
Antagonist Oppose prime mover Chest press, Overhead press, Row, Squat

The Endocrine System

The endocrine system is a system of glands that secrete hormones into the bloodstream to regulate a variety of bodily functions, including the control of mood, growth and development, tissue function, and metabolism. The endocrine system consists of host organs (known as glands), chemical messengers (hormones), and target (receptor) cells. Once a hormone is secreted from a gland, it travels through the bloodstream to target cells designed to receive its message. The target cells have hormone-specific receptors ensuring that each hormone will communicate only with specific target cells.

Hormones produced by the endocrine system virtually affect all forms of human function including triggering muscle contraction, stimulating protein and fat synthesis, activating enzyme systems, regulating growth and metabolism, and determining how the body will physically and emotionally respond to stress.

The Cardio-Respiratory system

The cardio-respiratory system is composed of the cardiovascular system and the respiratory system. Together, they provide the body with oxygen, nutrients, protective agents, and a means to remove waste products. The cardiovascular system is composed of the heart, blood, and blood vessels. The heart is located in the mediastinum and is made up of involuntary cardiac muscle, which contracts according to a built-in rhythm to regularly pump blood throughout the body. It is divided into four chambers: two atria (which gather blood from the body) and two ventricles (which pump blood out to the body) on each side.



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