NASM-CPT

Every program a trainer designs rests on how the body senses, moves, fuels, and adapts. This section reviews the nervous, skeletal, muscular, and cardiorespiratory systems, the energy systems that supply ATP, the body's acute responses and chronic adaptations to training, and the nutrition and hydration principles a personal trainer may share within scope of practice.

The nervous system is the body's control center, coordinating all movement and physiological responses. It has two primary divisions.

Division	ComponentsandFunctions
Centralnervoussystem(CNS)	Brainandspinalcord Thebrainprocessessensoryinformation initiatesmotorcommands and . , ,
	coordinatescomplexmovement;thespinalcordcarriessignalsbetweenthebrainandbodyand
	coordinatesreflexesonitsown .
Peripheralnervoussystem	AllneuraltissueoutsidetheCNS Thesomaticdivisioncontrolsvoluntarymovement;the .
(PNS)	autonomicdivisionregulatesinvoluntaryfunctionsthroughthesympathetic("fightorflight")and
	parasympathetic("restanddigest")branches .

The neuron is the functional unit of the nervous system: a cell body (soma), dendrites that receive signals, and an axon that transmits them. Sensory (afferent) neurons carry information from receptors to the CNS, motor (efferent) neurons carry commands from the CNS to muscles, and interneurons connect the two within the CNS.

Proprioception and Mechanoreceptors

Proprioception is the body's sense of joint position and movement. Specialized mechanoreceptors supply this information:

Lie parallel to muscle fibers and sense

Located at the musculotendinous

Located in and around the joint

changes in muscle length and the rate

junction and sense changes in tension.

capsule; respond to pressure,

of change. When stretched too far or

Excessive or prolonged tension causes

acceleration, and deceleration of the

too fast, they trigger the stretch reflex,

the muscle to relax (autogenic

joint and signal extreme positions.

causing the muscle to contract.

inhibition) to prevent injury.

The Motor Unit

A motor unit is one motor neuron and all the muscle fibers it innervates. Small motor units control fine movements; large motor units generate powerful movements. Recruitment follows the size principle: smaller units activate first, and larger units are recruited as force demands increase. Neuromuscular efficiency is the ability of the nervous system to recruit the right muscles, with the right force, at the right time.

The adult skeleton has 206 bones. Bones provide structure and support, protect vital organs, serve as attachment sites for muscles, store minerals (particularly calcium and phosphorus), and produce blood cells in the bone marrow.

Cortical (compact) bone forms the dense outer layer that gives strength and protection. Trabecular (cancellous or spongy) bone forms the inner mesh that provides strength while reducing weight.

Bone remodeling continues throughout life: osteoblasts build new bone, and osteoclasts break down and resorb bone. According to Wolff's law, bone adapts to the loads placed on it, so regular weight-bearing exercise increases bone density.

The axial skeleton (80 bones) includes the skull, vertebral column, and thoracic cage. The appendicular skeleton (126 bones) includes the pectoral girdle, upper extremities, pelvic girdle, and lower extremities.

ANTERIOR →

Cervical

7 vertebrae · lordosis (curves inward)

Thoracic

12 vertebrae · kyphosis (curves outward) · ribs attach

Lumbar

5 vertebrae · lordosis · largest, weight-bearing

Sacrum

5 fused vertebrae · joins the pelvis

Coccyx

4 fused vertebrae · tailbone

Simplified schematic for study purposes; not to anatomical scale. The adult column has 33 vertebrae (24 movable plus the fused sacrum and coccyx). Normal curves distribute mechanical stress; excessive lordosis, excessive kyphosis, and lateral curvature (scoliosis) are abnormal.

Joints

Synovial joints are freely movable and most relevant to exercise. They contain a joint capsule, synovial membrane, synovial fluid, and articular cartilage.

SynovialJoint	Example
Hinge	Elbow knee(ankle) ,
Ball -and -socket	Hip shoulder ,
Pivot	Atlantoaxialjoint(betweenthefirsttwocervicalvertebrae)
Saddle	Carpometacarpaljointofthethumb
Condyloid	Wrist(radiocarpaljoint)
Gliding(plane)	Intercarpalandintertarsaljoints

Planes of Motion

Human movement is described in three planes. Most exercises emphasize one plane, and a well-designed program trains all three.