Components of Fitness:

VO2 MAX: Maximal oxygen uptake.  The maximum amount of oxygen that can be consume per minute during maximal exercise.  VO2 max is probably the best overall determinate of fitness.  It takes into account all aspects of the cardiovascular and muscular system.

ENDURANCE: Time to fatigue.  Endurance is the ability to perform activities for extended periods of time.

ANAEROBIC THRESHOLD/LACTIC THRESHOLD: The point where the exercise intensity increases to a level that the body’s energy requirements can no longer be met completely by the aerobic energy pathway.  When exercisers reach their anaerobic threshold, which essentially is the same as the lactic threshold, lactic acid is produced at a faster rate than it can be cleared from the body and as a result exercisers have that uncomfortable ‘out of breath’ feeling.

STRENGTH: The ability of a muscle to exert force against a resistance.  Muscular strength is the maximum force that can be applied by a muscle during a single maximal contraction.  Muscular endurance is the ability to perform repetitive muscular contraction against some resistance.  Strength is also related to agility, or the ability of the body to make a rapid, coordinated change in direction.

Building Blocks of Fitness:

PROPRIOCEPTION:  Proprioception is the sense of bodily movement and position in space.  The major receptors responsible for this sense are the muscle spindle stretch receptors that occur in skeletal muscles.  They respond to both the absolute magnitude of muscle stretch and the rate at which the stretch occurs.  Mechanoreceptors in the joint, tendons, ligaments, and skin also play a role in proprioception. 
            “Sense of effort” refers to an awareness of the amount of muscle contraction being exerted in a given situation.  For this awareness, we rely on information in pathways that descend to the motor neurons from the parts of the brain controlling motor behavior.  Collateral branches from these descending pathways synapse with parts of the brain involved in sensory function, which simultaneously leads to an awareness that one is exerting muscular effort.

FLEXIBILITY: Flexibility is the ability to move joints and use muscles through their full range of motion.  Flexibility can be gained through stretching, massage, and relaxation.  These all aid in preventing injuries, increasing range of motion, restoring the length of the muscles, allowing maximal contraction, and improving biomechanical efficiency.  Stretching can be passive (someone else loosening the tissue surrounding the joint and holding a static or non-moving stretch for a period of time), static (you stretch your own muscle to the point of tension and hold it), active (stretch the muscle by flexing the antagonistic muscle group), ballistic (bouncing or jerking movements—not necessarily the safest form of stretching, although effective for some individuals).
            Techniques such as massage and rolfing are effective ways to stretch the muscles, increase circulation and flexibility and sometimes relax.
            Proprioceptive neuromuscular facilitation (known as PNF) is a method of stretching similar to active stretching .  The nervous/muscle reaction activated by flexion are used to stretch a particular muscle further.   PNF is done with assistance of a partner or physical therapist.

Some popular “training” methods used to help build your fitness base by stimulating your proprioceptive sense, increase flexibility, and build strength (core strength in particular) include:
YOGA PILATES        TAI CHI         MARTIAL ARTS      BALLET         DANCE          CERTAIN TEAM SPORTS         PLYOMETRICS

Exercises used for rehabilitation and used by physical therapists incorporate balance, stretching, and strengthening drills.