Segments of the body move because a force causes them to move.  There are different types of forces that can cause a body part to move.  Gravitational force, muscle force, an object or another person can influence the movement of a body part.  Points to understand about force and gravity and its affect on muscular movement are listed below:

• Gravity always acts downward toward the center of the earth, thus whenever a body part is moving towards the floor, gravity usually is causing the motion.
• If  a body part is moving upward, away from the floor, muscles or some other force is causing the action.
• If gravity were the only force acting on our body, the body would crash to the floor.
• Fortunately, certain muscles (opposing muscles) resist the movement caused by the initial force.

Types of Muscular Contractions:

1. Concentric Contraction: causes the muscle to shorten in length as it develops tension.  This is a dynamic contraction.  Example: an arm curl requires the bicep to work against gravity and the muscle shortens during the contraction.  This would be considered the “positive” movement.
2. Eccentric Contraction: is the opposite of a concentric contraction by causing the muscle to lengthen as it develops tension.  Lowering movements or resisting movements cause the muscle to contract eccentrically.  This is a dynamic contraction.  Example: lowering your arm from a bicep curl causes the bicep to contract eccentrically and lengthen.  This would be considered the “negative” movement.
3. Isometric Contraction: during an isometric contraction, the muscle develops tension but does not change in length or produce any movement.  Often referred to as a static or tonic contraction. 

Roles of the Skeletal Muscle:

     Muscle groups work together in producing an exercise movement.  There are about 75 pairs of muscles involved in general posture and movement of the body.  Muscles act as Agonists, Antagonists and Stabilizers.

• Agonist: or “prime mover” is the muscle that contracts concentrically (away from the floor) to produce the movement.  Examples: the biceps are prime movers for flexing/bending the arm.  Pectoralis major and triceps are prime movers for a push up.
• Antagonist: is usually located opposite the prime mover (agonist).  The antagonist is a muscle where contraction produces the opposite movement to that of the primary mover.  Examples: the tricep is the antagonist of the bicep curl because it extends/straightens the arm.  The tricep (antagonist) must relax and lengthen as the bicep (agonist) contracts and shortens·        
• Stabilizer: is a muscle which supports, anchors and stabilizes a body part so that another active muscle may produce movement in other body parts.  Example: the abdominals serve as a stabilizer during a push up.  They prevent undesirable sagging of the hip and trunk region.  This enables the triceps and pectoralis major to produce the exercise movement.

The Structure of the Muscles

• What is muscle made of? 
• 75% water
• 20% protein
• 05% salts and other substances
  
•  How is muscle structured?
• Skeletal muscles are made up of thousands of muscle fibers.  Each fiber is composed of smaller functional units.  Women’s fibers are generally smaller than men’s fibers.
• Each fiber is composed of myofibrils.  Each myofibril is composed of myofilaments.
• Actin and myocin are the two main myofilaments that cause the muscle to contract.  These two myofilaments actually intertwine to form what is often referred to as “crossbridging” of the muscle.
• Wrapping of fibrous connective tissue surround the muscle fibers to form the muscle body.
• Toward the end of the muscle, the connective tissue continues so that it attaches the muscle to the bone.  This connective tissue is called the tendon.  Connective tissue is a very important element of strength and structural integrity to the muscular system.  Also, it is thought to be involved with delayed muscle soreness.

• Are there different types of muscle fibers?
  Distribution of muscle fiber type is determined by genetic make-up.  However, all types of muscle fibers are highly trainable which means they are capable of adapting to specific metabolic demands.
• Fast-Twitch Fibers:  These fibers are generally used in short-term, sprint activities that depend on anaerobic metabolism for their energy source.  They have a faster contraction time and are important in stop and go movements or movements which change direction quickly.  Weight training works more with these types of fibers, which are more capable of hypertrophy (increase in size of muscle cell).
• Slow-Twitch Fibers:  These fibers are generally used for endurance type of activities that depend on aerobic metabolism for energy.  Both types of muscle fibers will show improvement in aerobic ability, the slow-twitch fibers will be more responsive to aerobic training and show more improvement in aerobic capacity.
• Intermediate Fibers:  These have properties of both fiber types.
• Most people have about 50/50 of each fiber type.  Fiber type affects the performance of top athletes.  Endurance athletes have at least 80% of slow-twitch fibers whereas, body builders have a higher % of fast-twitch fibers.

• What are the effects of weight training on muscle fibers?
• Fast-twitch fibers enlarge as much as 45%.  This is called hypertrophy.  The actin and myocin filaments get larger and can store more glycogen for energy.  People with unusually large muscular builds very likely have a greater % of fast-twitch fibers.  Generally, women do not experience hypertrophy to the same extent that men do, because of the male hormone, testosterone.
• Slow-twitch fibers get more aerobically efficient, but do not grow in size.
• Current research is supporting the idea that muscle fibers increase in number as well as in size.

• Where does the energy come from to make a muscle contract?
  Muscles require a continuous supply of energy in order to function.  Ultimately, the food we eat supplies this energy. However, the food we eat is then broken down in to a chemical compound called adenosine triphosphate or ATP.
  
  Foods we eat are made up of carbohydrates, fats and proteins.  The process of digestion breaks down these nutrients into simple components (glucose, fatty acids, and amino acids) which are absorbed into the flood, then transported to metabolically active cells and muscles.  All energy systems require the breakdown of ATP.
  
  Extremely intense, powerful muscle contractions require a metabolic system that provides instant energy, therefore the rate of energy production must be extremely high. The short term energy system or ATP system lasts about 10 seconds.  The anaerobic energy system or glycogen system lasts about 3 to 5 minutes.  The aerobic energy system or oxygen system lasts for hours.

• Why do my muscles burn and shake?
  The muscles produce a bi-product called lactic acid which can cause a burning sensation and fatigue in the muscles.  Generally, the more intense the exercise, the more lactic acid the muscle may produce.  When the muscle is conditioned, it can tolerate a 20% to 30% higher level of lactic acid production.  Shakiness or lack of coordination in the muscle after working out is caused by a decrease in nerve conduction of the contractile mechanisms and to the lack of oxygen to that particular muscle.

• Common causes of injuries in weighted workouts:
• Repetition speed to fast/loss of control
• Too much weight
• Overuse
• Hyperextension
• Muscular imbalance
• High impact activity with weights