Muscle Physiology: Contraction, Energy Systems and Training Adaptations

Muscle Physiology and Exercise Fundamentals

Insertion: the attachment point of a muscle to bone.
Muscle belly: the fleshy central part of a muscle.
Tendon: the tissue that connects muscle to bone; its tension leads to muscle contraction.

Types of Muscle Tissue

Skeletal muscle: promotes voluntary movement and constitutes the skeletal muscle system.
Cardiac muscle: the heart is the main part of the cardiac muscle and controls the circulation of blood.
Visceral muscle: internal organs and the parts of blood vessels; it controls involuntary movement.

Muscle Movement Terminology

• Agonist: the muscle that acts as the prime mover for a movement.
• Antagonist: the muscle that opposes a movement.
• Fixator: a muscle that stabilizes the origin of a prime mover so that it can act correctly, often across two joints.

Types of Muscle Contraction

Isometric contraction: the muscle length does not change, but tension develops in the muscle.
Isotonic contraction: the muscle develops constant tension while changing length. Eccentric isotonic contractions lengthen the muscle, while concentric isotonic contractions shorten it.

Energy Systems for Muscle Work

ATP is the molecule that transports and produces energy through its hydrolysis. Muscles store only small amounts of ATP, which provides energy for about 5 seconds.

Anaerobic Alactic System

ATP is regenerated from creatine phosphate (CP) stored in the muscle. The breakdown of CP sustains effort for about 20 seconds, and it is most useful when the intensity of the activity is high.

Anaerobic Lactic System

Through the breakdown of one molecule of glucose, the body obtains 2 molecules of ATP and lactic acid. This system provides energy for up to about 2 minutes.

Aerobic System

Glucose and fats are broken down in the presence of oxygen. The aerobic system becomes active from about 1 minute of exercise and can sustain exercise lasting 90 minutes or more, serving as the main source of energy for prolonged activity.

Muscle Fiber Types

Fast-twitch fibers: produce rapid, forceful contractions over short periods.
Slow-twitch fibers: produce endurance and sustained contractions during prolonged periods.
Mixed fibers: contain characteristics of both fast-twitch and slow-twitch fibers.

Cardiorespiratory Responses to Exercise

• Maximum respiratory capacity: increases by up to 50% of its resting level, providing an additional margin of safety through extra ventilation in case of need.
• Pulmonary diffusion capacity: in trained athletes it can rise to 3 times or more its normal resting value during rapid exercise.
• Muscle blood flow: blood flow to the muscles can reach up to 25 times resting values and increases further during exercise.
• Heart rate (beats per minute): in a normal resting state, it increases during exercise. In trained individuals, resting heart rate may drop up to 6 beats per minute.

Training Adaptations

Muscle hypertrophy: during 6 to 8 weeks of structured resistance training, muscle size can increase by 30%, which also stabilizes and increases muscular strength.

VO2 max: the maximum rate of oxygen consumption during exercise, which is a fundamental measure of aerobic fitness. It can increase by 30% with continuous training.

Cardiac hypertrophy and VO2 max: endurance training can increase the size of the heart and maximal oxygen uptake. A trained person can reach up to 40% of maximal VO2 capacity, and cardiac output can increase by 40%. Cardiac muscle hypertrophy occurs in endurance or long-duration training.

Nutrition Fundamentals

Individual dietary plan: a plan should include basic knowledge of nutrition, the characteristics of the person, total caloric value, weekly monitoring of the diet, definition of the objective (caloric deficit, caloric surplus or maintenance), elaboration of a weekly menu, and control and monitoring.

Classes of Nutrients

• Energy nutrients: support the maintenance of essential functions (fats, carbohydrates and proteins).
• Constructive nutrients: support growth, maintenance and renewal of tissues (proteins, minerals, water, fat and carbohydrates).
• Protective nutrients: support the body's defense mechanisms (vitamins, minerals, proteins and fats).