Muscle Contraction, Energy Pathways, and Exercise Adaptations
Muscle Contraction and Movement
The brain sends signals that instruct muscles to contract or relax, producing movement. The nervous system transmits these signals through the spinal cord and peripheral nerves to the muscle fibers. The neuromuscular junction is where the nerve meets the muscle, enabling nerve impulses to trigger muscle fiber contraction.
Muscle contraction can be:
- Isometric: The muscle develops tension but cannot shorten because it is fixed.
- Eccentric: Muscle tension is created during joint movement (e.g., stretching a muscle while resisting).
- Concentric: The muscle shortens during contraction (e.g., climbing stairs).
ATP: The Energy Source for Muscle Contraction
ATP (Adenosine Triphosphate) is a substance that fuels muscle contraction. However, muscles store only small amounts of ATP, providing power for just a few contractions and lasting only about 5 seconds.
Anaerobic Pathways for ATP Production
Anaerobic pathways produce ATP without oxygen, leading to an oxygen deficit.
Anaerobic Alactic Pathway
In this pathway, phosphocreatine, a substance stored in the muscle, is broken down. This process, called ATP transphosphorylation, allows for longer activity, but at a lower intensity than direct ATP degradation.
Anaerobic Lactic Pathway
This pathway uses glucose from carbohydrates. Muscles store glucose as glycogen. Anaerobic glycolysis yields 2 ATP molecules and lactic acid from each glucose molecule.
Aerobic Pathway for ATP Production
The aerobic pathway requires oxygen and is activated during continuous, less intense exercise, typically after 1 minute. The body relies on this pathway to generate energy based on the intensity of the exercise.
Temporary Adjustments to Exercise
Muscular System Adjustments
- Fast or White Fibers: These fibers contract rapidly over short periods.
- Slow or Red Fibers: These fibers have long endurance and can contract for extended periods.
- Mixed Fibers: Individuals have inherited differences in the proportions of these three muscle fiber types. Training does not significantly alter these proportions, even with specialized sports training. However, mixed fibers can adapt to become more like slow or fast fibers depending on the training. Some individuals are genetically predisposed to be marathon runners, while others are better suited for sprinting.
Respiratory System Adjustments
- Increased Maximum Breathing Capacity: Maximum breathing capacity can increase by up to 50%, providing a safety margin for athletes by allowing extra ventilation.
- Enhanced Oxygen Diffusion: Capillaries fill to maximum capacity, increasing the surface area for oxygen diffusion into the blood.
Cardiovascular System Adjustments
- Increased Muscle Blood Flow: Blood flow to muscles increases significantly during exercise, especially after muscle contraction.
- Increased Cardiac Output: Cardiac output is related to increased heart rate. An untrained person’s heart rate can increase more than four times their resting heart rate, while a well-trained athlete’s heart rate can increase up to six times.
Body Adaptations to Chronic Exercise
Muscular System Adaptations
- Muscle Hypertrophy: Strength increases by approximately 30% in the first 6-8 weeks of training and then stabilizes. Muscle volume also increases (muscle hypertrophy).
Respiratory System Adaptations
- Effects of Training on Maximum Oxygen Consumption: This measures an individual’s aerobic fitness and is defined as the maximum amount of oxygen consumed during exercise.
Cardiovascular System Adaptations
- Effect of Training on Cardiac Hypertrophy and Performance: The heart also undergoes hypertrophy during training, improving its performance.