Muscles and Movement

Muscles Structure and Properties

Muscles are composed of two types of fibres: fast-twitch and slow-twitch.
Fast-twitch fibres contract quickly, producing short bursts of power. They utilise anaerobic respiration and fatigue more rapidly.
Slow-twitch fibres contract more slowly and are resistant to fatigue. They are more efficient at utilising aerobic respiration.
All muscles consist of muscle fibres, made of individual myofibrils.
Each myofibril is segmented into sarcomeres - the basic functional units of skeletal muscle.

Muscle contraction is caused by the sliding action of actin and myosin filaments within sarcomeres.
This is known as the sliding filament theory.
The process is driven by ATP and regulated by calcium ions and proteins tropomyosin and troponin.
ATP provides energy for the myosin head to change shape and pull the actin filament, leading to muscle contraction.

Muscle contraction requires ATP; however, muscles store only a small amount of ATP.
ATP is quickly regenerated from ADP through three metabolic pathways: aerobic respiration, anaerobic respiration (lactic acid fermentation), and creatine phosphate breakdown.

Muscles need to respire to produce ATP, this can be done through aerobic or anaerobic respiration.
Aerobic respiration is efficient, producing a large amount of ATP, but requires oxygen and glucose. It is used during slower, less powerful muscle activity.
Anaerobic respiration does not require oxygen and is faster, but much less ATP is produced. It also leads to the production of lactic acid, which can cause muscle fatigue. This form of respiration is used for quick, explosive muscle activity.

Muscles are attached to bones by tendons, and their contraction allows movement about a joint.
Muscles only pull and cannot push, so they usually come in antagonistic pairs. When one muscle contracts (the agonist), the other relaxes (the antagonist). This allows motion in different directions.
The type of joint determines the range and direction of movement possible. For example, hinge joints (like the elbow) only allow movement in one direction, while ball-and-socket joints (like the shoulder) allow rotation and movement in multiple directions.