The Muscular System

The muscular system consists of various types of muscle that each play a crucial role in the function of the body. It consists of three types: Cardiac Muscle, Smooth Muscles and Skeletal Muscles.

Cardiac Muscle, also known as myocardium or The Heart Muscle is an involuntary muscle constantly contracting to provide fresh, oxygenated blood to the entire body.

Smooth Muscles are present within internal organs of the human body, such as stomach, liver, kidney, etc.

Skeletal Muscles are the muscles which are attached to bones of the human body and are responsible for movement. They are are of two types:
Type 1 – Slow Twitch muscle fibres (smaller in size, more in number, more mitochondria); used Predominantly in Endurance training.
Type 2 – Fast Twitch muscle fibres (larger in size, less mitochondria); used predominantly in Power / Anaerobic training.

Muscle Fibres or Myofibrils

The epimysium is the dense connective tissue that surrounds the entire muscle tissue. The epimysium usually contains many bundles (fascicles) of muscle fibres.
The perimysium is the connective tissue that surrounds each bundle of muscle fibres.
The endomysium is the connective tissue that covers each single muscle fibre or myofibril or muscle cell. Within each myofibrils are sarcomeres which contain the contractile proteins of muscles: Actin & Myosin. Actin and Myosin are responsible for the contraction of the muscle fibres, Actin is the thin filament and Myosin the thick filament.

Skeletal Muscles – Origin & Insertion
Most skeletal muscles have two ends, origin and insertion. Muscle origin at the proximal end of a muscle and is more stable. Muscle insert at the distal end and are more mobile, contraction of the distal end of a muscle is responsible for joint movements.

Tendons and Ligaments
Tendons are connective tissues responsible for attaching bones to muscles, however cartilage and ligaments attach bones to bones.

Types of Muscle Contractions

Isotonic Contractions
A contraction of a muscle which involves shortening and lengthening of a muscle, There are two isotonic contractions: Concentric Contractions and Eccentric Contractions

Concentric Contractions
A concentric contraction is a type of muscle contraction in which the muscles shorten while generating force, overcoming resistance and moving against gravity. For example, when lifting a heavy weight, a concentric contraction of the biceps would cause the arm to bend at the elbow, lifting the weight towards the shoulder. Cross-bridge cycling occurs, shortening the sarcomere, muscle fiber, and muscle.

Eccentric Contractions
An eccentric contraction results in the elongation of a muscle while the muscle is still generating force; in effect, resistance is greater than force generated. Eccentric contractions can be both voluntary and involuntary. For example, a voluntary eccentric contraction would be the controlled lowering of the heavyweight raised during the above concentric contraction. An involuntary eccentric contraction may occur when a weight is too great for a muscle to bear and so it is slowly lowered while under tension. Eccentric contractions occur when moving towards gravity.

Isometric Contractions
In contrast to isotonic contractions, isometric contractions generate force without changing the length of the muscle, common in the muscles of the hand and forearm responsible for grip. Using the above example, the muscle contraction required to grip but not move a heavy object prior to lifting would be isometric. Isometric contractions are frequently used to maintain posture.
Isometric contractions are sometimes described as yielding or overcoming.

Isometric contraction During a plank

Yielding
A yielding contraction occurs when a muscle contraction is opposed by resistance. For example, when holding a heavy weight steady, neither raising or lowering it.

Overcoming
An overcoming contraction occurs when a muscle contraction is opposed by an immovable object, such as the contraction generated in the muscles when pushing against a wall.

In both instances of Yielding and Overcoming, cross-bridge cycling is maintaining tension in the muscle; the sarcomere, muscle fibers, and muscle are not changing length.

Muscles as Movers
Agonist – Prime movers of a movement
Antagonist – Opposing muscles of the prime movers
Synergist – Supporting muscles of a joint movement