Muscles are always attached at two ends by tendons - an origin and an insertion - which allow them to pull against bones and create movement. They work in pairs, with an agonist muscle contracting to cause movement and an antagonist relaxing to prevent injury. Inside, muscles are made of fascicles containing bundles of fibers called myofibrils. Myofibrils contain sarcomeres, the contractile units, which shorten when cross-bridges on myosin filaments pull actin filaments inward. This process occurs along the entire muscle fiber, causing contraction.

1. +
Understanding the muscle
Pairings, microscopic structure and function

2. +
Understanding the muscle

Muscles never work on their own, and always work in teams.

A muscle belly will have tendons at either end to attach them to
bone

A muscle is always attached to 2 or more bones (by tendons) to
allow a stable and strong surface to pull against. These
connections are called the origin and insertion.

3. +
Muscle connections to bones
Origin

Is the place where the muscle is
attached to the most stable surface
– which is usually a flat bone such
as the scapula or pelvic girdle.

This provides the muscle with a
strong surface to pull against
Insertion

Is at the other end of the muscle at
attached to the bone where the
movement occurs.

4. +
Origin and insertion example

Bicep

Its origin is at the scapula and
insertion is the radius

So when the bicep shortens, the
lower arm (including the radius)
will move towards its origin

5. +
Reciprocal inhibition

When muscles on one side of a
bone or joint relax to accommodate
contraction on the other side of the
bone or joint

Biceps and triceps

Quads and Hamstrings

Gastrocnemius and tibialis anterior

6. +
Agonist – Antagonist relationship

Muscles always work in pairs

The agonist is the muscle primarily responsible for producing a
movement

The antagonist is the muscle which relaxes while the movement
takes place to prevent injury

When a muscle contracts, it is critical for the muscle which
performs the opposite movement to relax to prevent an injury
occurring (muscle tear)

7. +
Stabilisers

Provide stability to the origin so maximal contraction force can
be applied.

EXAMPLE: During elbow flexion the trapezius contracts to
stabilise the scapula and provide a strong, rigid base for the
bicep to pull on.

8. +
Inside the Muscle Belly
Microscopic structure of a muscle

9. +
The structures

The muscle belly
consists of thousands
of muscle fibres known
as fascicles which run
side by side along the
length of the muscle

Each of these fibres is
encased in and
surrounded by
connective tissue
known as
perimysium, which
assists in keeping the
fascicles together.

11. +
Muscle Fibres

Each fascicle is made up of several
muscle fibres, which are made up of
even smaller fibres called
myofibrils, which are similar to the
many wires within a telephone
cable. These have many
units, known as sarcomeres, which
are arranged end to end for their
entire length of the myofibril

12. +
The Sarcomere

Is a contractile unit, and each end
is designated by a line called a Zline.

Each sarcomere consists of two
proteins myofilaments called actin
and myosin.

Actin is a thin filament which is
attached to the Z-line

Myosin is a thick filament which is
situated between each of the actin
filaments

13. The Lines and Zones of a Sarcomere
+

14. +
The Lines and Zones of a
Sarcomere

The Z-line: Marks the two ends of a sarcomere

The I-band: Where only actin is found

The A-band: Where both actin and myosin are found and
equates to the length of the myosin filaments

The H-zone: Where only myosin is found and is the gap
between the ends of the actin

15. +
A muscle contraction

The myosin filaments have cross bridges (oar-like structures)
that are attracted to the actin filaments

At rest, there is little contact between the actin and the myosin

However, when the sarcomere contracts, the cross bridges
attach to the actin filaments and pull them into the centre of
the sarcomere in a ‘rowing’ action

The cross bridges continue to detach and reattach themselves
from the actin filaments, shortening the sarcomere.

Every sarcomere along the muscle fibre shortens, leading the
whole muscle to contract.

The muscle will relax when the actin and myosin filaments lose
contact with each other that is, when the cross bridges detach
from the actin.

18. +
Muscle Tone

Not all the myosin filaments
detatch themselves from the acti.
Some may stay in contact, so the
muscle is never completely relaxed.

If this is the case, the muscle is said
to have ‘tone’

The advantage of muscle tone is
that the actin and myosin filaments
are always ready to contract