1. Muscle Properties
Irritability - A muscle irritability refers to the ability of the muscle to respond to a stimulus.
Contractility - A muscle contractility refers to the muscle’s ability to shorten in length.
Elasticity - This refers to the muscles ability to stretch and return to normal length.
Extensibility - This refers to the muscle’s ability to extend in length.
Conductivity - This refers to a muscle’s ability to transmit nerve impulses.

3. Sarcomeres separated by narrow zones of dense material called Z lines
within a sarcomere is a dark area called the A band (thick myofilaments)
ends of the A band are darker because of overlapping thick and thin myofilaments
the light coloured area is called the I band (thin myofilaments)
the combination of alternating dark A bands and light I bands gives the muscle fibre its striated appearance

4. Muscle Contraction Muscle structure under a microscope Muscle fibres
skeletal muscle viewed under a microscope contains thousands of these elongated, cylindrical cells
Sarcolemma
the plasma membrane that covers each muscle fibre
Myofibrils
found within each skeletal muscle fibre
cylindrical structures which run longitudinally through the muscle fibre
consist of two smaller structures called myofilaments
Myofilaments
thin myofilaments and thick myofilaments
do not extend the entire length of a muscle fibre
they are arranged in compartments called sarcomeres

5. Myofilaments Thin myofilaments
thin myofilaments are anchored to the Z lines
composed mostly of the protein actin
actin is arranged in two single strands that entwine like a rope
each actin molecule contains a myosin- binding site
thin myofilaments contain two other protein molecules that help regulate muscle contraction (tropomyosin and troponin)
Thick myofilaments
composed mostly of the protein myosin which is shaped like a golf club
the heads of the golf clubs project outward
these projecting heads are called cross bridges and contain an actin- binding site and an ATP binding site

6. Sliding Filament Theory
during muscle contraction, thin myofilaments slide inward toward the centre of a sarcomere
sarcomere shortens, but the lengths of the thin and thick myofilaments do not change
myosin cross bridges of the thick myofilaments connect with portions of actin on thin myofilaments
myosin cross bridges move like the oars of a boat on the surface of the thin myofilaments
thin and thick myofilaments slide past one another
as thin myofilaments slide inward, the Z lines are drawn toward each other and the sarcomere is shortened
myofilament sliding and sarcomere shortening result in muscle contraction
this process can only occur in the presence of sufficient calcium (Ca++) ions and an adequate supply of energy (ATP)

8. Contractile Machinery: Sarcomeres
Contractile units
Organized in series ( attached end to end)
Two types of protein myofilaments:
- Actin: thin filament
- Myosin: thick filament
Each myosin is surrounded by six actin filaments
Projecting from each myosin are tiny contractile myosin bridges
Longitudinal section of myofibril
a) at rest

9. High microscope magnification of a single sarcomere within a single myofibril

10. Contractile Machinery: Crossbridge formation and movement
Cross bridge formation: a signal comes from the motor nerve activating the fibre the heads of the myosin filaments temporarily attach themselves to the actin filaments
Cross bridge movement:
- similar to the stroking of the oars and movement of rowing shell
- movement of myosin filaments in relation to actin filaments
- shortening of the sarcomere
- shortening of each sarcomere is additive
Longitudinal section of myofibril
b) contraction

11. Contractile Machinery: Optimal Crossbridge formation
Longitudinal section of myofibril
Sarcomeres should be optimal distance apart
For muscle contraction: optimal distance is ( mm)
At this distance an optimal number of cross bridges is formed
If the sarcomeres are stretched farther apart than optimal distance:
- fewer cross bridges can form  less force produced
If the sarcomeres are too close together:
- cross bridges interfere with one another as they form  less force produced
c) Powerful stretching
d) Powerful contraction

12. Contractile Machinery: Optimal muscle length and optimal joint angle
The distance between sarcomeres is dependent on the stretch of the muscle and the position of the joint
Maximal muscle force occurs at optimal muscle length (lo)
Maximal muscle force occurs at optimal joint angle
Optimal joint angle occurs at optimal muscle length

13. Muscle tension during elbow flexion at constant speed