NERD ALERT: the following reading is filled with words not used in daily-common language and if read may result in droopy eyelids and drooling.
Basic Structure of a Muscle
A Closer Look at a Muscle Fiber
Inside the sarcolemma the on-looker will be able to be that the space with-in is filled with a gelatin like substance called the sarcoplasm, which is the fluid part of the muscle fiber. The sarcoplasm contains mainly dissolved proteins, minerals, glycogen, fats, and necessary organelles. Yet sarcoplasm differs from cytoplasm of most cells because it contains high levels of glycogen and myoglobin instead of hemoglobin. Within the sarcoplasm is housed an extensive network of transverse tubules which are extensions of the sarcolemma that pass laterally through the muscle fiber. These tubules are interconnected and pass among myofibrils allowing nerve impulses received by the sarcolemma to be transmitted to individual myofibrils. Another network of tubules is housed within the sarcoplasm called the sarcoplasmic reticulum. This system runs next to around the myofibrils. These tubules serve as storage site for calcium which plays essential role in muscle contraction.
While observing the sarcoplasm one with also notice that their are smaller subunits of bundled fibers within the muscle fiber, which are called myofibrils. These are the contractile elements of skeletal muscle. They appear as long strands of smaller subunits called sarcomere. The sarcomeres are the basic functional unit of a myofibril. Each myofibril is composed of numerous sarcomeres joined end to end. Each sarcomere is defined as everything from one Z disk to another Z disk which includes: An I band, A band, H zone, rest of A band, and second I band. Here is where we stumble upon the most important structures for muscle movement. Under a much more powerful microscope one can differentiate two types of small protein filaments in a myofibril. The thinner of the two filaments are actin and the thicker of the two are myosin. Each myosin molecule is composed of two protein strands twisted together and one end of each strand is folded creating a globular head referred to as the myosin head and each filament contains several of these heads which protrude from the whole filament to form cross bridges that can interact with other filaments. Each actin filament contains active sites that interact with the myosin filaments.
I know probably more information than you wanted and a little bit of an anatomy nerd overload, but you know can now understand what functions will what that creates movement and causes the muscle to shorten and contract.
How Muscle Creates Movement
And finally there you all have it the structure of a muscle and a muscle fiber and the components of these structures that work together to create movement. I apologize for the anatomical overkill, but I think it is imperative you understand what structures are being referenced in the sliding filament theory and how they integrate into the overall picture.