Skeletal muscle power is nothing without lever action. The bone acts as a rigid bar, the joint is the fulcrum, and the muscle applies the force. Levers are divided into the weight arm, the area between the fulcrum and the weight; and the power arm, the area between the fulcrum and the force.
When the power arm is longer than the weight arm, less force is required to lift the weight, but range, or distance, and speed are sacrificed. When the weight arm is longer than the power arm, the range of action and speed increase, but power is sacrificed. Therefore, 90 degrees is the optimum angle for a muscle to attach to a bone and apply the greatest force.
Three classes of levers are at work in the body:
Class I, or seesaw: The fulcrum is located between the weight and the force being applied. An example is a nod of the head: The head-neck joint is the fulcrum, the head is the weight, and the muscles in the back of the neck apply the force.
Class II, or wheelbarrow: The weight is located between the fulcrum and the point at which the force is applied. An example is standing on your tiptoes: The fulcrum is the joint between the toes and the foot, the weight is the body, and the muscles in the back of the leg at the heel bone apply the force.
Class III, or removing a nail with a hammer: The force is located between the weight and the fulcrum. An example is flexing your arm and showing off your biceps: The elbow joint is the fulcrum, the weight is the lower arm and hand, and the biceps insertion on the lower arm applies the force.
The direction in which the muscle fibers run also plays a critical role in leverage. Here are the possible directions:
Longitudinal: Fibers run parallel to each other, or longitudinally, the length of the muscle. Example: sartorius.
Pennate: Fibers attach to the sides of the tendon, extending the length of the muscle. These come in subcategories:
Unipennate: Fibers attach to one side of the tendon; example: tibialis posterior
Bipennate: Fibers attach to two sides of the tendon; example: rectus femoris
Multipennate: Fibers attach to many sides of the tendon; example: deltoid
Radiate: Fibers converge from a broad area into a common point. Example: pectoralis major.
Sphincter: Fibers are arranged in a circle around an opening. Example: orbicularis oculi.
The three types of fasciae, which Gray’s Anatomy describes as “dissectable, fibrous connective tissues of the body,” are as follows:
Superficial fasciae: Found under the skin and consisting of two layers: an outer layer called the panniculus adiposus containing fat; and an inner layer made up of a thin, membranous, and highly elastic layer. Between the two layers are the superficial arteries, veins, nerves, and mammary glands.
Deep fasciae: Holds muscles or structures together or separates them into groups that function in unison. It’s a system of splitting, rejoining, and fusing membranes involving
An outer investing layer that’s found under the superficial fasciae covering a large part of the body
An internal investing layer that lines the inside of the body wall in the torso, or trunk, region
An intermediate investing layer that connects the outer investing layer and the internal investing layer
Subserous fasciae: Located between the internal investing layer of the deep fasciae and the peritoneum. It’s the subserous membrane that lines the abdominopelvic cavity, also known as the peritoneal cavity.
