How skeletal muscles produce movement

Muscles cross at least one joint, called an articulation, and are attached to bones to cause movement. Muscles only pull when they contract; they do not push. Each muscle has an origin (stationary end) and an insertion (moving end), though this terminology is shifting to proximal and distal attachments for better description. Facial muscles are unique, attaching to a bone on one end and to the fascia of the skin on the other, allowing for facial expressions and eye movements.

During movement, bones act as levers and joints function as a fulcrum (fixed point). A lever is a rigid structure that moves around a fulcrum, and it's acted upon by effort (muscle contraction) and load (resistance). The video explains three classes of levers using real-life and anatomical examples. A third-class lever, like lifting a book with your forearm, has the effort between the fulcrum and the load (FEL).

A second-class lever, similar to a wheelbarrow, has the load between the effort and the fulcrum (ELF). An example in the body is standing on your tiptoes, where the weight of your body is the load, your toes are the fulcrum, and your calf muscles provide the effort. A first-class lever has the fulcrum between the effort and the load (EFL), such as raising your head, where the neck muscles are the effort, the joint between your atlas and occipital bone is the fulcrum, and the weight of your head is the load.

The second-class lever produces the most force because the load is closer to the fulcrum, sacrificing speed and range of motion for power. Muscles are arranged in bundles called fascicles, which can have different patterns (parallel, fusiform, triangular, bipinnate). While muscle fibers shorten to about 70% of their resting length, the power and range of motion depend on the fascicular arrangement. Longer muscle fibers offer a greater range of motion, but muscle power depends on the number of fascicles per cross-sectional area. Pinnae and bipinnate muscles, with many short fibers in a small area, provide great strength (e.g., thigh muscles), while parallel muscles with long fibers (e.g., sternohyoid) offer a greater range of motion but less power.

Muscles are arranged in opposing pairs at joints to create various movements like flexion, extension, adduction, and abduction. An agonist (or prime mover) is the muscle that causes the motion, while an antagonist causes movement in the opposite direction. For example, during elbow flexion, the biceps brachii is the agonist, and the triceps brachii is the antagonist. During arm extension, their roles reverse, with the triceps becoming the prime mover and the biceps becoming the antagonist. Synergists are muscles that assist the prime mover.