Prelude
Humans can hold their bodies erect, vertically above the earth, because our bodies are solid objects capable of supporting our own weight. The human skeletal system is a collection of 206 bones, connected by soft tissues—cartilage, ligaments, tendons, and muscles—that together provide a mechanical support system for the human body.
Humans are also capable of movement. Muscles—connected to the solid bone framework—contract to generate forces that result in motion. Dancers, high jumpers, and surgeons learn to control these movements precisely to accomplish tasks and transport their bodies with precision (Figure 10.1). Strength, agility, and stamina can all be enhanced by training and, as a species, our understanding of the effects of training improves each year. As a result, humans continually improve performance on certain tasks, such as Olympic events (Figure 10.2).
This chapter describes some elements of the human body structure and mechanics. To aid in description, the chapter begins with some basic concepts about the mechanical properties of materials. These concepts will be used throughout the chapter to provide quantitative descriptions of the behavior of the biological materials that make up the human body.
Mechanical properties of materials
Biological materials, such as bones, muscles their cells, are mechanical objects and, therefore, are subject to forces that occur because of the world around them. The human body and its components can be studied by force and moment analysis, which is among the first concepts students in physics learn (Box 10.1). Physical forces are present throughout the body. Some forces are large: The head of the femur regularly experiences forces that are 2–3 times the weight of the whole body or 1, 800–2,700 N (for a 200-lb man). Much smaller forces have important biological effects: Muscles, which enable the macroscopic movements on body parts, typically generate forces of ~1 N. Nonmuscle cells—when attached to solid substrates (see Figure 5.5C)—can each generate forces of 100–800 nN (1).